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REDUCING THE FLASH HAZARD

For Presentation at theIEEE/PCA Cement Industry Technical Conference

Phoenix, AZ

April, 2006

By:IEEE-IAS Cement Industry Committee

Timothy B. Dugan, P.E.Electrical Engineer

PENTA Engineering Corp.

Abstract

This paper reviews the need for arc-flash hazard awareness, analysis, and personnel protection. It looksprimarily at the low voltage electrical system and what can be done to reduce incident energy. It furtheroutlines methods to protect workers from the devastating effects of arc-flash, and possible ways to reduceincident energy levels through review of various electrical design choices.

Arc-Faults: Arc-Flash and Arc-Blast

Electrical workers are commonly exposed to three types of hazards: Electrical shock, electrical burnsresulting from contact and arc-flash, and arc-blast [1]. Electrical burns that result from contact withenergized parts occur due to current flow through the body. Burns as a result of arc-flash occur due toradiant heat given off by an electrical arc. Lastly, arc-blast is the result of the heating effects of an arc-flash that produce tremendous pressures through the expansion of destroyed circuit materials. Of thesethree, electrical shock and electrical contact burns are the hazards of which most personnel are aware.

An arc-fault is the result of current flowing through air between conductors in phase-to-phase, singlephase-to-ground, or multiple phase-to-ground configurations (i.e. a 3-phase bolted fault). Arc-faults canrelease large amounts of energy in an extremely short amount of time in the form of radiant heat, intenselight, and high pressure waves. The radiant heat given off travels at the speed of light and can reachtemperatures of 35,000F [1]. Due to these high temperatures, circuit components directly involved in thearc can explosively change physical state from solid to vapor. Once the vapor state of the conductivemetal is reached the arcing fault can quickly proceed from a single-phase fault to a three-phase fault inless than 1ms [1]. The metal vapor, or conductive plasma cloud, superheats the surrounding air causingfurther explosive activity resulting in large pressure and sound waves. The pressure waves can destroyelectrical components outside the vicinity of the arc, causing significant amounts of shrapnel to beexpelled from the fault location at speeds greater than 700mph [1]. In general, an arc-fault is primarilycomposed of the release of thermal energy referred to as arc-flash, and large pressure waves referred to

as arc-blast.

In recent years the dangers and impact of arc-flash have been studied and are now becoming moreunderstood. However, the electrical industry is still in the process of studying the effects of arc-blast andways to mitigate its risks. This paper is limited to the review of arc-flash hazards, their analysis, andcorresponding ways to reduce incident energy in low voltage systems.

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Causes of Arc-Flash

Steps can be taken to reduce the level of available energy that can be released in an arc-flash at anypoint in an electrical system. Nonetheless, it is important to recognize that the main contributors to arc-flash events are human error and electrical equipment failure.

Human errors can include the dropping of tools or loose parts inside equipment, inadvertently contactingenergized conductors with tools, parts, or cables, or improper alignment of equipment while beinginserted into live electrical bus such as MCCs or Switchgear. Factors related to equipment failure caninclude lack of equipment maintenance, equipment or cabling insulation failure, or continued electricalfault occurrences that go uncorrected causing premature component failures (i.e., overvoltage, overload,and overcurrent conditions). Therefore, it is important to establish and maintain regular preventivemaintenance schedules for electrical equipment and systems, as well as take immediate action to correctrepetitive problems.

It is also important to point out that a suitable environment for electrical equipment is key to prevent theaccumulation of dust, or buildup of corrosion and condensation, all of which can lead to improperequipment operation and/or premature equipment failure. Therefore, a clean, conditioned electrical roomis warranted in places where major equipment such as low voltage substations, MCCs, and switchgearare located. A clean equipment environment also promotes safer working conditions for electrical

maintenance personnel who must service electrical equipment while energized.

Reduction of Arc-Flash Energy

According to NFPA 70E, Standard for Electrical Safety in the Workplace, incident energy is defined as theamount of energy impressed on a surface, a certain distance from the source, generated during anelectrical arc event [2]. Methods for reducing incident energy are aimed solely at changing the factorsthat determine the amount of energy that an arc-flash can produce. Namely, these are voltage, current,and time. If one or more of these parameters can be modified, it is possible to reduce the energyavailable to an arcing fault. The voltage level in a new plant or existing facility is determined by thedistribution system being used, and therefore is not easily modified. Hence, it is simpler to find ways tochange the current and time factors.

