Molded-Case Circuit Breakers Reduce Arc Flash Hazard Impact

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Molded-Case Circuit Breakers Reduce Arc Flash Hazard Impact

Introduction

When an arc fault occurs with personnel in the area, the harmful results can be

devastating and deadly for those who are not properly prepared. In addition equipmentdamage is usually considerable, frequently resulting in extended down time for theinstallation. Industry codes and standards have recently included measures to counterthe effects of arc flash. These codes and standards help facility operators to takepreventive steps.

The purpose of this paper is to identify the impact on arc flash by molded-case circuitbreakers !""#s$ and methods of determining it.

The arc flash hazard

%&'( )*+-***, tandard for +lectrical afety equirements for +mployeeWorkplaces, defines arc flash ha/ard as0 1a dangerous condition associated with therelease of energy caused by an electric arc.2 It is an explosion involving an electric arcoperating at temperatures of several thousands degrees "elsius and a pressure wavecreated by the arc. Within a few milliseconds of arc ignition, the energy from thisexplosion can cause molten metal particles, equipment parts and other loose items tobe expelled from the arc area in addition to the expulsion of hot, ioni/ed gas. +xtensiveequipment damage frequently results in extended down time for an installation. !oredevastating are the trauma, hearing and eyesight loss and burns to personnel in thearea which can result in catastrophic in3uries or even death.

Installed equipment can have a significant impact on the degree of ha/ard present.When equipment is expected to be serviced or opened while not in an electrically safework condition, an electrical safety program is required for such maintenance includingtraining, practices and analysis. 4uidelines for practices and training are included in56(, 7 "ode of &ederal egulations 'art 878*, ubpart with 1how to2 detail in%&'( )*+. %&'( )*+ also provides basic information regarding arc flash analysis.I+++ 89:;-**, 4uide for 'erforming (rc-&lash 6a/ard "alculations, supports %&'()*+ and provides a dependable method of performing the calculations.

ection 88*.8< of the ** %ational +lectrical "ode %+"$ requires switchboards,panelboards, industrial control panels and motor control centers be field marked with awarning of possible flash ha/ard. 'roposals have been made for the **9 %+" thatcould require additional marking of the flash protection boundary distance and ''+"ategory.

Flash Hazard Protection

There is only one sure way to protect against the potential devastating effects of arcflash and that is to de-energi/e the equipment before approaching it for the purposes of

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opening it or for working on it. %&'( )*+ describes several key steps in the process ofplacing the equipment in an 1electrically safe work condition.2 Those steps includeturning off the supply, locking it off, measuring to verify that it is de-energi/ed, andassuring that stored energy such as from capacitors or induced voltage does not impactworkers.

These steps are done while the equipment is not yet considered to be in a safecondition, which requires that appropriate protective precautions including use ofpersonal protective equipment ''+$ are applied during the de-energi/ing process. ''+includes the clothing, gloves and headwear that help to mitigate the effects of an arcflash event for a worker who is exposed. ''+ is generally determined to protect thehead and body against thermal effects that would cause severe burn. It does notnecessarily protect from the possible impact of any harmful light, sound, or pressureimpulses, toxic gas by-products or e3ected debris.

''+ is required by 56( and %&'( )*+ for operations that must be done with

equipment energi/ed, including the steps to place the equipment in an electrically safework condition. #oth 56( and %&'( )*+ acknowledge that some electrical work mustbe done with equipment energi/ed when it is either infeasible to de-energi/e or whende-energi/ing would cause additional ha/ards. In those cases in which work is done onenergi/ed equipment, there is increased risk of arc flash.

To address those cases, %&'( )*+ requires among other things that employees who dothat work be trained and knowledgeable regarding the task and its ha/ard, that aspecific work plan be made and used and that appropriate ''+ be used based on aflash ha/ard analysis.

Arc Flash Ener!

+nergy is a critical factor in evaluating the potential effect of an arc flash occurrence.These three definitions from %&'( )*+ relate to energy0

Arc flash hazard 0 ( dangerous condition associated with the release of energy causedby an electric arc.

Flash hazard analysis0 ( study investigating a worker=s potential exposure to arc flashenergy, conducted for the purpose of in3ury prevention and the determination of safework practices and appropriate levels of ''+.

Incident energy 0 The amount of energy impressed on a surface, a certain distance fromthe source, generated during an arc event. Incident energy is measured in >oules?cm or "alories?cm$

The magnitude of energy available during an arc flash event is proportional to theproduct of the current flowing times the system voltage times the duration of the event.

(n analysis of the flash ha/ard must identify these three elements. #y raising or

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lowering any of these elements, the available energy is also raised or loweredproportionally.

