CT 149, first issued octobre 1992 Frédéric Vaillant Mr. Vaillant graduated from the Ecole Polytechnique in 1984 (X81) and was awarded a PhD in microelectronics in 1987 (thesis prepared under a CIFRE contract with the Saint Gobin company). His career at Merlin Gerin began in 1987 within the Research Depart- ment, where he was in charge of a project concerning static current interruption techniques for medium voltage applications. Since the end of 1988 he has been in charge of Electromagnetic Compatibility within the Electronics Proficiency Centre of the Research and Development Management Division. n° 149 EMC: electromagnetic compatibility
Transcript
CT 149, first issued octobre 1992
Frédéric Vaillant
Mr. Vaillant graduated from theEcole Polytechnique in 1984 (X81)and was awarded a PhD inmicroelectronics in 1987 (thesisprepared under a CIFRE contractwith the Saint Gobin company).His career at Merlin Gerin began in1987 within the Research Depart-ment, where he was in charge of aproject concerning static currentinterruption techniques for mediumvoltage applications. Since the endof 1988 he has been in charge ofElectromagnetic Compatibility withinthe Electronics Proficiency Centre ofthe Research and DevelopmentManagement Division.
n° 149EMC:electromagneticcompatibility
Cahier Technique Merlin Gerin n° 149 / p.2
Cahier Technique Merlin Gerin n° 149 / p.3
EMC: electromagnetic compatibility
summary
1. Introduction Electromagnetic Compatibility - p. 4EMC - a characteristic anda disciplineToday, EMC is indispensable p. 4EMC theory is complex p. 5
2. The source The importance of identifying p. 5the sourceAn exemple of a continuous p. 6source of conducted disturbancesin power electronicsAn example of radiated p. 7disturbance sources:circuit closing in MT andTHT substations
3. Coupling Different coupling modes exist p. 8Common or differential mode p. 8field to wire couplingCommon impedance coupling p. 10Differential mode wire to wire p. 11coupling or crosstalk
4. The susceptor Equipment malfunction p. 12Solutions to the problem p. 12
5. Installation Installation is an important factor p. 14in the overall system EMCDesign phase p. 14Installation phase p. 14Pratical examples p. 14
6. Standards, test facilities and Standards p. 17Test facilities p. 17Tests p. 18
7. Conclusion p. 23Appendix 1: glossary p. 24Appendix 2: impedance of a conductor at high frequencies p. 25Appendix 3: the different parts of a cable p. 26Appendix 4: tests performed at the Merlin Gerin EMC laboratory p. 27Appendix 5: bibliography p. 28
For all electrotechnical equipment,EMC must be considered right from theinitial design phase and the variousprinciples and rules carried on throughto manufacture and installation.
This means that all those involved, fromthe engineers and architects thatdesign a building to the technicians thatwire the electrical cabinets, includingthe specialists that design the variousbuilding networks and the crews thatinstall them, must be concerned withEMC - a discipline aimed at achievingthe "peaceful" coexistence of equip-ment sensitive to electromagneticdisturbances alongside equipmentemitting such disturbances.
This publications is a compilation ofmore than the years of acquiredexperience at Merlin Gerin andpresents various disturbancesencountered and provides somepractical remedies.
tests standards
Cahier Technique Merlin Gerin n° 149 / p.4
1. introduction
electromagneticCompatibility -EMC-a characteristic and adisciplineEMC is a characteristic of equipment orsystems that mutually withstand theirrespective electromagnetic emissions.
According to the International Electro-technical Vocabulary IEV161-01-07,EMC is the ability of an equipment orsystem to function satisfactorily in itselectromagnetic environment withoutintroducing intolerable electromagneticdisturbances to anything in thatenvironment.
This definition has also been adopted inthe NF C 15-100 standard, chapter 33.
EMC is now also a discipline aimed atimproving the coexistence of equipmentor systems which may emit electro-magnetic disturbances and/or besensitive to them.
today, EMC isindispensableEquipment or systems are alwayssubjected to and, to some extent,generate electromagnetic disturbances.These disturbances are generated inmany ways. However, the main under-lying causes are sudden variations incurrent or voltage.The most common electrical distur-bances (see fig.1) in the low voltageelectrotechnical field are discussed inCahiers Techniques Publicationno. 141. Cahiers Techniques Publi-cation no. 143 discusses disturbancesgenerated when operating mediumvoltage switchgear.These disturbances can be propagatedby conduction along wires or cables orby radiation in the form of electro-magnetic waves.
Disturbances cause undesirablephenomena. Two examples are radio
class type originhigh energy voltage dips power source switching
short circuits starting of high power motors
medium frequency harmonics systems with power semi-conductors electric arc furnaces
high frequency overvoltages direct or indirect lightning strikes switching of control devices breaking of short-circuit currents byprotection devices
electrostatic discharge of static electricitydischarges stored in the human body
fig. 1: the most common electric disturbances.
wave interference and interference withcontrol and monitoring systems causedby electromagnetic emissions.
In recent years, several trends havetogether made EMC more importantthan ever: disturbances are becoming strongerwith increasing voltage and currentvalues, electronic circuits are becomingincreasingly sensitive, distances between sensitive circuits(often electronic) and disturbing circuits(power circuits) are becoming smaller.
In the development of its new products,Merlin Gerin foresaw the necessity ofunderstanding and applying EMCprinciples. In modern electricalswitchgear and controlgear, low andhigh currents, control and powerelectronics, electronic protection andelectric power devices all reside inclose proximity.
EMC is therefore a fundamentalcriterion that must be respected in allphases of product development andmanufacture (see fig. 2), as well asduring installation and wiring.Moreover, EMC is now included instandards and is becoming a legalrequirement.
fig. 2: EMC application example: a medium-voltage Fluair panel containing a circuitbreaker designed to interrupt hundreds ofampere at tens of kilovolts and a SEPAMprogrammable control, monitoring andprotection unit. The complete assembly mustremain operational under all circumstances.
Cahier Technique Merlin Gerin n° 149 / p.5
The experience and achievements ofMerlin Gerin are not limited to thesatisfactory operation of electrical and/or electronic systems in their usualelectromagnetic environment:Merlin Gerin designs and buildsequipment capable of withstandingharsher conditions such as electro-magnetic radiation generated by highaltitude nuclear blasts.
The necessary radiation hardening,i.e. improvement of the immunity ofsystems exposed to electromagneticpulses from nuclear sources, requiresthe most advanced EMC techniques.
EMC theory is complexAny work involving EMC involves theanalysis of a three component system: the disturbance generator or source, propagation or coupling, the device or system affected or thesusceptor.Strictly speaking, the three entities arenot independent but for all practicalpurposes are assumed to be.Note that installation, described inchapter 5, plays the most important rolein the propagation of disturbances.Theoretical analysis is difficult because itmust deal with the propagation of
electromagnetic waves described by aset of complex differential equationsknown as Maxwell’s equations.
Generally speaking, they cannot besolved to yield an analytical solutionfor real devices and dimensions. Evenwith powerful computer systems, aclose numerical solution is oftenextremely difficult to obtain.
In practice, EMC problems musttherefore be dealt with via simplifyingassumptions, the use of models and inparticular conducting experiments andtaking measurements.