The current involved in an arc-flash is the available short-circuit current as determined by a short-circuitanalysis. Possible ways to reduce the level of short-circuit current at the point of fault are:

1. Limit transformer size. The short-circuit current available in a system is primarily determined by thesize of the system transformer. At 5.75% impedance, with a 480V secondary, and assuming aninfinite bus on the primary, a 2,000kVA transformer can produce approximately 42kA. Under thesame conditions, a 1,000kVA transformer can produce approximately 21kA. Furthermore, moreconservative figures would approach 52kA and 26kA, respectively, when considering that a significantpercentage of the current could possibly be contributed by motors connected to the system (100%assumed for simplicity). Therefore, it is easily seen that the use of multiple smaller transformers isadvantageous over a single large one when comparing levels of short-circuit current.

In the past, it was standard practice in the cement industry to install a maximum of a 1,000kVAtransformer for secondary unit-substations [3]. However, in recent years it has become standardpractice to install maximum 1,500kVA transformers [4], and occasionally 2,000kVA unit-substations.Generally, installation of a single large transformer has a lower capital cost for installation whenconsidering the additional primary and secondary equipment, and electrical room space required toinstall multiple smaller transformers. Yet, installing larger transformers poses an increased risk to theelectrical worker in the form of increased available incident energy.

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2. Install higher impedance transformers. Secondary unit-substation transformers are commonly

manufactured with an approximate impedance of 5.75%. And, in the recent past, it was common toinstall transformers that were both smaller in size (1,000kVA) and higher in impedance, typically 8%.Today 8% impedance transformers will likely cost more than 5.75% transformers given that theyrequire more materials to manufacture and are not commonly produced. Nevertheless, specifying ahigher impedance transformer will provide significant reduction in available short-circuit current.

Again, a 1,000kVA transformer with 5.75% impedance would source approximately 21kA. Anequivalent transformer with 8% impedance would produce approximately 15kA. Again,conservatively including 100% motor contribution, current could possibly approach 20kA. Therefore,it is easily seen that the use of higher impedance transformers is also advantageous when reviewingways to reduce incident energy.

It is possible that using higher impedance transformers can cause voltage regulation problems whenstarting several large motors at full voltage. In applications where this is potentially problematic theengineer should perform a motor starting analysis to determine if installation of reduced voltagestarting equipment is warranted. The costs associated with both higher impedance transformers andreduced voltage starting equipment can be greater when compared to installations with lowerimpedance transformers. But, if higher impedance transformers can help reduce incident energyexposure to electrical workers, they are certainly worth consideration by both the facility owner and

the design engineer.

3. Use high-resistance grounding for low voltage systems. A high-resistance grounding system canreduce incident energy because of the addition of a resistance between the neutral connection pointon a wye-connected transformer and the system ground. Phase-to-phase faults in high resistancegrounded systems will behave similarly to solidly grounded systems. However, the resistancereduces the short-circuit current level in a phase-to-ground fault situation. And, since most arc-faultevents start out as phase-to-ground, it is possible that using high-resistance grounding can reducethe possibility of faults escalating into phase-to-phase or bolted fault situations. An additional benefitof this system is that it is designed to remain in service under phase-to-ground situations, therebyincreasing process uptime and allowing maintenance crews to fix faults before they escalate tophase-to-phase or bolted faults.

The time factor in an arc-flash event is the time duration that spans from when the arc occurs to when theOCPD opens or clears the fault. The following are design changes that can be made to reduce theamount of time required to clear a given fault.

1. Size overcurrent protective devices (OCPDs) as low as possible. By design, lower ampere ratedOCPDs will let through less current than higher rated ones prior to opening during a fault. Therefore,OCPDs for individual pieces of equipment such as motor starters or large electrical equipmentfeeders such as MCCs should be applied as tightly as the application will permit. For example, whenan 800A MCC is protected by a switchgear feeder breaker with an 800A frame, the trip setting shouldbe adjusted as low as the total running MCC load will permit. Future trip setting adjustments can bereviewed and made as the MCC load increases.

2. Use multiple feeders by splitting loads into smaller groups. For example, instead of installing a single

MCC with a 2,000A continuous rating, install three 800A MCCs. Or install three 600A MCCs insteadof a single 1,200A. By designing in this manner, the individual feeder OCPDs can be sized and setlower, again decreasing available energy as mentioned above.

