Whitepaper_Common Sources of Arc Flash Hazards in Industrial Power Systems of IEC Plants

7/23/2019 Whitepaper_Common Sources of Arc Flash Hazards in Industrial Power Systems of IEC Plants

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Common Sources of Arc Flash Hazards in Industrial Power Systems in IEC Plants

COMMON SOURCES OF ARC FLASH HAZARD ININDUSTRIAL POWER SYSTEMS IN IEC PLANTS

Abst ract - Electrical design of industrial facilities addresses basic design considerations such asreliability, flexibility, cost and safety. In Europe, arc flash hazards are not always dealt with in great detailbeyond reliance on intrinsic equipment safety. Reality is, however, that even with the best equipment

design, arc flash accidents still occur, often as a result of human error during work activities.

IEEE Standard 1584 has been published as a result of many years of research and empirical testing ofthe arc flash phenomenon. The standard provides guidance in calculating the arc flash incident energy,taking into account multiple design factors. Incident energy calculations require relatively accurateassessment of the short circuit levels and the characteristics of the protective devices. Such input isconveniently provided by conventional short-circuit and protective device coordination studies. Thecombination of factors influences incident energy levels rendering some common estimation techniquesuseless, perhaps even dangerous. For example, a lower short-circuit current can lead to higher incidentenergy levels because some types of protective devices take longer time to trip for lower current.

This paper presents some typical scenarios in industrial facilities that are prone to yield high incidentenergy levels. The information presented is based on authors’ practical experience with arc flash hazardassessments.

I. INTRODUCTION

In the United States, electrical safety is covered by the National Fire Protection Association (NFPA) in itsStandard for the Electrical Safety in the Workplace (NFPA 70E-2009 [1]). In this standard, arc flashhazard is defined as “a dangerous condition associated with the release of energy by an electric arc”.The hazard is to be considered when work is performed near energized conductors, because of thelikelihood of an electrical arc occurring due to human interaction with the equipment, or condition of thevery equipment. An arc flash hazard analysis quantifies the risk of human exposure to the energy of anarc event under worst-case conditions, as well as effectiveness of measures of reducing incident energyand/or additional protective measures.

Many multinational corporations today implement a global safety program based on universal set ofminimum safety requirements and procedures. When such a program originates from the US, arc flashhazard assessment is often included as a requirement. Increasingly, industrial plants operating inEurope are introduced to arc flash hazard in this fashion.

In the United States arc flash has been a recognized electrical hazard since 1991 [2], with the fifthedition of the NFPA 70E published in 1995 specifically addressing arc-flash hazard. In this edition therequirements for protective clothing and a definition for the arc-flash boundary are included. Laterrevisions provided additional guidance for calculation of arc flash incident energy and protectiveboundary. The first edition of IEEE Standard 1584, Guide for Performing Arc-Flash Hazard Calculations[3] was issued in 2002. Empirical equations were derived from statistical analysis of extensive test data.The standard also provides a procedure for conducting a complete arc flash hazard analysis. In 2006the IEEE and NFPA agreed to collaborate on a joint research initiative to increase the understanding of

arc flash phenomena. Part of the plan is working with the international community for global adoption ofa new standard.

European codes and standards do not require assessing arc flash hazard to the detail the NFPA 70Edoes. European Directive 89/391/EEC requires an employer to evaluate risks and take the necessarypreventive safety measures. The general principles of the Directive put emphasis on avoiding risks,combating them at the source and taking collective protective measures over individual ones. An arcflash hazard analysis therefore will fit into the European standards framework, while risk analysis andsubsequently taken preventive measures will most likely differ from US approach.




II. ARC FLASH HAZARD IN EUROPEThe goal of an arc flash hazard assessment is to determine the energy released when an arc flashoccurs. The calculated energy can be used to select Personal Protective Equipment (PPE) that canprotect workers against the hazard.

European electrical equipment design and work safety standards seek to provide passive safety againstshock hazard with preference of collective over personal safety measures. Although the arc flash hazardis not explicitly addressed in such standards, this approach often does offer protection against arc flashrisk by reducing the odds of arc initiation. An arc flash hazard assessment should not serve as a meansof substituting these measures with mere personal protection, but rather as a way to protectworkers in the case when passive and other measures do not adequately protect against arc flash risk.

