104
Klaus Stimper
The physical fundamentals of
LOW-VOLTAGE INSULATION CO-ORDINATION
Content
1 Introduction ................................................................................. 5
1 . 1 Terms ............................................................................................................... 6
1.2 Aims of stress-adequate insulation co-ordination ....................................... 7
2 Parameters for the dimensioning of insulations .......................... 9
2.1 Long-term voltage stresses .................................................................... 9
2.2 Short-term voltage stresses ..................................................................... 10
2.2.1 Overvoltages in the distributing system ......................................................... 10
2.2.2 Overvoltages in low-voltage systems ............................................................ 13
2.3 Environmental conditions ......................................................................... 16
2.3.1 Humidity ........................................................................................................ 20
2.3.2 Air pressure ................................................................................................... 28
2.3.3 Corrosive components in air ......................................................................... 28
2.3.4 Dust depositions .......................................................................................... -30
2.3.5 Influence of the electrodes ............................................................................ 34
2.4 Insulation material .................................................................................... 35
2.4.1 Tracking resistance ...................................................................................... -36
2.4.2 Surface roughness ........................................................................................ 38
2.4.3 Water adsorption ........................................................................................... 40
2.5 Electrode shaping .................................................................................... 42
3 Voltage strength of the clearances ............................................ 43
3.1 Strength at steady-state voltage stress ..................................................... 44
3.1.1 Breakdown under homogeneous field conditions ......................................... 44
3.1.2 Breakdown and partial discharges under inhomogeneous field conditions ... 46
3.1.3 Influence of the frequency ............................................................................. 51
3.2 Strength under short-term voltage stress .................................................. 52
3.2.1 Breakdown under homogeneous field conditions ........................................ -54
3.2.2 Breakdown under inhomogeneous field conditions ...................................... -55
4 Voltage strength of the creepage distances .............................. 57
4.1 Strength under steady-state voltage stress ................................................ 57
4.1 . 1 Flashover ..................................................................................................... -58
4.1.2 Partial discharges ......................................................................................... 58
4.1.3 Tracking ....................................................................................................... -60
4.1.4 Minimum insulation resistance ...................................................................... 65
4.2 Strength under short-term voltage stress ................................................... 75
4.2.1 Flashover ..................................................................................................... -75
4.2.2 Influence of ribs and grooves ........................................................................ 79
5 Physically founded dimensioning rules ..................................... 82
5.1 Parameters for the dimensioning ............................................................... 82
5.1.1 Long-term voltage stresses ........................................................................... 82
5.1.2 Overvoltage categories ................................................................................. 83
5.1.3 Micro-environmental categories .................................................................... 84
5.1.4 Insulation material categories ........................................................................ 87
5.1 -5 Electrode shaping ......................................................................................... 89
5.2 Dimensioning of the clearances ................................................................. 89
5.2.1 Dimensioning for steady-state voltage stresses ........................................... -89
5.2.2 Dimensioning for short-term voltage stresses .............................................. -92
5.3 Dimensioning of the creepage distances ................................................. 94
5.3.1 Dimensioning for steady-state voltage stresses ............................................ 94
5.3.2 Dimensioning for short-term voltage stresses ............................................... 96
5.4 Co-ordination of the insulation distances ................................................... 98
6 References ............................................................................... 100
7 Index ........................................................................................ 102
1 Introduction
Appropriate dimensioning of the insulations of electrical equipment must provide sufficient electrical strength and the required minimum insulation resistance during the entire service life. In the best possible case dimensioning rules have to quantify minimum distances for a given failure risk. Little experience has been gathered as to such rules so far. The publications IEC 66411980 and IEC 664Al1981 [ l ] for low-voltage insulation co-ordination determine insulation distances which have proved suitable during years of application. It is not evident however, which safety margins are included in these standards and in how far they differ from the minimum distances required. The determination of the smallest-possible insulation distances assumes a thorough knowledge of the mechanisms leading to insulation failure as well as their dependency on e.g. electrode spacing and voltage. This has been examined in several scientific projects during the last years [g, 10, 11, 12, 13, 151. The results of these investigations are completed by the evaluation of field trials on insulations stressed in a way similar to the service conditions [14]. This more recent knowledge has already been taken into account in the fundamentals of insulation co-ordination dealt with in this book. It gradually is introduced into new dimensioning rules by way of the revisions of publication IEC 664 [ l ] . Besides the physically founded requirements, however, particular considerations of standardization which are not taken into account in this book will inevitably also be included into the revised standards.
The investigations described in this book were carried out without exception approximately at sea-level (MSL). Accordingly all data mentioned in the following and all data which can be the basis for the standards revisions refer to this level. The values for dimensioning rules derived from this though are usually converted to 2000 m in the standard, assuming without closer investigation that Paschen's Law is valid for all types of gaseous discharges, including the inhomogeneous field as well as boundary layers.
1 .l Terms
The insulations considered in this book consist of at least two conductors, the electrodes, on an insulation material. The clearance is the shortest distance in air between the two conductive parts. It must be long enough to avoid a dielectric breakdown under the permitted operating conditions. In most cases also partial- discharges in air have to be avoided, i. e. discharges in the inhomogeneous field which bridge only part of the clearance. The creepage distance, the shortest distance along the surface of the insulating material between the two conductive parts, has to be designed in a way that no flashover occurs in the surface boundary layer and a specific minimum insulation resistance is maintained. As the solid insulation shows much greater voltage strength than clearances and creepage distances under the same stress, its dimensions can usually be left out of consideration in insulation co-ordination, if it is guaranteed that no partial- discharges in the insulating material will occur, i. e. in this case discharges in inner voids or gas inclusions.
The nominal voltage of low-voltage equipment is limited to AC 1000 V and DC 1500 V [l]. However, in the internal circuits even significantly higher voltages may occur. These voltages, decisive for dimensioning the insulations, have to be divided into long-term and short-term stresses. The working voltage is the highest r.m.s value of alternating or direct voltage which may occur (locally) across the insulation in question at rated supply voltage, transients being disregarded, in open circuit conditions or under normal operating conditions [l]. The according strength of the insulation is the r.m.s. withstand voltage, which is the highest r.m.s. value of a voltage which does not cause breakdown of insulation under specified conditions [l]. Short-term stresses are mainly caused by transient overvoltages due to switching operations, fault or lightning discharges. These transients are usually of short duration of a few milliseconds or less, oscillatory or non-oscillatory, usually highly damped [l]. The according strength of the insulation is the impulse withstand voltage, which is the highest peak value of impulse voltage, of prescribed form and polarity, which does not cause breakdown of insulation under specified conditions [l]. Functional overvoltages are deliberately imposed overvoltages necessary for the function of ,
a device [l]. It is always necessary to distinguish between nominal voltages and rated voltages. Besides the withstand voltage only the rated voltage is relevant for the judgement of insulation performance as described in this book. The rated voltage is the value of voltage, assigned by the manufacturer to a component, device or equipment, to which operation and performance characteristics are referred [l]. The nominal voltage only serves to classify and characterize components, devices or equipment.
The immediate ambient conditions surrounding the insulation have great influence on dimensioning. They are divided into the micro-environmental categories, which, in addition to the climatic conditions, include all solid, fluid or gaseous materials whose influence on the insulation is to be expected. The conditions in the surroundings are described in the macro-environmental categories.
The currents flowing through the surface layers on the insulations are called creepage currents. Under unfavourable conditions they can cause tracking, which is the local decomposition of the insulating material. This decomposition may lead to material erosion or to the formation of conductive carbonized residues, so-called tracks. The resistivity of an insulating material against the development of tracks is called tracking resistance.
1.2 Aims of stress-adequate insulation co-ordination
Insulation co-ordination implies the selection of the electric insulation characteristics of equipment with regard to its application and in relation to its surroundings [l]. Considerations must be given to the voltages which can appear within the system, to the location and characteristics of the overvoltage protective devices, to the continuity of service desired and to the safety of persons and property, so that the probability of undesired incidents due to voltage stresses is reduced to ensure an economically an operationally acceptable performance [l].
Insulation co-ordination requires that solid or liquid insulation materials involved have an impulse withstand voltage value in excess of the the breakdown value of either the voltage limiting device or the actual clearance on which the co- ordination is based [l].
Characteristic for a stress-adequate insulation co-ordination is the direct adjustment of the insulation strength to the occurring stresses by appropriate dimensioning. The stresses are mainly caused by the short-term and long-term voltages applied to the insulation. The environmental conditions of the insulation have great influence on them. The stresses can therefore be decreased by protective measures which either reduce the occurring voltages or the influential environmental conditions. The overvoltage protective devices may be special equipment including means for the storage or dissipation of energy or simple air gaps (clearances), under defined conditions, capable of dissipating without harm the energy of the overvoltages expected at the relevant location [l]. A reduction of pollution may be provided by effective use of enclosures, encapsulations or hermetic sealing. The insulation strength depends at first on the insulating
material. When the latter has been chosen, the strength can only be influenced by the dimensioning and configuration of the clearances and creepage distances.
The procedure of a stress-adequate insulation co-ordination has to be organized accordingly. After the determination of the parameters according to chapter 2 the clearances and creepage distances have to be adjusted independently to the different stresses according to chapter 3 and 4. Chapter 5 summarizes the physically founded dimensioning rules deducted from the investigations in the preceding chapters. It includes the co-ordination of the previously determined clearances and creepage distances.
2 Parameters for the dimensioning of insulations
In order to obtain a stress-adequate co-ordination of insulations first of all the parameters determining the insulation characteristics have to be judged. These parameters result from the intended service conditions and the given design requirements. Important for insulation co-ordination are long-term and short-term voltages, the environmental conditions, the insulating material and the configuration of the electric field determined by the electrodes. Only the optimization of these parameters can secure that the following dimensioning of the insulation distances leads to the smallest possible distances.
2.1 Long-term voltage stresses
The long-term voltage stress on insulations by the working voltage is the only unavoidable electrical stress. Temporary overvoltages which can occur e.g. due to defects and disturbances in the mains or the operation of the supply system, also belong to the long-term voltage stresses.
For the dimensioning rules a difference between direct voltage and alternating voltage is only relevant in so far as direct voltage may significantly influence interelectrode surface layers. Compared to alternating voltage a considerably increased amount of ions is drawn from the pollution. This leads to the often observed improvement of insulation resistance in comparison with 50 Hz- alternating voltage. It is unknown however, how this "cleaning effect" depends on the frequency.
All gas discharges being relevant for dimensioning depend on the peak value of the applied voltage. There is no indication that in this case the r.m.s. value of the voltage applied also has influence on the insulation characteristics in addition to the peak value. Therefore dimensioning rules for alternating voltage stress have to be based on the amplitude. This is relevant to a wide frequency range up to about 1 kHz. There are single investigations on the effect of higher frequencies, general dimensioning rules however cannot be derived so far. Particularly problematical is the valuation of short overvoltage impulses which sporadically or periodically overlap a lower, e.g. sinusoidal voltage. It is unknown by now, to which extent these repetitive overvoltages belong to the short-term voltage stresses, respectively at which frequency they need to be considered as long-term voltage stresses.
2.2 Short-term voltage stresses
Short-term voltage stresses are caused by transient overvoltages which are superimposed on the working voltage. According to the place of origin a distinction is made between external overvoltages which came into the low- voltage installation from the distributing system and internal overvoltages which are produced inside the low-voltage installation.
2.2.1 Overvoltages in the distributing system
Overvoltages in the distributing network can be caused by - direct strike of lightning into an overhead line, - rearwards flashover, - induction resulting from a strike of lightning near an overhead line, - influenced charges, - transmission of overvoltages from the primary distributing network via
transformers, - switching operations. They are limited by flashovers at the conductor insulation and by protective equipment against overvoltages, e.g. surge arresters or spark gaps [6].
The highest overvoltages in low-voltage networks are caused by a direct lightning stroke into the conductors of the overhead lines. In this case voltages in the megavolt range can easily occur. The lightning stroke into the mast of a low- voltage overhead line can produce a rearwards flashover to the conductors, if the voltage caused by the lightning current at the impulse impedance of both, mast and grounding, exceeds the dielectric strength of the insulation. Direct lightning strokes into the lines though are comparatively rare. Induced lightning overvoltages due to lightning strokes near an overhead line or influenced lightning overvoltages due to charges set free after a cloud to cloud lightning are much more probable. In comparison with the direct lightning stroke significantly lower overvoltages with an average between 10 and 50 kV are obtained. Usually the front comes to between about 1 and 10 ps; particularly steep impulses down to a 0.1 ps front or particularly slow impulses up to a 100 ps front are observed more rarely. The time of one-half crest value on the wave tail usually comes to between 10 and 50 ps. However, from time to time values up to over 200 ps are obtained.
As experience has shown overvoltages from the medium voltage network can be transmitted with a maximum of 40 % on low-voltage overhead lines and a maximum of 5 % on low-voltage cables via the coil capacities of the transformers.
In rare cases dangerous overvoltages due to lightning strokes into the vicinity of buried cables can also be induced in low-voltage cables by the currents flowing off nearby in the ground .
All overvoltages mentioned comply with the laws of traveling waves in their propagation in the network. They are reflected and refracted, if the impedance changes along the line. An increasing of the voltage up to two times the incoming wave can occur at the end of open circuits due to the total reflection. When changing from the higher wave resistance of an overhead line to the lower wave resistance of a connected system an overvoltage wave is reduced considerably at least for a short time. The traveling waves produced by reflection and refraction are always overlapping further and can e.g. be determined by graphical methods.
The produced overvoltages are at first limited by flashovers to the grounded anchoring material of the pylons and masts as well as to the neutral conductor. The resulting traveling surge is further reduced by the surge arrestors being installed along the overhead line. These surge arrestors are typically designed for a protection level of 2 kV at an extinction voltage of approximately 300 V in German 2301400 V networks.
A rough estimate of the overvoltages, which are to be expected at the origin of systems supplied by 2301400 V networks, has to be based on this overvoltage protection level of 2 kV. In addition, the up to 1 kV voltage drop at the earthing resistance of the arrestor must be taken into account. The resulting 3 kV traveling surge could be at worst doubled by reflections. This leads to expected transient overvoltages of roughly estimated 6 kV at the supply mains of systems energized by 2301400 V overhead transmission lines.
The estimate is in agreement with a literature study [l21 of more than 50 publications on transient overvoltages. It concluded that lightning-caused overvoltages measured in European 230/400 V systems hardly exceeded 6 kV. Only very few publications dealt with nominal voltages different from 2301400 V. The study can therefore give no evidence for the philosophy stated in the first edition of publication IEC 664 [l], which expects increasing overvoltages with increasing nominal voltage. Only for the 60 V-telephone systems a lower overvoltage level has been verified. However, this presumably has to be traced back less to the lower working voltage than to the lower height of the overhead line.
The temporal voltage development of lightning overvoltages is mostly reproduced by a standardized impulse as shown in figure 2.2.1 with a front ts = 1.2 ps and a
time of one-half crest value on the wave tail tr = 50 ps. These characteristic values of the impulse are implied by the designation 1.2150 ps. The temporal voltage development is described by 181
For the reproduction of switching overvoltages longer impulses like 10/700 ps or 20012000 ps are also used.
crest
wave tail
Figure 2.2.1. Standardized test impulse with the wave front ts and the one-half crest time tr
2.2.2 Overvoltages in low-voltage systems
The term "low-voltage system" includes the fixed installation and the equipment connected with it. With respect to the propagation of overvoltages the most unfavourable case in question is the unloaded installation under open circuit conditions where the incoming overvoltages are not attenuated by connected equipment. For good reasons the habit of switching on all lamps during a thunderstorm is established at some places.
First of all the insulation between phase and neutral is stressed by the external overvoltages coming from the mains. Overvoltages in the low-voltage system can also be produced by lightning strokes into the direct stroke protection. The lightning-stroke current flowing off causes a voltage at the impulse grounding resistance of the direct stroke protection which is transmitted into the installation via the ground bar. The voltages which are induced in metal loops by the lightning stroke current have to be added.
The mentioned external overvoltages are superimposed by the so-called internal overvoltages. They are mainly non-periodical switching overvoltages, originated by switching off currents with inductive share. To an increasing extent also functional overvoltages are occurring. They can overlap the working voltage periodically e.g. in switching power supplies.
The theory of traveling waves is principally valid for the spreading of overvoltages in low-voltage installations as well as in the distributing network. Every change of the wave resistance along the installation causes voltage alterations due to refraction and reflection. However, it has to be considered that all of theses impulse alterations only stay unchanged until the reflections of these alterations running to either side come back. In installations which are not loaded with their wave resistance this traveling time until the response returns comes to a few microseconds at most.
The wave conductor properties of the lines are generally only important when those are long in comparison with the spatial dimensions of the shortest flanks of the considered overvoltage impulses. Otherwise the installation can essentially be described as lumped capacitance which is charged via a wave conductor by a more or less distant overvoltage source. For the commonly used PVC-lines the propagation velocity in the installation is between 150 and 190 m/ps. Presuming a voltage wave of the type 1.2/50 ps the wave front extends over 120 to 160 m. This makes clear that only on very unfavourable conditions a doubling of the
overvoltage impulse due to the reflections at open ends has to be taken into account.
