Dynamic Arc Recognition and Termination

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Intrinsic safety is a worldwide-accepted type of ignition protection,

which offers many advantages over other types of ignitionprotection. The dynamically acting intrinsically safe energy supplyconcept DART is a means of facilitating considerably higher directpower, with simultaneous intrinsically safe energy limitationthrough rapid disconnection. This paper explains the principle ofoperation of DART as well as two areas of industrial application. Italso illustrates the essential technical safety aspects necessary forthe demonstration of intrinsic safety and explains the impact ofthese on the relevant international standards. In conclusion,practical areas of application in the process industry are examined.

Prepared by:

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Udo Gerlach, Thomas Uehlken, Ulrich JohannsmeyerPhysikalisch Technische Bundesanstalt

Martin Junker, Andreas HenneckePepperl+Fuchs

Paper presented at the 2008 5th European Conference on Electricaland Instrumentation Applications in the Petroleum and Chemical

Industry, Weimar, Germany, June 11-12, 2008


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DART The New Dimension In Intrinsic Safety Introduction

Table of Contents

4.1 The Power Supply ............................................................................................................... 44.2 The Loads ............................................................................................................................54.3 The Decoupling Module........................................................................................................5

7.1 Decoupling The Field Devices .............................................................................................. 87.2 Communication................................................................................................................... 8

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Introduction DART The New Dimension In Intrinsic Safety

1 IntroductionIn an area endangered by the likelihood of an ex-plosion (hazardous area) the type of protectionknown as intrinsic safety offers recognized advan-tages, such as its worldwide acceptance and thesimple connection and installation technology. Inaddition, it is possible to carry out work on circuitsand devices for the purpose of re-equipment, plantextension and maintenance, during actual opera-tion and without a hot-working certificate. Theintrinsic safety class of ignition protection is basedon the principle, that sparks occurring in an electri-cal circuit are always limited in terms of their en-ergy, so that they cannot cause an ignition to takeplace in an existing potentially explosive atmos-phere.

The intrinsic safety type of protection is currently

achieved by limiting the available power. This limi-tation of power usually to less than 2 W pro-vides intrinsic safety (Ex i) and is therefore mainlyemployed in the area of control and instrumenta-tion in the power supply to actuators and sensorswith low connected load.

A significantly higher direct power with the simul-taneous safeguarding of all the positive character-istics of intrinsic safety offers the user a new andessentially wider scope of application. These aimsare achieved through DART technology (DART: Dy-namic Arc Recognition and Termination). DART is a

means of instantaneous tripping, which dynami-cally detects an undesired condition or a fault inthe electrical system precisely as it occurs andinstigates an immediate transition to a safe condi-tion before any safety-critical parameters are ex-ceeded. DART is based on the detection of faultconditions and their characteristic rate of rise ofcurrent.

Through the use of DART, systems can be operatedat drastically increased direct power output com-pared to current intrinsic safety solutions. Moreavailable direct power opens the door to the use of

the intrinsic safety type of protection in many ap-plications relevant to the process industry. Thefollowing are some examples: Weighing equip-ment, lighting systems, valve control systems andfieldbus systems such as FOUNDATION Fieldbus H1and PROFIBUS PA.

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DART The New Dimension In Intrinsic Safety Basic Operating Principles

2 Basic Operating PrinciplesIn the normal operating condition the DART powersupply feeds the full nominal power, which depend-ing on the application, can be greater by a factor ofbetween 4 and 25 (8 to 50 W) compared to stan-dards-related permissible values. DART detects atthe very instant of the onset of a fault incident, duefor example to the opening of the circuit, the result-ing change in current and immediately switches offthe power supply. In this way, the energy from theelectrical system is effectively limited in just a fewmicroseconds and thus a spark capable of causingan ignition is prevented.

