by james phillips

7/23/2019 By James Phillips

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by James Phillips

What started as a slow drip a decade ago has turned into something more like a tidal

wave. I’m not talking about a leaky faucet or a failing dam; I am referring to arc-ash

ha!ard calculation studies. "ears ago# only a few mostly larger companies performed

these comple$ studies. %hen little by little# the &drip' of studies turned into a steadystream and today# the arc-ash ha!ard calculation study ()*+, has become an

integral part of many electrical safety

programs.

It was ten years ago that I/// 0123#

known as &I/// 4uide for Performing )rc

*lash +a!ard ,alculations#' was 5rst

introduced. %his landmark document

de5nes e6uations and methods that

have become key components of the)*+,.

)lthough the &drip' might have started in

the 7nited tates# the &tidal wave' has now washed across the entire globe and arc-

ash studies are becoming more common in other countries as well.

Arc-Flash Hazard Calculation Study

*igure 0. ) simple line diagram can be used to de5ne di8erent operating scenarios

When you 5rst attempt to perform the arc-ash study# it can seem 6uite intimidating.

/ven with the help of commercially available computer programs# knowing what data

to use# how to model the system and how to interpret the results can leave you

scratching your head.

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%he concept of the study is really 6uite simple. )t each piece of electrical e6uipment

that is part of the study# calculations are performed to determine the prospective

incident energy that could be available to a worker during an arc ash. %he magnitude

of incident energy# given in terms of calories per s6uare centimeter (cal9cm:# is used

to de5ne the severity of the arc ash as well as to determine which protective clothing

and personal protective e6uipment (PP/ to use.

In addition to the incident energy calculations# a distance known as the arc-ash

boundary ()* must also be determined. %he )* de5nes the distance from a

prospective arc source where the incident energy drops to 0.: cal9cm:. %his level of

incident energy can produce the onset of a second degree burn which is also called a

&<ust curable burn' and is the threshold where protection is re6uired. =*P) >?/

re6uires the use of properly rated protective clothing and PP/ when people are

working within this boundary and an arc-ash ha!ard is present.

IEEE 1584 Guide for Perforin! Arc Flash Hazard Calculations

I/// 0123 is the most commonly used method for calculating incident energy and the

arc-ash boundary. %he I/// e6uations were empirically derived from hundreds of arc-

ash tests and are valid for systems operating from :?2 volts up through 01 k@ with

short-circuit currents ranging from >?? amps up through 0?A#??? amps.

/ven though many arc-ash events begin with contact from only one phase to ground

or one phase to another phase# it is possible that the conducting plasma that develops

from the arc could 6uickly engulf the other phases and escalate into a larger three-

phase arc ash. )s a result# to be conservative# the I/// e6uations assume the arc ash

involves all three phases.

"here to #e!in$

)lthough the arc-ash ha!ard calculation study can appear to be overwhelming# it can

be more easily managed if the entire process is broken down into smaller simpler

steps. )ttempting to look at the entire study process all at once# especially if this is

your 5rst one# can bring on a feeling of panic. %ake it one step at a timeB

Step One – Data Collection



*igure :. %ime-current curves help de5ne the arc ash duration

%o accurately model the power system under study# a signi5cant amount of data is

re6uired. Cepending on the system’s si!e# age# and comple$ity# as well as what data is

readily available from previous studies and documents# this e8ort could re6uire a

signi5cant amount of manpower.

Cata re6uirements typically include information about the utility company’s availableshort-circuit current as well as their protective devices. )lso re6uired is impedance

data from components such as conductors and transformers as well as from other

sources of short-circuit current such as motors and generators. Protective device data#

the type of e6uipment such as whether it is a panel# switchgear or switchboard# as well

as other information such as working distances and gap distances# give you an idea of

how much e8ort will be re6uired.

Step Two – Single-Line Diagram and System Modeling

)n up-to-date single-line diagram is also necessary to document and organi!e the data.If a single line already e$ists# it must be veri5ed and updated with any changes that

may have occurred over time. Where a single-line diagram does not already e$ist# one

will need to be created.

Dany power systems can be operated under di8erent con5gurations such as a bus tie

open or closed# or operating under normal source conditions or from an emergency

generator. %his can result in a di8erent level of incident energy depending on how thesystem is con5gured at the time. %he single-line diagram can be used to assist in

de5ning di8erent operating scenarios as shown in 5gure 0. In addition to the normal

con5guration# &what if' scenarios may also be necessary to determine whether any of

the alternate con5gurations could produce results worse than the base case.