Figure 2.2.2. Voltage U, at the installation terminal and voltage U2 at the end of each installation branch, if a rectangular voltage pulse U enters the installation according to figure 2.2.3 at the moment t = 0 through a infinitely long cable. t2 is the propagation time of a traveling surge along one of the installation branches.
Usually the installation does not consist of only one line, as assumed for the consideration of the worst case, but of several branches which are joined in distribution busses. In these cases, the distribution of the traveling surges to the different branches has to be taken into account. The consequence is always that the voltage rise at the end of the line and at the mains becomes less steep. The voltage maximum determined for single lines is obtained with a delay if at all. Figure 2.2.2 shows this influence for a rectangular voltage pulse coming in from a long cable in dependence of the number of branches. Figure 2.2.3 shows the according schematic diagram. It can be seen that the distribution of the traveling surge into several branches increases the time constant of the voltage rise at the end. Therefore it becomes less likely that the originating overvoltage is long enough present at the installation terminal to really reach the superimposed
voltage maximum by the reflections at end of the installation branches. Moreover the waves are attenuated by the connected equipment. In addition there is the attenuation due to line losses which is, however, only of importance for very steep impulses though.
Figure 2.2.3. Schematic diagram of an installation with mains of the length I1 and installation branches of the length l2 = 2 l1 each. The mains are energized through a infinitely long cable. The wave impedance of both, the cable as well as the installation is assumed to be 50 Ohms.
In most cases the conduits of lines are of no important influence on the attenuation of overvoltage waves. Consequently there hardly is a difference for the spreading of overvoltage impulses if the lines are installed - without conduit - with metal, non-magnetic conduit or - with metal, magnetic conduit. In all cases a significant attenuation along the line only occurs when the impulses are very steep [16].
Measurements of overvoltages in low-voltage installations including the switching processes in motors determined in most cases peak values between 1.0 and 2.2 kV [12]. The greatest stresses with overvoltages up to over 3 kV were produced by circuit-opening fuses of low nominal current. The overvoltages occurring thereby show in addition very long one-half crest values of up to 3 ms. Higher peak voltages than approximately 2 kV were observed only for shorter impulses than 0.3 ms though. The highest voltages were measured when the short circuit occurred near the fuse. Significantly lower stresses occurred when the locality of the short circuit was situated several meters away, as it is mostly the case. Fuses with a higher nominal current are generally less critical, since the stronger fusible parts interrupt the current more slowly when melting. The energy content of the impulses produced by circuit-opening fuses is (in contrast to other switching impulses) such high that the peak value of the overvoltages is not influenced by the input current of the equipment.
2.3 Environmental conditions
Environmental conditions diminishing the insulation qualities have to be taken into account as an important parameter for the dimensioning of insulations. For this purpose it is unavoidable to determine the physically very different relevant influences of the immediate environment on the insulations comprehensively. This can be done in a simple way, most convenient for usage, by allocating the actual situation to one of few micro-environmental categories describing the conditions of the immediate environment.
First of all, surface layers from the environment are to be considered, which are deposited on the insulation surfaces during operation, e.g. by free sedimentation or by electrostatic attraction. The components of these contaminants can be classified by their conduction qualities as non-conductors, electronic-conductors and ionic-conductors. To a small extent these agents may be produced by corrosive gaseous components in the air. The most important contributors to surface deterioration are the ionic conductors which derive from salts, acids, and bases [19]. In the dry condition at ambient temperatures these materails are virtually non-conductors. However, in the presence of water they dissociate into positively charged cations and negatively charged anions. The water around the ions allows them to become mobile and follow the applied field. Because the electrolyte-water-solution may spread all over the insulation surface, this ionic conductivity is not limited to the location of the originating electrolyte deposition.
Electronic-conducting particles are usually metal or carbon but may also be semi- conductors. They transport current by free electrons which move from molecule to molecule. At the usual temperatures the particles are aggregates of many molecules. They are mostly insoluble in water and thus remain stationary on the insulator surface unless acted upon with sufficient external force. Electronic- conductors contribute to the surface deterioration by distorting the electric field, thereby causing a region of higher than normal voltage gradient [19]. They are also preferred footpoints of surface gas discharges.
The non-conductors can be either soluble or insoluble in water. They contribute to the surface deterioration only indirectly by providing the nucleus for condensation [19]. This not only increases absorption of moisture but also reduces its removal.
Besides the contaminations caused by the environmental conditions, also surface layers caused by the manufacturing process as well as surface layers caused by insulation ageing must be taken into consideration. The manufacturing of printed wiring boards uses for example caustics and soldering fluids. If residues of these mostly electrolyte containing agents remain on the insulation surface, the insulation resistance especially under the influence of humidity may significantly be reduced. Also components of the insulating material which reached the surface by diffusion may contribute to deterioration [17, 181. Such manufacturing or insulation specific contaminants cannot be covered by general dimensioning rules. The judgement on their influence depending on the individual case must therefore remain in the responsibility of the designing engineer.
The two gases air and water vapor contribute to the total pressure through their respective partial pressures which are functions of the volumetric content of the mixture. The sum of the two partial pressures equals the ambient pressure. Furthermore the partial pressure of the water vapor cannot be larger than the saturation vapor pressure. This saturation vapor pressure decreases with decreasing ambient temperature. Because of these physical relations moisture can be deposited on insulation surfaces by condensation from the water vapor available in the ambient air. This process is generally characterized by a temperature decrease below the dew point of the moist air. At temperatures below the dew point the partial pressure of the water vapor is temporarily larger than the saturation vapor pressure which leads to condensation of the excessive water
[l 91.
A temperature decrease below the dew point can be originated by putting cold equipment into a warmer environment, e.g. a heated room (with more or less moist air). In this case water vapor from the immediate surrounding of the
equipment condensates as long as the equipment temperature is below the dew point. In the other case condensation may occur if the equipment and the environment are at the same temperature at the beginning but the equipment cools down faster than the surrounding moist air. This is possible, e.g. in the case of heat transmission by radiation. A well known example is condensation to leaves and grass as well to outdoor equipment in the early hours of a summer day with clear sky.
Table 2.3.1. Relative humidities above the surface of saturated salt-water-solutions at room temperature 1131
characteristic re1 ative humidi ti es
[% relative humidity]
(approx. 100) 8 5
In addition to insulation acquiring moisture by the fact that its temperature is below the dew point, certain pollutants which may be on the surface hygroscopically remove water from the air. Many salts are hygroscopical which is characterized by the fact that the partial pressure of the water vapor above the surface of their salt- water-solution is lower than the partial pressure of the water vapor above the surface of pure water. These 'characteristic relative humidities' are for some saturated salt-water-solutions listed in table 2.3.1. They are to be compared with the relative humidity above pure water, which is 100%. If the relative humidity in the environment of a salt polluted surface is higher than its characteristic value according to table 2.3.1 the salt crystals absorb water and become more and more liquid. The originated salt-water-solution is diluted as long as its partial pressure of the water vapor equals the partial pressure of the water vapor in the surrounding air [19]. If the relative humidity of the environment is lower than the characteristic values according to table 2.3.1, the salt pollution totally dries out.
Mixtures of different salts are only dry, if the relative humidity of the environment is lower than the 'characteristic relative humidity' of the most soluble salt.
Comparable to the water acquisition by hygroscopical substances is the condensation into the pores up to 100 nm of the insulation surfaces. In this case the tension of the water surface reduces the partial pressure. Hygroscopical components of the insulating material may support the water acquisition in addition [g, 13, 251.
The so-called "industrial atmosphere" contains corrosive gases, which may decrease insulations in addition to dust depositions. Most important are the gases sulphurdioxide SO2, sulphurhydrogen H2S and nitrogen oxides NOx [ l 31. They all are relatively good soluble in water and form acids which corrode many metals and metal oxides. In moist air these gases support water condensation.
The combined stress by humidity, dust and corrosive atmospheres has the strongest influence on the insulation performance. The insulation decrease is significantly higher as it was to be expected if the influences of the different stresses were only added together. As an example, the insulation decrease caused by corrosive atmospheres can be relatively low as long as there are no additional influences such as salty dust or manufacturing residual matter. The conductor material of printed wiring boards also influences the effect of corrosive atmospheres: printed wiring boards with pure copper conductors showed lower insulation decrease than boards with copper conductors having solder on the surface but not on the edges. This performance is probably caused by local chemical elements [ l 31.
The practical experience showed that it is sufficient to establish four environmental categories for the purpose of insulation design. Two of these categories are already defined and are comparably easy to describe: - The category 1 describes the situation of clean insulations which are not
significantly deteriorated by environmental conditions during the entire service life.
- The category 4 describes insulations under outdoor conditions which were only investigated in high voltage applications by now.
This reduces the task to allocating the usual environmental conditions to the two categories 2 and 3 and defining suitable parameters to distinguish these categories. However, the numerical data necessary for these definitions cannot be easily gained by e.g. linearly dividing the range of the relevant environmental parameters. The definition has to be based on the different effects on the insulation which are described in the following.
Most of the cited investigations are based on data which were gained during a large field exposure of electrical insulations in co-operation between the USA and Germany [14]. Recordings of the climatic conditions at three test sites and municipal measurements of corrosive gases in the air at twelve test sites together with a chemical analysis of the specimen's surface layers provided detailed information on the environmental conditions at the test sites. The influence of these conditions on the insulation performance could especially be judged by evaluating the measurements of the specimen's insulation resistances. The test specimens in the shape of printed wiring boards without components produced no heat themselves. Therefore, it can be assumed that the micro-environment at the test specimens corresponded to the macro-environment which was defined by the above-mentioned measurements. However, in practical applications the micro-environment may be significantly different from the macro-environment. For example causes operational heat within the enclosure of electrical equipment a reduction of the relative humidity at a given absolute humidity. This improvement is intentionally used in many cases.
2.3.1 Humidity
Figure 2.3.1 shows a plot of the temperature and the relative humidity which were measured every 30 minutes in an outdoor container. It illustrates the anti-cyclic 24- hours period of temperature and humidity being typical for the investigated test sites. In many cases the absolute humidity remains constant for several days. The periodical changes of the relative humidity are therefore mostly caused by the daily temperature changes due to sunlight radiation [13].
The humidity influences nearly all of the mechanisms concerning the insulation co- ordination. Most significant is its influence on the insulation resistance which is described in the following. Further it is dealt with the influence of humidity on the flashover-, breakdown- and partial discharge onset voltage, which is essentially lower.
Figure 2.3.1 shows that the relative humidity comparably often varies within a wide range. Since the 'characteristic relative humidity' of many salts according to table 2.3.1 is between 50 and 80%, it is to be expected that insulation surface layers are wetted and dried out fairly often. The effect of humidity changes on the surface resistance of polluted insulations can for example be shown in a climatic chamber whose humidity is increased and later decreased step by step. The investigations should preferably be carried out with several insulations and the measurements put together in a geometric mean. If these results are entered into a linear
conductivity/humidity diagram the curve will show a steep increase of the conductivity at about 75% relative humidity. Figure 2.3.2 shows a plot of the results in a io~arithmic conductivity/humidity diagram in order to provide a better overview. For an example specimens of type 802 polyester resin moulding material are chosen, which were without voltage stress exposed in a container besides a busy road during 12 months. The extent and the rate of the humidity changes corresponded to the situation in nature: at the temperature 23 OC the relative humidity was beginning from 50% relative humidity increased by 5% relative humidity every hour until 95% relative humidity were reached. Afterwards it was reduced to 50% relative humidity in the same steps. Figure 2.3.2 only shows the performance of 0.16 mm and 6.3 mm creepage distances; the measured curves of the other distances are inbetween. The branch of the curve for humidity decrease is above of the branch for humidity increase because the surface layer needs a certain time to set the acquired moisture free [l 31.
Figure 2.3.1. Typical plot of the relative f humidity and the temperature 6 in an outdoor container of the field trial "creepage distances". It significantly shows the anti-cyclic 24-hours period [13]. Note: The time intervals do not start at midnight
Investigations on the specimens of the German-American field trial "creepage distances" [l41 showed that the pollution conductivity increases for about four powers of ten if the humidity increases from 50 to 95% relative humidity as usual in nature [13]. This can be used to define the threshold between the environmental categories 2 and 3 for the environmental parameter "humidity': it is the relative humidity at which the surface conductivity is increased for two powers of ten compared to the conductivity at 50% relative humidity. At this threshold the surface conductivity is also two powers of ten below the value at 95% relative humidity and therefore reaches 1% of it's considered maximum [13]. Table 2.3.2 shows the thresholds evaluated with this definition for the three investigated insulating materials after 12 months of exposure at a busy road and in the Black Forest for the creepage distances 0.1 6 mm and 6.3 mm. As a result of table 2.3.2 the humidity threshold 75% is recommended for defining the difference between environmental category 2 and 3 with some safety margin [13].
Table 2.3.2. Relative humidity reducing the insulation resistance for two powers of ten depending on insulation material, creepage distance and pollution after 12 months of exposure [l 31.
pol lut ion
clean
vented cabinet a t busy road
vented cabinet a t B1 ack Forest
percent r e l a t i v e humidity a t t he material S
g1 as f i bre epoxy f ab r i c
and a t the creepage di s tances
polyester res in moulding
and a t the creepage dis tances
phenol i c r e s i n moulding
and a t the creepage dis tances
Figure 2.3.2. Dependency of the conductivity G of a polluted insulation on the relative humidity f during a step by step increase and later decrease of the surrounding humidity [13]. - insulation material: type 802 polyester resin moulding material - creepage distances: 0.1 6 mm and 6.3 mm - pollution: dust deposition during 12 months of exposure in a
container close to a busy road - humidity changes: 5% relative humidity per hour
The long-term averaged humidity itself is not significant for defining environmental categories [13]. For the three investigated test sites the long-term averaged humidities are between 60 and 70% relative humidity which is below the above- defined humidity threshold. However, the insulations were significantly deteriorated which is definitely not acceptable for the environmental categories 1 or 2. It is obviously important, how frequent and how long high humidities occur 1131. The humidity recordings at the test sites showed that for several times the relative humidity was considerably high during hours. This explains the observed significant insulation deteriorations even at a lower long-term averaged humidity
[W
The humidity also influences the flashover voltage of clean and polluted insulation surfaces. However, the dependency is by far not as significant as the dependency of the insulation resistance. The relevant parameter is again the relative, not the absolute humidity because the moisture acquired by an insulation surface only depends on the relative humidity. Figure 2.3.3 and figure 2.3.4 show as an example for specimens of the German-American field trial "creepage distances" [l41 the dependency of the flashover voltage on the humidity. The measurements were used to determine the dielectric strength after 15 years of service life by means of statistical regressions and extrapolations. Since the influence of humidity depends on the water adsorption of the insulation surface, figure 2.3.3 shows the results at epoxy specimens and figure 2.3.4 at polyester specimens as examples for materials with high and low water adsorption.
Polluted epoxy surfaces show for increasing humidity at small and medium spacings up to about 1 mm in nearly all of the cases a larger decrease of the flashover voltage than clean surfaces. This decrease is significant even at the light indoor pollutions [12]. At polyester surfaces the dependency of the flashover voltage on the humidity is even under the presence of pollution considerably lower than at epoxy specimens. This can be explained by the reduced water adsorption of polyester resin moulding material which causes reduced water acquisition of the surface [12]. Under microscopic view the particles of the surface layer cover mostly only parts of the insulation surface. Therefore the influence of the water adsorption remains significant even under the presence of pollution.
clean power plant busy road Black Forest
Figure 2.3.3. Influence of the humidity f on the 15-years flashover voltage U of clean and polluted test specimens without voltage stress for the example of glassfibre reinforced epoxy fabric, which has a high water adsorption. The curve of the flashover voltage was extrapolated to 15 years of exposure by means of statistical regression [12]. Note: The measurements at 50 and 83% relative humidity were carried out at 23 'C, the measurements at 92% relative humidity at 40 'C. However, this difference has no significant influence on the measuring results [12].
clean power plant busy road Black Forest
Figure 2.3.4. As figure 2.3.3, but for the example of type 802 polyester resin moulding material which has a low water adsorption.
Figure 2.3.5 shows for spacings between 1 and 10 mm that also the partial discharge onset voltage of clearances is slightly decreased with increasing
humidity. This might be caused by e.g. oxide surface layers which increase the water adsorption at the electrodes [12].
The breakdown voltage of clearances is in opposite to the above-mentioned mechanisms slightly increased with increasing absolute humidity. This is mainly caused by the fact that charge carriers are lost by accumulation to H20- molecules. This depends on the number of H20-molecules and therefore on the absolute humidity. Figure 2.3.6 shows this influence at a nearly homogeneous plate-plate-arrangement under direct or alternating voltage stress. At 20 OC the change of the relative humidity from 50% to 100% is equivalent to a change of the absolute humidity from 9 to 17.5 g/m3. Usually 11 g/m3 is used as a standardized value [8].