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This procedure is possible due to a very character-istic and therefore easily detectable change incurrent di/dt during the onset of a fault condition.The reaction of the power supply takes place very

quickly in approximately 1.4 s. On such a fastreacting system, an additional factor to be consid-ered is the propagation time on the cable. The en-ergy released is determined by the power convertedat the point of the fault integrated over the time upto the effective disconnection. The following physi-cal parameters are principally responsible for this:

The power determined by the supply volt-age and the load current

The time comprising the signal propaga-tion delay in the cable and the reaction timeof the power supply

The energy stored in the connection cable The load behavior.

The energy liberated in the spark is determined bythe power available, integrated over time. The rela-tionships are explained below. Fig. 1 shows thearrangement of the power supply, cable and de-vices in the hazardous area.

3 Detecting The Ignition Of ASpark

The determination of the intrinsically safe ignition

limit values is made with the spark test apparatusspecified in the standard IEC 60079-11 in whichthese values are subjected to a specified ignitionprobability. It is important to distinguish makesparks and break sparks. Only break sparks areconsidered in this context.

A typical example of the behavior of the electricalparameters of a break spark is shown in Fig. 2. Abreak spark commences with the voltage U

F= 0 V

and usually ends on reaching the open circuit volt-age at U

F= U

0, in which the steady increase of the

spark voltage is directly associated with a reduc-tion in the spark current I

Fin a linear circuit. The

period of time in between depends on the circuit

and is referred to as the spark duration tF. Typical

spark duration tF: 5 s < t

F< 2 ms.

At the start of a break spark the spark voltage UF

jumps within a very short time (t 1 s) from 0 V toUF

10 V. The voltage change is directly linked witha characteristic and easily evaluated current jumpdi/dt (see curve I

F). Directly after this jump in cur-

rent the spark current and spark voltage remainrelatively constant for approximately 1 to 5 s. Dur-ing this period there is definitively no possibility ofignition due to the extremely low available sparkenergy W

Fand it is referred to as the initial phase.

There then follows a longer period of time, which asa maximum, persists up to the end of the sparkduration t

F. This range is the critical phase during

which an ignition can occur. During this period the

spark draws the necessary ignition energy from thesystem, i.e. from the source, the cable and theconsumer loads.

From the knowledge of these variations with time itcan be seen, that the rapid detection of sparks incombination with a means for the rapid disconnec-tion of the source can be employed to reliably pre-vent the ignition of an explosive mixture. The taskis principally to evaluate the current jump di/dt,while giving due consideration to the characteristicsafety values.

Fig. 3 shows the time history of a spark interruptedby a DART power supply. The current jump is clearlyevident, which triggers the transition of the circuitinto the safe condition. It is clear, that with DART afault condition is not only already detected andevaluated within the initial phase, but that it alsoleads to the disconnection of the power supply. Theswitch-off time available during this process de-pends on the system. A frequently used value,based on the physics of the spark is 5 s.

Due to the very short rise times of current and volt-age during the onset of a spark, the connecting

cable between the power supply and the load actsas a wave guide even when the cable lengths are

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Function Of DART Components DART The New Dimension In Intrinsic Safety

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very short. The information that a spark is in exis-tence propagates as a traveling wave or surge onthe connecting cable. Thus the power supply re-ceives the information delayed by up to one cablepropagation delay period. The reaction of the powersupply in turn becomes effective at the position of

the spark only after one cable propagation delayperiod.

This delay is an important safety parameter. In atypical cable used for instrumentation electricwaves travel at approx. half the speed of light or160,000 km/s. Available power is approximatelyinverse proportional to the cable length. Furtherinfluencing factors to be considered are, for exam-ple, the stored energy in the connection cable andin the load.

4 Function Of DART ComponentsA DART power system is comprised of three compo-nents the power supply, the connecting cable/sand one or more loads. A system shall basicallyconsist of only one source, which can however beprovided in a redundant form for reasons of avail-ability. The loads are connected to the power sup-ply via a connecting cable with a fixed, definedsurge impedance.

