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WWW.ECMWEB.COM APRIL 2010

THE MAGAZINE OF ELECTRICAL DESIGN, CONSTRUCTION & MAINTENANCE

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Wiring methods:Chapter 3, the sequel

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FAQ on HRG: Arc flashsafety as a three-letter

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Arc Mitigation Techniques7EBINARDRAWSRECORDNUMBEROFQUESTIONSFROMTHElELDWITHAUDIENCEMEMBERS

n October 13, 2009,EC&Mhosted a free we-binar on Arc Mitigation A Total Systems Ap-proach. The 1-hr eventwas sponsored by i-Gard,

a Mississauga, Ontario, Canada-baseddeveloper and manufacturer of ground-fault protection and power-resistor-based solutions. The 45-min. presenta-tion was led by Daleep Mohla, principaland owner of DCM Electrical ConsultingServices in Missouri City, Texas. In fact,

you can still view this archived webinaron EC&Ms Web site. Access it by clickingon the link under the EC&M webinarheader in the right rail of our home page(www.ecmweb.com).

In addition to reviewing mitiga-

tion technologies, such as arc-resistantswitchgear, current-limiting devices, andthermal ionization equipment, Mohlashared his expert insight on NFPA 70Eand discussed the pros and cons of high-resistance grounding (HRG) systems.The event drew more than 1,000 partici-pants as well as an impressive numberof questions submitted by attendeesthroughout the webinar. In fact, so manyquestions were posed that it took severaldays to follow up with attendees and

provide them with answers.As a follow-up to this webinar, wethought it would be worthwhile to pres-ent a summary of some of the more inter-esting Q&A items. From the magazinesviewpoint, it appears many electricalengineers, contractors, and maintenanceprofessionals are searching for answerson how best to address the dangers of arcflash events as well as what types of arcflash mitigation technologies are availablein the market. Many of the questions re-volved around the use of HRG equipment

and design configurations. We hope thissummary will help you better understandthis very important topic.

O

The following answers were developedby Mohla and i-Gard technical represen-tatives.

Q.What are your thoughts on arc-proofswitchgear, which a lot of vendors are of-fering these days? To me, when you openthe panel doors to work on the gear, itdefeats the purpose of arc proofing it!

A. Arc-resistant switchgear is de-signed to contain the energy producedby an arcing event and to direct that

energy away from personnel operatingthe switchgear. If compartment doorsare open or if panels are removed

the energy can no longer be directedaway from personnel. It is also possibleto have arc-resistant switchgear thatis protected by arc flash mitigationrelays. This combination not onlyhelps minimize arc flash damage, butalso allows you to place the damagedsection of switchgear back in servicemore quickly.

Q. Does the NEC make any referenceson where I can use HRG systems?

A. Section 250.36 of the NEC allowsthe use of HRG on systems up to 1,000V.Make sure you note the three conditions

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that are listed. Section 250.186 allowsHRG at higher voltages.

Q. What are your thoughts or recom-mendations regarding the placement of480V drives or motor starters in the samecontrol panel as PLC and network-typeequipment?

A. You may find your customers re-questing that control equipment and480V drives or any other 480V equipmentbe placed in different compartments orenclosures to ensure that the personneloperating or testing the PLC and/or net-work equipment will not need to wear

personal protective equipment (PPE).

Q.Why is HRG considered a method toreduce arc flash hazard, since the IEEE1584 equations for arc flash incident en-ergy and all IEEE group testing are basedon 3-phase bolted fault currents?

A. HRG is considered an arc flashmitigation technique in NFPA 70E andCSA Z462, because it reduces the risk ofdeveloping arcing faults from phase-to-ground faults. According to our research,

anywhere from 80% to 98% of all electri-cal faults begin as ground faults.

Q. Where is the best place to locate apressure arc detection device?

A. There are several places where youcan locate a pressure sensor. You can in-stall them in the cable compartment, buscompartment, or breaker compartment.You can also place these pressure sensorsin the SF

6bottles of a circuit breaker.

Q. Why is the relay detection time soimportant, especially when the circuitbreaker opening time for medium-voltage units can be as high as eight cyclesin the instantaneous setting range?

A. It has been documented that if theduration of an arcing fault is less than100 msec, then the energy produced bythe fault will not burn through the steelenclosure. Eight cycles in an instanta-neous setting range is 133 msec. So if the

breaker opening time (plus the detectionsystem) is less than 100 msec, then lessdamage is likely to occur.