Figure 2.3.5. Partial discharge onset voltage U of a needle--plate+ -arrangement in air with a needle radius of 20 pm under direct voltage stress depending on the relative humidity f at different spacings [l 21.
Figure 2.3.6. Dependency of the direct or alternating breakdown voltage U of a plate-plate- arrangement on the absolute humidity a at different spacings 1121.
2.3.2 Air pressure
The process of gas discharges depends on the field strength to the same extent as on the air pressure. A 10% reduced air pressure therefore requires a 10% increased electrode spacing to obtain the same dielectric strength under homogeneous field conditions. At sea level (MSL) the air pressure usually varies between 985 and 1040 hPa which results in a 3% variation of the breakdown voltage. The differences become more significant if measurements at MSL are compared to measurements at locations with high elevation. The dimensioning rules of the first edition of publication IEC 664 [l] take this into consideration by the requirement that insulations are to be designed for an elevation of 2000 m above MSL. The measurements on which all of the IEC 664 specifications are based were converted from the laboratory elevation to 2000 m MSL by use of the Paschen equation (see chapter 3.1 .l). This is strictly correct only for homogeneous field conditions. Regardless of the rated elevation the dependency on the air pressure causes a problem for the impulse withstand voltage test: a testing laboratory at low elevation will always gain better test results than a testing laboratory at high elevation.
2.3.3 Corrosive components in air
As a part of the German-American field trial "creepage distances" [l41 the municipal measuring data of corrosive components of the atmospheric air at locations close to the test sites were evaluated. Table 2.3.3 lists the mean and peak values of the measured concentrations of different corrosive gases. As to be expected the test sites 'Westerland' on Sylt and 'Schauinsland' in the Black Forest which were far from industry showed the lowest content of corrosive components.
The data in table 2.3.3 can be judged in comparing them with concentrations which were used in laboratory corrosion test procedures and whose effect on the insulation performance is known by that. This leads to the conclusion that the concentrations of corrosive components in the surrounding air are relatively low from the viewpoint of insulation co-ordination. For example even the sulphurdioxide peak values are for orders of magnitude below the steady-state stress of 10 ppm = 26.7 mg/m3 which caused in laboratory experiments a measurable insulation deterioration only after several weeks [13]. The measured concentrations of the observed components stay in general below the limits of the class 3C2 of IEC publication 721-3 [2], which are as an extract listed in table 2.3.4. Only the peak values of nitrogen oxide measured in Munich and Frankfurt
exceed these limits. The class 3C2 can therefore at least for the observed components be used for defining the usual environmental conditions.
Table 2.3.3. Mean and peak concentrations of corrosive air components in pg/m3. The data were gained during the field trial [l41 from November 1981 until December 1983. The number before the slash V) is the mean, the number after the slash is the peak value 1131.
concent ra t ion i n micrograms pe r m 3
Munich E f f n e r p l a t z K a r l sp l a t z
Mannheim B1 ack Forest Leverkusen Wester1 and/Syl t Ludwigshafen F r a n k f u r t
Cl-,F-,03 ( t o t a l )
Table 2.3.4. Some limits for corrosive components in air according to class 3C2 of IEC publication 721 121
component
so2 "402 c h l o r i n e ch l o r i ne hydrogen ozone
l i m i t s i n micrograms pe r m 3
average va l ue peak value
Since the long-term insulation deterioration is caused by relatively slow- progressing chemical reactions the mean concentration values are probably more significant than the peak values. If the progression of the deteriorating chemical
processes is proportional to the concentration of the corroding gases it can be assumed that the measured relatively low concentrations have no considerable influence on the insulation performance. This is confirmed by the fact that the data of corrosive gases evaluated during the field trial "creepage distances" [l41 showed no correlation to the chemical analysis of the pollution deposition on the test specimens. The regarded insulation deterioration during the test is obviously mainly caused by the deposited dust [13].
2.3.4 Dust depositions
The chemical analysis of the specimen pollutions during the field trial "creepage distances" [l41 came to the result that mainly unsoluble substances are deposited. The pollution contains only a small portion of soluble salts which are dissolved into ions under the presence of humidity and provide by that the electrical conductivity of the surface layer. The deposited dust totals between 0.1 7 g per m* and month in a manufacturing hall and 1.7 g per m2 and month besides a busy road [13]. The salt content amounts mostly between about 1 and 6% per weight [13]. Exceptions are the test sites Sylt with about 16% per weight and a subway tunnel with about 0.6% per weight [l 31.
Since the reduction of the insulation resistance is mainly caused by the electrolytical components of the surface layers the classification of the environmental conditions must also be based on a determination of the deposited salt and its effect on the insulation properties. For this purpose the pollution analysis of the field trial "creepage distances" [l41 was used to calculate the deposited mass of highly soluble salts related to the deposition area for the different test sites. Table 2.3.5 shows the results. The according influence of pollution on the insulation resistance can be determined by means of the measuring results at specimens exposed without voltage stress. The investigation concentrates on the organic insulation materials epoxy, polyester and phenolic, which are most important in practical use. Figure 2.3.7 shows for the example of 6.3 mm creepage distances at polyester insulations the averaged insulation resistances under the climate 23 OC / 83% relative humidity depending on the salt deposition. It is not regarded at which location and during which time period the pollution was originated. It can be seen that even small salt depositions may drastically reduce the insulation resistance in moist climate. However, the statistical correlation coefficient between the pollution conductivity and the salt deposition is too low to allow a sufficiently ensured regression [13]. Figure 2.3.7 provides the additional information that under strong pollution further dust
deposition have only reduced influence. This phenomenon was already earlier regarded at polluted insulators for high voltage applications.
Table 2.3.5. Salt depositions calculated by use of the chemical pollution analysis of test specimens from different locations and with different time of exposure in the field trial of the research association "creepage distances" [14]. Besides the different dust depositions at the different locations the investigations show also significant differences between the exposure in vented enclosures and in vented containers.
l o c a t i o n
Mannheim, power p1 a n t
Miinchen, subway tunnel busy road
B1 ack F o r e s t , Schaui nsl and
Leverkusen, chemical p1 a n t
Ludwigshafen, shower room
Frankfu r t , product ion ha1 l
Hamburg, i n d u s t r i a l a r e a
S y l t , c o a s t a l a e r ea
enc losu re
c a b i n e t c o n t a i n e r
c a b i n e t c a b i n e t c o n t a i n e r
c o n t a i n e r
c a b i n e t
c a b i n e t
c a b i n e t
c a b i n e t
c a b i n e t
t ime o f exposure
i n months
measured d u s t
d e p o s i t i o n i n g/m2
ca l cu l a t ed s a l t
d e p o s i t i o n i n g/m2
0.473 0.204 0.369
0.128 0.387 0.246
0.057
0.163 0.280
0.071
0.021 0.044 0.050
0.255 0.824
0.515
Figure 2.3.7. Insulation resistance R of polluted specimens exposed without voltage during the field trial of the research association "creepage distances" [l41 depending on the salt deposition m. Each point refers to one of the locations and exposition durations listed in table 2.3.5. The lines and arrows explain the evaluation of the salt deposition threshold [l 31. - insulation material: type 802 polyester resin moulding material - creepage distance: 6.3 mm - measuring climate: 23 'C / 83% relative humidity
For the purpose of classifying the environmental influences it is suggested to evaluate the threshold for the parameter salt deposition equivalent to the threshold of the parameter humidity according to chapter 2.3.1. This threshold is determined by the reduction of the insulation resistance for two powers of ten compared to the clean condition. In order to consider the worst case, at first the points with the lowest insulation resistances were connected by a polygon. In the second step the insulation resistance reduced by two powers of ten was determined, which leads to the required salt deposition. The procedure is explained in figure 2.3.7 by the lines and arrows. In Table 2.3.6 shows the gained
limits of the salt depositions at the investigated insulation materials were listed depending on the climate during the resistance measurement.
Table 2.3.6 Limits of the salt deposition in g/m2 which reduce the insulation resistance for two powers of ten [l 31.
creepage dis tance
l i m i t s in g/m2 a t t he insula t ion material
phenol i c r e s in moulding
i m
g1 asf i bre epoxy f ab r i c
in the neasurin c l imate
23/50 23/83 l
The large variation of the determined values makes it difficult to deduct a generally valid threshold of the parameter "salt deposition". It is surely impossible to average the gained salt depositions of the different insulating materials or use other mathematical methods. Most safe was the minimum of 0.035 g/m2. However, it is obvious that this limit is too low, because such light pollution was exceeded during the field trial "creepage distances" [l41 even at the most clean location already after 12 months. It must also be noted that these evaluated small salt depositions at epoxy are mostly caused by the very high insulation resistance of the clean material. It therefore seems to be adequate to define for the parameter "salt deposition" the threshold between the environmental categories 2 and 3 at about 0.1 g/m2.
polyester r e s in moulding
i m measurin c l imate
23/50 23/83 l
However, the effect of the accumulated salt and dust deposition on the insulation performance is surprisingly significant depending on the history of the climatic conditions. The table 2.3.5 shows that during the field trial "creepage distances" [l41 the pollutions in the manufacturing hall were not significantly different from those at the Black Forest. However, the averaged insulation resistance after 12 months of exposure in the manufacturing hall is significantly higher and the number of total insulation breakdown is significantly lower compared to the
specimens which were exposed for 12 months in the Black Forest. At the latter location the insulation performance is comparable to the test sites near a power plant and near a busy road. As explanation is to be assumed that in the Black Forest large climatic variations with relative humidity up to 97% (probably even up to condensation) occurred, whereas at the manufacturing hall 75% relative humidity were never exceeded. This results in the conclusion that the influence of the humidity on the insulation performance significantly exceeds the influence of the salt deposition. It is supported by the fact that the salt deposition gradually increases to its maximum influence at the end of the service life whereas the humidity is effective from the first day of service.
2.3.5 Influence of the electrodes
Corrosive components of the surrounding air have no considerable direct influence on the insulation resistance. However, these components may as well as the surface layer itself corrode the electrodes and influence the insulation performance by the originated corrosion products which spread over the insulation surface.
Table 2.3.7 shows that the various conductor materials are different sensible against atmospheres containing sulphur, hydrogen and sulphur dioxide. An extremely bad combination is obviously the stress of copper electrodes by sulphur hydrogen. Solder shows the overall best performance.
Also at surface layers containing chloride chemical reactions with the electrodes were observed. By x-ray quantum analysis e.g. corrosion products of copper electrodes were detected on the insulation surfaces 1211. The investigation of the relevant mechanisms resulted, that the metal is corroded electrochemically which first leads to a surface contamination by Cu20. The microscopically small gas discharges (scintillations) as a consequence of the creepage currents through the surface layers may ignite this Cu20 and further oxidizes it to CuO. This locally sets free energy which is added to the thermal energy release of the scintillations and accelerates tracking [21]. The calculated maximum temperature increase caused by the oxidation processes is approximately 1200 K [21]. Of course this calculation neglects heat dissipation to the electrolyte, heat dissipation to the surface of the insulation, heat dissipation by radiation and so forth. However, there can be no doubt that the release of heat caused by this reaction may stress the surface of organic insulation materials far above their thermal stability.
Table 2.3.7. Time to failure of A120g-insulations with 0.1 mm creepage distance depending on the electrode material and the corrosive atmospheres. A failure was defined as a reduction of the insulation resistance from 10" Ohms at the beginning below lo7 Ohms.
t es t ing atmosphere
1 cm3 H2S / m 3 a t 23 'C / 75 % r e l . humidity
moist (non-corrosive) climate 23 'C / 75 % re1 . humidity
electrode materi a1
copper brass solder
copper brass solder
copper brass solder
time t o f a i lu re in days
Experiments with another water unsoluble oxidable surface layer, pulverized ferrous sulphide, showed that also in this case the thermal stress by scintillations was significantly increased by additional exothermal reactions [21]. The Cu20 is therefore no single case.
2.4 Insulation material
In most cases the selection of materials for low-voltage insulations will be mainly determined by economical aspects and by the mechanical properties, the processability and the flame resistance. Only in some cases, especially at electronic circuits, additional requirements will be put on the surface or insulation resistance which are generally sufficient at modern materials. With respect to the insulation co-ordination the performance of the insulation material under the influence of creepage currents is important, as well as the influence of the surface on the flashover voltage. The design of solid insulations must additionally take the withstand capability against partial discharges into account.
2.4.1 Tracking resistance
Moist and polluted insulations with applied voltage can be thermally stressed to a high extent by the Joulean heat of the creepage current [19], by electrolysis processes [l31 and by microscopic gas discharges, so-called scintillation. Except at extreme pollution only the scintillation is of importance [19]. The organic insulation material, which is mainly used for low-voltage applications can be damaged by these thermal processes. This thermal decomposition will result either in erosion or in carbonized tracks. The latter will eventually result in a carbonized path between both electrodes and cause a short-circuit. The withstand capability of insulation materials against the thermal stresses under the presence of creepage currents is called 'tracking resistance'.
Tests intended to determine the tracking resistance must accelerated apply the operationally occurring surface stresses to the insulation material and judge its withstand capability. The test results depend on the procedure and can therefore be only used to compare different insulation materials among each other or to evaluate a material ranking (X better than Y better than Z ...). Precise and absolute test results can only be gained in a procedure which simulates the essential parameters precisely according to the processes of polluting and moisturing under (natural) environmental conditions.
The International Electrical Commission defined the procedure IEC 112 [3] for determining the tracking resistance of low-voltage insulations. In this procedure the investigated materials are stressed with surface scintillation of increasing intensity and it is determined to which extent the material withstands without tracking.
The procedure uses two platinum electrodes in the distance of 4 mm on the insulation surface. Every thirty seconds a drop of NH4CI-solution is applied onto the inter-electrode surface of the insulation. As a measure for the tracking resistance the maximum voltage is determined at which the specimen withstands fifty drops during five subsequent tests. The definition of failure is that the surface current exceeds 0.5 A for more than two seconds or that the material burns. The fifty-drop-voltage is called 'comparative tracking index' and is recorded with the unit "CTI" and not "V". This syntax is intended to avoid the misunderstanding that the procedure qualifies materials for the use at certain voltages. Table 2.4.1 shows the tracking resistance of some usual electrical insulation materials. It should be noted that the fifty-drop-voltages may be significantly reduced if the material contains agents to improve the flame resistance, colours or fillers. Pure
polybutylenterephthalat has for example the CTI 600 and can be only by including glass fibers reduced to CTI 300.
Table 2.4.1. Fiidrop-voltages of different insulating materials according to the test procedure
materi a1
IEC 112
50-drop-v01 tage -
ceramics A1 *03, 97% FR 4 g1 ass f i ber reinforced epoxy f ab r i c type 802 polyester res in moul di ng materi a1 type 31.5 phenolic res in moulding material polyimide f i lm FR 2 phenolic res in impregnated paper GPO I11 polyester res in laminate Type 150 me1 ami ne res in moulding materi a1 Polybutene terephthal a t e Polycarbonate
The problem of the procedure IEC 112 which is used since more than twenty years is the considerable variation of the results [22]. It is caused by the fact that scintillation occurs only in nearly dry surface layers which requires that all of the exceeding water of the dropped on aqueous solution must be evaporated before the intended thermal stress is originated. The boiling accidentally distributes the drying-out pollution over the surface before the scintillation starts. As an improvement the procedure IEC 11 2 requires the additional evaluation of the 100- drop-voltage which has to be noted in parenthesis besides the CTI if it is more than 25 V below the 50-drop-voltage.
The test procedure IEC 112 is very time consuming. A complete test which consists of at least ten single tests requires at minimum six to seven hours. Testing laboratories with high demand on CTI tests therefore often use three or five test equipments in parallel.
Many scientific investigations were concerned with the improvement of the IEC 11 2 test procedure. However, the publications give evidence that improvements are hardly possible with little modifications. Apparently the testing experience in the past led already to an optimum in the test specifications. The variation of the test results can be reduced as far as possible by carefully cleaning and preparing the electrodes before each test. Additional improvements only seem to be
possible if the thermal stresses caused by the creepage currents can be simulated closer to the reality in a totally different procedure.
The test of insulation materials for some specified applications might be closer to the service conditions if the influence of copper corrosion products according to chapter 2.3.5 is considered by use of copper electrodes. In this case the diagram of the drops to failure versus voltage is significantly changed 1221. Besides a comparable small transition area the drops to failure are either very low or reach the maximum number of drops limited by the test procedure. In consequence the 50-drop-voltage is determined with considerably reduced variation. Besides this the the required testing time is considerably shorter. Another advantage are the low costs of the copper electrodes compared to platinum. For each test new electrodes can be used which avoids the time consuming cleaning and conditioning.
However, in opposite to the use of platinum electrodes the insulation material is not only stressed by heat radiation caused by scintillation, but also by the initiated exothermal chemical reactions according to chapter 2.3.5. The mechanism of the insulation damage is changed and more difficult to define. Results which are gained with copper electrodes are in consequence not directly comparable with results from the standardized test with platinum electrodes. However, the experiences indicate so far that the ranking of the insulation materials remains unchanged [22].