2. the source
the importance ofidentifying the sourceThe identification and measurement ofthe source is essential since the typeof source will determine which of thefollowing measures must be taken: limit the disturbances generated(e.g. on a contactor, by installing aninterference suppressing RC unit inparallel with the A.C. coil, or a diodeon the D.C. coil), avoid cross-coupling (i.e. physicallyseparate two highly incompatibleelements), desensitize potential susceptors(e.g. using shielding).
Main causesAny device or physical/electricalphenomenon that emits anelectromagnetic disturbance, eitherconducted or radiated, qualifies as asource. The main causes ofelectromagnetic disturbances areelectric power distribution, radiowaves, electrostatic discharge andlightning.
in electric power distribution, a largenumber of disturbances are created bycircuit switching operations. in the low voltage field, the opening ofinductive circuits such as contactor coils,motors, solenoid valves etc. generatesvery high surge voltages (up to severalkV across the coil terminals) that containhigh frequency harmonics (ten tohundreds of MHz). in the medium and high voltage fields,the opening and closing of disconnectorsproduces waves with a very fast rate ofrise (a few nanoseconds). These wavesare particularly harmful to micro-processor-based systems.
radio waves emitted by remotemonitoring systems, remote controls,radio communications, television sets,walkie-talkies etc. are, for someequipment, sources of disturbances inthe range of several volts per meter. Allof these disturbance emitters arenowadays increasingly common andsusceptible equipment must therefore beprovided with increasingly effectiveprotection.
an electrically charged human body:for example, a person walking oncertain types of carpet in a cold and dryclimate can be charged up to more than25 kV ! Any contact with equipmentproduces a discharge with a very fastrise time (several nanoseconds) whichenters the device by conduction andradiation, generating a majordisturbance.
Disturbance characteristicsSources may be intentional (e.g. radiotransmitters) or not (e.g. arc weldingunits). However in general they can bedistinguished by the characteristics ofthe disturbances they produce: spectrum, waveform, rise time or envelope ofthe spectrum, amplitude, energy.
the spectrum, i.e. the frequency bandcovered by the disturbance can be verynarrow as for the case of mobiletelephones, or very wide, as for electricarc furnaces.
Cahier Technique Merlin Gerin n° 149 / p.6
Pulse type disturbances cover a parti-cularly wide spectrum extending up to100 MHz or more (see fig. 3). To thislast category belong almost exclusivelysources such as: electrostatic discharges, switching of relays, disconnectors,contactors, switches and circuitbreakers in the LV, MV and HV range, lightning, nuclear electromagnetic pulses (aspecial domain).
Since the degree of coupling is directlyproportional to frequency, EMC usesthe frequency domain to characterizedisturbances. This type of represen-tation, for a periodic signal, is similar toa Fourier series decomposition (like asum of harmonics).
the waveform describes the charac-teristics of the disturbance with timeand can, for example, be a dampedsine wave or double exponentialfunction. It is expressed as a rise timetr, an equivalent frequency 1/π.tr orsimply the disturbance frequency for anarrow band signal or as a wavelength lrelated to frequency by l = c/f, where cis the speed of light (3.108 ms-1).
the amplitude is the maximum valuethe signal reaches in terms of voltage(Volts), electric field (Volts/Meter), etc.
the energy is the integral of theinstantaneous energy over the time thedisturbance lasts (Joules).
an example of a continuoussource of conducteddisturbances in powerelectronicsIn power electronics, the principalsources of disturbances are more oftenvoltage rather than current transients.The voltages can vary by hundreds ofvolts in a matter of a few nanosecondsgiving dV/dt’s in excess of 109 V/s.Pulse Width Modulation (PWM)(see fig.4), for example, used togenerate a sine wave voltage from aD.C. voltage, works with voltagechanges from 0 to Udc (660 V forrectified three-phase) occurring in avery short time, nano to microsecondsdepending on the technology used.Rapid voltage changes are the sourceof various disturbance phenomena, the
most problematic of which is, based onexperience, the generation of currentsflowing through any parasiticcapacitances.
Taking only the parasitic capacitanceCp into account, the common modecurrent: Icm = Cp . dV/dT.
With the rise times mentioned earlier, aparasitic capacitance of 100 pF issufficient to generate currents ofseveral hundred milliamperes.This disturbance current will flowthrough chassis ground (0 V reference
fig. 3: spectral characteristics of disturbances.
fig. 4: a source of disturbances in powerelectronics: pulse width modulation.a: principle,b: even with the time scale not well chosenfor this type of phenomenon tr ≈ 2 to 3 tf(10 ns to 1µs) while the sine wave covers20 ms.
(a)
U
Ucc
Uca
t
(b)
Ucc
t t f
t
AC curve
(part of a sine wave)
rRadio wave
amplitude ofdisturbance
0time
T
amplitude ofdisturbance
0time
Indirect lightning effect
t r
wide band
spectraldensity
01/ t rπ
narrow band
frequency1/T0
spectraldensity
frequency
Cahier Technique Merlin Gerin n° 149 / p.7
of the unit) of the electronics and can,via coupling, modify signals (infor-mation or controls), be superimposedon sensitive measurements and disturbother equipment by injecting thedisturbance back into the publicdistribution network.
One way of dealing with this type ofphenomenon, i.e. of ensuring EMC, isto increase the voltage rise time.However such a solution wouldconsiderably increase the switchinglosses in the transistors, producingharmful thermal stresses. Anothereffective way of reducing commonmode currents consists of increasingthe common mode impedance. Forexample, when mounting electronicpower devices, two methods arecommonly used:
either leave the heat sinks floating(no electric connection), (see fig. 5), ifsafety regulations are not violated, or reduce the parasitic capacitancebetween the device and the heat sinkusing an insulator with a low dielectricconstant (see fig. 6).
In the field of UPS’s - UninterruptiblePower Supplies- for instance, theabove precautionary measures makethe difference between a «polluting»system and a «clean» system.
For UPS’s, note that the low levelelectronics in the static inverter must beprotected against disturbances createdby its own power circuits.
It is necessary to understand andcontrol the phenomenon at the sourceto effectively and economically limitconducted emissions.
Other less frequent sources ofconducted disturbances exist such aslightning and switching surges that cangenerate large dV/dt’s and dI/dt’s.These disturbances also radiate.
an example of radiateddisturbance sources:circuit closing in MT andTHT substationsThe substation environment, especiallyin medium and very high voltageapplications, can contain very strongpulsed electromagnetic fields.
Certain switchgear operations cangenerate voltages much higher than therated value in a very short time. Forexample, when a 24 kV switch isclosed, the preignition phenomenoncauses voltage variations of tens ofkilovolts in a few nanoseconds (10-9 s).
This is discussed in greater detail inCahiers Techniques Publicationno. 153: «SF6 Fluarc circuit breakersand MV motor protection».
Measurements performed at the MerlinGerin laboratories have shown thatduring the switching of a 24 kV mediumvoltage circuit breaker, dampedsinusoidal pulsed fields reach peakvalues of 7.7 kV/m with a frequency of80 MHz at a distance of one meter fromthe cubicle.The field strength is enormous whencompared to that of a 1 W portable twoway radio (walkie-talkie) whichgenerates 3 to 5 V/m measured at adistance of one meter.