4.1 The Power SupplyThe output voltage is galvanically isolated from thestation supply and limited by multiple redundantcircuits. The DART specific behavior is achievedthrough the functions represented in the blockdiagram in Fig. 4.

Coordination of functions integrated in the DARTpower supply leads to the output characteristics, inwhich the output voltage U

outis represented against

the output current Iout

described below. In additionto the safe permitted highest values U

limand I

limthe

characteristic is divided into the two operatingranges A and B:

This range, which is called the start-up and fold-back range, represents the characteristic curve of alinear voltage source with safe values. After switch-ing on the source switch S1 is open (Point 1). A verylow current of a few mA, the so-called trickle cur-rent (Point 2) is made available at the output ter-minals across the resistance R

Start. When the load

resistance due to the combination of cable and loadis sufficiently large (R

Last> R

L1) it means that no fault

is present. The output voltage reaches or exceeds afixed threshold value U

thr(Point 3) and the source

switches after a necessary safety period of approx.3 ms to Range B, the operating range. However, this

is only possible if the current variation di/dt due tothe load lies below the prescribed detectionthreshold during the switch-on phase.



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DART The New Dimension In Intrinsic Safety Testing DART

Range B represents an almost ideal voltage source

with an internal resistance Ri 0 . In the operating

range the source can provide the optimum power tothe load, by which means the maximum powerconversion is possible at Point 4 with R

Last= R

L2. Any

variations in the load condition including that dueto faults are associated with an immediate cur-rent variation di/dt. If at this point the prescribedmaximum value of the current variation is exceededin actual value, the source switches off and theoperating point returns immediately from Range Bto the safe Fold-Back Range A. This likewise takesplace if the maximum permissible load current I

limis

exceeded. (see Point 4).

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In summary, the dynamic control behavior of aDART source can be characterized as follows: Bycontrast with customary electronic current limita-tion there are the following differences in the caseof DART made from a safety viewpoint: a transitioninto the optimum operating range in the ms rangeand rapid turn-off to the safe Fold-Back Range inthe s range in the event of faults.

4.2 The LoadsThe following prerequisites have been taken intoaccount in the DART concept with regard to theloads:

The spectrum of loads that can be usedshould be as comprehensive as possible.

It should be as simple as possible to inte-grate the loads into the system.

It should be possible to operate already ex-isting components / loads (including the

customary field devices) with this technologyin the same manner as is possible with pre-

viously customary technologies e.g. FISCO(protection of stocks).

In order to keep the safety considerationsstraightforward, only a line topology is en-visaged.

The loads must not have a negative influenceeither on the functional or the safety capabil-ity of the DART source or other loads (includ-ing the cable).

The following particularly applies to the loads: Theymust not restrict or absorb the propagation of in-formation on the formation of sparks. In this con-text the load behavior must be accepted as notbeing exactly defined. The following two examplesillustrate safety-critical cases, which demand addi-tional measures.

4.3 The Decoupling ModuleA decoupling module ensures a well-defined elec-trical behavior both from a functional as well as asafety perspective. It permits operation of practi-cally any load with DART. A decoupling module isintegrated into the explosion-proof housing of theload and connected in series with it. The decoup-ling module essentially fulfills the following tasks:

Soft start-up of the load with limited currentrise di/dt

Well-defined electrical behavior Optional disconnection in the case of faults

through di/dt detection.

5 Testing DARTAll the safety limit values for spark ignition given inthe basic standard on Intrinsic safety IEC/EN60079-11 are based on the spark test apparatusdefined there. This apparatus generates both breaksparks and make sparks under prescribed con-straints. During the time that passes up to the igni-tion of the explosive mixture, statistically evaluatedpredictions can be made on the ignition capabilityof different circuits. The ignition limit values ob-tained by this method can be found in the directcurrent reference curves and in the tables in IEC/EN60079-11. In addition to this evaluation, the stan-dard now permits the execution of various testswith the spark test apparatus in accordance withAppendix B of IEC 60079-11. A software test is alsopossible with the ISPARK program.