2.4.2 Surface roughness
The roughness of the insulation surfaces influences on principle the insulation resistance by dust collection and the flashover voltage by disturbances of the electrical field. Figure 2.4.1 shows the averaged roughness RZ according to DIN 4768 [4] for usual insulation materials.
However, the roughness according to figure 2.4.1 has no considerable influence on the insulation performance. A reduction of the flashover voltage was only observed, when the roughness exceeded approximately 30 pm [l l].
material
Figure 2.4.1. Surface roughness RZ according to DIN 4768 [4] as an average of three specimens of usual insulation materials. A: ceramics AI2O3, 97% (unglazed) B: FR 4 glass fiber reinforced epoxy fabric C: type 802 polyester resin mould. mat. D: type 31.5 phenolic resin mould. mat. E: polyimide film laminated to B F: FR 2 phenolic resin impregnated paper G: GPO Ill polyester resin laminate H: type 150 melamine resin mould. mat. I: polybutene terephthalate K: polycarbonate N: CU 96 laminated paper
2.4.3 Water adsorption
The reduction of the flashover voltage of polluted and unpolluted insulations with increasing humidity depends at small creepage distances up to about 1 mm on the water adsorption of the surface [12]. Figure 2.3.3 shows as an example the flashover voltage of creepage distances on epoxy which has a comparably high water adsorption. In comparison shows figure 2.3.4 the performance of creepage distances on polyester which has a comparably low water adsorption. The averaged flashover voltage of polyester specimens is even at high humidity nearly unchanged or even slightly increased. Only at very small spacings below about 0.5 mm a slight decrease of the dielectric strength can be noted. Totally different is the performance of epoxy specimens. They show at creepage distances up to about 1 mm a considerable reduction of the flashover voltage when the humidity is increased. At larger spacings the influence of the water adsorption is negligible for all investigated materials.
Figure 2.4.2. Critical humidity fk depending on the creepage distance S for different insulation materials which are allocated to four adsorption groups [ I l l . The investigated insulation materials and the allocations to the groups are listed in table 2.4.2.
The water adsorption can be quantified by use of the 'critical humidity' fk. It indicates the relative humidity at which the flashover voltage of clean material is reduced for 5% [l l]. However, the critical humidity depends on the creepage distance. This requires that a complete comparison of different insulation materials must include tests at the whole variety of spacings.
Figure 2.4.2 shows the 'critical humidities' of the insulation materials used in the German-American field trial "creepage distances" 1141 depending on the electrode spacing. It was suggested to allocate the different materials to the four adsorption groups according to table 2.4.2 [12]. However, the differences between the investigated insulation materials are not very significant. The classification is therefore not very precise. The ranking of the materials was found to be unchanged even under the presence of pollution [l 21.
Table 2.4.2. The allocation of the insulation materials of figure 2.4.2 to the adsorption groups one (low) to four (very high) according to the "critical humidity" [12].
I material
C: type 802 polyes ter res in moulding material H: type 150 melamine res in moulding material
D: type 31.5 phenolic res in moulding material K: polycarbonate
E: polyimide f i lm F: FR 2 phenolic r e s i n impregnated paper G: GPO I11 polyes ter res in laminate I: polybutene t e reph tha la te
A: ceramics A1 03, 97% B: FR 4 g lass p i b e r reinforced epoxy f a b r i c
adsorption group
2.5 Electrode shaping
The shape of the electrodes determines the degree of uniformity of the electric field and, therefore, the breakdown, flashover and partial-discharge voltages. In the homogeneous field, e.g. between two large plates, the field strength along the entire insulating distance is constant. In the inhomogeneous field, however, the field strength is location-dependent. Compared to all other electrode arrangements with the same radius of rounding of the point, the point-plate arrangement exhibits the most inhomogeneous field and hence the lowest electric strength. It therefore serves as a basis in the following consideration of the worst case, based on a needle-plate arrangement with a needle rounding radius of 25 to 30 pm. Smaller radii can hardly be expected with today's usual deburring methods for punched parts.
3 Voltage strength of the clearances
Clearances have to be sufficiently long in order to avoid dielectric breakdown at the voltages which are to be expected. The long-term applied working voltages are to be taken into consideration as well as the short-term occurring transient overvoltages. In many cases appropriate clearance design must also avoid partial discharges in air. This chapter only deals with the performance of clearances all around in air. The following chapter 4 describes the physical performance of clearances in the boundary layer along the insulation surface as well as the creepage distances.
Mainly by cosmic and radioactive radiation about 50 molecules per cm3 and second are ionized in air under standard conditions. The life-term of these ions until they are recombined amounts to about 17 seconds on the average. This leads to a balance of about 1000 ions and electrons per cm3 [7]. Electrons can also be expelled from the metal surface of the electrodes. The field emission which is supported by the microscopical roughness of the electrode surface needs at usual electrode materials a minimum field strength of 1 o7 to 10* volts per cm [l 21. To impulse tests it might be advantageous to provide reliably the necessary first electrons. For this purpose the cathode might be eradiated by a quartz lamp or by the light of an arc.
The first electrons with the charge e move in the electric field of the strength E towards the anode and gain along the distance X the energy
If the energy of an electron is sufficient, another electron can be set free during a collision with a molecule. The necessary ionization energy in air is Wi = 15 eV. The sequence of many ionizations like this might lead to electron avalanches which finally form a conductive plasma between the electrodes and introduces the dielectric breakdown. A very fast avalanche growth is combined with a high number of charges in the avalanche head. This leads to an increase of the field between the avalanche and the cathode as well as between the avalanche and the anode. The avalanche head emits photons which first originate new avalanches around the anode and then form a conductive channel towards the anode. If this channel comes close enough to the anode, a streamer towards the cathode is formed also by photoionization. The following avalanches contribute more and
more to the highly conductive plasma channel and lead to the final dielectric breakdown.
The dielectric strength depends especially in the homogeneous field on the condition of the electrode surface, on the temperature and on the humidity. Polluted electrode surfaces disrupt the homogeneous field by a local field peaks. Thin oxide layers influence the surface ionization [28]. Water molecules in air reduce the movability of the air ions and the amplification of avalanches by attracting the electrons to the water dipoles. Also the origination of electrons by photoionization is reduced with increasing vapor content.
3.1 Strength at steady-state voltage stress
The design of clearances must ensure that the long-term applied voltages during operation cause no dielectric breakdown. It has to be taken into consideration whether the field between the electrodes is homogeneous or inhomogeneous. Also the frequency of the applied voltage is of importance. In most cases not only the breakdown but also partial discharges must be avoided which might occur under inhomogeneous field conditions at spacings above about 1 mm. The gases (e.g. ozone and nitrogen oxides) originated by partial discharges may be aggressive to the adjacent insulating material as well as the electrodes and may contribute to a reduction of the long-term voltage strength.
3.1.1 Breakdown under homogeneous field conditions
The breakdown voltage of clearances under homogeneous field conditions at steady-state voltage stress is described by the Paschen equation
This equation shows that the breakdown voltage at a given surrounding gas and a given electrode material is a function only of the product of the gas pressure and the distance between the electrodes. Figure 3.1.1 pictures the according Paschen curve in air at the pressure of main sea level, which is proven in numerous measurements. The Paschen curve can be used as a basis for dimensioning clearances under homogeneous field conditions at steady-state
voltage stress. A collection of the standardized and in several investigations measured Paschen curves in air is given by Dakin in [24].
The equation (3.1.1) shows a minimum for UD at a small product of pressure p and clearance S. In air of standard conditions this so-called 'Paschen minimum' is given at a spacing of about 7 pm with UD = 327 V. Thus the reduction of spacing reduces the breakdown voltage only to a certain extent. Below the Paschen minimum the reduction of spacing increases the strength because the short distance allows less and less electrons to gain the necessary ionization energy.
Partial discharges do not occur in the homogeneous field because the discharge conditions are identically everywhere between the electrodes either given or not.
Figure 3.1 . l . Paschen curve in air at 1013 hPa: Dependency of the breakdown voltage U on the spacing s under homogeneous field conditions at MSL [24].
Changes of temperature and air pressure cause a change of the air density. Its influence on the breakdown voltage in comparison with the standard conditions To = 293 K and p. = 1013 hPa can be approximated by
At electrode distances below about 10 mm also the influence of their spacing is to take into consideration for the transformation. In this case the equation
can be used which is deducted from the Paschen equation (3.1.1) [l l]. By means of the equations 3.1.2 and 3.1.3 it is possible to transform the breakdown voltages which are measured at any temperature and any air pressure to the equivalent breakdown voltage at standard conditions.
3.1.2 Breakdown and partial discharges under inhomogeneous field conditions
The calculation of the breakdown voltage in an inhomogeneous field is only possible if ideal conditions are assumed. The real performance especially of small distances cannot be calculated. Dimensioning rules have to be based on laboratory experiments.
With positive polarity at the needle the breakdown voltages of figure 3.1.2 are for small spacings between 0.01 and 0.2 mm up to 20% higher than the Paschen curve. The positive space charges in front of the needle seem to reduce the spacing. This apparently moves the performance of the configuration into the increasing branch of the Paschen curve at very small distances according to figure 3.1.1. Only at larger spacings the increase of the field at the cathode by positive space charges in front of the needle causes better ionization conditions. Approximately at 0.2 mm the voltage strength is equivalent to the Paschen curve. At spacings around 1.5 mm the withstand voltages of both polarities, positive and negative, are nearly the same. Larger spacings show a reduced voltage strength of the positive needle compared to the negative polarity. Fine plasma channels are formed directed to the cathode which quickly bridge the remaining gas area [23]. At larger spacings between 2 and 5 mm the steepness of the measured curve is significantly increased. The range of the fine plasma channels is now limited by
recombination processes or due to increased negative space charges in the avalanche head [l 21.
Figure 3.1.2. Breakdown voltage U of clearances of a needle-plate-arrangement under direct voltage stress depending on the spacing S and the voltage polarity [l 21 needle: radius 25 to 30 p m plate: squared steel, 1 m2
At a negative needle and spacings up to 0.1 mm the voltage strength in figure 3.1.2 is like the Paschen curve. Only at larger spacings the inhomogeneity of the field gains increased influence which results in reduced breakdown voltages compared to the homogeneous field. The start-up electrons must pass an area of high electric field and fast avalanche development at the beginning. Therefore it is fairly unlikely that the discharge is terminated by losing the start-up charge carriers by recombination which is the case at positive needle polarity. This is the reason for the voltage strength of the arrangement with negative needle being lower compared to the positive needle. With larger electrode spacing the discharge performance is more and more determined by positive space charges in front of the needle. In the case of further increased spacings the configuration becomes extremely inhomogeneous and the effect of these positive space charges is
compensated by negative space charges which is originated by the recombination processes [12]. This leads to the increased steepness of the measured curve above about 1.3 mm which is in this area approximately in parallel to the Paschen curve.
In many cases the designer cannot determine whether the polarity of the voltage at the electrode arrangements with inhomogeneous field is positive or negative. Therefore the clearance design must be mostly based on the worst case in which according to figure 3.1.2 the negative needle dominates below about 1.5 mm spacing and the positive needle above. For alternating voltage stress the peak value and not the r.m.s. value is to be considered.
The clearance design for avoiding continuing partial discharges can be based on the measurements of the partial discharge onset and extinction voltage according to figure 3.1.3 which were gained in the inhomogeneous field of a needle-plate- arrangement with 25 pm needle. The onset threshold was determined by use of a direct voltage increasing with 20 V per second at a sensitivity of about 0.1 pc. The extinction threshold during stepwise decreased voltage was defined as the voltage at which during 1 minute no partial discharge was detected. Needles with lower radiuses cause lower partial discharge onset and extinction voltages. The worst case under direct voltage stress is given with negative polarity at the needle, because the negative needle shows the first partial discharges. In this case the onset and extinction voltages are the same as the peak values under alternating voltage stress [12]. Short spacings below about 1 mm show never partial discharges but a sudden total breakdown. In this area the partial discharge curves join the curves of the figure 3.1.2. The range above about 1 mm is characterized by a second, negative area of space charges in front of the plate. It prohibits a total breakdown and allows only partial discharges with short current impulses rrichel impulses) [8]. Above about 50 mm electrode spacing the extinction voltages are significantly lower than the onset voltages. This is presumably caused by dust particles attracted to the needle during the experiment which took up to 10 minutes for the determination of the extinction voltage [12]. The increased steepness of the onset and extinction voltage curves above about 200 mm electrode spacing can be explained by the fact that the dimensions of the plate are no more large compared to the electrode spacing. In this case the needle-plate-arrangement starts slightly changing to the needle-needle- arrangement with increased voltage strength [12]. However, it is fairly unlikely to have in practice of low-voltage insulations larger plates as used for the measurements.
Figure
extinction
3.1.3. Partial discharge onset and extinction voltages of needle--plate+-arrangements under direct voltage stress U depending on the electrode spacing S compared to the breakdown voltage [l 21 needle: radius 25 to 30 pm plate: squared steel, 1 m2
In the cases in which partial discharges are to be avoided, the design of clearances should generally be based on figure 3.1.3. However, smaller spacings are possible if partial discharge measurements proved a less inhomogeneous field. Figure 3.1.4 shows to which extent partial discharges are influenced by the needle radius. Changes in the geometry of the needle are most efficient at small radiuses.
0.1
Figure 3.1.4. Onset voltage U of partial discharges at the needle--plate+-arrangement under direc voltage depending on the needle radius and the electrode spacing S [l21 plate: squared steel, 1 m2
Conductive particles may significantly reduce the effective needle radius. As an example, a 40 km needle which carries a 10 pm nickel particle behaves like a 10 pm needle [l 21.
3.1.3 Influence of the frequency
The breakdown voltage decreases with increasing frequency. Figure 3.1.5 shows the results of breakdown voltage measurements at ball electrodes depending on the electrode spacing and the frequency carried out by Hermstein. The influence of the frequency is under inhomogeneous field conditions more significant as in the homogeneous field. This can be judged by comparing figure 3.1.5 with figure 3.1.6 which shows the breakdown voltages of a needle-plate-arrangement as a result of laboratory experiments by Hermstein. The needle radius is different from the other arrangements mentioned in this book 1.5 mm. Another series of experiments under inhomogeneous field conditions at 75 kHz resulted in a reduction of the voltage strength for more than 50% compared to 50 Hz. Increasing frequency also reduces the partial discharge onset voltages of clearances significantly [l 21.
Figure 3.1.5. Breakdown voltage U at ball electrodes depending on the electrode spacing s under alternating voltage stress with 50 Hz, 3 MHz and 21 MHz. ball: diameter 50 mm
Figure 3.1.6. Breakdown voltage U of a needle-plate-arrangement depending on the electrode spacing S under alternating voltage stress of 50 Hz, 3 MHz and 21 MHz. needle: radius 1.5 mm
3.2 Strength under short-term voltage stress
The design of clearances must also ensure that the expected transient voltage stresses cause no dielectric breakdown. The design again needs to take into consideration whether the field between the electrodes is homogeneous or inhomogeneous. With respect to the short duration of transient overvoltages partial discharges need no concern.
In the discharge development to the total breakdown the following steps can be distinguished according to figure 3.2.1:
Figure 3.2.1. Breakdown development under impulse voltage stress
"DO: statistical breakdown voltage ts: statistical delay time t,: avalanche development time
4: discharge development time b: total discharge delay time
a) statistical time delay In most cases a certain time is necessary until a sufficient number of start-up electrons are present. This time is naturally object to statistical alterations. It significantly influences the breakdown probability in the case that the voltage, e.g. an impulse voltage, is only applied during a short time. The minimum breakdown voltage can only be evaluated in several experiments by use of statistical methods. However, it is possible to nearly totally eliminate the delay time by use of a sufficient pre-ionizing, which provides, e.g. with ultra-violet light, instantly sufficient start-up electrons.
b) avalanche development time As soon as a sufficient number of start-up electrons is present, another time interval is needed until either the critical number of avalanches is reached or a streamer is developed. Under homogeneous field conditions and spacings of several centimeters the avalanche development time amounts to about 300 ns, at millimeter spacings to about 200 ns [l 21.
c) breakdown development time The total breakdown finally needs additional breakdown development time to forward the thermo-ionization. This time of about 30 ns is relatively short [12]. Thus, under sufficient pre-ionization the total delay time is mainly determined by the avalanche development time.
The total breakdown is only possible, if sufficient voltage is applied to the electrodes during all phases of the discharge development. For this reason, the strength of a given clearance can be higher for short-term applied voltages than for long-term applied voltages.
The standardized impulse voltage 1.2150 ps according to figure 2.2.1 is suitable for simulating most of the transient overvoltages. However, under certain circumstances, e.g. simulation of surges as consequence of a blowing fuse, significantly longer impulses are possible and e.g. simulated by 101700 ps impulses.
3.2.1 Breakdown under homogeneous field conditions
The Paschen equation (3.1 .l) which describes the voltage strength of clearances with homogeneous field is strictly valid only for long-term applied constant voltages. However, laboratory experiments [Ill ascertained nearly the same voltage strength for 1.2150 ps impulses at typical low-voltage insulation spacings if only sufficient pre-ionization was provided. Dimensioning of clearances under this impulse stress can therefore be based on figure 3.1.1. The presumption of sufficient pre-ionization which is usual in IEC 664 provides the minimum clearances with a certain safety margin since most insulations show the statistical discharge delay as described in chapter 3.2.