The transients propagate alongconductors, busbars, cables andoverhead lines. At the frequenciesinvolved, i.e. the rapidity of thephenomenon, the conductors(especially busbars) behave likeantennas and the characteristics of theelectromagnetic fields they emit arehighly dependent on the design of themetal enclosures (partitioning,cubicles).
In metal clad very high voltagesubstations, the electromagnetic fieldsare particularly strong.
Metal clad SF6-insulated substationshave a coaxial shape and thereforedisplay a constant characteristicimpedance. Rapid voltage changesinside the tubular metal enclosuresgenerate standing wave phenomena.They are created by reflectionsoccurring at impedance mismatchesdue to conic outgoing feedthroughs thatcross the shielding for example. Themagnitude and duration of thephenomenon is also increased by thiseffect.
fig. 5: the parasitic capacitance of the heatsink (for cooling of electronic devices) istaken into account in the design of UPSinverter stacks.
fig. 6: dielectric constants for the most common insulators used in mounting electronic devices.
insulating washer thickness parasitic capacitancefor TO3 case (mm) (pF)
Mica 0.1 160
Plastic 0.2 95
Alumina 2 22
@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ @@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ
ground
semi-conductorinsulator heat sink
Cp I CM
V
Cahier Technique Merlin Gerin n° 149 / p.8
The electronic environment at mediumand very high voltages requires indepth electromagnetic compatibilitystudies for the design and installation ofrelay systems and control andmonitoring devices.
This is particularly important because inaddition to the radiated disturbances,conducted voltage transients are alsogenerated in substations as discussedat the beginning of this section(see fig. 7).
fig. 7: SEPAM and Masterpact units; MV and HV protection and control and monitoring deviceswith digital electronics developed by Merlin Gerin and designed taking full advantage of EMCresearch.
3. coupling
different coupling modesexistCoupling refers to the linking, transferor transmission of electromagneticdisturbances from an emitter to asusceptor.
Coupling is expressed in terms of acoupling coefficient k, expressed in dB(e.g. -75 dB), which can be seen as thetransmission efficiency of thedisturbance from the emitter to thepotential susceptor(k = 20 log A (received)/A (transmitted),where A is the amplitude of thedisturbance).
It is important to define this coefficientfor EMC since the lower the coefficient(the larger its absolute value indecibels) the weaker the disturbancevoltage received by the susceptor andthe better the EMC.
This coefficient k is only meaningfulwhen the transfer of electromagneticdisturbances is proportional tofrequency, which is often the case inpractice.Three well known coupling modes canbe distinguished: common and differential mode field towire coupling, common impedance coupling,
differential mode wire to wire couplingor crosstalk.
common or differentialmode field to wire couplingAn electromagnetic field can couple intoany kind of wire-like structure andgenerate either common mode (withrespect to ground) or differential mode(between wires) voltages or, as isgenerally the case, both. This type ofcoupling is called field to wire couplingand is also known as the antenna effectof wiring, printed circuit board traces,etc.
Cahier Technique Merlin Gerin n° 149 / p.9
The equations that govern the couplingbetween the electromagnetic field(impedance of an arbitrary wave) anda wire-like structure (which can also bearbitrary) are very complex. In mostcases they can neither be solvedanalytically nor numerically.
Nonetheless, one of the simpler andmost common types of coupling canbe expressed analytically: the couplingbetween the magnetic component ofan electromagnetic field and a loop ofarea A formed by the conductors(see fig. 9).The magnetic component H of the fieldinduces in the loop a series voltageequal to:e = µ0 'A' dH/dt,with µ = the permeability in a vacuum(4π 10-7 H/m).
For example, in a medium voltagesubstation, a loop (of wire or cable)covering 100 cm2 placed 1 m from the
common mode coupling generatescommon mode disturbance voltages orcurrents.
A conducted common mode voltagedisturbance (VCM) is a voltage thataffects all active conductors.It is referenced to chassis or earthground (typically in electrical systems):all common mode isolation tests on lowvoltage circuit breakers are thereforeperformed between earth ground andall phases.
A common mode current (ICM) is acurrent that flows through all activeconductors in the same direction(see fig. 8). The current induced in a LVline by a lightning impulse is a commonmode current.
differential mode coupling involvesvoltages and currents in the classicsense, for example, between twophases of a circuit breaker or betweentwo wires which transmit sensor data tothe electronics.
fig. 8: common mode voltage and current between two relays of a low voltage compartment ina medium voltage cubicle.
disturbancegenerator
PEI
CM
VCM
ICM
Cp Cp
@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀfig. 9: an example of differential mode fieldto wire coupling.
electromagneticfield
e = voltage induced by theelectromagnetic field
E
H
e
surface exposedto electromagneticfield
Cahier Technique Merlin Gerin n° 149 / p.10
cubicle (see fig. 10) and exposed to apulsed field of 5.5 kVrms/m (laboratorymeasurement) will generate (by induc-tion) a series transient voltage of 15 V.The above equation holds as long asthe largest dimension of the loop does
not exceed a tenth of the wavelength ofthe disturbance. Note that such agreen/yellow wire loop (see fig. 10) iseasily created in the «relay compart-ment» when the wires are connected ina star configuration to ground.
common impedancecouplingAs the name implies, commonimpedance coupling results from animpedance that is shared by two ormore circuits. The common impedancecan be the ground connection, the earthground network, the power distributionnetwork, the return conductor shared byseveral low power signals etc...
An example follows showing the effectsof this type of coupling (see fig. 11): Adisturbance current in circuit A in thetens of mA range is sufficient togenerate disturbance voltages in the voltrange in circuit B. If circuit B uses pointM as its reference (possibly ground),then the reference can vary over severalvolts. This certainly influencesintegrated circuit electronics that workwith volta-ges of the same order ofmagnitude.The example in figure 11 shows that acommon impedance can be formed by awire a few meters in length and which iscommon to both circuits A and B.The disturbance has a magnitudeUc = Ia . Zc where Ia is the disturbance current and Zc is the common impedance(see fig. 12).At low frequencies the commonimpedance is usually extremely small.For example, safety requirementsdictate minimum cross-sectional areasfor the PE conductors, i.e. the green/yellow wires, of grounding networksdepending on the prospective short-circuit current. The impedance at 50 Hzbetween two points in the network istherefore always much lower than oneOhm.
fig. 10: example of a ground loop in a low voltage compartment of a medium voltage cubicle.
fig. 11: the quantities measured by the operational amplifier will be incorrect because thedisturbance current in circuit A (power supply) is high enough to create a disturbance voltage incircuit B (measurement). fig. 12: common impedance diagram.
@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ
0 volt
cubicle ground
0 volt
@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ
common Z
+
-
0 volt
input
I supply +I measurement
measurement circuit B
ground ofmeasurement device
supply circuit A
@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ@@ÀÀ supplycircuit
measurementcircuit
la = i1 + i2
E1 E2
Uc
Z1 Zc Z2
i1 i2
Cahier Technique Merlin Gerin n° 149 / p.11
But that same impedance can bemuch larger at the typical frequenciesof the disturbances discussed earlier.Impedances can reach several kilo-ohms or more (see appendix 2).
differential mode wire towire coupling or crosstalkCrosstalk is a mode of coupling thatresembles the field to cable coupling. Itis called capacitive or inductivecrosstalk, if a change in current orvoltage respectively is its cause.