A. Internal arc proof switchgearIEC equipment guidelines define requirements and test methods for internal arc in metal clad switchgearand controlgear. These are IEC 62271-200 [Fig. 6] for medium voltage and IEC/TR 61641 [5] for lowvoltage. Although these are not mandatory requirements and, for low voltage, the guidelines do not havethe weight of a standard, the testing methods clearly demonstrate how a particular design limits the riskof injuries to personnel and damage to switchgear in case of an internal arc. Care needs to be exercisedin how such tests are interpreted. For example, when the test is performed with all doors closed, theinternal arc classification will not apply when doors are opened while work is performed.

B. IP codes and forms of separationEnclosures of electrical equipment are designed to prevent access to hazardous parts, and specificallythe contact with low-voltage live parts, mechanical parts or approach to high-voltage live parts. The IPcode indicates the degree of protection provided by the enclosure. IEC metal-enclosed switchgearequipment standards for low voltage (IEC 61439-1 [6]) and medium voltage (IEC 62271-200 [4])prescribe minimum degree of protection for enclosures depending on the type of assembly and theenvironment.

For low voltage switchgear different forms of separation are defined where compartments can separatebusbar, functional unit and cable connection. These compartments will have a minimum degree ofprotection expressed by an IP code.

It is important to keep in mind that there is no relationship between the degree of mechanical protectionand the internal arc flash rating of an enclosure. Although higher degrees of ingress protection may bethought of as reducing the likelihood of an arc initiation from external sources, the safety hazard may notbe reduced due to other effects of arc flash such as release of flying shrapnel.

C. Arc detection and reduction systemsVarious systems are available that detect internal arcs in switchgear and act to reduce the arcingduration, either by fast tripping of protective devices or by creating an intentional short-circuit to quenchthe arc [7].

These systems do not work to prevent an arc flash from occurring, but are often an effective way ofreducing the hazard. As these are active means of suppressing the effect of an internal arc, routineperiodical testing and maintenance of these systems is necessary.

D. Safe work practi cesFor CENELEC countries minimum requirements for electrical safe working are laid out by EuropeanStandard EN 50110, Operation of Electrical Installations [8]. The standard is endorsed by nationalstandards and regulations. The standard divides working procedures into three categories: deadworking, live working and working in the vicinity of live parts.

Dead working is considered to be the base safe working condition where essential requirements aresatisfied to ensure that the electrical installation at the work location is safely de-energized. Live working




and working in the vicinity of live parts are subject to national regulations, often requiring additionalmeasures as well as training of the workers.

Arc flash risk may be present in all three conditions. Some examples include verifying that theinstallation is de-energized using voltage detectors, connecting of portable meters to the live parts,phase comparison checks, placing of the shorting grounds, replacing fuses that are energized and livefault finding. It is therefore prudent to evaluate each work activity individually and consider how existingmeasures protect against arc flash hazard and if additional measures can be taken. Performing an arcflash hazard assessment helps to select adequate PPE when the risk is unavoidable.

III. ARC FLASH HAZARD ASSESSMENT WITH IEEE STD 1584-2002

The IEEE Standard 1584-2002 provides techniques in determining arc flash hazard due to an arc eventin 3phase ac systems. Two models are presented for arc flash incident energy and boundarycalculation; an empirically derived model for voltages from 208V up to 15kV, and a theoretically derivedmodel for all other voltages (also called the Lee method). All calculations in this paper were carried outwith the empirically derived model.

Hazards are calculated as arc flash incident energy, or radiated energy density, that is impressed on aworker’s skin in joules per cm2. A researcher named Stoll determined by experiment that it takes 5

joules per cm2 of heat energy for human skin to reach the threshold between a first degree burn and asecond degree burn. Therefore, the value of 5 J/cm2 has been established as the maximum amount ofenergy to be allowed to contact the worker’s skin during an arc event.

Incident energy is always calculated at a certain work distance. IEEE 1584 provides default distancesfor different types of switchgear and voltages. Incident energy is in indirect proportion to work distance;therefore increasing the distance can be an effective solution to high arc flash hazard. Moving a workerto twice the original work distance reduces the energy level by a factor four.

The arc flash boundary is defined as the distance from the arc where incident energy has attenuated to5 J/cm2. The boundary is a function of arc flash incident energy and the work distance. It is essentiallythe minimum working distance for injuries not to exceed first degree burns.