3.2.2 Breakdown under inhomogeneous field conditions
While needle-plate-arrangements with 0.16 mm spacing and a 25 pm needle are still like arrangements with homogeneous fields, the voltage strength of needle- plate-arrangements with 7 pm needle is already for about 35% lower [l l]. This shows that especially at small spacings already relatively low increases of the electric field h$ &ignificant influence on the breakdown voltage.
Figure 3.2.2 shows a collection of the measured breakdown voltages under the stress of 1.2150 ps and 10/700 ps impulses. The curves and the physical mechanisms are similar to those under direct voltage stress according to chapter 3.1.2 and figure 3.1.2. At spacings below about 1.5 mm the impulse withstand voltage is slightly increased compared to the DC stress. This indicates short discharge delays of up to some micro-seconds [12]. Only at increased spacings above about 2 mm the voltage strength under the investigated impulses is significantly different from the direct voltage strength. Now the breakdown is determined by the relatively slow formation and reduction of the ion space charges [12]. This causes fairly long discharge delays which results in increased impulse withstand voltages especially for short impulse stresses.
Even at needle-plate-arrangements with the pointed 7 pm needle was proven that as a consequence of the short discharge delay at small spacings below about 1 mm no significant differences in the breakdown voltage under 1-2/50 ps impulse, 1 O/7OO ps impulse and 10 ms half-wave stress are to be expected [l l]. For larger spacings and positive needle the shorter wave tail time of the 1.2/50 ps impulse causes a 10% increased breakdown voltage compared to the 10 ms half-wave. At negative needles an increase of even 25% was ascertained [l 21.
The measuring results are influenced by the roughness of the plate electrode [12]. A more rough surface causes at short spacings a change of the performance towards the needle-needle-arrangement with higher dielectric strength [23]. The influence of this field disturbance becomes lower for larger spacings.
Figure 3.2.2. Breakdown voltage U of clearances with inhomogeneous field under 1.2/50 ps and 10/700 p s impulse stress with pre-ionization depending on the electrode spacing S.
The + and - identifies the polarity of the needle [l21 needle: radius 25 to 30 p m plate: squared steel, 1 m2
In most cases the polarity of the impulse voltage stress is not predictable. The dimensioning must therefore be based on the worst case. In general can be stated that for larger spacings above about 1.5 mm the arrangement with positive needle dominates, whereas for low distances the arrangement with negative needle is to be considered [l 21.
4 Voltage strength of the creepage distances
Creepage distances must be such that with the voltages developed in operation no flashover occurs, no creepage path is created and a specified minimum insulation resistance is maintained. Both the long-term working voltages applied and the short-term transient overvoltages must be taken into account.
In general, creepage distances exhibit a lower electric strength than the breakdown strength of clearances in air with the same electrode geometry. Approximately the same performance can only be expected with clean surfaces under ideal conditions. The reduction of the flashover voltage is essentially caused by local increases of the electric field strength [l 1, 121. At clean surfaces these disturbances of the electric field are mainly originated by the adjacent materials of different permittivity. Under the presence of pollution the uneven distribution of the surface layer conductivity has the strongest influence [l 1, 121.
If metal dendrite growth and other electrode mechanisms are not taken into account, the steady-state voltage strength is mainly determined by the thermal strength of the insulating material with respect to the thermal stresses caused by the surface currents (creepage currents) under the working voltage. If the thermal strength is insufficient, the material is carbonized or eroded: tracks are formed, which irreversibly degrades the insulation. The formation of carbonaceous tracks might cause a short-circuit. The real-time test of steady-state voltage strength is very time consuming, because tracking proceeds very slowly.
Without a complete creepage path between the electrodes it is not possible to doubtlessly detect tracking by means of changes of the insulation resistance. To realize early stages of tracking visual examination is generally necessary.
4.1 Strength under steady-state voltage stress
Appropriate dimensioning of creepage distances must at any rate provide a sufficient electric strength against the long-term applied working voltages. Neither flashover in the surface boundary layer must occur nor tracking under the influence of partial discharges or creepage currents. In addition is for many applications a certain minimum insulation resistance required during the entire service life of the equipment.
4. l .l Flashover
The investigations on the flashover strength of low-voltage insulations were concentrated in the past on short-term impulse voltage stresses. Systematical investigations of the flashover mechanisms under the steady-state stress of direct or alternating voltage, especially at polluted surfaces, have not been published so far. Some single measurements indicated, as to be expected, that the strength under steady-state stress is lower than under the short-term voltage stresses according to chapter 4.2.1.
4.1.2 Partial discharges
During some investigations on polluted insulations among others the partial discharge onset voltages were detected by means of the usual partial discharge measurement technique [l 21. The value of the gained results is controversial because the measurement not only detects the partial discharges caused by the electrostatic field but also the switch-off discharges and current changes caused by the numerous drying out and changing events in the surface layer [10]. However, some results put together in figure 4.1.1 are interesting, which were gained at specimen of the German-American field trial "creepage distances" [14]. The investigated specimens had been exposed inside a workhall under 250 V d.c. stress. It can be assumed that under the given situation scintillations and partial discharges do not occur below the detected thresholds in figure 4.1 .l.
Figure 4.1 .l shows the lowest values of the lower tolerance boundaries of the partial discharge onset voltages of different insulations after 3 years exposure under 250 V d.c. voltage stress in the conditions of the micro-environmental category II. The lower tolerance boundaries were determined for the statistical safety s=95% and the quantile h=95%. The onset of partial discharges seems to be influenced by the water adsorption like the flashover strength. Figure 4.1.1 therefore distinguishes between materials of the category WC1 with low water adsorption and materials of the category WC2 with medium and high water adsorption according to chapter 5.1.4. Only for insulations with negligible water adsorption the average partial discharge onset voltage is for spacings above about 1 mm equivalent to clearances with the same spacing under inhomogeneous field conditions. Medium and high water adsorption results for all creepage distances in 15 to 30% reduced partial discharge onset voltages compared to clearances with inhomogeneous field. The lower tolerance boundaries are significantly reduced as well. At spacings above about 2.5 mm the
partial discharge onset voltage can only be slightly improved by increasing spacing, which is equivalent to the performance of clearances.
Figure 4.1 . l . Lowest averages of the partial discharge onset voltages U and lowest values of the lower tolerance boundaries (S = 95%, h = 95%) at several insulations after 3 years of exposure under 250 V direct voltage in the conditions of micro-environmental category II dependin on the creepage distance S. The measurements were carried 8 out in the climate 23 C / 83% relative humidity. It is distinguished between materials of low water adsorption (WC1) and materials of medium and high water adsorption
(WC4 v21.
4.1.3 Tracking
One of the heat sources stressing the insulation material is the Joule heat of the creepage currents. It may increase the surface temperature at maximum to the boiling temperature of the moist surface layer. However, this maximum will only be reached, if the heat density is sufficient for covering the thermal dissipation by conduction, radiation, convection and water evaporation, which requires either high voltage gradients or a thick layer of contaminants. In addition, sufficient energy is to be provided for charging the adjacent thermal capacitors. These conditions are rarely met, even in very inhomogeneous electrical fields. Thus, in most applications, the thermal stress caused by the Joule heat of the creepage currents can be neglected [19].
Another source of heat are exothermic chemical reactions at the electrodes in the course of the electrolysis processes. For example, if all discharged chlorine ions at the copper anode react to copper-chloride, a heat of reaction of 1.14 mW/mA creepage current is generated at the metal surface. Normally this heat is dissipated by the evaporation of water; however, the water of the moist surface layer can be separated by an area of greater heat transmission resistance and the heat adds to the energy stored in the electrode. As a result the temperature of the electrode exceeds that of the contaminated insulation surface and is mainly limited by the thermal electrode capacity. This effect might enable temperatures of several hundred centigrades provided that very thin electrodes and a large quantity of contaminants are involved. However, under usual circumstances this heat source also can be neglected.
The most obvious heat source is given by the many, minute, white-hot micro-arcs (scintillation) which may occur in drying out surface layers under voltage stress. In contrast to the thermal stress by Joule dissipation and exothermic reactions, scintillation can occur even on relatively clean (but moist) insulations. The high energy density makes the micro-arcs very effective in damaging the insulation. The origin of scintillation is promoted by inhomogeneous distribution of the contaminant conductivity. This causes many more or less connected current paths through the surface layer. Areas of reduced conductivity are subject to increased power dissipation which results in increased water evaporation. By that, these areas are drying out and interrupting the current, while the other parts of the path are still moist and conductive.
It is known from circuit-breakers that circuit interruption may cause transient arcs at the separated contacts. If arcs occur or not, depends on the current over the contacts before separation and the voltage between the contacts after separation.
The minimum voltage which is necessary for arcing is very low and can be even less than 20 V. Even though the "contacts" of a drying out creepage path are liquid, similar physical laws are applicable as for metal contacts. This explains, why the minimum voltages at which scintillation can be observed increases as well with decreasing contamination as with increasing electrode spacing. Both results in lower interrupted creepage currents, which increases the voltages being necessary for the occurrence of switching-off arcs.
5 10 k ohms 15
R - Figure 4.1.2. Minimum voltages U for discharges in a single drying-out current path of a moist
surface layer depending on the insulation resistance R immediately before the discharge occurrence [g].
Laboratory experiments with varied voltages UI and recorded currents lk lead to a set of UI-lk couples which can mark in a U-l diagram the threshold curve at which switch-off discharges are to be expected [19]. In an earlier publication [g] such a diagram was determined for a single drying out current track in the moist pollution of an insulation surface. Figure 4.1.2 is deducted from this diagram and shows the minimum insulation resistance in order to avoid switching-off discharges in the pollution depending on the applied voltage. However, in practice it is unlikely that
only one single current path is formed in the surface layer. In most cases many electrically more or less decoupled paths exist between the electrodes to which the creepage current is distributed. The resistances of these single paths are naturally significantly larger than the measurable total insulation resistance. Therefore figure 4.1.2 cannot be used as a basis for dimensioning rules. It would lead to tremendously exaggerated dimensions in most cases [g].
For estimating the thermal stress caused by scintillation, the visible light being emitted from the micro-arcs can be used. Comprehensive investigations gave the evidence, that the temperatures of the discharges reach several thousand degrees and the power density varies between 1 and 100 w/cm2. The duration differs, but comes in general to few microseconds. The area covered by the micro-arcs is at least for lower voltages nearly circular with a diameter of 0.2 mm not depending on the other parameters [IO]. The energy content of bright discharges increases with increasing voltage [l 01.
The thermal stress of a polluted insulation surface increases with the number of drying out processes. Such a drying out process can be either initiated if a polluted insulation being applied to voltage becomes moist or if the voltage is applied to a moist and polluted insulation. The first case leads to a reduced thermal stress because the drying out can already start with the very first penetration of moisture into the pollution. Therefore the surface conductivity and the creepage current never gets as high as in the second case immediately after the application of voltage.
Under long-term applied voltage and constant relative humidity a drying out of the surface layer may reduce the creepage currents to such an extent that the occurrence of scintillations becomes very rare or is even terminated. Significantly increased stress is therefore caused by periodical drying out processes e.g. caused by the daily changes of the relative humidity. The worst case is generally given if the voltage is not constantly applied. During the periods without voltage humidity may penetrate the pollution which causes after voltage application an increased creepage current and an increased number of scintillations.
In order to avoid tracking it is necessary to either reduce the thermal stresses to a harmless extent or to use sufficiently tracking resistant insulating materials. The exothermal processes in the surface layers depend beside the voltage on the pollution as well as on the creepage distance. The determining voltage is the peak of the alternating voltage or the direct voltage which is applied for longer time. In most cases transient overvoltages have no influence on tracking. However, in
some cases temporary overvoltages or functional overvoltages might to be taken into account.
Figure 4.1.3. Minimum creepage distances S in order to avoid scintillations at different dust depositions on insulations of glass fiber reinforced epoxy fabric under condensation. The mixture of the surface layers applied in the laboratory is equivalent to the pollution measured on specimen which were exposed in a vented cabinet near a power plant. Deposition 1 : 13.2 g/m2 Deposition 2: 22.9 g/m2 Deposition 3: 48.3 g/m2
Dimensioning rules in order to avoid tracking may generally be deducted from the measurements of the scintillation onset voltages of polluted insulations. Figure 4.1.3 shows for specimen of glass fiber reinforced epoxy fabric the minimum creepage distances depending on the applied voltage in order to totally avoid pollution scintillations. Three different curves show the influence of the deposited pollution quantity. The mixture of the deposited salty dust is comparable to the pollution of specimen which were exposed in the vicinity of a coal-fired power plant in a vented cabinet. The salt content amounted to totally 3.8% per weight. After 12 months of exposure a dust deposition of 11.7 g/m2 was found. This
roughly corresponds to the upper curve in figure 4.1.3. It is obvious that higher dust deposition significantly increases the minimum creepage distances. Under strong pollution and high relative humidity the onset voltages of scintillations can hardly be influenced by increasing the creepage distance.
Figure 4.1.4. Minimum creepage distances S in order to provide less than 1% failures under the conditions of environmental category Ill in the vicinity of a power plant during 15 years of service life. The curves distinguish the insulating materials A: ceramics A1203, 97% (unglazed) B: FR 4 glass fiber reinforced epoxy fabric C: type 802 polyester resin moulding material D: type 31.5 phenolic resin moulding material
Dimensioning rules can also be based on the total breakdown of insulations as carried out during the German-American field trial "creepage distances" [l 41. This project aimed at the investigation of the performance of about 20,000 creepage distances under typical environmental conditions of electric equipment during up to 36 months. All of these creepage distances were protected by a 32 mA fuse. Among others, blown fuses were recorded. The problem of this data base is that
by far not all of the fuses were blown during the exposure. In order to deduct at least some estimates the data needed to be processed by statistical methods. In a first step only the data of small creepage distances and high voltage stress were regarded at which a high portion of fuses were blown. The time-to-failure performance of these creepage distances was approximate'd by the Weibull- function. On the basis of these sufficiently founded functions and the more or less numerous data of blown fuses at the other combinations of creepage distance and voltage a complete set of Weibull functions was deducted. On this basis the minimum creepage distances which ensure less than 1% failures during 15 years of service life were evaluated. Figure 4.1.4 shows the result for the conditions according to micro-environmental category Ill in the vicinity of a power plant. A significant influence of the insulation material cannot be recognized. The importance of the results in figure 4.1.4 is unfortunately not very high because of the small data base. However, the results show under these circumstances a fairly good correspondence to the micro-environmental category Ill dimensioning curve which is deducted in the following chapter in order to provide a minimum insulation resistance of 100 kOhms for 15 years of service life.
All of the cited investigations indicate that for creepage distances which are designed in order to provide the minimum insulation resistance according to chapter 4.1.4 also the formation of tracks is avoided.
4.1.4 Minimum insulation resistance
Insulation resistances are usually evaluated by measuring the current after applying a constant direct voltage to the insulation. In most cases this procedure leads to a continuously increasing insulation resistance during the measurement. It therefore is usual to record the insulation resistance after a previously determined measuring time, e.g. 5 S or 30 S.
Operationally applied direct voltages can also increase the insulation resistance. This is - of course - only possible if no tracking occurs. The insulation resistance under alternating voltage stress remains comparably lower [13, 151. The increase of the insulation resistance during direct voltage stress is therefore not caused by drying of the pollution but by electrolysis processes. The ions move under the constant influence of the applied electric field towards the anode or cathode. This leads to a continuous rate of discharged ions at the electrodes and thereby a continuous decrease of charge carriers and of conductivity of the pollution around the electrodes [13]. Under alternating voltage such electro-chemical reactions are significantly reduced and the insulation resistance is not increased.
In order to leave the insulation resistance unchanged by the measurement itself the resistance measurement with alternating voltage is to be recommended. However, such instruments are complicated because they must distinguish between the comparably large capacitive currents of the measuring arrangements from the mostly low ohmic current over the insulation.
In most cases the insulation resistances of electrical low-voltage equipment are tested only when they are new. However, it is often necessary that certain minimum insulation resistances are provided during the entire service life. This requires the knowledge on the pollution which is to be expected under operation conditions and on the ageing of the used insulation materials in practice. The German-American research project "creepage distances" [l 41 aimed at gaining these informations. The following gives a short summary of some results.
Figure 4.1.5 shows a plot of the insulation resistance versus time. It is subjected to periodic changes which are obviously caused by changes of the relative humidity of the surrounding air.
In general, the courses of the insulation resistances of insulations with same creepage distances being on the same specimen, show only little differences during the entire exposition. The insulation resistances vary within one order of magnitude. Also the courses of the insulation resistances of insulations with same creepage distances on different but comparable specimens are qualitatively and in most cases even quantitatively similar. Therefore, these creepage distances can be regarded as being on the same specimen.
