Arc Flash Energy Computation Methods

7/21/2019 Arc Flash Energy Computation Methods

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ARC FLASH ENERGY COMPUTATION METHODS

Industrial electric power systems are typically radial connected and fed from medium voltage

systems. Transformers reduce this voltage to secondary medium voltages for operating large

motors and further distribution to low voltage motor control centers (MCC’s).

Industrial low voltage is typically 44 !. Industrial power systems have low impedance and

operate at low voltage resulting in high fault currents and arc energies.

Current standards re"uire arc flash analysis and ha#ard classification labeling throughout

industrial power systems.

$ystem voltage% fault current levels% time% and pro&imity are all factors in determining arc flash

intensities.

'elays% fuses% moldedcase (MCC) and lowvoltage power circuit brea*ers (+!,C) provide

fault protection in most industrial power systems. In radial systems% fault current magnitudes

diminish as the fault location moves further from the source.

$ystem designers achieve protection coordination by selecting devices that have timecurrentcharacteristics that intentionally introduce delay in device operation.



-igure shows fault current decay in a radial system as a function of distance from a source.

,roper protection coordination re"uires that the highest current levels at brea*er / have the

longest time delay (Mason% /012).

ARC HAZARD ANALYSIS USING NFPA-70E-2004



oth arc flash standards include algorithms for computing incident energy. The detailed

algorithms in each standard re"uire short circuit calculations and protective device settings to

determine incident energy levels. The 3-, 564 standard includes tables of common

tas*s and simple formulas to determine the ,,6 category without detailed calculations.

The first step in the 3-,564 algorithm is to determine the tas* personnel will perform.

The ne&t step is to calculate the flash protection boundary and determine if wor* on energi#ed

e"uipment falls within this distance. This standard uses the product of bolted threephase fault

current and the total clearing time to determine the flash protection boundary. If the timecurrent

product is less than 7 *cycles then the flashprotection distance is 4 feet. 6"uations / and compute the flashprotection boundary for electrical system and transformer faults respectively.

where8

9c : distance that will ;ust cause a seconddegree burn on a person (ft)%

M!bf : bolted threephase fault power (M!)%



M! : power rating of transformer (M!).

-or transformers with power ratings below 51 *!% multiply transformer power rating by /.1

t : total clearing time ($ec)

<nce the flash boundary is computed% one of three methods finds the necessary level of ,,6 for

wor*ers within this distance. -igure 7 shows the basic steps in these methods.

Methods / and are tabular techni"ues for finding the ha#ard ris* category (='C).

Method / uses Table /75(C)(0)(a) in the 3-, 564 to associate common wor* tas*s

with an ='C (4).

Method uses a simplified table that determines the appropriate level of ,,6. These tabular

methods can substitute for more detailed arc flash analyses but they must be applied carefully.



The tables only apply to listed tas*s under the specified assumptions and cannot be e&trapolated

to other situations and circumstances. These assumptions place limits on fault current and protective device operating time.

nalysis using method three is necessary when fault currents and device operating time violate

these limits.

The third method for conducting an arc flash analysis in 3-, 564 re"uires detailed

system data but gives the most precise results.

This algorithm finds the incident energy level in calories>cm. The resulting incident energy

then determines the ,,6 category from Table /.



The steps for detailed arc flash analysis using the 3-, 564 standard are8

/. ?se 6"uations (/) or () to determine the flashprotection boundary.

. 9etermine the minimum wor*er approach distance to electrical e"uipment for the designated

tas*. If the minimum approach is within the boundary then continue with the analysis.7. -ind the bolted threephase fault current at the wor* location. ?se ma&imum and minimum

arcsustaining current values for the remaining steps. (3-, 564 defines minimum arc

sustaining current at 4@! as 7@A of available fault current.)

4. -ind total fault clearing time for the values in step 7.

1. 9etermine if wor* will be done in open air or inside an enclosure. ?se the appropriate

formula below to compute incident energy

Bhere8 6M : incident energy in open air (calories>cm)

6M : incident energy for enclosed bo& (calories>cm)9 : distance from electrodes (inches)

t : ma&imum arc clearing time ($ec)

- : short circuit current (*% range /21 *)



2. If incident energy calculated from above is less than /. Cal>cm% flame retardant clothing

may not be re"uired to prevent burns although protection may be needed for other ha#ards

(raham% =odder% D ates% @).

5. 9etermine the ='C and select the proper level of ,,6 from the incident calculations.

Calculations for the 3-, 564 standard produce conservative results for incident energy

that tend to overprotect wor*ers

6"uations (7) and (4) are based on theoretical concepts and models derived from a small test

data set.