Incident energy is often expressed in calories because US standards such as the NFPA 70E did not fullyadopt the metric system. The IEEE 1584 calculation methods do provide results in joules, but becausetesting was conducted with calories, both terms appear in the standard. Clothing and other PPEavailable in the market today use ratings in cal/cm2. The widespread use of calories as a measure forincident energy can make it difficult to exclusively use joules when dealing with arc-flash, but it shouldbe the unit of choice by virtue of being a SI unit. The following relations can be used to convert betweencalories and joules:

1 calorie = 4.184 joules1 joule = 0.239 calories

There are many of factors influencing incident energy levels, adding to the complexity of calculations [9].Two main factors are the three-phase fault current and the fault clearing time, which are taken from a

short-circuit study and a protective device coordination study, respectively.

Most protective devices applied in the electrical distribution systems operate based on sensing theovercurrent condition at the point of their placement. A time-current characteristics (TCC) graph of theovercurrent protective device will be used to explain the effect of the fault current and clearing time,see Fig. 1. In a TCC diagram the time it takes the protective device, in this case a low voltage circuitbreaker, to interrupt a certain current is plotted on a log-log scale. The scales show time vertically from0.01s to 1000s and the current horizontally from 500 A to 100 kA. In a system with several voltage levelsthe current is conveniently shown for all protective devices upon selecting the referencevoltage level.




.

Fig. 1: TCC diagram showing constant energy lines

Low voltage air circuit breaker (ACB) trip units are commonly equipped with different protectivesegments; each segment can be independently set to provide protection of equipment and coordinationwith other protective devices. Although different names are used for these segments amongmanufacturers, they will generally fall within the following categories:

• Long Time – provides protection against overload currents and mimics a thermal type protectionwith an I2t characteristic. The long time segment has a pickup (LTPU) value ranging from 0.5xto 1.0x of the sensor rating, and a time delay (LTD) defined at some multiple of the pickup value,calibrated in seconds.

• Short Time – This segment is commonly used for backup protection. Shorter tripping time is

required for close faults while longer tripping time is used for the backup of the downstreamdevices. The short time segment has a pickup value (STPU) of about 2x to 10x the sensor (orLTPU) rating and a time delay (STD) of typically 0.1 to 0.5s.

• Instantaneous – the instantaneous stage has no intentional time-delay and it will trip as soon asthe current reaches or exceeds the pickup value, ranging from 2x to 16x the sensor rating. Thissegment provides maximum protection against close faults for both the protective equipmentand the circuit breaker itself, but may lead to miscoordination as two instantaneous protectionstages in series cannot discriminate unless separated by significant impedance.




A short-circuit study yields a so-called bolted fault current for each location; this is the maximum value ofthe short-circuit current based on a calculation using only the system and source impedance. The arcingcurrent in low voltage systems is always lower than the bolted short circuit current because of the finiteimpedance of the plasma cloud conducting the arc current. This reduction is different at different voltagelevels (see Fig. 2) and is significant in low voltage systems. Shown in Fig. 1 are five arcing current levelsas Ia1 through Ia5. The variance of the arcing current in a real distribution system is typically not asdramatic as shown in this picture where the current levels are chosen arbitrarily only for the purpose ofdemonstration.

Fig. 2: Relationship between bolted fault current and arcing current for two vol tage levels

Constant energy is shown as diagonal dotted lines, showing the maximum fault clearing times for theentire range of currents. The energy values chosen and the corresponding hazard categories are thereference incident energy levels based on NFPA 70E, for reason of simplifying PPE selection. Thisillustration can be useful for understanding the relationship between arcing current and clearing timewhen determining incident energy.

Table I shows the calculated incident energy for each of the arcing current levels. Higher arcing currentcan result in either lower or higher incident energy, depending on the protective device characteristics.The highest energies in this case occur at the lowest arcing currents.

The challenge of an arc flash hazard analysis is to find the worst-case incident energy occurring in thesystem. As we have seen, this is not simply a case of finding a scenario with the highest (or lowest)short-circuit levels. All possible scenarios, from changing network topology to the minimum andmaximum level of short-circuit contribution should be evaluated for incident energy at every location.

TABLE I CALCULATED INCIDENT ENERGY FOR ARCING CURRENTS IA1 THROUGH IA5




IV. COMMON SOURCES OF ARC FLASH HAZARD

Fig. shows a simple model of an industrial system that will be used to demonstrate some commonsituations that can lead to high arc flash hazard. Buses LV1 and LV2 are being evaluated, each fed byits own 630 kVA transformer. A standby (emergency) generator is connected at Bus LV2. It is onlyallowed to operate when the normal power from utility is not present.

Fig. 3: Single Line Diagram

In the normal operating condition the tie-breaker and generator breaker are open. Typical values havebeen assumed for MV level utility contribution and transformer impedance. LV-I1, LV-I2 & LV-G are1000A air circuit breakers with adjustable trip units with the following settings: LTPU/LTD of 0.9x/2s,STPU/STD of 6x/0.2s, Instantaneous disabled. LV-F is a typical slow acting current limiting BS88 / IEC269 fuse.