In opposite to the expectations, in most cases the total insulation failure is not preceded by any significant changes of the insulation resistance. The failure occurs as well at low insulation resistances under temporarily high humidity of the surrounding air, as at high insulation resistances under temporarily dry conditions. This is reasoned by the fact that tracking as well as metal dendrite growth only influences the total insulation resistance if they bridge nearly the whole creepage distance. It may also explain that the insulation failures recorded in figure 4.1.5 occur at the temporarily reduced insulation resistances under the presence of high relative humidity as well as at the increased resistances under low humidity.
Figure 4.1.5. Course of the relative humidity f and of the insulation resistances R of four 0.4 mm creepage distances on epoxy resin under 660 V d.c. The humidity was recorded every 30 minutes, the insulation resistance every 10 hours. During the figured period from day 45 until day 128 of exposure in the vicinity of a power plant three of the four fuses of the investigated creepage distances were blown (see arrows). After the fuse was blown the resistance was not recorded any longer. The upper range limit of the insulation resistance measurement amounted 1 . 5 . 1 0 ~ ~ Ohms [26].
The increasing pollution on the insulation surfaces during the years of service life reduces the insulation resistance. Under direct voltage stress this reduction is lower compared to alternating voltage because the electrolysis processes around the electrodes deprive more charge carriers from the pollution in the direct field. However, direct voltage is only better than alternating voltage as long as the insulation level is high enough to safely avoid total insulation failure. Otherwise insulation failures with blowing fuses occur significantly earlier under direct voltage stress than alternating voltage stress [26] which is probably caused by the increased thermal stress by scintillations in the pollution.
1E+14 ohms
material
Figure 4.1.6. Averaged insulation resistances R of the Imm creepage distances of the 10 investigated materials under increasing pollution. The four bars of each material allow the comparison of the measurements at the beginning, after 1 month, 6 months and 12 months of exposure [27]. Laboratory measurement climate: 23 'C / 83% relative humidity Pollution: Average of all tests sites used in the field trial
"creepage distances" Voltage stress: 250 Vdc Materials: A: ceramics AI2O3, 97% (unglazed)
B: FR 4 glass fiber reinforced epoxy fabric C: type 802 polyester resin moulding material D: type 31.5 phenolic resin moulding material E: polyimide film laminated to B F: FR 2 phenolic resin impregnated paper G: GPO 111 polyester resin laminate H: type 150 melamine resin moulding material I : Polybutene terephthalate K: Polycarbonate
Figure 4.1.6 shows the averaged insulation resistances of the Imm creepage distances of the 10 investigated materials exposed under 250 V d.c. Each group of bars shows subsequently the measuring results in the laboratory climate 23 OC
/ 83% relative humidity before exposure, after 1 month of exposure, after 6 months and after 12 months. According to the expectations, the insulation resistances of the different materials are very different before the exposure. During exposure and with increasing contamination of the surfaces, the differences between the materials get smaller, but, however, keep in most cases qualitatively significant [IS, 261. The latter cannot be explained by the fact that a more or less high insulation resistance at the beginning is shunted by a decreasing "pollution resistance" because these two resistances are too different. It is rather to be supposed that the insulation material itself influences a deposition of pollution [26]. A correlation between the water adsorption of the materials according to table 2.4.2 and the course of the insulation resistance is not significant.
Nearly all of the specimens of the German/American research association [l41 showed a relatively steep decrease of the insulation resistance within the first month of exposure. For smaller creepage distances, this performance was particularly evident. The continuing course of the insulation resistance can be approximated by linear regression in a double logarithmic resistance versus time diagram.
After the first month of exposure, it can be significantly distinguished between insulations with rapid decrease of the insulation resistance and insulations with only slight changes. Increased voltage stress requires increased minimum creepage distances, if only slight insulation degradation is permitted. Provided that the creepage distances are larger than these minimum values, the insulation resistance can be improved, if direct voltage instead of alternating voltage is applied. In this case, direct voltage is even better than no voltage. This is due to the fact, that the electrolysis processes under direct voltage result in a reduction of the surface conductivity, as long as no total insulation failure occurs.
A long-term comparison between the insulation materials ceramic, epoxy resin, polyester resin moulded and phenolic resin moulded resulted in polyester having the best and phenolic having the worst insulation properties. A long term comparison of the test sites "power plant", "busy road" and "Black Forest" showed only little differences.
The insulation measurements were used for deducting a proposal for dimensioning rules. In the first step the time of sufficient reliability was estimated from the extrapolated regression of the insulation resistance course for each
creepage distance, voltage, insulation material and location of exposure. lnsulations were considered sufficiently reliable as long as less than 1% were below a given minimum insulation resistance with a probability of 95%. For insulations in the micro-environmental category l1 was required that a minimum insulation resistance of 10 MOhms was met in the climate 23 OC / 50% relative humidity. The given relative humidity of 50% is approximately the long-term humidity average in rooms. For insulations in the micro-environmental category I l l was required that the minimum insulation resistance of 100 kOhms was met in the climate 23 OC / 83% relative humidity. The given relative humidity of 83% is approximately the average in outside rooms in the temperate climate zone.
In the second step, the time of sufficient reliability was demanded to be at least 15 years. Finally, the minimum creepage distances which meet these conditions were evaluated [15, 261. A s an additional condition it was required that the creepage distances evaluated by that method for a certain voltage passed the exposure of the field trial [l41 without any insulation failure at all. Figure 4.1.7 shows the deducted minimum creepage distances depending on the applied voltage and the micro-environmental categories I I and I l l . It came out, that the minimum creepage distances in the environmental category I l l were only determined by the fact that no blown fuses were allowed [15]. The course of the insulation resistances had led to significantly smaller spacings. The minimum creepage distances in category I l l are based as well on the extrapolation of the insulation resistance as on the blown fuses.
For creepage distances in category 1, the steady-state voltage needs no special consideration. The influence of the CTI on the insulation performance appeared to be less than expected. Thus, the shown dimensioning curves apply to most of the investigated insulation materials with only few exceptions. Alternating voltage requires increased creepage distances compared to direct voltage.
The curves in figure 4.1.7 must inevitably meet the rough grid of the creepage distances 0.16 mm, 0.4 mm, 1 .O mm, 1.5 mm and 6.3 mm which were investigated in the German-American research project "creepage distances" [14]. Every point on the curves means that at the given voltage the next smaller distance does not meet the requirements. The rough grid leads to the fact that the results contain a safety factor up to 2.5.
Figure 4.1.7. Minimum creepage distances S in order to achieve the minimum insulation resistance - lo7 Ohms for pollutions of the micro-environmental category 2 (MEC 2)
measured in the climate 23 'C / 50% relative humidity or - 10' Ohms for pollutions of the micro-environmental category 3 (MEC 3)
measured in the climate 23 'C / 83% relative humidity during 15 years of service life, depending on the voltage U and the type of voltage stress (d.c. or a.c.).
The minimum creepage distances given in figure 4.1.7 are based on the investigation of the following insulating materials, taking into account the worst case at all installation sites examined: - ceramics A1203, 97% (unglazed) - FR 4 glass fiber reinforced epoxy fabric - type 802 polyester resin moulding material - type 31.5 phenolic resin moulding material - polyimide film - FR 2 phenolic resin impregnated paper - GPO Ill polyester resin laminate
- type 150 melamine resin moulding material - Polybutene terephthalate - Polycarbonate For micro-environmental category 3, however, it should be noted that for the polyimide film insulating material the minimum creepage distance obtained was 6.3 mm instead of 2.5 mm, when operated near a major chemical plant with a direct voltage stress of 250 V. For the - FR 4 glass fiber reinforced epoxy fabric, and - polycarbonate insulating materials, minimum creepage distances of 6.3 mm are not sufficient when allowing for the worst case at all installation sites examined for micro- environmental category 3. It should be noted, however, that the rated values for 15 years were derived by extrapolation of the maximum of 3 years' operation within the scope of the field trial of the research association "creepage distances" [14]. It is possible that this method is not appropriate for assessing insulating materials with a very high initial resistance and a correspondingly pronounced insulation decrease in the first months. As a result of long-term operational experience, the above restrictions relating to insulating materials with the highest initial values may prove to be unfounded [l 51.
For alternating voltages with r.m.s. values of between 240 V and 660 V, the minimum creepage distances chosen should be greater by a factor of 2.5 than with the corresponding direct voltage stress 1151. No test results are available for lower alternating voltage stress.
A significant insulation deterioration cannot be expected in micro-environmental category 1; no minimum creepage distances need therefore be specified here. Rated values cannot be stipulated for the conditions of micro-environmental category 4 because no trial results are available.
Furthermore, as expected, the field trial of the research association "creepage distances" [l41 indicated that in the case of insulation without voltage stress for a long time, the insulation resistance developed is less than that with low direct voltage applied. The required minimum insulation resistances can then only be achieved with creepage distances which are in some cases considerably increased. The effect of short voltage interruptions is already covered by the rated values of figure 4.1.7, because the voltage stress on specimens in the field trial of the research association "creepage distances" [14], taken as a basis, was switched on and off in a ten-hour cycle [l 51.
Table 4.1.1 indicates the minimum insulation resistances to be expected for the ratings according to figure 4.1.7, when the humidity does not correspond to the basic values but fluctuates within the permissible limits of the relevant micro- environment category.
Table 4.1.1. Minimum insulation resistance which is to be expected at the end of 15 years of service life if the suggested creepage distances of Figure 4.1.7 are used. The micro-environmental categories 2 and 3 are defined in table 5.1 .l.
MEC
2
3
relative humidity (r.h.1
short time, in total max. 100 h/a: 75% cr.h.2 85%
max. 30 h, in total max. 7 d/a: 95% <r.h.< 100%
conden- sation
no
short time
precipi- tation
minimum insulation resistance to be expected
relative humidity above 95% the insulation re- sistance may fall below-104 R, especially towards the end of service life. However, in most cases it will stay above 103 R.
In micro-environmental category 2, it can be assumed that the minimum insulation resistance determined for 50% relative humidity is lower by about a factor of ten at 75% and is approximately 1 megohm. At 85% relative humidity, the measured values of the 23 OC / 83% relative humidity climate can be taken as a basis; these exhibit a minimum insulation resistance of 100 kilohms for most insulating materials examined. Under the environmental conditions of a industrial laboratory, only the - polyirnide film - FR 2 phenolic resin impregnated paper, and - GPO Ill polyester resin laminate
insulating materials result in lower expected minimum values, although the resistance does not drop below 100 kilohms under the conditions of a industrial workshop. In micro-environmental category 3, it can be assumed that the minimum insulation resistance determined for 83% relative humidity is lower by about a factor of ten at 95% and is about 10 kilohms. Laboratory trials [l31 have shown that at very high humidity, up to moisture condensation, it is quite possible that the insulation resistance is reduced by a factor of 100 compared to the measured values at 83% relative humidity. With all values given, however, it should be noted that they are only reached with the low statistical probability taken as a basis, at the extreme humidity values and at the end of the service life.
The resistance to tracking (CTI) of the insulating materials used does not initially contribute to the dimensioning rules, because the design according to Figure 4.1.7 avoids a significant flow of leakage currents. Under particularly unfavourable environmental conditions, however, or in service other than that prescribed, the CTI also contributes to the insulation performance.
4.2 Strength under short-term voltage stress
With short-term voltage stress, the objective of rating the creepage distances must merely be to establish that no flashover occurs in the surface boundary layer. As a rule, tracking and partial discharge can be discounted. Among others the flashover voltage strength can be influenced by the shape of the insulation surface.
4.2.1 Flashover
Figure 4.2.1 shows the statistically ensured lower boundary values of the impulse withstand voltages of creepage distances in comparison to the voltage strength of clearances with inhomogeneous field according to figure 3.2.4. The considered creepage distances are located at clean and dry plane insulation arrangements like printed circuit boards. The flashover voltage strength was measured under pre-ionization by use of 1.2150 ps impulse voltages in the climate 23 OC / 50% relative humidity. The differences of the voltage strength at the different insulating materials is caused by the different manufacturing processes of the specimens which leads to a microscopically different shape of the conductors [12]. The flashover voltage strength of the investigated clean and dry arrangements shows in total only little differences compared to the voltage strength of clearances with inhomogeneous field. Only at the electrode spacing 6.3 mm the needle-plate- arrangement in air has a significantly higher strength than the comparable clean creepage distance between two conductors. This performance is possibly caused by the dielectric stress of the insulation surface in the immediate vicinity of the electrodes which increases compared to the remaining creepage distance at enlarged spacing [12]. At larger spacings the significant increase of the discharge delay time must additionally be taken into consideration. The flashover may already be preceded by strong partial discharges because the partial discharge onset voltage is significantly below the flashover voltage [l 21.
material
Figure 4.2.1. Lower tolerance boundary values (s=95%, h=99%) of the impulse withstand voltage U of unpolluted creepage distances on plane surfaces of 11 different insulation materials (A to N) compared to the impulse withstand voltage of clearances (UD) with inhomogeneous field according to figure 5.2.4. For each material the strength of the spacings 6.3 mm, 2.5 mm, 1.0 mm, 0.4 mm and 0.16 mm are entered. The measuring results which were gained under 1.2150 ps impulse voltage stress in the climate 23 'C / 50% relative humidity are related to main sea level [12]. The figure distinguishes between the materials A: ceramics AI2O3, 97% (unglazed) B: FR 4 glass fiber reinforced epoxy fabric C: type 802 polyester resin mould. mat. D: type 31.5 phenolic resin mould. mat. E: polyimide film laminated to B F: FR 2 phenolic resin impregnated paper G: GPO Ill polyester resin laminate H: type 150 melamine resin mould. mat. I: polybutene terephthalate K: polycarbonate N: CU 96 laminated paper
The investigation of the impulse withstand voltages at polluted creepage distances are also based on the printed-wiring-board insulation arrangements of the German-American research project "creepage distances" [l41 which were exposed to different environmental conditions. At the beginning of the project and several times during the in total up to 36 months of exposure the strength under
1.2150 ps impulse voltage stress was determined under pre-ionization. The laboratory climate during measurement was as best as possible adapted to the worst climate of the according environmental category. The choice of the measuring climate took into consideration that the flashover in opposite to the long-term insulation degradation is determined by the actual climate and not by its average. It came out that the flashover voltage strength decreased during the exposure and that the curve of this decrease can be linear approximated in the double logarithmic scale. Comparable to the investigation of the insulation resistance the lower confidence boundaries have been determined above which 99% of the results are to be expected with 95% probability. These lower confidence boundaries were extrapolated from the at maximum 36 months of exposure to a operation time of 15 years. Figure 4.2.2 shows the impulse withstand voltages determined by this procedure in dependency of the creepage distance. For the conditions of the micro-environmental category 1 the measuring results of figure 4.2.1 are entered. The figure distinguishes by the water adsorption of the insulating materials according to table 5.1.3. The high statistical probability under which the data shown in figure 4.2.2 were gained need no more safety margins for design rules.
The rated values of figure 4.2.2 are based on examination of the following insulating materials, taking into account the worst case at all installation sites examined: - ceramics A1203, 97% (unglazed) - FR 4 glass fiber reinforced epoxy fabric - type 802 polyester resin moulding material - type 31.5 phenolic resin moulding material - polyimide film - FR 2 phenolic resin impregnated paper - GPO Ill polyester resin laminate - type 150 melamine resin moulding material - Polybutene terephthalate - Polycarbonate A distinction is made according to insulating material categories WC2 and WC1 according to chapter 5.1.4. For the - ceramics A1203, 97% (unglazed), and - FR 4 glass fiber reinforced epoxy fabric insulating materials in micro-environmental category 3, lower impulse withstand voltages were determined and indicated by Curve A, allowing for the worst case at all installation sites examined. These insulating materials exhibit a comparatively high water adsorption.
Figure 4.2.2. Lower tolerance boundary values (s=95%, h=99%) of the impulse withstand voltage U of polluted creepage distances on plane surfaces after extrapolation to the end of 15 years life and with respect to the worst of the investigated test sites [14]. The figure distinguishes between the micro-environmental categories according to chapter 5.1.3: MECl (climate 23 'C / 50% relative humidity during measurement): dotted MEC2 (climate 23 ' C / 83% relative humidity during measurement): dashed MEC3 (climate 40 'C / 92% relative humidity during measurement): solid The figure also distinguishes between the insulation categories WC2 (considerable water adsorption) and WC1 (negligible water adsorption) according to table 5.1.3. The curve WC2A applies to the materials A203 ceramic (unglazed) and epoxy resin laminated glass fabric FR 4 in the micro-environmental category 3. All of the values are related to main sea level [ l 21.
The rated values were determined from measured values obtained on specimens similar to printed circuit boards, between printed conductors of a few micrometers in thickness. The electric field of these arrangements is comparatively non- uniform; it is therefore hardly likely that the electric strengths with the usual arrangements will drop below the values determined. However, it is uncertain whether making the field uniform could improve the electric strength, particularly
under the effect of pollution. If creepage paths develop with inadequate long-term electric strength, the electric field becomes less uniform and the impulse strength of the creepage distances is reduced [l 21.