NFPA 70E Protection Boundaries

Flash Protection Boundary

• $erious in;ury due to arc flash burns can occur within this area unless appropriate ,,6 isused.

• nyone within this area must wear appropriate ,,6 regardless of what they are doing.

• The distance from the arc source at which the onset of a second degree burn occurs.

•

/. Cal>cmE F ./ sec. is considered a second degree burn threshold.• Medical treatment may still be re"uired if bare s*in is e&posed to this level of flash. -ull

recovery e&pected.



Limited Approach Boundary

• 9efines a boundary around e&posed live parts that may not be crossed by Gun"ualifiedH

persons unless accompanied by G"ualifiedH persons.

• May be closer than flash boundary.

• 9efined solely based on the nominal voltage.

Restricted Approach Boundary

• oundary near e&posed live parts that may be crossed only by G"ualifiedH persons using

appropriate shoc* prevention techni"ues and e"uipment.

• Concern is a shoc* ha#ard.

• 9efined solely based on the nominal voltage.

Prohibited Approach Boundary

shoc* protection boundary to be crossed by only G"ualifiedH persons using same protection as if direct contact with live part is planned. 9efined solely based on the nominal

voltage.



The theoretical ma&imum arc power in MB is half the bolted 7phase fault M!/4%/1. Thisoccurs when the arc current is 5.5A of the bolted fault current. ased on this% the flash

protection boundary is calculated as8

Arc in open air – 0.6 kV or below, 16-50 kA short circuit current

E = 5271D−

1.9593

t [ 0.0016 * Ibf 2 − 0.0076 * Ibf + 0.8938 ] (7.5)

Arc in box – 0.6 kV or below, 16-50 kA short circuit current

E = 1038.7 D−

1.4738

t [ 0.0093 * Ibf 2 − 0.3453 * Ibf + 5.9675 ] (7.@)

Arc in open air – Above 0.6 kV

E = 793 D−

2 V Ibf t (7.0)

where

6 : incident energy (cal > cm)

Ibf : bolted fault current (*)

t : arcing time (seconds)

9 : wor*ing distance from arc (inches)

Equatio! "3.7# a$ "3.8# are %art of the 2000 e$itio& a$ equatio "3.9# wa! %ro%o!e$ i the 2003 $raft.



3.3.2 Annex C Method (23!

Table 3.3 !"uations #or arc in box #or calculatin$ arc current, inci%ent ener$& an% #lash protection boun%ar&.

! / *! / *! ! 1 *! ! F 1 *!

Ia : 0.85 Ibf ' 0.004 Ibf 2

.0@ I bf I bf

6 :416 Ia t D

(1.6

/.@ Ia t 9.55

/2.1 Ia t 9.55

9 : (4/2 Ia t > /.).21

(/.@ Ia t > /.)/.7

(/2.1 Ia t > /.)/.7

The e"uations in Table 7.7 apply only to arc in box for short circuit currents between 0.6 kA an% 106 kA.

where

6 : incident energy (cal > cm)

I bf : bolted fault current (*)

Ia : arc current (*)

t : arcing time (seconds)

9 : wor*ing distance from arc (inches)

9 : distance of the flash protection boundary from the arcing point (inches)



NFPA 70E Tables

Flash Protection Boundary

Table .()()(C) of the proposed 3-, 56 J 7 '<, provides a simple method of determining flash protection boundary.

'i(ple (etho% o# %eter(inin$ #lash protection boun%ar& as per Table ))0.)*+*)* o# the propose% /A 0! – )003

rc +ocation $ystem !oltage -lash ,rotection oundary (feet)

rc in ir to / volts 4

rc in 6nclosure to / volts /

rc in 6nclosure / volts and up



3.".2 #a$ard%Ris& Cate'ory Classiications

Table .2()(0)() =a#ard 'is* Category Classifications/2. This table does not provide theflash protection boundary% but only prescribes the ha#ard>ris* category number. The table alsospecifies the re"uirement of !rated gloves and !rated tools. The classification of ris* categoryis based on several factors such as voltage% type of e"uipment% type of wor* to be performed%available short circuit current% circuit brea*er tripping time or fuse clearing time and the

position of the enclosure doors. The various types of wor* mentioned in the table areK operatingcircuit brea*ers or fuses% wor*ing on live parts% voltage testing% removing and installing bolted

covers% applying safety grounds% wor*ing on control circuits% etc.

n e&ample of what 56 (4) Table .2()(0)() may loo* li*e is summari#ed for two

items in Table 7.48 wor*ing on live parts and voltage testing. This table is preliminary and is for

reference purposes only. 'efer to 3-,56 (4) for final application guidelines.