A rapid voltage change between a wireand a ground plane or between twowires (see fig. 13) generates a fieldthat can nearby, with someapproximations, be considered anelectric field only.This field can couple into any otherparallel wire-like structure. This iscalled capacitive crosstalk.
Similarly, a current change in a wire orcable generates an electromagneticfield that with the same approximationscan be considered a magnetic fieldonly.The field can couple into a pair ofwires and induce a disturbancevoltage. This is called inductivecrosstalk (see fig. 14).
Capacitive and inductive crosstalkexists whenever conductors are routedin parallel or reside in close proximityto each other.Crosstalk can occur in cableways andtroughs and especially between powercables carrying high frequencydisturbances differentially and twistedpairs used by digital networks such asBatibus.The crosstalk will be stronger thelonger the parallel paths, the smaller
the distance between wires or pairs ofwires and the higher the frequency ofthe disturbances.
For example, using the notation infigure 13, the voltage couplingcoefficient (capacitive crosstalk) canbe expressed as:
V N
V1 =
j 2 π f C 12
(C 12 + C 20)
j 2 π f C 12
R (C 12 + C 20)
where: V1: voltage source, VN: disturbance voltage induced bycoupling, C12: coupling capacitance betweentwo wires which is proportional to thewire length and the distancecoefficient Log [1 + (h/e)2] where h isthe distance between the two wires ofthe pair and e the distance betweenpairs, C20: leakage capacitance betweenthe two wires of the pair creating thedisturbance, R: load impedance of the susceptorpair.
To be more specific, consider twopairs with wires of 0.65 mm diameterrunning 10 meters in parallel; thewires in the pair are 1 cm apart andthe pairs 2 cm away from each otherand R = 1 k Ω. For a 1 MHz signal, acoupling coefficient of - 22 dB isfound, therefore
V N
V1 = 1
12
In practice, capacitive and inductivecoupling of this type is considerablyreduced by the use of twisted pairsand shielded cables.
fig. 13: a rapid change in V1 creates a fieldwhich at a short distance can be assumed tobe purely electric and induces a voltage VNin another wire-like structure which runs inparallel; this mode of coupling is calledcapacitive crosstalk.
fig. 14: a current change in the cablegenerates an electromagnetic field which ata short distance can be considered to bepurely magnetic and induces a disturbance(voltage) in wires that form a loop; this modeof coupling is called inductive crosstalk.
e
V1
H
h
R
NV C
C12
20
HI
power cable
low power pairof wires
Cahier Technique Merlin Gerin n° 149 / p.12
4. the susceptor
The susceptor is the third participant inthe source/coupling/susceptor systemand refers to any equipment that maybe affected by a disturbance.
It is typically equipment containingsome electronics which malfunctionbecause of electromagneticdisturbances occurring in anunexpected frequency band.
equipment malfunctionEquipment malfunctions are dividedinto four categories and can be: permanent and measurable, random and non-repetitive, appearingwhen the disturbances appear, random and non-repetitive, remainingafter the disturbances vanish, permanent equipment failure(components physically destroyed).
The above types characterize theduration of the fault but not its severity.
The severity of a fault is a matter offunctionality or, in other words howcritical the equipment is. Certainmalfunctions may be acceptable for alimited time such as the temporary loss
of a display; others may not beacceptable such as security equipmentmalfunctions.
solutions to the problemNumerous solutions in terms of howequipment is to be built exist to provideeffective and low-cost immunity toelectromagnetic disturbances.Precautionary measures can be takenin: the design of printed circuit boards(functional partitioning, trace layouts,interconnects), the choice of electronic devices, the ground interconnections, the wiring.
The choices involve many differentdisciplines and should be made duringthe design phase of a project to avoidadditional costs which are always highfor modifications after the design iscompleted or when the product isalready on the market.
Implementing all of these precautionarymeasures requires know-how which
goes far beyond the standard filteringand shielding techniques oftenrecommended to increase immunityeven if their effectiveness has notbeen proven.
Printed circuit boardsThe designer of printed circuit boardsmust follow certain rules that concernfunctional partitions and layout.
Starting with component placement, itis already possible to reduce couplingeffects related to proximity.
For example, the grouping together ofelements that belong to the samecircuit category (digital vs - analoguevs - power circuits), according to theirsusceptibility, reduces interferences.
Furthermore, the layout of circuitboard traces (routing) has a dramaticeffect on susceptibility: the sameelectrical schematic implemented indifferent ways can display orders ofmagnitude different immunity levels.For example, a «minimum etch»circuit board layout (see fig. 15)reduces radiation effects andsensitivity.
fig. 15: the circuit layout can reduce the electromagnetic susceptibility of a PCB:either by minimizing impedances (minimum etch),or by reducing the coupling of the electromagnetic field (ground place).
0 volt
thin circuit layout minimum etch layout layout with ground plane
Cahier Technique Merlin Gerin n° 149 / p.13
Electronic devicesNumerous devices are available toprovide effective protection againstconducted disturbances. Selection isguided by the power level of the circuitto protect (power supply, control andmonitoring, etc.) and the type ofdisturbance. Consequently, forcommon mode disturbances in a powercircuit, a transformer will be used if thedisturbances are at low (< 1 kHz)frequencies and a filter if they are athigh frequencies.
The table in figure 16 gives a non-exhaustive list of protection devices. Allare not equivalent: a filter does notprotect against surges, and a surgeprotector does not protect against highfrequency disturbances.
ShieldingEnclosing sensitive equipment in aconductive shield provides protectionagainst electric fields. To be effective,the thickness of the conductive shieldmust exceed the skin depth at thefrequencies of the disturbanceencountered (see fig. 17).The choice of material is of littleimportance. In some cases aconductive lacquer can be used as ashield. The metal or metal-coatedinsulator shield constitutes the«ground».
Ground interconnectionsWhen it comes to grounding, goodelectric conductivity between differentparts of the housing is extremelyimportant. They must be carefully andcorrectly interconnected, for exampleprotecting contact areas from any paintand also by using short, wide wirebraids (to reduce impedance to aminimum).
type device example applicationssurge arrester power supply, control and monitoring
spark gap in installationslightning arresterlimitervaristor electronic devicesZener diode
filtering transformer power supply, control and monitoringinductors (installations and electronic devices)capacitorsfilters
shielding wire grid data transmissiondoor braid (cabinet in disturbed area)shielded cableshigh frequency gasketscurrent finger
fig. 16: list of protection devices.
fig. 17: screening effect of a metallic shield.
shield depth
transmission
reflection
incidentwave
absorption
skin depth
conductivity ( /cm )Ωσ 2
Cahier Technique Merlin Gerin n° 149 / p.14
WiringThe shielding of wires, sometimescalled screening, can be seen as anextension of the conductive envelopeplaced around sensitive systems.It therefore has the shortest possibleconnection and if possible all around itsperimeter to protect against highfrequency disturbances.
Just as with the coupling between anelectromagnetic field and a wire-likestructure (see section 3), the theorygoverning wire shielding is verycomplex and too vast to be covered in
this paper. References to specialliterature are given in the bibliography.
When all design and manufacturingrules are respected, the system will besufficiently immune to electromagneticdisturbances in the environment it wasbuilt for.
Nevertheless, this immunity can only bevalidated by actual measurements thatdetermine the effectiveness of differentshielding techniques. At Merlin Gerin,for example, different prototype modelsof electronic trip units for circuitbreakers are exposed to rigorous tests
representative of the largestdisturbances to which they may beexpected to be subjected to.