3. Reduce time delay settings. Decreasing long time, short time, or instantaneous trip settings on lowvoltage power circuit breakers can help to reduce incident energy by decreasing the amount of time abreaker takes to open. Yet, the engineer should be cautious to consider any effects this may have onexisting OCPD selective coordination prior to making any adjustments.

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4. Install ground fault sensing equipment. As stated previously, most arc-faults start out as phase-to-ground faults. Therefore, installation of ground fault detection equipment can help to detect arc-faultsquicker. This is primarily because ground fault sensors are more sensitive than overcurrent sensors.Ground fault sensing equipment should be installed in multiple places such as on low voltageswitchgear mains and feeders, and possibly on individual MCC feeders or motor starters. The moreground fault sensing there is, the lower the settings can be for each feeder or piece of equipment,thereby decreasing clearing time. Ground fault sensing installed only on the mains of switchgearprovides some means of protection, but can be of little benefit when troubleshooting for fault isolation.

5. Use current limiting OCPDs. The use of current-limiting circuit breakers, fuses, or combinationsthereof can greatly reduce the amount of incident energy in a fault. Generally, current-limitingdevices have quicker clearing times than non current-limiting devices. Current-limiting fuses cantypically interrupt a fault within the first half cycle whereas non current-limiting circuit breakers cantake several cycles to open. Generally, when current-limiting devices are applied in conjunction withthe above mentioned methods, further reduction can be achieved with some exceptions.

Current-limiting devices must operate within their specified current limiting range to provide thegreatest reduction in incident energy. When current limiting fuses do not operate in their currentlimiting range, clearing times can be significantly longer than cycle [5]. Therefore, when usingcurrent-limiting devices it may be best to apply lower impedance circuit components, such as

transformers and/or cabling, in lieu of components with higher impedances [6]. The idea is to achievehigher available short-circuit current to allow current-limiting devices to operate in their respectivecurrent-limiting range, thereby clearing faults quicker and reducing incident energy exposure.

6. Use Type 2 motor starters. IEC 947-4-1 and UL508E provide for two different levels of coordinationand damage protection. During a fault, a Type 1 coordinated starter will interrupt the fault current, butis not required to provide protection for the internal components. Therefore, components in a Type 1starter can be severely damaged and/or destroyed during a fault. A starter that adheres to Type 2coordinated protection must interrupt the fault current, and still be operational after the fault occurs.Type 2 coordination is referred to as No-Damage Protection, and is typically achieved throughcurrent-limiting devices such as current-limiting fuses. Therefore, since current-limiting devices areused, Type 2 starters can not only provide a means of reducing incident energy, but can also providelower maintenance costs and less downtime because damage does not occur to the starter during a

fault.

Protection from Arc-flash

The best protection against the dangers of arc-flash is to prevent exposure by not working on or nearenergized equipment. This is easily achieved through utilization and enforcement of documented lockoutprocedures as mandated by OSHA or MSHA. However, NFPA 70E recognizes that many circumstancesmay warrant working on equipment while energized, such as when deenergization would createadditional hazards or would be infeasible due to equipment design or operational limitations.

In cases when work must be performed on or near live parts, the following general requirements shouldbe considered:

Assign qualified worker(s) to the task.

Train workers for the specific task they are undertaking.

Keep workers not involved in the task outside the flash protection boundary.

Limit the number of persons performing the work.

Utilize appropriate PPE including fire resistant (FR) clothing for protection against arc-flash, andvoltage rated gloves and/or leather protectors for protection against electrical shock.

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Importance of Protection

The danger and corresponding injuries caused by an arc-flash have been with us since theimplementation of electrical power systems. It is an ever-present danger for the personnel charged withtheir maintenance and operation of electrical systems. Many electrical maintenance activities involvework on energized equipment. Additionally, working on equipment when cubicle doors are open is aregular occurrence when voltage or current testing on energized circuit parts is required. Therefore, it ishighly probable that electrical workers could be involved in arc-flash incidents.