Step Three – Arcing Short-Circuit Current

I/// 0123 provides e6uations for calculating the &arcing' short-circuit current based on

using a known &bolted' short-circuit current obtained from a traditional short-circuit

study. %he &bolted' condition means that no additional impedance is at the point of thefault. It acts as if there is a bolted or welded connection. %he &arcing' current results

when the short-circuit current <umps across an air gap# typically created from a

conductor or conducting ob<ect either melting or being blown back. %he additional

impedance of the air gap means the &arcing' short-circuit current will always be less

than the &bolted' value.

Step Four – Arc Flash Duration

*igure E. %able of arc ash study results

Incident energy is not only dependent on the arcing short-circuit current# it is directly

dependent on the duration of the arc ash. %he longer the arc ash lasts# the greater

the total incident energy e$posure. =ormally# the duration is de5ned by how long it

takes a protective device upstream from the arc ash to operate. *or this evaluation#

time current curves such as illustrated in 5gure : are used. %he calculated arcing short-

circuit current is located on the hori!ontal a$is of the graph. Crawing a vertical line# the

point at which the current value intersects the protective devices time current curve

de5nes the duration in seconds.

Step Five – ncident !nergy



Incident energy calculations are used to determine how much energy can reach a

person located at a speci5c distance# known as the &working distance#' from the source

of the arc. %he speci5c working distance used depends on the type of e6uipment and

is typically de5ned as either 02# :3 or EA inches# although other distances may also be

used.

%he total incident energy available during an arc ash is a direct function of the short-

circuit current owing through the air gap and the time it takes an upstream protective

device to clear the fault. In general# the greater the short-circuit current# the greater

the incident energy. +owever# this is not always the case.

It is a common belief that the greater the available short-circuit current is at a

particular location# the more damage can occur. When it comes to evaluating a

protective device’s interrupting and withstand capability# this is a correct statement.

+owever# in the case of an arc ash# it is 6uite possible that a lower short-circuit

current could cause the upstream protective device to take longer to operate andactually increase the overall incident energy e$posure.

*igure 3. %he arc ash warning label contains very important information

%he incident energy is also dependent on whether the arc ash occurs in open air or in

a bo$ type of environment such as an e6uipment enclosure. When an arc occurs in

open air# energy can radiate spherically in all directions# and less incident energy is

concentrated towards the worker. +owever# when an arc occurs in a bo$# the energy is

focused out of the opening towards the worker# resulting in much higher incident

energy.

Step Si" – Arc-Flash #oundary

%he arc-ash boundary is considered to be the minimum distance from a potential

source of an arc ash where the incident energy falls to 0.: cal9cm:. ince this energy



level is the threshold of a second degree burn# it is the minimum distance that people

not wearing appropriate PP/ should be located when an arc-ash ha!ard e$ists.

%he results of an arc-ash calculation study will often yield many di8erent )*s. %his is

based on each piece of e6uipment’s location and uni6ue characteristics as illustrated

in the results shown in 5gure E. With so many di8erent boundaries there is a potentialof causing confusion.

)lthough the individual calculated values are often used# a simpler approach to reduce

the confusion is to adopt a more standardi!ed )*. %his re6uires reviewing the various

)* results and adopting the largest boundary within reason. Feviewing the results of

the )*+, shown in 5gure E indicates the largest )* is A.A1 feet at the main

switchgear. Founding this value up to > or 2 feet could provide the basis for a

standardi!ed boundary that can be used at all locations. Femember the )* does not

directly a8ect the person performing the work. It only a8ects the person that is not

performing the work# i.e.# de5ning how far they need to be from the potential arc ashsource.

%he term &within reason' is used because it is possible to have an unusually large )*

that may not be realistic. %he e$isting I/// 0123 e6uations use a protective device’s

clearing time as one of the many input variables. If the arcing short-circuit current is

low# a protective device’s time current characteristic may indicate an unusually long

clearing time# perhaps tens of seconds resulting in an unusually large )* calculation.

Step Seven – $$! Selection

)rc-rated ()F clothing and PP/ are designed to protect the worker against the thermalenergy e$posure from an arc ash. %o properly select protective clothing and

e6uipment# the arc rating of the protection must be suGcient for the calculated

incident energy.

imilar to the situation where a study has many di8erent arc ash boundaries# each

piece of e6uipment will have its own calculated incident energy. %he simplest approach

for selecting the protective clothing and e6uipment is to determine the largest incident

energy value within reason and select a standard arc rating based on this value. *or

the study results in 5gure E# 0: cal9cm: would be suGcient based on the largest

incident energy value of 0?.A cal9cm: at Panel 0). ometimes there may be a few

locations where the incident energy is greater than the arc rating that was selected. Inthose cases# protection with a higher arc rating can be necessary.