With none of the listed evaluation methods is itpossible to carry out an objective safety assess-ment of dynamic, intrinsically-safe power sources -like DART because the achievable ignition limitvalues with this new concept are way above the

values in the standards. The intrinsic safety ofthese sources can only be ascertained by means of



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DART The Power Concept DART The New Dimension In Intrinsic Safety

their dynamic principle of operation, i.e. their im-mediate reaction to fault conditions.

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The necessary demonstration of proof demands theintroduction of new types of test methods. Thesemust target and reproduce the most critical cases

that can be encountered in practice. In order toassess the ignition behavior of dynamically operat-ing sources these have to be loaded by means ofhardware before the occurrence of the fault (spark)with precisely defined scenarios for the especiallycritical conditions, i.e. a defined spark history mustbe created. The definition of a worst-case sce-nario is already available. However, due to thecomplexity of the relationships further investiga-tions are necessary.

In the 6th edition of IEC 60079-11, due for publica-tion around 2010, section 10.1.2 will be supple-

mented. In cases, in which the spark test apparatuscannot be used such as in the case of dynamicallyacting sources considered here - alternative testmethods will be permissible. The test methods tobe used will be incorporated into the standard at alater stage, when further assured knowledge ofthese is available. Thus the 6th edition will open up

the way for the international application of theDART technology.

6 DART The Power ConceptThe DART Power solution will be used as the focal

point for the point-to-point supply from the powersupply to the load. The resulting simple topologyconsists of the power supply, cable and the cus-tomary loads at the end of the cable, renderedpossible by a simple means of the consideration ofthe complete system, to provide high intrinsically-safe direct power supplies to the loads.

The decoupling module enables both, the safety ofthe system and its functional operation to beachieved independently of the characteristics of therespective loads. Fig. 7 shows an example of theinterconnection of a DART-High-Power power sup-

ply with three loads via a connection cable and adecoupling module.



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DART The New Dimension In Intrinsic Safety DART The Power Concept

The decoupling module incorporates soft start andload. Due to the safety-related, easily describedbehavior of the system, at the point in time of thispublication maximum output data is achievable asfollows: U

max= 50 V and I

max= 1.2 A with a cable

length of 100 m. Fig. 8 shows the block diagram for

a decoupling module.

The soft start and load switch-on components aswell as a reservoir condensator, which provide forfault-free and straightforward switch-on of the load.The reservoir condensator takes care of switch-onover-currents and short periods of strong currentfluctuations. From a safety perspective the combi-nation of reservoir condensator, AC and reversepolarity protection provides for a defined DARTsystem.

In order to be able to cover the widest possible

range of applications, the possibility of the transferof data on the power supply line was anticipated in

the basic concept. The decoupling elements re-quired for this and the cable terminations toachieve a BPSK data transfer > 500 kbit/s are al-ready available in the power supply and in the in-terface circuitry. A 500 kbit/s data transfer via aDART High-Power System has already been suc-

cessfully tested. Further detailed information onthe data transfer can be obtained via the PTB.

The following applications can be achieved withDART Power in the explosion group Ex ib IIC:

Industrial PC, operating terminals and dis-plays

LED illumination system Sensors with high power requirements, e.g.:

Coriolis flowmeters Analytical devices Magnetic actuators and high power solenoid

valves Electrical heating systems

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DART For Fieldbus DART The New Dimension In Intrinsic Safety

7 DART For FieldbusIn the area of process automation the two fieldbussystems FOUNDATION Fieldbus H1 and PROFIBUSPA (MBP) as defined in IEC 61158-2 have been es-tablished as de facto standards.

A trunk-and-spur-topology is employed utilizing ahome run cable, also referred to as trunk. Fielddevices are connected via spur lines to wiring inter-faces with short-circuit protection, which can beconnected to the trunk at arbitrary points. Fig. 10shows the principle electric circuit of a topology.