4.2.2 Influence of ribs and grooves
Ribs and grooves are mainly used to increase the creepage distance at a given electrode spacing. They provide the additional advantage that in most cases less dust is deposited at the vertical compared to the horizontal surfaces. By that the influence of pollution on the insulation is reduced. In addition a part of the clearance is not in the surface boundary layer of the insulation material. For this reason the reduction of the flashover voltage strength compared to the clearance breakdown voltage strength takes only place in a part of the spacing. The remaining part is as a clearance not influenced by pollution. Thus, ribs and grooves not only reduce the dimensions of a insulation arrangement but also increase the dielectric strength of the creepage distance under long-term and especially under short-term voltage stress.
Figure 4.2.3. Clearances (L) and creepage distances (K) at insulation arrangements with ribs and grooves
a) insulation arrangement with rib: b: width of the rib, h: height of the rib, S: electrode spacing
b) insulation arrangement with groove: b: width of the groove, h: depth of the groove, S: electrode spacing
Figure 4.2.3a shows a insulation arrangement with rib, figure 4.2.3b a arrangement with groove. At a given electrode spacing a groove increases the creepage distance whereas the clearance remains unchanged. A rib increases the creepage distance as well as the clearance.
At clean insulation arrangements the flashover always takes the "line of sight" between the electrodes. The dielectric strength of the discharge part which is free of the surface boundary layer increases with increasing relative humidity according to the performance of a clearance. At insulation arrangements with sufficiently deep grooves the voltage is additionally divided according to the capacitive fractions 1121. This leads to the fact that at a given electrode spacing the potential difference at the groove is increased by an increased groove width, which reduces the voltage stress of the remaining creepage distance. This reduces the locally increased electric field under inhomogeneous field conditions at the electrodes. The field becomes more homogeneous and the dielectric strength is increased. The same result is gained by coatings which also equalize the field by capacitive voltage division [12]. Compared to the impulse withstand voltage of clean and plane insulations clean arrangements with ribs and grooves show accordingly always increased dielectric strength under the influence of humidity [12]. The maximum increase of dielectric strength at clean insulations is ,
gained with grooves which are as wide as possible. However, the width of the grooves should not exceed about 90% of the electrode spacing [12]. Clean electrode arrangements with sufficiently deep grooves showed at least for the investigated electrode spacings between 2 and 20 mm a impulse withstand voltage which was at least increased by 1 kV compared to the plane insulation arrangement with the same electrode spacing [l 21.
Less significant is the performance under electrolytical pollution. The following considers at first insulation arrangements with ribs. Because of the pollution the flashover follows more or less the contour of the insulation surface [Ill. By that the flashover is mainly influenced by the boundary layer of the polluted insulation surface. The part of the uninfluenced clearance and by that the gain of strength becomes marginal. At 50% relative humidity the pollution in most cases only adsorbs little water and the influence on the flashover which follows still the "line of sight" remains low [l l]. But in the climate 23 OC / 83% relative humidity the flashover follows partially and in the climate 40 OC / 93% relative humidity totally the contour of the polluted insulation surface [ I l l . This leads to the conclusion that ribs increase the dielectric strength compared to the plane arrangement only under the condition that the surrounding relative humidity and by that the conductivity of the pollution remains low.
With increasing width of the rib an accordingly increasing part of the flashover follows the insulation contour and is influenced by the surface boundary layer. In this part the voltage strength is decreased by increasing relative humidity. The maximum increase of dielectric strength is therefore gained if the width of the ribs is small and the height is large. However, the angle a between' the clearance and the side of the rib should remain large enough. At angles lower 15' was observed that the flashover followed the insulation contour even at low relative humidity 1121. By that the rib loses its effect and keeps only the advantage of reduced dimensions of the insulation arrangement.
At polluted insulation arrangements with grooves the voltage strength is less increased by the capacitive potential division but mainly by the fact that a part of the flashover must pass a free clearance [l 21. The gain of dielectric strength is as higher as wider the groove is. Small grooves up to 0.8 mm cannot increase the impulse withstand voltage at any rate [12]. They are presumably not able to interrupt the surface layer sufficiently. Under moist conditions only insulation arrangements with at least 2 mm wide grooves show an increased voltage strength compared to the plane arrangement of the same electrode spacing [12]. Under strong pollution the grooves should therefore have a width of at least the half clearance and they should not be smaller than 2 mm [12]. The depth of the grooves should be at minimum the same as the width [12]. In this case the impulse withstand voltage of the insulation arrangement with groove is even under conditions of high relative humidity for more than 50% increased compared to the plane arrangement with the same spacing [12].
Physically founded dimensioning rules
The experimental results and the physical fundamentals described in the preceding chapters may lead to the following recommendations for physically founded dimensioning rules for low-voltage insulations. Even though these data include all of the major investigations, it is inevitable that the finally issued revised standard IEC 664, which is also based on these investigations, exhibits some different specifications. However, the recommendations of the following chapters may be helpful to judge the safety margin between the physically founded dimensioning rules and the standard's specifications which include the particular considerations of standardization. It is important to notice that in accordance with the experimental results, but in opposition to the specifications of IEC 664, all of the recommended distances are bases on the MSL air pressure.
5.1 Parameters for the dimensioning
The smallest possible insulation distances can only be obtained, if the following parameters, which determine the insulation characteristics, are optimized as far as possible.
5.1 .l Long-term voltage stresses
Mainly the working voltage and temporary overvoltages need to be considered as long-term voltage stresses. The dimensioning of creepage distances is based on the direct voltage or on the r.m.s. value of the alternating voltage.
Dimensioning rules for alternating voltage stresses of clearances have to be based on the amplitude. This is relevant to a wide frequency range up to about 1 kHz. Physically founded dimensioning rules for higher frequencies cannot be given by now.
It is also unknown by now, to which extent repetitive overvoltages belong to the short-term voltage stresses, respectively at which frequency they need to be considered as long-term voltage stresses.
5.1.2 Overvoltage categories
The classification of overvoltages for the purpose of insulation co-ordination has to take into account - the requirements of the equipment concerned with regard to reliability and
availability - the expected maximum voltage values. If the rating is based on high overvoltages, there is a correspondingly low probability that rare higher voltages will cause an insulation breakdown and therefore a failure of the equipment.
Where a low-voltage system is energized from an extended cable system, lightning-induced overvoltages can be discounted. Firstly, overvoltages on the overhead line system are greatly reflected upon entry into the cable system with a considerably lower impedance; secondly, lightning strokes in the vicinity of buried cables usually only induce low overvoltages. In extended cable systems, a considerable attenuation of the overvoltages can also be expected.
The studies described in chapter 2.2 justify the classification of expected overvoltages in 2301400 V systems under the following four categories. The worst case is considered, i.e. the transient overvoltage occurs at the peak of the steady- state voltage.
Category I: Sites which are protected against transient overvoltages. Examples might be protected electronic circuits. The rating of insulation is governed by the level of protection of the surge suppressor in use.
Category II: Sites at which external overvoltages are not expected, e.g. installations fed via cable. Examples might be appliances, portable tools and other household and similar loads. The insulation here must be rated for short-term voltages of up to 2.5 kV.
Category Ill: Sites of Category II at which particular demands are made for reliability and availability. A lower probability of failure can be achieved here by means of a higher, rated impulse withstand voltage. Examples might be switches in the fixed installation and equipment for industrial use with permanent connection to the fixed installation. The rating could be based on short-term voltages of up to 4 kV, for example.
Category IV: Sites at which external overvoltages are expected, e.g. unprotected installations which are fed via an overhead line. Examples might be electricity meters and primary overcurrent protection equipment. The insulation here must be rated for short-term voltages of up to 6 kV.
More precise stipulations, which also take into account the dependence of overvoltages on the rated system voltage, can only be made after completion of the overvoltage measurements in low-voltage networks and systems, currently in progress.
5.1.3 Micro-environmental categories
The overall results obtained according to the chapters 2.3.1 to 2.3.5 may serve to allocate the ambient conditions relevant to low-voltage insulation to micro- environmental categories. The classification should be based on the assumption that the effect of manufacturing residual matter on insulation performance is negligible, compared to the effect of environmental pollution. Furthermore, it can be assumed that the long-term mean concentration values of corrosive air pollution are under the limits of Class 3C2 of Publication IEC 721-3 [2]. Metallically conductive pollution is not taken into account. Additionally, the definition of environmental categories must be based on the usual time-of-day and seasonal fluctuations in air humidity, i.e. continuous operation in the upper humidity region is not taken into account.
In detail, this results in the following classification with which the severity of stress caused by environmental effects rises in the order of te micro-environmental categories MEC 1 to MEC 4. This classification, which is mainly oriented on the air humidity expected under service conditions, is summarized in Table 5.1 .l.
Table 5.1 -1. Definition of the Micro-environmental Categories (MEC)
MEC relative humidity (r.h.1
short time, in total max. 100 h/a: 75% cr.h.5 85%
short time, in total max. 100 h/a: 75% <r.h.l 85%
max. 30 h, in total max. 7 d/a: 95% cr.h.5 100%
max. 30 h, in total max. 7 d/a: 95% cr.h.5 100%
salt deposition
conden- sation
short time
short time
precipi- tation
The micro-environmental category 1 covers conditions under which insulation always remains so clean and dry that pollution has no effect. There must be no depositing of salt-laden pollution during the entire service life. The relative air humidity at the insulation must always remain below 75%, with no moisture
condensation. Such environmental conditions are encountered with insulating configurations which are fully encapsulated in the dry state.
The micro-environmental category 2 covers indoor conditions in which the relative air humidity immediately at the insulation does not usually exceed 75%. This can be exceeded for a short time up to a maximum of 85% and totaling up to 100 hours a year.
The effect of pollution can be classified additionally according to whether its salt deposition exceeds a limit of about 0.1 g/m2 during the entire service life. However, this classification is only of secondary importance compared to that relating to the air humidity encountered.
The micro-environmental category 3 covers locations which are sheltered but subjected to circulating outside air with cyclic humidification and dehumidification, approximately daily. The relative humidity at the insulation frequently exceeds 75%; the total duration above 95% may amount to one week in a year, but not more than 30 hours without interruption.
The effect of pollution can be classified additionally according to whether its salt deposition exceeds a limit of about 0.1 g/m2 during the entire service life. However, this classification is only of secondary importance compared to that relating to the air humidity encountered.
The micro-environmental category 4 covers unsheltered locations.
This definition of environmental categories is based on the exclusion of extreme pollution such as conductive deposits or highly salt-laden deposits.
The definition of micro-environment categories also meets the limits of the usual classes of IEC 721-3 with respect to air humidity in rooms. The micro- environmental conditions for open, unprotected insulations are the same as the macro-environmental conditions if the influence of the equipment itself can be discounted.
The requirements of micro-environmental category 1 relating to humidity are met by climatic category 3K2 of IEC 721 -3-3 for open, unprotected insulations. For example, a room for a computer system with air conditioning and dust filtering of the air falls in this category.
The requirements of micro-environmental category 2 relating to humidity are met by climatic category 3K2 of IEC 721-3-3 for open, unprotected insulations.
Most sites described with category 3K3 also meet the condition. For example, laboratories and closed workshops fall in this category.
The requirements of micro-environmental category 3 relating to humidity are met by climatic categories 3K5 or 3K6 of IEC 721-3-3 for open, unprotected insulations. Outdoor equipment which is not fully encapsulated may also fall in this category.
The humidity conditions of micro-environmental category 4 apply to open, unprotected insulations of climatic category 4K3 of IEC 721 -3-4.
The micro-environmental conditions can be improved with respect to external environmental conditions by means of enclosures, encapsulation, ventilation or heating. In this way, even the micro-environmental categories 1 or 2 can be achieved at unfavourable locations. The micro-environmental conditions may be worsened with respect to the external environmental conditions by the equipment itself, e.g. by self-created humidity, abrasion, arcing or manufacturing residual matter.
5.1.4 Insulation material categories
The informations of chapter 2.4 may be used to define insulation material categories. For an assessment of thermal stability under the effects of leakage currents, it is sufficient to distinguish between three categories of insulating material as shown in Table 5.1.2:
Table 5.1.2. Classification of the insulation materials with respect to the tracking resistance
category tracking resistance
I T C 1
The limit of the lowest category corresponds to experience gained thus far in practice. The limit of the highest category is given by the method of IEC 112 which
C T I 1 6 0 0
T C 3 C T I < 175
uses 600 V as the maximum test voltage. The variation and precision of the IEC 1 12 procedure definitely justifies no finer distinction as it is established in the 1980 edition of IEC 664.
With insulating materials of the lowest category TCI, it is necessary to ensure that no scintillations occur at the pollution layer during operation. Owing to the low tracking resistance, creepage paths would otherwise form rapidly and result in insulation failure. With the insulating materials of categories TC2 and TC3, sporadic scintillations at the pollution layer may occur without unacceptable impairment of the insulating property. However, application under rough environmental conditions is definitely an assignment for the insulating materials of category TC3.
For classification of the water adsorption, it should be noted that this has a relatively low effect on electric strength and, furthermore, there is no established method of determining it. It is therefore advisable to distinguish between only two categories in accordance with Table 5.1.3:
Table 5.1.3. Classification of the insulation materials with respect to the water adsorption.
category water adsorption
water adsorption has no influence on the flashover voltage
water adsorption decreases the flashover voltage
The category WC2 includes the materials of the adsorption groups II to IV according to chapter 2.4.3, the category WC1 the materials of the adsorption group l.
5.1.5 Electrode shaping
Compared to all other electrode arrangements, the point-plate arrangement exhibits the most inhomogeneous field and hence the lowest electric strength. It therefore serves as a basis in the consideration of the worst case. The following recommended insulation distances for inhomogeneous field conditions are based on a needle-plate arrangement with a needle rounding radius of 25 to 30 pm. Smaller radii can hardly be expected with today's usual deburring methods for punched parts.
5.2 Dimensioning of the clearances
The design of clearances has to take into account that very small spacings can be significantly reduced by dust particles which adhere to the electrodes. In these cases increased clearances might be necessary. According to IEC 75(Sec)94 [5] the following clearance excesses can be recommended: - In micro-environmental category 11: 75 pm, - In micro-environmental category 111: 150 pm. In micro-environmental category IV it might be necessary in addition to take precipitation into consideration, such as rain or snow.
The following recommendations for appropriately designed clearances are expressed in graphs. In opposite to the previously described physical fundamentals these graphs use the voltage for the x-axis and the distance for the y-axis. This follows the use in the revised standard IEC 664 and is intended to facilitate the comparison.
5.2.1 Dimensioning for steady-state voltage stresses
The design of clearances for steady-state voltage stresses under homogeneous field conditions are recommended according to figure 5.2.1 which shows the same data as figure 3.1.1.
Figure 52.1. Physically founded minimum clearances S under homogeneous field conditions depending on the peak value of the steady-state voltage stress U at MSL [24].
Partial discharges do not occur in the homogeneous field because the discharge conditions are identically everywhere between the electrodes either given or not.
The design of clearances for steady-state voltage stresses under inhomogeneous field conditions are recommended according to figure 5.2.2 which is deducted from figure 3.1.2.
Figure 5.2.2. Physically founded minimum clearances S in order to avoid dielectric breakdown under inhomogeneous field conditions depending on the peak value of the steady- state voltage stress U at MSL [l 21.
The dimensioning rules the first edition of publication IEC 664 [l] for clearances with inhomogeneous field under steady-state voltage stress are based on measurements of Hermstein [23]. He used in his laboratory experiments a needle- plate-arrangement with a fairly pointed needle of presumably about 7 pm [l21 (specified is 25 pm). However, according to chapter 5.1.5 it is sufficient using the smallest radius of 25 to 30 pm as a basis for dimensioning rules. The design of clearances can therefore be based on figure 5.2.2 which leads to slightly reduced spacings compared to the values in the first edition of publication IEC 664 [l]. An edge-plate-arrangement with the same edge radius provides a higher voltage strength.
In the cases in which partial discharges are to be avoided, the design of clearances should generally be based on figure 5.2.3, which is deducted from
figure 3.1.3. However, smaller spacings are possible if partial discharge measurements proved a less inhomogeneous field.
Figure 5.2.3. Physically founded minimum clearances S in order to avoid partial discharges under inhomogeneous field conditions depending on the peak value of the steady-state voltage stress U at MSL [12].
5.2.2 Dimensioning for short-term voltage stresses
Since no explicit experimental results are available, the design of clearances short-term voltage stresses under homogeneous field conditions can also based on figure 5.2.1, which was deducted for steady-state voltage stresses.
for be
Clearances for short-term voltage stresses under inhomogeneous field conditions can be based on the 'Case A' curve of IEC 664 which is reproduced in figure 5.2.4 for main sea level. Even though not all of the measurements cited in the appendix of the first edition of publication IEC 664 [l] could be confirmed in detail, the described newer investigations showed that clearances dimensioned
according to this curve can mostly withstand even 10/700 ps impulses. However, significantly longer impulses may require increased clearances.