The e&act short circuit currents for three phase bolted faults can be calculated using commercial

software. simple appro&imation described in nne& . of proposed 3-, 56 J 7 '<,draft is using the upstream transformer data in the following e"uation. The actual short circuit

current will be less than this calculated value due to the impedance of the system upstream to

transformer .



)V +a!e100I,-

= (7./)

/1.732 V

where

I$C : 7phase bolted fault current

M! ase : rated M! of the upstream transformer

! : linetoline voltage at the secondary side of the transformer

AL : percentage impedance of the transformer.



Table 3.2 *a a4ar% isk ate$or& lassi#ication #or 7orkin$ on 8ive arts as per Table ))0.6*+*9*A

-ault

$hort Circuit Clearing .55 to .7 to / to 7@

6"uipment Type 6"uipment $ide Current (*) Time (s) .4 *! .2 *! 5. *! *!

,anel oard 4 .7 /

/ .7 /

72 ./

MCC .2 *! Class +oad $ide of 21 .7

rea*er > fuse / .7 /

us 4 .77 4

us 1 . 4

us 21 ./ 4

us 2 .77 1

us 52 . 1

us / ./ 1

us / ./ 7

us / .77 4

$witchgear .2 *! Class 71 .1 4

4 .77 4

1 . 4

21 ./ 4

1 .77 7

2 .77 1

52 . 1

/ ./ 1

<ther 6"uipment .2 *! Class 71 .1 4

4 .77 4

1 . 4

21 ./ 4

36M 6 Motor $tarters M! 11 .71 1

Metal Clad $witchgear% M! 1

<ther 6"uipment 1



Table 3.2 *b a4ar% isk ate$or& lassi#ication #or Volta$e Testin$ as per Table ))0.6*+*9*A

-ault

$hort Circuit Clearing .55 to .7 to / to 7@

6"uipment Type 6"uipment $ide Current (*) Time (s) .4 *! .2 *! 5. *! *!

,anel oard 4 .7 /

/ .7 /

72 ./

MCC .2 *! Class +oad $ide of 21 .7

rea*er > fuse / .7 /

us 4 .77

us 1 .

us 21 ./

us 2 .77

us 52 .

us / ./

us / ./ /

us / .77 /

$witchgear .2 *! Class 71 .1

4 .77

1 .

21 ./

1 .77 /

2 .77

52 .

/ ./

<ther 6"uipment .2 *! Class 71 .1

4 .77

1 .

21 ./

36M 6 Motor $tarters M! 11 .71

Metal Clad $witchgear% M! 1

<ther 6"uipment 1



ARC HAZARD ANALYSIS USING IEEE-1584-2002

The I666 /1@4 standard presents another method for detailed arc flash analysis. -igure 4

shows the steps in this algorithm with the following e&planation.

on%itions #or which the :!!! 15;2 e"uations are applicable,arameter pplicable 'ange

$ystem voltage (*!) .@ to /1 *!

-re"uencies (=#) 1 or 2 =#

olted fault current (*) .5 to /2 *

ap between electrodes (mm) /7 to /1 mm

6"uipment enclosure type <pen air% bo&% MCC% panel% switchgear% cables

rounding type ?ngrounded% grounded% high resistance grounded,hases 7 ,hase faults





)tep *+ ,stimate o Arcin' Current

-or low voltage systems (/ *!)% the arc current is given by e"uation

Ia = 10+0.662 o"Ibf # +0.0966V +0.000526 + 0.5588V *o"Ibf # ( 0.00304*o "Ibf #

Bhere

log is the log/

Ia : arcing current (*)

: J./17K open configuration

: J.05Kbo& configuration

I bf : bolted fault current for threephase faults (symmetrical 'M$) (*) ! : system voltage (*!)

: gap between conductors% (mm)

-or medium voltage systems (F/ *!)% the arc current is given by e"uation (7.).

Ia = o 0.00402 + 0.983 o"Ibf #



)tep 2+ ,stimate o -ormali$ed ncident ,ner'y

The normali#ed incident energy% based on . second arc duration and 2/ mm distance from the

arc% is given

E = 10 1 + 2 + 1.081 * o"Ia # + 0.0011

where

6n : incident energy normali#ed for time and distance (N >cm)

/ : .50K open configuration

: .111K bo& configuration

: K ungrounded and high resistance grounded systems

: .//7K grounded systems

: gap between conductors (mm)



)tep 3+ ,stimate o ncident ,ner'yThe normali#ed incident energy is used to obtain the incident energy at a normal surface at a given distance and arcing time with e"uation (7.4).

t 610

E = 4.184 -f E

D0.2

where

6 : incident energy (N > cm) <istance /actor *x #or various volta$es an% enclosure t&pes

6nclosure Type .@ to / *! F/ to /1 *!