Step !ight – Arc Flash %arning La&els

Presently there are only minimal re6uirements regarding the content and format of

arc ash warning labels. )=I H1E1 provides guidance about signal words such as

,aution# Warning and Canger as well as the appropriate colors to use. %he =ational

/lectrical ,ode and =*P) >?/ both re6uire the use of labels to warn about the

potential arc ash ha!ard. %he :?0: edition of =*P) >?/ takes it further and re6uires

speci5c information to be listed on the label. )ccording to =*P) >?/ 0E?.1(,# the label

must contain at least one of the following



• )vailable incident energy and the corresponding working distance

• Dinimum arc rating of the clothing

• Fe6uired level of PP/

• +ighest ha!ard9risk category (+F, for thee6uipment

In addition# the nominal system voltage and arc-ash boundary must also be listed on

the label. %his information is critical so the 6uali5ed person performing the work can

properly address the ha!ards. %he earlier :?? edition of =*P) >?/ had fewer

re6uirements for arc-ash labels so the new :?0: edition allows an e$ception for

labels created prior to eptember E?# :?00# as long as they have the available incident

energy or level of PP/ listed.

Dany di8erent label formats have evolved with some preferring to use only theminimum information re6uired while others prefer to include additional data.

)lthough not oGcially re6uired# the additional information may include items such as

the limited# restricted and prohibited approach boundaries found in =*P) >?/ as well

as other information illustrated in 5gure 3.

Step 'ine – (eport and (ecommendations to (educe ncident !nergy

)fter the arc-ash ha!ard calculations have been completed# a formal report should be

developed that documents as a minimum# the data used# study assumptions#

calculation results# PP/ recommendations# single-line drawing# a description of the

study procedure# as well as recommendations on how to further reduce incidentenergy e$posure.

Fecommended solutions for reducing incident energy can be divided into two

categories depending on the cost and ease of implementation. %he 5rst category could

contain low-cost or no-cost solutions# such as changing overcurrent device settings.

%he second category could contain changes re6uiring some level of e$penditure in

order of priority based on their costs and bene5ts.

!lectrically Sa)e – The #est Step

)s comple$ as an arc-ash study could be# breaking it down into smaller steps can go a

long way towards making it more manageable. +owever# the best step that can be

taken to protect the worker against electrical ha!ards is to only permit work on

e6uipment that is placed into an electrically safe condition. %his means it has been de-

energi!ed# locked out# tested for the absence of voltage and safety grounds installed if

necessary. Knly then is the system truly safe to work on.

,opyright L :?0: rain5ller# Inc.

Fig. 1. Connectors and terminations for conductors more finely stranded than Class B and

Class C stranding must be identified for the conductor class.

Per UL Standard 486 A-B, a terminal/lug/connector must be listed and marked for use witconductors stranded in oter tan !lass B" #it no marking or factor$ instructions to tecontrar$, terminals ma$ be used onl$ wit !lass B stranded conductors"

Arc flash

As %art of te &'(( !ode cange %rocess, te re)uirements for arc flas warning markings

a*e been increased again, and te title of (('"(6 as been re*ised" +is section doesnt%ro*ide an$ %rotection. as te %re*ious title flas %rotection0 im%lied" 1nstead, it %ro*idesfor a warning against te a2ards associated wit an arc flas"

3lectrical e)ui%ment in oter tan dwelling units0 must be field-marked to warn )ualified%ersons of te danger associated wit an arc flas from sort circuits or ground faults" +efield-marking must be clearl$ *isible to )ualified %ersons before te$ ins%ect or work on tee)ui%ment Fig. 20" 5PA '3, Standard for 3lectrical Safet$ in te #ork%lace,. %ro*idesassistance in determining te se*erit$ of %otential e7%osure, %lanning safe work %ractices,and selecting %ersonal%rotecti*e e)ui%ment"

Fig. 2. Electrical equipment (in other than dwelling units) must be fieldmar!ed to warn qualifiedpersons of the danger associated with an arc flash from short circuits or ground faults.

Fig. 2. Electrical equipment (in other than dwelling units) must be field-marked to warnqualified persons of the danger associated with an arc flash from short circuits or groundfaults.

+e &''8 3! used te term oter tan dwelling occu%ancies. in tis section"!onse)uentl$, te warnings re)uired b$ tis section didnt a%%l$ to multifamil$ dwellings,e*en toug suc dwellings migt a*e remarkabl$ larger ser*ices tan some nondwellingoccu%ancies" +o address tat issue, te !ode now re)uires te marking on multifamil$dwellings but not te indi*idual dwellings of a multifamil$ dwelling unit building0"

A"ailable fault current

A new section re)uires some e)ui%ment to be marked wit te a*ailable fault current andre)uires u%dating of tat marking if modifications of te electrical s$stem occur (('"&49"

5ield :arking" Ser*ice e)ui%ment in oter tan dwelling units must be legibl$ field-markedwit te ma7imum a*ailable fault current, including te date te fault current calculationwas %erformed, and be of sufficient durabilit$ to witstand te en*ironment in*ol*ed Fig.#0"

Fig. #. $a% close attention to what the field mar!ing must list& including the date the fault currentcalculation was performed& and ma!e sure it's durable enough to withstand the en"ironment around it.