Next, a single incident energy is calculated for each case of protective devices settings, based on theassumption that work is performed on the ‘bus side’ of the protective device, which in this case is themain device for the panels LV1 or LV2. This means that the arcing current is interrupted by theprotective device being evaluated, as long as the space that containing the device doesn’t include the‘line side’ bare conductor whereby the fault would need to be interrupted by an upstream (backup)

device. Hence for such assumption to be valid, ‘line side’ elements would have to becompartmentalized,or separated from the work area with barriers containing the arc flash.

A. High pickup sett ings, high tr ip timeMaximum settings for protective devices lead to high potential arc flash hazard. This is illustrated on theTCC diagrams in Figs. 4 and 5, where the pickup setting of theShort Time segment has been increased from 6x to 10x (typical maximum setting for air circuitbreakers).




Table II shows the resulting arc flash hazard. The fault clearing time for the second case increaseddramatically with incident energy increased by almost a factor of four. This also demonstrates that in theabsence of instantaneous protection or fuses, the Short Time segment settings of the air circuit breakersare pivotal in managing the arc flash incident energy levels.

When dealing with outgoing (feeder) sections of the switchgear, the calculated ‘bus side’ incident energyapplies. When looking downstream of the switchgear, the feeder protective devices will be responsibleof clearing the arc fault currents. As these devices are sized well below the main and bus tie devices,the incident energy and arc flash hazard levels downstream of the branch circuit protective devicesdecrease in relation to faster fault clearing and current-limiting effects of the cables.

TABLE II HAZARD INCREASE DUE TO A CHANGE IN SETTING

Next example shows the effect of sizing the low voltage fuses that are for the purpose of illustrationassumed to be connected at panel LV1. In table III three fuse sizes are compared, sized at 18%, 44%and 69% of the nominal current (In) of the upstream transformer. Fig. 6 shows their time-currentcharacteristics together with the arcing current at LV1. It is interesting to note a dramatic increase ofincident energy for the 630A fuse as this fuse clears the fault by the I2t part of the fuse curve and not inthe current limiting mode as the smallest fuse in this example.

In this example, an arc fault clearing by the 400 A fuse may require a little closer scrutiny. In fact, whilethe effect of the cable impedances can often be neglected in most short circuit estimates, as it is often

considered to be within ‘margin of error’, arc flash assessment may demand a second look.

Fig. 4 ACB with low STPU Fig. 5 ACB with high STPU Fig. 6 Three fuse sizes




In the radial system, a difference in the bolted fault current between both ends of the low voltage feedercan be as much as 5 % but more commonly less than 3 %. Even this relatively small difference cancause the fuse, such as the 400 A fuse in our example, to clear the fault in the current limiting mode atthe beginning of the feeder, while clearing the fault at the remote end of the same feeder in its I2t mode,resulting in higher incident energy at the remote end compared to the beginning of the radial circuit.

TABLE III HAZARD FOR DIFFERENT FUSE SIZES

B. Low arcing current, high trip time

Another, much more dramatic effect of the low fault levels can be seen on an example with the systemsupplied from the emergency generator. The emergency generators are typically sized at the fraction ofthe normal system capacity for the economical reason and the fact that not all loads in the systemrequire emergency power supply. The source reactance represented by the typical standby generatorcan be therefore many times higher than the source reactance largely represented by the utility and thesupply transformers. From what was demonstrated earlier, it follows that if, instead of a utility, the mainLV2 bus is now supplied by the backup generator, arc flash hazard may change and it needs to beclosely investigated. The change will depend on the response of the protective devices to lower boltedfault and therefore arc fault currents. Also demonstrated earlier was the difference in response betweenair circuit breakers and standard fuses.

In this case bus LV2 is evaluated with incoming circuit breakers LV-I1 & LV-I2 open, generator Grunning and generator circuit breaker LV-G closed. This is the most common emergency scenarioemploying standby generator system. Paralleling design is sometimes employed only for generatortesting and source transitions. In emergency mode, a fault on bus LV2 will only see the generator’scontribution. Overcurrent settings of the generator circuit breaker are kept the same as the transformerincoming circuit breakers (Long Time at 0.9x, 2s, Short Time at 6x, 0.2s, Instantaneous disabled).