<pen air

$witchgear /.457 .057

MCC and ,anels /.24/Cable

Cf : Calculation factor

: /. !oltage F / *!

: /.1 !oltage / *!

t : arcing time (seconds)9 : wor*ing distance from arc (mm)

& : distance e&ponent as shown in Table



)tep "+ Flash Protection Boundary

The flash protection boundary is the distance at which a person without personal protective e"uipment (,,6) may get a seconddegree burn that is curable.

1

t 1 6

D+ = 610 * 4.184- f E (7.1)

0.2 E+

where

9 : distance of the boundary from the arcing point (mm) Cf :calculation factor : /.K voltage F / *!

: /.1K voltage / *!

6n : incident energy normali#ed

6 : incident energy at the boundary distance (N>cm)K 6 can be set at 1.

N>cm (/. Cal>cm) for bare s*in.

t : arcing time (seconds)& : the distance e&ponent from Table 7.. I bf :

bolted fault current (*).



Detailed Steps

/. ather power system and electrical e"uipment data.

. 'eview system topology to determine different operating modes.

7. Calculate minimum and ma&imum fault currents and O>' ratios at wor*

locations.

4. -ind arc fault currents. This value is different from fault currents due to arc

resistance. 6"uations (1a) and (1b) compute this value for system voltages under / *!.



pplying specific case values of clearing time and wor*ing distance converts

this value to actual incident energy values. 9istance e&ponents for different

types of e"uipment model energy dissipation with distance The followingformula computes the actual incident energy for specific arcing time and

personnel distance.





Bhere8

9 : oundary distance from arc (mm)6 : Incident energy level at boundary (N> cm). This is usually set to the value

of 1 N> cm% which is the burn threshold energy.

0. $elect proper ,,6 category based on incident energy and flash protection

boundary.

The I666 /1@4 method is "uite comple& and re"uires e&tensive cal

culations. The standard comes with spreadsheet software for ma*ing thesecalculations.

The e"uations in I666/1@4 derive from fitting e&tensive test data

statistically to a model. The relationship of the variables produces a good fit to

the data but also results in anomalous results for certain ranges of parameters

The algorithms omit contributions from induction and synchronous motors.

They also use symmetrical fault current values that ignore 9C offsets.



The e&isting algorithms rely on symmetrical% threephase fault current% but most

faults start as linetoground faults and progress into a threephase fault.

$ignificant energy dissipates during the transition that is damaging to e"uipmentand dangerous to personnel.



Comparison on I666 and 3-, methods









ARC FLASH MITIGATION TECHNIQUES

rc flash energy depends on three *ey factors8 system voltage% fault current

magnitude and arc time. Industrial power system owners have limited control

over the first two factors% leaving only arc time as a controllable variable.

,rotective relays% fuses% +!,C% and MCC are the devices commonly used to

provide fault protection in industrial power systems. These devices all have an

inverse time relationship where higher currents cause the devices to operate

faster

properly designed protection scheme will clear system faults with minimum

interruption to electric supply. This re"uires a time delay between protective

devices that increases as the device nears the fault current source. properly

coordinated protection system has the longest time delays nearest the utility

source% which is where the highest fault currents occur. -igure shows the

relationship between fault current magnitudes and protection system time delay.

=igh fault currents produce large incident energies and re"uire the most

stringent level of arc flash ,,6.Table 4 summari#es arc flash mitigation techni"ues commonly applied in

industrial power systems. Most of these techni"ues reduce arc flash incident

energy e&posure by shortening the protection system response time to the fault

current. us differential relaying% fast bus tripping% current limiting fuses% and



arc flash detectors all reduce incident energy by shortening the tripping time for

a fault current. rc flash detectors respond to the high intensity light emitted

from arc flashes and give the fastest response of all these schemes. Current

limiting fuses can reduce faultclearing times to @.7 ms or less% but only within a

specified range of fault currents

<ther techni"ues in Table 4 modify wor* rules or e"uipment settings to reduce

the energy e&posure a wor*er encounters. Increasing the distance between the

wor*er and live electrical protective e"uipment reduces the incident energy asthe s"uare of the distance. G=ot stic*s%H remote control tripping% and rac*ing of

brea*ers are all methods that reduce arc flash ha#ards.