The true objective of these tests is tocheck that the trip unit does not operateinadvertently and that the circuitbreaker opens correctly and in therequired time.
The «product» standards now includethese specifications: a document iscurrently being discussed at the IEC ,representing an EMC appendix to theIEC 947-2 standard concerningindustrial circuit breakers.
5. installation
installation is an importantfactor in the overall systemEMCEvidence of this fact can be found inthe NF C 15-100 generalLV installation standards whichdevotes an entire chapter (33) toelectromagnetic compatibility.
The two previous chapters haveshown that installation plays animportant role in EMC; this is true forboth the design and layout phase andthe actual installation phase.
design phaseDuring the design and layout phase twomajor factors govern EMC: the choice ofequipment and their relative locations(see fig. 18).
The first factor concerns the choice ofboth emitters and susceptors: a givenpiece of equipment can to some extentgenerate disturbances and/or besusceptible.
For example, if two units are to operateclose to each other they must: either combine an emitter thatgenerates low levels of disturbancesand an «ordinary» (i.e. not overlysensitive) susceptor, or combine an «ordinary» emitter thatgenerates moderate levels of
disturbances and a low sensitivitysusceptor, or form a compromise between theabove two extremes.
The second factor that depends directlyon the first concerns the positioning ofequipment, already selected withrespect to their individual charac-teristics, to satisfy EMC requirements.
It is obvious that this selection must takeinto account the cost of equipment andof its installation.
installation phaseElectrical and electronic installation workshould follow the guidelines alreadydiscussed in the previous chapters. Inpractice, the different coexistentcoupling modes must be studied andreduced to satisfy the EMCrequirements. Different techniquesshould be applied: the circuits and the chassis/earthgrounds must be laid out in a grid, the circuits must be physicallyseparated, the wiring must be carefully planned.
practical examples:Grid layout for circuits and chassis/earth groundsToday, equipment can be susceptible tovery low energy levels. It contains
interconnected electronics sensitive tohigh frequencies. Common impedancecoupling frequently occurs and to avoidit, the best possible equipotentialgrounding system or to be more precisea ground grid, is essential.
This is the first step in providingprotection against disturbanceproblems. In a factory powerdistribution network, all protection (PE)wires must be joined together andconnected to the existing metalstructures as specified in NF C 15-100(see fig. 19).
Similarly, within equipment, all groundsand frames must be connected to agrid-like grounding system in theshortest possible way using lowimpedance (at high frequencies), wideand short electrical connections (wiresor braids).The wiring of an electrical cabinet is atypical example: all grounds must beconnected together.
There is a change to be noted here: themethod involving the connection of allgrounds to a central point (starconfiguration), sometimes used forelectronic equipment sensitive to 50/60 Hz hum, has been replaced by gridswhich are far more effective in reducingdisturbances that affect todays digitalsystems, protection relays and controland monitoring systems.
Cahier Technique Merlin Gerin n° 149 / p.15
fig. 18: example of electrical equipment layout respecting EMC.
fig. 19: the grids forcircuits and for chassis/earth grounding systems are often combined inelectrical cabinets.
low voltage for "workshops"distribution switchboard
MV/LV substationmain low voltage switchboard
switchboard withisolation transformer
low voltage for "machines"
electricwelding sets
Laboratory
Production
distribution switchboardand UPS
low voltage for "offices"
Sales departmentComputer department
PEPE
M
@À@À@À@À@À@À@À@À@À@À@À@À
@À@À@À@À@À@À
Cahier Technique Merlin Gerin n° 149 / p.16
Separation of electrical circuitsThis technique consists of separatingthe energy sources (usually 50 or60 Hz). The aim is to avoidinterference on a sensitive devicecaused by conducted disturbancesgenerated by other systems connectedto the same power source. Theprinciple is to create two separatepower sources isolated by impedancesthat are high at the frequency of thedisturbances.
Transformers (not auto-transformers)are effective isolators, especially at lowfrequencies: MV/LV transformers,isolation transformers and any inputtransformer for electronics stopconducted disturbances.
Sometimes an isolating filter isrequired to eliminate high frequencydisturbances. If the sensitiveequipment also requires emergencypower, it can be supplied by anuninterruptible power supply (UPS) aslong as the UPS contains the requiredisolation transformer(s).
Rational wiringThe effects of the three couplingmechanisms discussed earlier can bereduced if the wire and cable routingadheres to the following rules:
in all systems that cannot beseparated physically for economicreasons, wires/cables must be groupedtogether by category. The differentcategories should be routed separately:in particular, power cables should be onone side and low power cables(telephone, control and monitoring) onthe other.
If a sufficient number of cableways ortroughs are available, power cablescarrying more than a few amperes at220 V should be routed separately from
the low power signal cables. Otherwise,a minimum distance of at least20 centimeters must be kept betweenthe two.
Any element common to these twocategories of cables must be avoided.
Circuitry using low level signals shouldhave, whenever, possible its own returnwire (0 Volts) to avoid commonimpedance coupling. The majority ofsystems that communicate over busesrequire pairs of wires reservedexclusively for data exchange.
in any case, the overall loop areaformed by the conductor and its returnmust be minimized. In datatransmission, twisted pairs reduce thesusceptibility to differential modecoupling. The twisted pair is to bepreferred over straight wires.
cables used for measurements andlow signal level data transmissionshould be shielded, if possible, andunless specially instructed by themanufacturer, their shield connected toground at a maximum number ofpoints.
the cable routing troughs should be, ifat all possible, made out of metal. Thetroughs should be correctly electricallyinterconnected, e.g. screwed togetherand connected to the grounding grid(see fig. 20).
the most sensitive cables (e.g. thoseused in measurements) should beplaced in the corner of the troughwhere they can benefit from maximumprotection against electromagneticradiation. Their shielding, if any, shouldbe connected to the trough at regularintervals.
The use of prefabricated cable trunkingassemblies in which the cables arepositioned and connected correctly,such as Telemecanique’s Canalissystem with built-in control wires, arehighly recommended.
All these cabling techniques, whicheffectively avoid EMC problems, onlyincrease costs slightly when applied atdesign or installation time. Latermodifications of an existing installationshowing excessive electromagneticcoupling are far more expensive.
d
power cables shielded cableconductingmeasurementdata, possibilityconnected to thecable trough atregular intervals
control wires
d
d = a few centimeters
fig. 20: cable routing example.
Cahier Technique Merlin Gerin n° 149 / p.17
6. standards, test facilities and tests standards
standardsDocumented standards that regulateelectromagnetic compatibility ofsystems have long been in existence.
The first regulations were issued by theInternational Special Committee onRadio Interference (CISPR). Theseregulations covered only the maximumacceptable power level that could beemitted by different types of equipment,mainly to protect radio transmissionand reception.
National Committees and theInternational ElectrotechnicalCommission (IEC) have issueddocumented standards that cover allaspects of EMC emission andsusceptibility encountered in the civiliandomain.
Military standards on EMC have beencompiled in the GAM EG 13 series inFrance and in the MIL-STD series inthe United States.
The increasing importance of EMC andthe forthcoming unification of Europeare changing the landscape of civilianstandards.