Table IV compares the arc flash hazard for both modes of the system operation. In the emergencymode, the arc fault current at the LV2 bus is less than half of the fault current in normal condition, belowthe short time pickup of the main circuit breaker. Consequently this leads to very high incident energy,more than eight times higher than in the normal system condition.

TABLE IV HAZARD WITH A TRANSFORMER AND GENERATOR




C. High arcing currentsWhen evaluating arc flash hazard, fault levels from all possible system scenarios should be considered.When multiple sources exist in the system that can be paralleled together such as via system tiebreakers, arcing currents will increase. Such situation would occur if two utility sources were to be tiedtogether at the customer site in a ring or Ring Main Unit (RMU) configuration which is relatively rare inindustrial systems, or the site has another source of power, such as a generator used for combined heatand power generation (CHP).

We will demonstrate the effect of higher fault levels on paralleling of the two transformers in our samplecase. The arc flash hazard for both cases is shown in table V.

In our example, increasing the arcing current leads to somewhat moderate increase of incident energy.In this case the 100% increase of fault current leads to a mere 44% increase of incident energy. This isevidently because of the segmentation of the air circuit breaker trip curve discussed earlier and in thegiven situation, higher current will not cause trip unit to act slower. In fact, it could only result in fastertripping if the instantaneous segment were employed.

TABLE V HAZARD WITH SINGLE AND PARALLEL TRANSFORMER CONTRIBUTION

D. Lack of a main protective deviceIn cases where no incoming protective device is present in the panel or switchgear, or energized partsof the bus are not separated by compartmentalization or barriers, arc fault must be cleared by theprotective devices upstream of the panel or switchgear.

The incident energy results for the faults at ‘bus side’ versus ‘line side’ of the main low voltageswitchgear are compared in table VI. In this example, the upstream protective device is a mediumvoltage relay of ‘definite time’ characteristic, with a fault clearing time of 1.1 s. The effect of relying onupstream device, in the given example, leads to an increase of incident energy of over four times.

TABLE VI HAZARD WITH TRIP BY ACB OR UPSTREAM RELAY




V. CONCLUSIONS Arc flash hazard assessment is a relatively new development in Europe, but it fits the general frameworkof requirements imposed on employers in performing safety risk assessments. Equipment and worksafety standards seek to provide passive protection against shock hazard and although the arc flashhazard is not explicitly addressed, existing measures often can reduce the likelihood of an arc initiation.Despite these measures the arc flash risk cannot be completely avoided during some work activities. Anarc flash hazard assessment can be used to select PPE that protects against the hazard.

The level of arc flash hazard is difficult to predict and depends on many factors. A detailed studyfollowing the IEEE Standard 1584 procedures is in many cases required to reliably find the worst-caseincident energy in each location.

A few scenarios typically found in industrial facilities were shown to yield high arc flash incident energylevels. The high hazards were shown to have multiple sources: low or high fault contribution, protectivedevice characteristics and system properties. Performing an arc flash hazard assessment can readilyidentify these issues, as well as indicate how these issues may be addressed to reduce arc flashhazard.

VI. REFERENCES

[1] NFPA 70E-2009, Standard for Electrical Safety Requirements for Employee Workplaces.

[2] Ravel F. Ammerman, P. K. Sen, John P. Nelson, “Electrical Arcing Phenomena, A historicalperspective and comparative study of the standards IEEE 1584 and NFPA 70E”, IEEE Industry

Applications Magazine, May / June 2009.

[3] IEEE Std 1584-2002 IEEE Guide for Performing Arc-Flash Hazard Calculations.

[4] IEC 62271-200 AC metal-enclosed switchgear and controlgear for rated voltages above 1 kVand up to and including 52 kV.

[5] IEC/TR 61641 Enclosed low-voltage switchgear and controlgear assemblies – Guide for testingunder conditions of arcing due to internal fault.

[6] IEC 61439-1 Low-voltage switchgear and controlgear assemblies – Part 1: General rules.

[7] L. Kumpulainen and S. Dahl, “Minimizing Hazard to Personnel, Damage to Equipment, andProcess Outages by Optical Arc-Flash Protection” in IEEE PCIC Conference Record, 2010.

[8] EN 50110-1 Operation of electrical installations.

[9] H. Wallace Tinsley III, M. Hodder, “A Practical Approach to Arc Flash Hazard Analysis andReduction”, IEEE Industry Applications Magazine, Volume 41 Issue 1, pp 144-154, Jan/Feb2005.

Whitepaper_Common Sources of Arc Flash Hazards in Industrial Power Systems of IEC Plants

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