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In comparison with existing intrinsically safe field-bus solutions, DART enables four times as muchpower on the trunk line. Power is approximately thesame compared to the well-accepted High-Power-Trunk concept, without the disadvantage with in-

creased safety installation methods required forthe trunk.

Though highest voltage values would be beneficialfor maximum output power, the available power isselected to 24 V. Thus any existing field deviceconformant with the Entity concept defined in IEC60079-27 can be connected. Entity enables intrinsicsafety to be validated for any topology through asimple comparison of values.

Frequently the plants that are to be automatedextend over a wide area, which requires long cable

connections. If the cable length is determined asbeing 1000 m, this results in an available effectivepower of 8 W. This output power is suitable for up

to 24 loads per segment and corresponds with theavailable power on Fieldbus segments with thegenerally recognized High-Power Trunk concept.

7.1 Decoupling The Field DevicesAs already described in section 4 the dynamic be-

havior of loads is not defined from a safety stand-point. Decoupling circuits are built into the Seg-ment Protectors as shown in Fig. 9.

Irrespective of the actual electric characteristics ofthe field device, the storage capacitor ensures adefined load behavior at the cable input terminals.

7.2 CommunicationThe communication, in the form of a trapezoidalalternating signal with a peak-to-peak value of 18mA (+/- 9 mA) is superimposed on the direct currentsupply signal. The flanks of the signal can be sev-eral microseconds short. The system distinguishes

these current variations unambiguously and relia-bly from those that occur in the generation of thespark.



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DART The New Dimension In Intrinsic Safety Summary And Outlook

8 Summary And OutlookDue to DART, very high intrinsically safe power isavailable for new applications in the process indus-try, depending on the length of cable employed.The maximum possible power output is stronglydependant on the delay times on the transfer cable.Solutions exist for two application areas: DARTPower for maximum power output and DART for theFieldbus, optimized for Fieldbus applications.

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50 VDC app. 50 W 100 m

24 VDC app. 22 W 100 m

50 VDC app. 8 W 1000 m

24 VDC app. 8 W 1000 m

Suitable test methods have been developed for anexact safety evaluation of the energy-limiting be-havior of dynamically operating power supply con-cepts. Changes to the currently applicable stan-

dards have already been investigated. Furthersteps will follow.

DART enables the use of intrinsic safety in applica-tions with power requirements, which today neces-sitate other, typically inflexible or expensive typesof explosion protection. By means of DART operat-ing processes will become simpler and complexityis reduced. Operating safety will be increased.

9 AcknowledgementThe research project 14490 N was funded by budg-ets of the German Ministry of Economics and Tech-nology (BMWi) via the Association of IndustrialResearch Organizations (AIF). The following compa-nies collaborated on the committee accompanyingthe AiF-Project:

HIMA Paul Hildebrandt GmbH + Co KG,68782 Brhl

Gnnheimer Elektronic GmbH,67433 Neustadt

Pepperl+Fuchs GmbH,68307 Mannheim

Knick Elektronische Messgerte GmbH,14134 Berlin

Dipl.-Ing. Bender GmbH,35305 Grnberg

Dezidata GmbH,94469 Deggendorf

DART test procedures and technology have beendeveloped in close cooperation between Physi-kalisch Technische Bundesanstalt and Pep-perl+Fuchs.

10 Literature[1] IEC 60079-11: Explosive atmospheres Part

11: Equipment protection by intrinsic safetyi

[2] IEC 61158-2: Digital data communications formeasurement and control Fieldbus for usein industrial control systems Part 2: Physi-cal layer specification and service definition

[3] PTB-Bericht PTB-Ex-1, Braunschweig, Juni2007, 11. BAM/PTB-Kolloquium zu Fragender chemischen und physikalischen Sicher-heitstechnik; Beitrag von U. Gerlach und Th.Uehlken: Neue Herausforderungen bei Spei-sesystemen hoher Leistung in der Znd-schutzart Eigensicherheit



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