Fig. 3. Pay close attention to what the field marking must list including the date the faultcurrent calculation was performed and make sure it!s durable enough to withstand the

en"ironment around it.

:odifications" #en modifications to te electrical installation affect te ma7imum a*ailablefault current at te ser*ice, te ma7imum a*ailable fault current must be recalculated toensure te ser*ice e)ui%ment ratings are sufficient for te ma7imum a*ailable fault currentat te line terminals of te e)ui%ment" +e re)uired field markings0 in (('"&4A0 must bead;usted to reflect te new le*el of ma7imum a*ailable fault current"

E#ception$ 5ield markings arent re)uired for industrial installations were conditions ofmaintenance and su%er*ision ensure tat onl$ )ualified %ersons ser*ice te e)ui%ment"

All e)ui%ment must a*e an interru%ting rating or S!!< tats at least e)ual to tea*ailable fault current (('"= and (('"('9" As wiring s$stems age, electric utilities ma$cange transformers in an effort to become more efficient or to increase ca%acit$" +is caneasil$ cause an increase in te a*ailable fault current, often wit a noncom%liant anddangerous0 wiring s$stem" +e intention of tis new %ro*ision is tat owners re-e*aluate teratings of e)ui%ment wen te$ install on-site generation or wen an$one canges tesu%%l$ transformers"

>%%onents of tis cange argue tat often te ratings of e)ui%ment are based on worst-case. scenarios" #ile tis is suitable for designing a s$stem, it isnt suitable for %erformingte calculations re)uired to establis te %ro%er %ersonal %rotecti*e e)ui%ment PP30 forworking on te e)ui%ment" Using artificiall$ ig *alues of fault current for e)ui%mentratings often %roduces a lower PP3 rating"

or!ing space height

+e &'(( 3! re*ises (('"&6A0?0 to include all of te eigt re)uirements found in(('"&6 and adds a new e7ce%tion for meters in meter sockets"

+e eigt of te working s%ace in front of e)ui%ment cant be less tan 6@ ft, measuredfrom te grade, floor, %latform, or te e)ui%ment eigt wice*er is greater0, as sown inFig. " ou can install racewa$s, cables, or similar e)ui%ment abo*e or below electricale)ui%ment, but it cant e7tend more tan 6 in" into te working s%ace of tat e)ui%ment"

Fig. . *he height of the wor!ing space in front of equipment is of the utmost importance.

Fig. 4. %he height of the working space in front of equipment is of the utmost importance.

+wo e7ce%tions

+e minimum eadroom re)uirement doesnt a%%l$ to ser*ice e)ui%ment or %anelboardsrated &''A or less in an e7isting dwelling unit"

:eters can e7tend be$ond te oter e)ui%ment"

1n %re*ious !ode editions, eigt re)uirements were in (('"&6A0?0 and (('"&630"Because teres no reason to a*e two subsections gi*ing similar %ro*isions, (('"&630was deleted, and te te7t was incor%orated into (('"&6A0?0"

:eters are ob*iousl$ installed inside te working s%ace discussed in tis section"Pre*iousl$, te 3! allowed meters to %rotrude u% to 6 in" into te work s%ace" ow meterscan e7tend more tan 6 in" into te work s%ace"

+llumination

+e illumination for indoor ser*ice e)ui%ment, switcboards, %anelboards, and motorcontrol centers must not be controlled onl$ b$ automatic means (('"&6C09" Pre*iousl$, tisre)uirement a%%lied onl$ to electrical rooms" But e)ui%ment addressed b$ tis rule is ofteninstalled in s%aces tat arent electrical rooms". 1f $ou install %anels, ten %ro*ide a manualmeans of controlling te ligting for tem"

ow tat we*e sed some ligt on im%ortant as%ects of Art" ((', $ou can see w$ itsbeneficial to %eriodicall$ read troug tis Article" Because Art" ((' a%%lies to allinstallations, time s%ent understanding it %a$s off wit e*er$ installation"

,+-EA0 hat are 1onductor 1lasses

• !lass B stranding Standard0 features (6 strands of wire in an (8 A#D conductor,

&6 strands in (6 A#D, strands in si2es (4 A#D to & A#D, (= strands in si2es ( A#D to4/' A#D, and ? strands in si2es &E'kcmil to E''kcmil"

• !lass ! stranding features (= strands of wire in si2es (4 A#D to & A#D, ?

strands in si2es ( A#D to 4/' A#D, and 6( strands in si2es &E'kcmil to E''kcmil"

by james phillips

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