The European Council published aDirective (reference 89/336/EC) inMay 1989 on this subject. It relates tounifying the EMC legislation of themember countries. Every membercountry is committed to include it in itsnational legislation and make its useand application mandatory.
The European Directive not onlyimposes limits on emitted disturbancesbut also sets the minimum immunity toelectromagnetic disturbances. TheDirective makes reference to standardsnot yet ratified; standards that definemaximum acceptable disturbancelevels, minimal immunity levels andmeasurement methods. A TechnicalCommittee, TC 110, has been createdfor this purpose by the EuropeanCommittee for ElectrotechnicalStandardization (CENELEC). Its duty isto bring together the existing standards
that are in accordance with theDirective and to write or rewrite thosethat are not. Without anticipating thework of TC 110, it seems likely that itwill be based on existing standardsalready in use in the industrialcommunity (see fig. 21).
For emission tests, the Germanstandards VDE 0871 and VDE 0875were used for some time as areference. The recent Europeanstandards EN 55011 and EN 55022 arenow replacing them.
For immunity tests, the IEC 801publication is currently used as areference. It will be included in the IEC1000 publication which gathers allmaterial on EMC written by the IEC inthe framework of Committee 77.
Publication 801 contains several partsfor different types of disturbances thatmay affect a system or equipment. Theparts are respectively: 801-1: general introduction, 801-2: electrostatic dischargerequirements, 801-3: radiated electromagnetic fieldrequirements, 801-4: electrical fast transient/burstrequirements, 801-5: surge immunity requirements(proposed), 801-6: current injection (proposed).
Parts 801-2, 801-3 and 801-4 relate totypical disturbances encountered in themodern electrotechnical world. Theyare widely accepted in the internationalcommunity and Merlin Gerin hasdecided to adopt them for its products.
The following section describes in moredetail the tests that relate to thesestandards.
test facilitiesAs mentioned before, to respectregulations, standardizedmeasurements and tests must also beperformed.Due to its field of applications, MerlinGerin made EMC one of its majorconcerns long ago. Large installationssuch as Faraday rooms have been inuse since the seventies.In 1988 a new dimension was reachedwith the opening of the EMC laboratoryat the DTE Research and DevelopmentCentre in Grenoble. This centre makesfull use of skills and knowledge andpromotes the exchange of information.It also offers measurement servicesand is involved in special projects,training, and standards work as arecognized expert. As a centre offeringservices to outside customers, itperforms measurements in all EMCfields: electrostatic discharge,conducted and radiated emissions,susceptibility to conduction or radiation.As with any other measurements,electromagnetic compatibilitymeasurements must be reproducibleboth in time and in space, which meansthat two measurements performed attwo different laboratories must yield thesame results. In the EMC discipline,this means large facilities requiringconsiderable investments and a strictquality policy.
fig. 21: table of main standards in use in France and their international counterparts.
application french original internationalfield standards standards susceptibility NF C 46-02x IEC 801 - (x + 1)example NF C 46-022 IEC 801 - 3 emission NF C 91-0xx EN 55 0xxexample NF C 91-022 EN 55 022
Cahier Technique Merlin Gerin n° 149 / p.18
The quality program at the Merlin GerinEMC laboratory is based on a QualityManual and a set of procedures. Theseprocedures concern calibration and theconnection to calibrated standards inaddition to each type of measurementitself. The list of tests for standardsthat can be performed at the laboratoryare listed in appendix 4.
The fact that the laboratory has beenaccredited by the Réseau Nationald’Essais (National Testing Network)acknowledges the quality assurancepolicy.
testsElectrostatic dischargeThese tests are designed to check theimmunity of circuit boards, equipmentand systems to electrostatic discharge.
Electrostatic discharges are the resultof charge accumulated by a person, forexample, walking on a floor coveredwith an electrically insulating material.When the person touches an
electrically conducting materialconnected via an impedance toground, he discharges suddenlythrough the impedance.
Several studies have shown that thewaveform is a function of thecharacteristics of the emitter (thesource of the discharge) and of thecircuits involved, but also of otherparameters such as relative humidity(see fig. 22) or the speed at which thecharged body approaches, in ourexample the hand of the person etc.
This research has led to standardizeddischarge tests. They are performedwith an electrostatic gun that simulatesa human being in predeterminedconfigurations (see fig. 23).Discharges are performed on allaccessible parts of the device undertest, in its immediate environment andrepeated a sufficient number of timesto make sure that the device resistselectrostatic discharge.
These measurements require anappropriate test bench.
All tests are completely defined bystandard IEC 801-2 (revised in 1991)with severity levels shown in the tableof figure 24.
Conducted electromagneticsusceptibilitySusceptibility tests are used to verifythe resistance of equipment todisturbances reaching it via externalequipment cables (inputs, outputs andpower supply). As mentioned before,these disturbances differ depending onthe type and installation characteristicsof the cable. The electromagneticsignals or pulses used in these testshave characteristic amplitudes,waveforms, frequencies etc.
Disturbance measurements performedon numerous sites have led to theselection of two tests.
The first test, covered by IEC 801-4,simulates typical disturbancesgenerated by the operation ofcontrolgear. The test uses burstsconsisting of a number of fasttransients. The burst repetition
fig 22: the effect of relative humidity on theelectrostatic discharge voltage for threetypes of floor materials. fig. 23: electrostatic test site as defined by standard IEC 801-2.
161514131211109876543210
voltage(kV)
5 10 20 30 40 50 60 70 80 90 100
relative humidity (%)
synthetics
wool
anti-static
conductive surface
470 k resistors
ground reference plane
conductive surface
insulated table
@À@À@À@À@À@À
power supply
mains
insulator
equipment under test (EUT)
@À@À@À@À@À@À@À@À@À@À@À@À
Ω
Cahier Technique Merlin Gerin n° 149 / p.19
frequency is approx. 3 Hz. Each burstcontains approx. 100 transients every100 µs . Each transient rises steeply(5 ns) to an amplitude of several kV,depending on the required severitylevel (see fig. 25 and 26).
All cables can be subjected to fasttransients. This type of disturbancecouples into wiring very easily e.g.crosstalk (see the chapter on«coupling»). It takes only one cablegenerating such disturbances in a cableor wire trough to pollute all other cablesrunning along the same path. The testmust therefore involve all cables and
wires: a common mode test isperformed on all wires with artificiallyinduced disturbances (cables otherthan the power supply) and a commonand differential mode test on cablesconnected to the mains. Disturbancesare injected into the tested cableseither via direct capacitive coupling(power supplies), or via a couplingclamp consisting of two metal platesthat enclose the secondary cables(see fig. 27).
The equipment under test must notshow a malfunction over a prede-termined period (1 min). This test is the
most relevant one for device immunitybecause fast transients are the mostfrequent ones encountered.The second test is representative ofsecondary effects created byphenomena such as lightning. Itsimulates conducted disturbancesappearing on LV power lines afterlightning strikes (801-5 draft).These disturbances consist of energythat is transformed into: voltage impulses 1.2/50 µs, if theimpedance of the tested device is high,with amplitudes that can reachseveral kV,
severity applied test voltage (± 10 %) in kVlevel without malfunctions occurring (open circuit output)according to on power supply on input/output linesIEC 801-4 (signal, data, control)1 0.5 0.252 1 0.53 2 14 4 2x special special
level x is defined contractually between manufacturer and client.
fig. 24: electrostatic discharge voltages thatdevices must withstand to comply withstandard IEC 801-2. fig. 26: table of severity levels defined in IEC 801-4.
fig. 27: susceptibility to fast transients, measured on an Isis master control unit (test 801-4) in aFaraday room. This photo shows the disturbance generator being adjusted by an operator, thewooden case containing the coupling clamp and the Isis master control unit connected to theBatibus network.
fig. 25: shape of the bursts (a) and their fasttransients (b).