Injuries suffered by electrical workers are usually more severe than injuries suffered by workers of othertrade classifications. Yet, at some facilities there is no electrical safety program implemented other thanbasic electrical safety training and lockout or tagout procedures. In addition to destroying electricalcomponents, the effects of arc-faults can be highly destructive and devastating to the human body.Studies show 80 percent of documented electrical injury cases were burns resulting from exposure toelectrical arc-flash [1]. Furthermore, more than 2,000 people are admitted annually to burn centers in theU.S. with severe electrical burns [1]. However, in addition to burn injuries, victims of arc-faults may alsoexperience sight damage, hearing loss, and respiratory, muscular, skeletal, or nervous systemimpairments.

With respect to the need for arc-fault flash hazard analysis, the NFPA has adopted specific requirements

in its code standards. Article 110.16 Flash Protection was added to the 2002 edition of NFPA 70,National Electrical Code. It states that equipment that is likely to require examination, adjustment,servicing, or maintenance while energized shall be clearly and visibly labeled to warn persons of potentialarc-flash hazard [2]. Additionally, the 2004 edition of NFPA 70E, Standard for Electrical Safety in theWorkplace, states in Article 110.8(B)(1) that both a shock and arc-flash hazard analysis shall becompleted prior to work on or near energized equipment. The results of the study shall determine thelevel of PPE required to perform the task [2]. Since most electrical equipment in industrial manufacturingor processing facilities will eventually require some sort of maintenance while energized, it is safe toassume that all equipment should be marked or labeled per NFPA 70 and 70E requirements.

Overall, the importance of protection from arc-flash hazards, as well as reduction of incident energy isshown by the need to:

Decrease hazards to workers.

Avoid litigation expense which is often incurred after serious or fatal arc-flash events. Minimize process downtime.

Minimize equipment damage when faults occur.

Compliance with codes (OSHA, MSHA, NFPA).

Meet any particular insurance requirements.

Arc-Flash Study and Analysis

There are relatively few steps involved in performing an arc-flash analysis, assuming that the requiredinformation is available for use by the engineer, and is up to date. The following is a list of pre-requisiteinformation needed to complete an arc-flash analysis.

I. Short-Circuit Analysis. To perform an arc-flash analysis the level of short-circuit current at severaldifferent points in an electrical system must be known, and the protective devices and their settings mustbe identified. Therefore, the results of a short-circuit analysis are often required. Modern plants mayhave this information available in a previously completed short-circuit study. The results of this study maybe used for the arc-flash study. Still, the engineer should be cautious to review the validity of thisinformation if significant changes have been made to the system since the study was completed. Forthose facilities without an existing short-circuit study available, one must be completed using the generalinformation requirements as outlined below:

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1. One-Line Diagram(s). Industrial facilities are often in pursuit of increased production. Hence,modifications are made to electrical systems that differ greatly from their original design. Because ofthis, electrical systems can quickly become poorly documented, mis-coordinated, underrated, andmisunderstood as to their exact makeup. Therefore, it is critical that sufficient time be spent collectinginformation about the structure of an electrical system. A complete and accurate one-line diagramcreates an electrical picture of a facility by documenting the following:

a. The various voltage levels in a plant from Medium Voltage (i.e. 13.8kV / 4.16kV) down to LowVoltage (i.e. 480V / 120V).

b. Equipment types such as MV and LV Substations, Switchgear, MCCs, Transformers, andPanelboards, and their corresponding locations in the distribution system.

c. Rating information about the electrical distribution equipment such as fuse types and sizes,circuit breaker frame and trip ratings, short-circuit interrupting capacities, and transformerimpedances.

d. OCPD types, locations, ratings, and clearing times.

2. Protective Device Information. Detailed information about the protective devices used in an electricalsystem is equally important as an accurate one-line diagram. This includes information such as:

a. Device type: Electro-mechanical or Microprocessor based relays, circuit breakers, fuses, etc.b. Device manufacturer and model or part numbers.c. Existing protective device trip settings (i.e. LT, ST, Inst, etc., and/or CB/fuse curves).

3. Cable and Raceway Information. An electrical cable has an inherent impedance determined mainlyby size (diameter) and length. The cable impedance, along with transformer impedance, affects thelevel of short-circuit current at a given point in the system. The types of raceways cables are installedin, magnetic or non-magnetic, can also affect the short-circuit current. One-line diagrams often showcable sizes, but may or may not indicate length or installed raceway type. Therefore, it is important tosurvey this type of information as it is supplemental to the one-line diagram and corresponding short-circuit analysis.

4. Identify Possible System Operating Modes: Facilities configured as main-tie-main or similar can bepotentially problematic when the tie and both mains are closed concurrently. Configuration of theelectrical system in this manner is usually only used when required to start large motors. However,placing both transformers in parallel causes the available short-circuit current to increase significantly.