Modifying system protection settings either permanently or temporarily to

reduce operating times also reduces the arc flash energy (uff D Limmerman%

@). The use of maintenance settings on protective devices gives instantaneous ( ms) tripping of brea*ers while wor*ers are near energi#ed

electrical e"uipment. 'educing device coordination times can also reduce

incident energy but may produce small reductions relative to the cost.

+owering fault currents by using high impedance grounding can increase rather

than decrease arc energies. 'educed fault currents increase the response time of



inverse time protective devices. This increased time causes increased incident

energies that can lead to greater wor*er ha#ards. This techni"ue must be

accompanied by a detailed analysis of protective device coordination times.

3ew or redesigned industrial power systems can employ new e"uipment

technologies that reduce arc flash ha#ards. rc resistant switchgear redirects arc

blasts away from wor*ers. Installing main brea*ers in MCC’s adds another level

of protection and reduces faultclearing times% resulting in lower incident energy

levels (=opper D 6t#el% @). Incorporating arc flash safety into new designs

and retrofits of e&isting systems gives the best results with the least cost.





FUTURE TRENDS IN ARC FLASH ANALYSIS AND MITIGATION

The goal of arc flash analysis and mitigation is to provide wor*ers with enough

protection to prevent seconddegree burns but to avoid overprotecting wor*ers

so that they do not encounter a greater ris* of heat stress and other in;uries due

to poor visibility and limited movements. The current standards use algorithms

based on e&perimental data ac"uired from laboratory tests. These algorithms

include simplifying assumptions to ma*e the problem tractable and tend to over

estimate incident energy levels. This results in selecting higher ,,6 categories

that overprotect wor*ers. time domain representation that uses nonlinear

timevarying resistance to model arcs can give more precise estimates of arc

currents and incident energies.

Computer simulation programs such as the lternative Transients ,rogram

(T,) (Canadian>merican 6MT, ?ser roup% @) and Mat+ab with

$imulin* (Mathwor*s% 0) allow engineers to build comple& timedomain

representations of electrical networ*s. These tools also have control systems

modeling capabilities to represent protective device behaviors. It is possible tocreate a time domain models of an arc flash and system protective devices using

these tools% but fieldtesting must verify the results. Bor* should focus on the

evolution of linetoground faults into three phase faults so that fault e&posure

time can be minimi#ed.



Increasing the wor*ing distance is a simply way of reducing the incident energy

in an industrial electric system. $ecure wireless operation of brea*ers would

allow maintenance wor*ers to stay outside the flash protection boundary while

operating energi#ed electrical e"uipment with incident energies greater than 4

cal>cm. These devices will be part of the ne&t generation of electrical

maintenance tools.

Many of the mitigation techni"ues must have communication channels to

transfer tripping and tripbloc*ing information to other parts of the electrical

system. Innovations in secure wireless communications between protective

devices will help reduce the costs of implementing these schemes. ?sing ad hocwireless networ*ing would allow low cost e&pansion of protection schemes with

less setup time.

Continued development of digital relays can e&tend to +!,C’s and MCC’s

that have greater capabilities and more fle&ibility than today’s models.

pplication of lowcost microcontrollers to produce alternatives to time

overcurrent protection in industrial power systems will give designers morechoices in designing and retrofitting protection schemes. Implementing

impedance relays using this technology in low voltage systems can achieve high

speed tripping over @ to 0 percent of distribution feeders at low cost. =all

effect current sensors that do not saturate when sub;ected to high currents can



ta*e the place of current transformers with reduced cost and greater fle&ibility in

retrofitting.

Technology cannot ta*e the place of wor*er training and s*ill. 'educing the high

percentage of electrical accidents attributed to personnel errors must be a

priority. ll maintenance personnel and system operators must have continuing

training on electrical safety procedures and current industry practices regarding

loc*ing and tagging of industrial electrical e"uipment for deenergi#ed service.

Bor*er training should emphasi#e completing tas*s in the safest way% not the

easiest or fastest. Industrial maintenance supervisors and management must

ma*e electrical safety a priority. Industries should maintain electrical system

diagrams in an GasbuildH or GasoperatingH state to prevent accidents due to

undocumented system changes. These drawings should be available to wor*ers

as needed. Communication between maintenance% operations and engineering

personnel on the current state of the electrical system should promote a safe andefficient industrial operation and reduce the ris* of arc flash accidents



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Arc Flash Energy Computation Methods

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