15 ms
300 ms
u
t
5 ns
100 sµ
u
t
(a)
(b)
Cahier Technique Merlin Gerin n° 149 / p.20
current impulses 8/20 µs if theimpedance is low, with amplitudesreaching several kA.
The rise time of this type of disturbanceis in the order of a thousand timeslonger, in the microsecond range, thanfor bursts of fast transients (see fig. 28).Crosstalk type of coupling is thereforeless prevalent and this second type oftest only applies to cables directlyconnected to the mains. The commonand differential mode tests usecapacitive coupling and appropriatelevels. The procedure resembles thefast transients test: the equipmentunder test must not malfunction.
Susceptibility to radiated emissionThe susceptibility tests for radiatedemissions were devised to ensure thesatisfactory operation of equipmentwhen exposed to electromagneticfields.
Since these tests are particularlyenvironment sensitive, the meansdeployed and competency levelsrequired to produce reliable andreproducible susceptibilitymeasurements are very high. Thesurrounding environment must besufficiently «clean» and free of wavesnormally present, since (as discussedin the «source» chapter)electromagnetic fields with strengths inthe several V/m range are frequent(e.g. two-way portable radios) andpulsed electromagnetic fields witheven higher levels are common in
industrial environments. These testsmust therefore be conducted inFaraday rooms with walls covered byhigh frequency absorbing materials.The rooms are called anechoicchambers when all walls including thefloor are covered and semi-anechoicwhen the floor is not.
In the chambers, the fields aregenerated by different types ofantennae depending on the type offield, the frequency range andpolarization. The antennae are drivenby a wideband power amplifiercontrolled by a R.F. generator(see fig. 29).
The generated fields are calibratedusing broadband isotropic sensors
(field strength monitors). The diagram infigure 30 shows a typical test setup.Standards define the acceptabledisturbance levels. In particular,standard 801-3 (currently being revised)recommends tests using frequencies inthe range of 27 to 500 MHz at threeseverity levels. (1.3 and 10 V/m).Note that the test conditions that can becreated at the Merlin Gerin laboratoriesare much more severe: the frequencyrange that can be covered extends from10 kHz to 1 GHz. From 27 MHz to 1 GHzdevices can be tested against fieldsreaching 30 V/m and 80 % modulation.Standardized measurements for pulsedelectromagnetic fields do not yet exist. Inthis domain, Merlin Gerin uses its owninternal procedures.
fig. 29: Faraday room: semi-anechoic chamber and a several antennae of the Merlin GerinEMC laboratory.
severity test open-circuitlevels output voltagerraccording to (kV)draftIEC 801-51 0.52 13 24 4x speciallevel x is defined contractually betweenmanufacturer and client.
fig. 28: severity levels as defined in projectIEC 801-5 (generator impedance = 2 Ω).
Cahier Technique Merlin Gerin n° 149 / p.21
Conducted emissionConducted emission measurementsquantify the disturbances that theequipment under test reinjects into allcables connected to it.
The disturbance strongly depends onthe high frequency characteristics of theload connected to it since the equipmentunder test is the generator in this case(see fig. 31).
To obtain reproducible measurementresults and especially to avoid problemswith the characteristic impedance of thenetwork, the conducted emission mea-surements are performed with the helpof a Line Impedance Stabilizing Network(LISN). A high frequency receiver isconnected to the network to measureemission levels at each frequency.
fig. 30: typical test setup in a Faraday room. Measurements are performed in two stages:1 - calibration of the filed for a given frequency range, without the EUT,2 - verification of the EUT immunity.
equipmentunder test(victim)
network
filter
1 kW
10 kHzto1 GHz
broadbandamplifier
RFgenerator
Faraday roomsemi-anechoic
antenna
fig. 31: measurement configuration for conducted emissions. The EUT (equipment under test)is the generator, the line impedance stabilizing network is the load.
network
filter
Faraday roomsemi-anechoic
measurementdevice
line impedancestabilizing network
equipmentunder test(source)
Cahier Technique Merlin Gerin n° 149 / p.22
The level of disturbances reinjectedmay not exceed the limits defined in thestandards. These limits depend on thetype of cable and the environment. Thegraph below (see fig. 32) shows theresults of a measurement performedon an uninterruptible power supply andthe levels defined in standardEN 55 022 for comparison.
Radiated emissionsRadiated emission measurementsquantify the level of disturbancesemitted by a device in the form ofelectromagnetic waves.
Just as with radiated susceptibilitytests, radiated emission tests must beperformed in the absence of wavesnormally present such as CB, radio etc.and must not be modified by reflections
from surrounding objects. These twoconditions are contradictory and this isthe reason for the existence of two testmethods.
The first method consists of placingthe EUT in a field free of obstacleswithin a given perimeter. Theenvironment is uncontrolled.
The second method is implemented ina Faraday room; the reflections fromthe walls are deliberately attenuatedby high frequency absorbing materials(see fig. 29). The environment can beperfectly controlled.
The Merlin Gerin laboratory uses thesecond method. It offers a keyadvantage in that measurements canbe automated and that equipment
handling is minimized, since emissionand susceptibility level measurementscan be performed at the same site withjust few setup changes.
As for conducted emissions, theemission levels must be less than thelimits set by specifications or standards.
Measuring pulsed fieldsStandardized tests are performed tomeasure emission levels or test thesusceptibility of devices or systems tothe most common types of electro-magnetic disturbances encountered inan industrial environment.
However, the environment for devicesdeveloped by Merlin Gerin has certaincharacteristics not yet covered bystandards.
fig. 32: results of measurements performed on a Maxipac SX5000 uninterruptible power supply.
Cahier Technique Merlin Gerin n° 149 / p.23
For example, specific EMC testprocedures for equipment in mediumvoltage substations do not yet exist.This is why Merlin Gerin performs aseries of measurements to betterunderstand the typical disturbancesthat exist in the vicinity of the
equipment it manufactures, especiallynear low, medium and very highvoltage switchgear.In a second phase, in-house testsusing special test systems have beendeveloped. They allow testing of theelectromagnetic compatibility of
devices without having to revert to fullscale tests. These tests are easier toreproduce and less costly. They areperformed early in the design whichminimizes costs for EMC protection.
7. conclusion
The use of electronics in a largenumber of applications, and especiallyin electrotechnical equipment, hasintroduced a new and importantrequirement: electromagneticcompatibility. Trouble-free operation indisturbed environments and operationwithout producing disturbances areessential to product quality require-ments. To achieve both these goals,the complex phenomena involved in thesources, coupling and susceptors must
be well understood. A certain numberof rules must be followed in the design,industrialization and manufacture ofproducts.