Since most plants do not operate continuously in this manner, it is likely not a valid consideration.This should be confirmed by verification with people knowledgeable about the normal operation of theplant.

Once the above information has been collected, the engineer can complete a short-circuit analysis usingtraditional hand calculations or more often by using one of the many commercially available softwarepackages. The main purpose of a short-circuit analysis is to determine that a facilitys electricalequipment and cabling have been applied within their short-circuit ratings. The output of the short-circuitstudy forms the basis of the arc-flash study as described below.

II. Perform an Overcurrent Protection Coordination Study. It is common practice to perform an OCPDcoordination study after completion of a short-circuit analysis. Coordination studies are performed toallow OCPDs closest to an electrical fault to open first, thereby limiting electrical outages to affected

circuits only. The study output determines required fuse types, circuit breaker settings, and protectiverelay settings to achieve best coordination. Breaker and relay settings can directly impact the results ofan arc-flash study as they often involve the use of short time delay, long time delay, and instantaneoussettings to allow lower level OCPDs to open the circuit first.

III. Perform Shock Hazard Analysis. In addition to arc-flash analysis, Article 110.8(B)(1)(a) in NFPA 70Erequires a shock hazard analysis to determine the voltage to which personnel will be exposed, boundaryrequirements, and the personal protective equipment (PPE) necessary in order to minimize the possibilityof electric shock. The shock protection boundaries are determined by the nominal system voltage that apiece of equipment is rated for.

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Putting Study Results to Use

The following general steps can be used to implement the results of an arc-flash study.

1. Label equipment so that personnel will recognize the presence of possible arc-flash.2. Train personnel how to interpret labels and what actions should be taken by them to protect

themselves against the devastating effects of arc-flash.3. Implement an electrical safety program that addresses when and how to work on energized

equipment per guidelines in NFPA 70E.4. Review possible electrical system design changes and their corresponding effect on incident

energy exposure levels.5. Create an action plan for approved design changes to reduce incident energy and improve

personnel safety.

Conclusion

Much research and testing has been completed in recent years studying the effects of arc-flash as well asways to reduce incident energy. Additionally, code standards such as NFPA 70 and NFPA 70E have

adopted new requirements to warn personnel of the presence of arc-flash, as well as provide informationthat the electrical worker can use to select appropriate personal protection equipment. It is becomingmore apparent that facilities should implement workplace safety programs including those that educate,train, and protect workers against arc-flash. Furthermore, electrical system planners, designers, andengineers should work closely together to implement means of reducing incident energy throughappropriate electrical design changes.

References

[1] Safety BASICs, Handbook for Electrical Safety, Bussmann Awareness of Safety Issues Campaign ,Edition 2, Cooper Bussmann, 2004.[2] Standard for Electrical Safety in the Workplace, NFPA 70E, 2004.

[3] IEEE Recommended Practice for Cement Plant Power Distribution, IEEE Std. 277-1975.[4] IEEE Recommended Practice for Cement Plant Power Distribution, IEEE Std. 277-1994.[5] R.L. Doughty, T.E. Neal, T.L. Macalady, V. Saporita, and K. Borgwald, The use of low-voltagecurrent-limiting fuses to reduce arc-flash energy, IEEE Transactions On Industry Applications, vol. 36,no. 6, Nov./Dec. 2000.[6] Tim Crnko, Steve Dyrnes, Arcing flash/blast review with safety suggestions for design andmaintenance, in IEEE Pulp and Paper Industry Technical Conference Record, June 2000, pp. 118-126.[7] National Electrical Code, NFPA 70, 2005.[8] R.H. Lee, The other electrical hazard: Electrical arc-blast burns, IEEE Transactions On Industry

Applications, vol. IA-18, No. 3, May/June 1982.[9] Guide for Performing Arc-Flash Hazard Calculations, IEEE 1584-2002.

ANNEX

Attached are two general examples of program outputs and arc-flash warning labels that are availablethrough commercially available arc-flash analysis software packages.

1. 1000kVA (5.75%) secondary unit-substation, with secondary switchgear breaker feeding an MCC,with a total load of 500Hp.

2. 1000kVA (5.75%) secondary unit-substation, with secondary fused switch feeding an MCC, with atotal load of 500Hp

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