Figure 5.2.4. Minimum clearances s under inhomogeneous field conditions depending on the peak value of the impulse voltage stress U according to the first edition of publication IEC 664 [l]. Like the other recommendations in this book the figure is related to main sea level (MSL).
5.3 Dimensioning of the creepage distances
The following recommendations for appropriately designed creepage distances are expressed in graphs. In opposite to the previously described physical fundamentals these graphs use the voltage for the x-axis and the distance for the y-axis. This follows the use in the revised standard IEC 664 and is intended to facilitate the comparison.
5.3.1 Dimensioning for steady-state voltage stresses
Systematical investigations of the flashover mechanisms under the steady-state stress of direct or alternating voltage, especially at polluted surfaces, have not been published so far. Some single measurements indicated that the strength under steady-state stress is lower than under the short-term voltage stresses according to chapter 5.3.2.
However, the design of creepage distances need no further investigations on the long-term flashover strength as long as the minimum insulation resistances according to chapter 4.1.4 are aimed. In this case the creepage distances become automatically sufficient long to withstand the steady-state voltage stress without flashover. All of the cited investigations indicate in addition that for creepage distances which are designed in order to provide the minimum insulation resistance according to chapter 4.1.4 also the formation of tracks is avoided.
Figure 5.3.1 shows the recommendations for the design of creepage distance in order to provide a minimum insulation resistance. It uses the same data as figure 4.1.7. For creepage distances in category 1 no recommendations are given, since the steady-state voltage needs no special consideration. The influence of the CTI on the insulation has been proven to be negligible as long as the minimum insulation resistance is achieved. Thus, the shown dimensioning curves apply to most of the investigated insulation materials with only few exceptions. For micro-environmental category 3, however, it should be noted that for the polyimide film insulating material the minimum creepage distance obtained was 6.3 mm instead of 2.5 mm, when operated near a major chemical plant with a direct voltage stress of 250 V. For the - FR 4 glass fiber reinforced epoxy fabric, and - polycarbonate
insulating materials, minimum creepage distances of 6.3 mm are not sufficient when allowing for the worst case at all installation sites examined for micro- environmental category 3.
t
I I I l l I l l l I I 1 1 1 1
1 I l I I I I I I I I I 1 1 1 1 1 1 1 l l1
I I I l 1 1 i l l I l I 1 1 1 1 1 1 I I l I l I l l
Figure 5.3.1. Physically founded minimum creepage distances S in order to achieve the minimum insulation resistance - lo7 Ohms for pollutions of the micro-environmental category 2 (MEC 2)
measured in the climate 23 'C / 50% relative humidity or - 10' Ohms for pollutions of the micro-environmental category 3 (MEC 3)
measured in the climate 23 'C / 83% relative humidity during 15 years of service life, depending on the r.m.s value of the voltage U and the type of voltage stress (d.c. or a.c.). Curve 1 : direct voltage stress in MEC2 Curve 2: alternating voltage stress in MEC2 Curve 3: direct voltage stress in MEC3 Curve 4: alternating voltage stress in MEC2
The resistance to tracking (CTI) of the insulating materials used does not initially contribute to the dimensioning rules, because the design according to Figure 5.3.1 avoids a significant flow of leakage currents. Under particularly unfavourable
environmental conditions, however, or in service other than that prescribed, the CTI also contributes to the insulation performance.
A comparison between the rated values of figure 5.3.1 and the first edition of publication IEC 664A shows that the present stipulations include safety margins, some of which are substantial, at least for the conditions examined in the field trial [14]. Adequate insulation resistances should also be achievable here with considerably smaller creepage distances, particularly at low voltages and with good insulating materials [27]. The minimum creepage distances of figure 5.3.1 were determined with high confidence levels; safety margins are therefore not required.
5.3.2 Dimensioning for short-term voltage stresses
The short duration of voltages impulses needs no considerations concerning tracking and partial discharges. Figure 5.3.2 shows the recommendations for the design of creepage distance in order to avoid insulation flashover. It uses the same data as figure 4.2.2.
The minimum creepage distances of figure 5.3.2 were determined with high confidence levels; safety margins are therefore not required.
Figure 5.3.2. Physically founded minimum creepage distances s in order to avoid flashover during a service life of 15 years depending on the peak value of the impulse voltage stress U [14]. The figure distinguishes between the micro-environmental categories according to chapter 5.1.3: MEC1 : dotted MEC2 : dashed MEC3 : solid. The figure also distinguishes between the insulation categories WC2 and WC1 according to table 5.1.3. It was necessary to separate an additional category WC2A which applies to the materials A1203 ceramic (unglazed) and epoxy resin laminated glass fabric FR 4 in the micro-environmental category 3. All of the values are related to main sea level [ l 21. Curve 1 : material WC2A in MEC3 Curve 2: material WC2 in MEC3 Curve 3: material WC1 in MEC3 Curve 4: material WC2 in MEC2 Curve 5: material WC1 in MEC2 Curve 6: material WC1 or WC2 in MECl
5.4 Co-ordination of the insulation distances
During the definition of the parameters of the insulation co-ordination according to chapter 5.1 the following was to be determined: - the long-term applied working voltage U according to chapter 5.1 . l , - the overvoltage category OC according to chapter 5.1.2 which defines the stress
by short-term applied voltages, - the micro-environmental category MEC according to chapter 5.1.3, which
defines the environmental conditions and - the insulation categories TC and WC according to chapter 5.1.4, which classify
the properties of the used insulation material under the influence of creepage currents and flashovers.
In the second step the clearances and creepage distances were adjusted to the different stresses. This resulted in - the minimum clearance SLI under long-term voltage stresses according to
chapter 5.2.1, depending on U, - the minimum clearance s ~ k under short-term voltage stresses according to
chapter 5.2.2, depending on OC, - the minimum creepage distance SKI under long-term voltage stresses according
to chapter 5.3.1, depending on U, MEC and WC, - the minimum creepage distance s ~ k under short-term voltage stresses
according to chapter 5.3.2, depending on OC, MEC and WC.
It is trivial that the dimensioning is to be based on the larger one of the two determined minimum clearances and on the larger one of the two determined minimum creepage distances:
If this results in a larger creepage distance than clearance (sK > sL) the minimum design of the insulation arrangement requires ribs or grooves. At plane arrangements the electrode spacing is determined by the creepage distance in this case.
If a smaller creepage distance than clearance is resulted (sK < sL) the design is only determined by the clearance.
A comparison with the dimensioning rules according to the first edition of publication IEC 664 shows that insulation distances which are in some cases considerably smaller become possible using stress-related coordination of
insulation, particularly at low voltages and with good insulating materials. Under unfavourable service conditions, however, greater distances than those currently specified may also be required. This is no contradiction to the good experiences during the use of the dimensioning rules of the first edition of publication IEC 664 [l]. The reason is that in many cases the macro-environmental conditions were regarded during the choice of the pollution degrees instead of the micro- environmental categories. Therefore the insulations which were in most cases protected by an enclosure had often been operated in significantly better micro- environmental categories than assumed.
References
IEC Publication 664 (1980), 664A (1981): Insulation Co-ordination within Low-voltage Systems, including Clearances and Creepage Distances for Equipment. Genf 1980 und 1981 IEC Publication 721 : Classification of Environmental Conditions Part 3: Classification of Groups of Environmental Parameters and their Severities - 721 33 (1987): Stationary Use at Weather-protected Locations - 721-3-4 (1987): Stationary Use at Non-weatherprotected Locations
IEC Publication 112 (1979): Method for Determining the Comparative and the Proof Tracking Indices of Solid Insulating Materials under Moist Conditions.
DIN 476818.74: Errnittlung der RauheitsmeOgroOen R,, R,, R,, mit elektrischen Tast- schnittgeraten. Blatt 1: Grundlagen. Berlin & Koln: Beuth Verlag.
IEC 75 (Sec.) 94 / May 1988: Draft - Publication 721-2 Environmental conditions appearing in nature. 721 -2-5: Dust, sand, salt mistlwind.
Hosemann, G. (Hrsg.): Hutte, Elektrische Energietechnik, Bd. 3 Netze. Kapitel 1.2; Schneider, K.-H.; Stimper, K.: lsolationsbeanspruchung und -bemessung. Berlin: Springer 1988, pp. 68-1 01 Roth, A.: Hochspannungstechnik, 4. Auflage. Berlin: Springer 1959
Beyer, M.; Boeck, W.; Moller, K.; Zaengl, W.: Hochspannungstechnik: theoret. U. prakt. Grundlagen fur die Anwendung. Berlin; Heidelberg; New York; London; Paris; Tokyo: Springer l986
Stimper, K.: Thermische Beanspruchung verschmutzter Isolatoroberflachen. Dissertation Universitat Erlangen-Nurnberg, Erlangen 1981
Ocker, W.: Thermische Beanspruchung verschmutzter Isolatoroberflachen durch Gasent- ladungen bei Niederspannung. Dissertation Universitat Erlangen-Niirnberg, Erlangen 1986
Richter, K.: Die elektrische Festigkeit kleiner lsolierstrecken unter dem EinfluO natiirlicher Urngebungsbedingungen. Dissertation Technische Hochschule Darmstadt, Darmstadt 1986
Uhlemann, F.: Erarbeitung neuer Bemessungsregeln fiir Kriechstrecken in Niederspan- nungsbetriebsrnitteln. Dissertation Technische Hochschule Darmstadt, Darmstadt 1990
Sachsenweger, C.: Klassifizierung und Simulation der fur Niederspannungs-lsolierungen maRgebenden Urngebungseinflusse. Dissertation Universitat Erlangen-Nurnberg, Erlangen 1989 Schau, P.v., Middendorf, W.H.: An International Research Project to Determine New Dimensioning Rules for Creepage Distances. IEEE Trans. on Electrical Insulations, Vol.EI- 18, No.2, April 1983, pp. 158-162
Schwetz, S.: Bemessungsregeln fur Kriechstrecken unter langzeitiger Spannungsbean- spruchung. W-Bericht P46-1, Universitat Erlangen-Nurnberg, Erlangen (unpublished)
Martzloff, F.D: The propagation and attenuation of surge voltages and surge currents in low-voltage ac circuits. IEEE Trans. Vol. PAS-1 02 (1 983) No. 5, pp. 11 63-1 170
Institute for Interconnecting and Packaging Electronic Circuits: Technical Report IPC-TR- 476: How to avoid metallic growth problems on electronic hardware. Evanston/lllinois (USA) 1977
Institute for Interconnecting and Packaging Electronic Circuits: Technical Report IPC-TR- 468: Factors affecting insulation resistance performance of printed boards Evanston/lllinois (USA) 1979
Stimper, K; Middendorf, W.H.: Mechanisms of deterioration of electrical insulation sur- faces. IEEE Trans. on E.I. 19 (1984) No. 4, pp. 314-320
Stimper, K; Sachsenweger, C.; Middendorf, W.H.: Experiments on tracking enhancement by copper. IEEE Trans. on E.I. 23 (1988) No. 2, pp. 243-248
Stimper, K; Sachsenweger, C.; Middendorf, W.H.: The chemistry of insulation tracking with copper electrodes. IEEE Trans. on E.I. 23 (1988) No. 6, pp. 987-991
Middendorf, W.H.: The use of copper electrodes for the Comparative Tracking Index test. IEEE Trans. on E.I. 20 (1985) No. 3, pp. 537-542
Hermstein, W.: Bemessung von Luflstrecken, insbesondere fiir 5OHz-Wechselspannung. ETZ-A 90 (1 969), pp. 25 1 -255
Dakin, T.W.; et al.: Breakdown of gases in uniform fields; Paschen curves for nitrogen, air and sulphur hexafluoride. Electra 32 (1 974), pp. 61 -82
Holte, K.C.: Application of insulators in a contaminated environment IEEE Trans. on PAS 95 (1 979) NO. 5, pp. 1676-1 695
Stimper, K: Physical fundamentals of insulation design for low-voltage equipment. IEEE Trans. on E.I. 25 (1990) No. 6, pp. 1097-1 103
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UewellynJones, F.; Davies, D.E.: Influence of cathode surface layers on minimum spar- king potential of air and hydrogen. Brit. J. appl. Phys. 64 (1951), pp. 397-404
7 Index
air pressure 28
altitude 28
breakdown voltage
dependency on the temperature 46
in the homogeneous field 44, 54
influence of the humidity 29
influence of the frequency 51
in the inhomogeneous field 46, 55
influence of the frequency 51
influence of the needle radius 46, 55
influence of the polarity 46
influence of the voltage rise-time 55
with positve needle 46
with negative needle 47
influence of the air pressure 45
influence of the humidity 20, 28
influence of the frequency 51
clearances 6
co-ordination with creepage distances
98
dimensioning 43
breakdown 44,46,54,55
partial discharges 46
reduction by conductive paricles 50, 89
condensation 20, 38, 82
corrosion products 34, 35, 38
creepage currents 7
creepage distances 6
co-ordination with clearances 98
dimensioning 94
influence of grooves 79
influence of ribs 79
insulation degradation 70
to avoid flashover 58,94, 96
to avoid partial discharges 58
to avoid tracking 60
CTI 36, 87, 95
dielectric breakdown 6
dust 16, 19, 30, 50, 89
electrolytes 16
electronic-conductors 16
monthly deposition 30
salt content 17, 30
electrodes 6
material 19, 34
shape 42
surface 44,55
flashover 6
flashover voltage 24, 38, 40, 58, 75, 96
at grooves 79
at ribs 79
influence of the humidity 20, 24, 40
long-term changes 77
gases, corrosive 19, 28, 34, 84
grooves 79
humidity 18, 19, 40, 44, 73, 85, 86, 95
characteristic relative humidity 18
critical humidity 41
daily changes 20
influence on surface layers 33
medium humidity 23
thresholds for classification 22
industrial atmosphere 19, 28
insulation material 35
surface roughness 38
insulation material categories 87
insulation resistance 20, 22, 65, 95
without voltage 72
at sufficient spacings 73
under pollution 3
influence of the voltage 67
long-term changes 69
measurement 66
changes 66
insulation strength 7
insulation stress 7
MEC: see micro-environmental categories
micro-environmental categories 7, 70, 73,
77,84, 95, 97
moisture: see humidity
nominal voltage 6
overvoltages
at the system's origin 11
attenuation 15
functional overvoltages 6, 10
in low-voltage systems 13, 16
in the distributing system 10
lightning overvoltages 10, 12, 13
standardized test impulse 12
switching overvoltages 12, 13, 54
standardized test impulse 12
transient overvoltages 6
overvoltage categories 83
partial discharges 6, 25, 43,44, 46, 89
at creepage distances 58
extinction voltages 48
in the homogeneous field 90
in the inhomogeneous field 46, 89
influence of the needle radius 49
onset voltages 48
Paschen equation 28,44
Paschen minimum 45
pollution 16, 29
moisturing 22
by manufacturing 17
ribs 79
roughness 38
salt 30, 34, 38, 85, 86
threshold for classification 32
scintillation 60
soldering fluids 17
stress, thermal
amplification by pollution 34, 38
avalanche development time 53
development 43, 52
discharge development time 53
from chemical reactions 60
from Joulean heat 60
from scintillation 60
pre-ionization 43, 53
statistical delay time 53
surface discharges 60
surface roughness 38
temperature
daily changes 21
track 7
tracking 7, 60
tracking resistance 7, 36, 57, 87, 95
test method 36
travelling waves 1 1
voltage stress
short-term 10,52, 75, 83, 92,96
steady-state 9, 44, 57, 82, 89, 94
water adsorption 24, 40, 58, 77, 88, 96
working voltage 6
The author The author is general manager of AENA Angewandte Energie- und Automatisierungstechnik GmbH in Erlangen/Germany. From the end of his university studies until now he is concerned wi th dimensioning and testing of low-voltage insulations in the fields of research, design and standards development. Many publications and even the subjects of his dissertation and habi- litation works at the University of Erlangen-Nurnberg dealt wi th this topic. For more than ten years he was involved in the revision of the German insulation co-ordination standard DIN VDE 0110. He chaired a working groE9 which aimed at providing the fundamentals for a physically founded stress-adequate co-ordination of low-voltage insulations. The recently completed results of this working group are the basis for the current revisions of the publica- tions IEC 6 6 4 and IEC 664A.
The book The first editions of the publications IEC 6 6 4 and IEC 6 6 4 A are subject t o fundamental revi- sions in order t o settle a physically founded stress-adequate co-ordination of low-voltage insulations. It is aimed at obtaining the smallest-possible insulation distances by directly adapting the insulation strength to the certain insulation stresses whichare to be expected during operation of the according equipment. The basis for this revised insulation co-ordina- tion are next t o the operating experience the numerous scientific investigations carried out in co-operation with the German National Standards Committee during the last years. This book is intended t o explain the background of the requirements of the first edition publications IEC 6 6 4 and IEC 664A and the new requirements defined in the revision and the supplemen- tary drafts. It gives a revised summary of the fundamental investigations in order to make it easier for the user to judge the importance and safety of the requirements.
The adressed readers The book is written for design engineers, manufacturers and users of electrical low-voltage equipment, for members of standards committees, for engineers of testing laboratories and for students in electrotechnical faculties.