The site and installation characteristicsalso play an important role inelectromagnetic compatibility.This explains the importance ofcarefully considering the location andlayout of power components, cablerouting, shielding etc. right from theintitial design phase. Even if equipment
offers satisfactory EMC, a welldesigned installation can extend thecompatibility safety margins.Only measurements requiring a highlevel of expertise and sophisticatedequipment can produce valid resultsquantifying the electromagneticcompatibility of equipment.Compliance with standards thereforeprovides the certainty that equipmentwill operate satisfactorily in its electro-magnetic environment.
Cahier Technique Merlin Gerin n° 149 / p.24
appendix 1: glossary
Electromagnetic compatibility,EMC (abbreviation) (IEV 161-01-07)The ability of an equipment or systemto function satisfactorily in itselectromagnetic environment withoutintroducing intolerable electromagneticdisturbances to anything in thatenvironment.
(Electromagnetic) compatibility level(IEV 161-03-10)The specified maximum disturbancelevel expected to be impressed on adevice, equipment or system operatedin particular conditions.
Note: In practice the electromagneticcompatibility level is not an absolutemaximum level but may be exceededby a small probability.
Electromagnetic disturbance(IEV 161-01-05)Any electromagnetic phenomenonwhich may degrade the performance ofa device, equipment or system, oradversely affect living or inert matter.
Note: An electromagnetic disturbancemay be an electromagnetic noise, anunwanted signal or a change in thepropagation medium.
Disturbance level(not defined in IEV 161)Level of an electromagnetic distur-bance of a given form measured inparticular conditions.
Limit of disturbance(IEV 161-03-08)The maximum permissible electro-magnetic disturbance level, asmeasured in a specified way.
Immunity level(IEV 161-03-14)The maximum level of a givendisturbance incident on a particulardevice, equipment or system for which
it remains capable of operating at arequired degree of performance.
(Electromagnetic) susceptibility(IEV 161-01-21)The inability of a device, equipment orsystem to perform without degradationin the presence of an electromagneticdisturbance.
Note: Suceptibility is a lack ofimmunity.
DecibelThe decibel is a unit of sound pressurethat is also used to express amplituderatios according toX/Xo (dB@) = 20 . log10 X/Xo,withX = measured amplitude,Xo = reference amplitude,@ = mesure unit for X and Xo.A few sample values are given in thetable below (see fig. 33).
appendix 2: impedance of a conductor at high frequencies
The level of EMC in equipmentdepends on coupling between circuits.Coupling is directly related to theimpedance between circuits, especiallyat high frequencies. To improve EMC,these impedances must be determinedand then reduced.
A few approximating formulae exist todetermine the high frequencyimpedance of typical conductors. Theseformulae are cumbersome and theirresults meaningless if the exactposition of all involved elements isunknown. But who knows the exactposition of a wire with respect to theothers in a cable trough? The answersto this and similar questions come fromexperience together with basicknowledge of the theory of electricalphenomena.
First of all it is important to keep in mindthat the impedance of a conductor ismainly a function of its inductance andbecomes preponderant starting at a fewkilohertz for a standard wire.For a wire assumed to be infinitely long,the inductance per unit lengthincreases logarithmically with thediameter, therefore very slowly: forwires that do not exceed 1/4 of thedisturbance wavelength, an inductanceof one µH/m can be used irrespectiveof the diameter (see fig. 34).
This value is much lower when the wireis correctly run against a conductiveplane. It becomes a function of thedistance between the wire and theplane and the inductance can easily bedecreased by 10 dB. At very highfrequencies the wire must beconsidered as a transmission line witha characteristic impedance of aroundone hundred ohms.In this light, a common inductance ofseveral µH can easily be created, for
example, with a few meters of green-yellow (grounding) wire. This translatesinto a few ohms at 1 MHz and a fewhundred ohms at 100 MHz.
ConclusionA conducting metal plate represents theelectrical interconnect offering the
lowest impedance, independent ofthickness as long as it is greater thanthe skin depth (415 µm at 10 kHz forcopper). A copper plate displays aninductance of 0.6 nH (at 10 kHz) and aresistance of 37 µΩ per square.
fig. 34: at equal lenghts, the different impedances:a: wire in air (l ≈ 1 µH/m),b: cable placed on a metal plane,c: metal grid with electrical contact at each node (e.g. welded concrete rebar),d: metal plane,have a per unit lenght impedance Z1 > Z2 > Z3 > Z4.
(a) (b)
(d)(c)
Z3 Z4
Z2Z1
Cahier Technique Merlin Gerin n° 149 / p.26
appendix 3: the different parts of a cable
draining the capacitive current aswell as earth leakage fault currents(zero sequence short-circuits), protection of life and property in theevent of a puncture.
That is why it is generally made ofmetal and is continuous (lead tubing,braided wire, helically wound bands).
For cables carrying data, the screen,more often called a shield, consists ofcopper or aluminum wire bands orbraids, wrapped around to form a
The technical terms used to describedifferent parts of a cable can haveslightly different meanings dependingon the cable’s field of application(power transmission, telephone, dataor control and monitoring),(see fig. 35).
The IEC definitions are in italics.
JacketThe jacket’s most important role is toprotect the cable from mechanicaldamage. That is why it usuallycontains two helically stranded softsteel sheets (NF C 32-050).For data transmission cables, it alsoserves as an electrostatic and moreoften electromagnetic shield.
ShieldSame as a screen; i.e. devicedesigned to reduce the intensity ofelectromagnetic radiation penetratinginto a certain region.A jacket or screen of a cable, whetherfor power or data transmission, canform a shield.
ScreenA device used to reduce thepenetration of a field into an assignedregionIt has multiple functions: creation of an equipotential surfacearound the insulator, protection against the effects ofexternal an internal electrostatic fields,
shield against electrostatic orelectromagnetic fields.
It can be an overall shield, for allconductors in the cable, when thedisturbances are external to the cable.
It can also be partial, for a limitednumber of conductors, to protectagainst disturbances emitted by theother conductors in the cable.
InsulatorThe insulator renders the cable waterand/or air tight.
Telephone cable Medium voltage power transmissioncable
fig. 35.
insulator (PVC)
jacket conductor(two steel bands)
internal insulation(PVC)
metal screen(aluminum)
insulator (PVC)
core (copper wire)
insulator (PVC)
jacket conductor(two steel sheets)
cushion (paper)
metal screen(copper)
conductive ribbon
filler
insulator (PVC)
core (copper wire)
Cahier Technique Merlin Gerin n° 149 / p.27
appendix 4: tests performed at the Merlin Gerin EMC laboratory
The Merlin Gerin EMC laboratory hasthe required equipment and expertiseto perform a large number of tests.
NF C 63-850 (October 1982)Programmable controllers10-2-8-1 and 10-2-8-3:electromagnetic compatibility tests.
Emission EN 55 011 (to be published)Limits and methods of measurement ofelectromagnetic disturbancecharacteristics of Industrial, Scientificand Medical (ISM) radio-frequencyequipment.[conducted emission part]
EN 55 022Limits and methods of measurement ofradio interference characteristics ofinformation technology equipment.[conducted emission part]
VDE 0871 (June 1978)Disturbance suppression for Industrial,Scientific and Medical (ISM) orequivalent high frequency equipment.
Specific standards Telecommunications centres I 12-10 (1988)published by the Committee forEquipment Specifications (CSE),France Telecom.Electromagnetic environment ofequipment in a telecommunicationscentre.
[the parts on immunity of equipment toradiated disturbances, radiated andconducted disturbances created byequipment].