Q. Does installation of an HRG systemintroduce any problems with selectivecoordination of the power system?

A. The installation of an HRG systemactually assists in selective coordination.With HRG, you do not have to trip onthe occurrence of a ground fault. You canalso prioritize up to 50 feeders and selec-tively trip the feeder of lower priority onthe occurrence of a second fault.

Q. What is the difference between apressure detector and an optical detec-tor? Is one of these devices more selectivethan the other?

A. The main difference between anoptical sensor and a pressure sensor isthe time it takes for the respective deviceto detect and operate on the occurrenceof an arc. The light produced from an arctravels at the speed of light and can bedetected in less than 1 msec. The pres-sure wave produced from an arc propa-gates at the speed of sound. A pressuredetector is typically a mechanical devicethat will react to a pressure differencein 8 msec to 18 msec. In both cases, the

total reaction time, including detectionand actuation of the circuit-isolatingdevice, will be less than the 100-msecguideline for minimizing equipmentdamage.

Q. How do you determine the continu-ous current rating of a neutral groundingresistor?

A. The continuous current of a con-tinuously rated neutral grounding resis-tor is equal to the system line-to-neutralvoltage divided by the ohmic value of theresistor. The continuous current shouldbe equal to or greater than the systemcapacitive charging current. The formulais as follows: I

NGR= V

LN R (considering

INGR

IC, where I

Cis the system charging

current).

Q. If most arc faults start out as singlephase-to-ground faults, will an HRGsystem really be effective?

A. HRG will be extremely effective inthis case. The high-resistance devicewill limit the fault current to a low value

(i.e., 5A or 10A), as opposed to tens ofthousands of amperes. This will impedethe progress of the arc, and the fault willremain as a single phase-to-ground faultinstead of propagating into a phase-to-phase or 3-phase fault.

Q. Are arc flash requirements appliedto low-voltage switchgear and motorcontrol centers (i.e., 480V systems) ormostly medium-voltage equipment?

A. While arc flash requirements aremore commonly applied to medium-voltage equipment, given the higherincident energy levels available on todays

systems, they can be effectively applied tomany 480V and 600V systems.

Q.Does HRG reduce the amount of arcflash energy in a phase-to-phase fault?

A. No, HRG will not limit the faultcurrent in a phase-to-phase fault. It willonly limit the fault current of a singlephase-to-ground fault and reduce therisk of this type of fault developing intoa phase-to-phase fault.

Q. Where should HRG configurationsbe installed?

A. HRG systems can be employed innetworks that do nothave a distributedneutral. Thus, they can be used on 480V,600V, 2,400V and 4,160V systems. Youcan employ an HRG design on highervoltage systems, but consult with a pro-fessional engineer that has experiencein this type of design prior to venturingdown this path.

One limitation of the HRG systemis that line-to-neutral loads cannot beserved. Therefore, you will need to de-termine what percentage of the load isserved by line-to-neutral voltages andestimate the cost of adding a transformerin the circuit to service these loads.

Q. Does an HRG system have an ef-fect on the normal operation of thenetwork?

A. The HRG system does not have

any effect on normal short circuits orarc flash events, unless they are normalsingle phase-to-ground faults. In this

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case, the HRG system will limit the faultcurrent and prevent further damage anddestruction.

Q. Should we focus on the line side orload side of a breaker when calculatingthe potential arc flash energy levels?

A. It depends on which side of thebreaker you are working on. If you areworking on the breaker itself, you shouldconsider the nearest protective upstreamdevice when making your calculations.

Q. Would you go so far as to say that aresistance grounded system is always su-

perior to a solidly grounded system, or isthere a place for both? Mostly what I see inuse is the conventional 277/480V, 3-phase,4-wire solidly grounded power system.

A. There will always be a place forboth, as HRG is prohibited where thereare neutral loads. For example, youwill never see a 120/208V HRG system.However, if you have a 277/480V systemand 20% to 30% of your load consistsof 277V lighting equipment, you mayconsider using an HRG design. But

if 70% to 80% of your load is rated277V, then you should not consider anHRG design. The bottom line is thereare many variables that come into playwhen making this type of decision.Therefore, youll still need to considerarc flash mitigation protection on bothtypes of systems.

Q. When using an HRG system, howcan you be sure the equipment will cleara line-to-ground fault before a phase-to-phase arc fault occurs?

A.The benefit of an HRG system is thatyou do not have to clear a line-to-groundfault quickly. The equipment and toolsare available for you to determine thelocation of the fault in a reasonable time.If you are concerned with clearing thefirst fault before a second fault occurs,

you can install relays that will detect thisscenario and automatically isolate oneof the faults in a timely manner so thatonly one ground fault is present on the

system at any given time. You can also setthe system up to isolate the fault with thelowest priority.

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