Incident energy is determined by the three basic elements arc current, system voltageand duration$. 5ther factors include system grounding, bus bar spacing, and whether

the arc is in a box radiating in a single direction, or in open air radiating in all directions."omponents or equipment parts located between the arc and a worker may alsoprovide some shielding from the arc, if these items do not become pro3ectiles.

5vercurrent protective devices including !""#s have a significant influence on theincident energy. &rom their position in the system, they impact both the magnitude andthe duration of the arcing fault current.

Calculation method

( dependable method of performing the calculations is in I+++ 89:;, 4uide for

'erforming (rc-&lash 6a/ard calculations. This method is based on extensive testingand solid analytical work. It requires input of the following basic items0

• (ccurate bolted-fault current available at the equipment location

• ystem voltage

• @uration of the arc clearing time for the overcurrent protective device$

• Whether conductors are enclosed or in open air

• "lass of equipment switchgear, switchboard, motor control center, etc.$

• Whether the system is solidly grounded, impedance grounded or ungrounded

• @istance from worker to conductor

•

4ap distance between conductors

The last two items are optional with default values assigned for most commonconfigurations where specific information is not provided.

I+++ 89:; provides equations to output the following information with input of the aboveitems. The complexity of the equations makes solving them by hand difficult. 6owever,I+++ 89:; provides an +xcelA spreadsheet with each copy of the standard thatautomatically performs the calculations when basic information is input. Thisspreadsheet allows multiple calculations to be done rapidly.

"utput items#

Arc current . (rc current is quite different from bolted fault current, especially in low-voltage systems. &or example in a ;:* B system, a bolted fault current of 9* k(available will result in an arc current of only < k(. (rc current is used to determine thetime for the overcurrent protective device to clear the circuit. ince the time for theovercurrent protective device depends on the value of current flowing, it could be quitedifferent for an arc than for a bolted fault.

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Incident energy . This value, defined above, is used to determine the ha/ard category for selection of ''+.

Flash protection boundary . This boundary is defined in %&'( )*+ as 1(n approach limit

at a distance from exposed live parts within which a person could receive a second-degree burn if an electric arc flash were to occur.2 (n incident energy value of 8.calories?cm is the accepted maximum value at which a second-degree burn would beexpected. 5nly 1qualified2 workers are permitted within the flash protection boundaryand they are required to use appropriately rated ''+.

Hazard category . The ha/ard category is used to determine the kind of ''+ required. Itis directly related to incident energy as indicated in Table C-C.7.C of 'art II of %&'( )*+.

Input items

The most difficult input items to obtain are bolted fault current and arc clearing time orduration. (s we will see, it is essential to have accurate bolted fault current informationrather than a guess at maximum bolted fault current. 6igher incident energy mayactually occur for certain lower bolted fault current conditions.

To obtain accurate bolted fault current, it is necessary to obtain the value of availablepower from the utility. It is also necessary to obtain information about equipment andconductors installed in order to determine their impedance. !odes of operation shouldbe well understood to know whether there are multiple potential sources or currentpaths. With this information, a short circuit calculation can be made to determine thebolted fault current. (nother simple spreadsheet is provided with each copy of I+++

89:; to make this calculation.

Dnowing the bolted fault current and system voltage, a value for arcing fault current canbe determined from an equation in I+++ 89:;. It is essential to use arcing current rather than bolted fault current because there can be significant difference between the two.@uration of arcing is determined by knowing how long the overcurrent protective devicewill take to clear the circuit. This time can be taken from the time-current curves that areavailable from the manufacturer. ( typical, !""# time-current characteristic appears infigure 8. Esing the arcing current, read the highest time from the band presented.emember, it is essential to use the calculated arcing current when determining theclearing timeF

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Fiure $

Instantaneous %ersus thermal lon-time reion

(s we look at figure 8, it is useful to compare the clearing time in the instantaneousregion with that in the thermal region. %otice that instantaneous region time is roughlyone electrical cycle or *.*8< seconds. The actual value will change slightly with eachcircuit breaker. "ontrast that very short time with tens or hundreds of seconds in thethermal region. In considering the duration that must be taken into account, it is usefulto note this statement from I+++ 89:;0 1It is likely that a person exposed to an arc flashwill move away quickly if it is physically possible and two seconds is a reasonablemaximum time for calculations.2 ecogni/e that it is possible to have a bolted faultcurrent well into the instantaneous region, such as * times rating. 6owever, when anarc occurs, the current could be well below 8* times and into the thermal region.

&igure illustrates a typical relationship between incident energy and correspondingbolted fault current available, Ib. This chart takes into account the reduced arc currentthat would result from the available bolted fault current shown even though arc currentis not shown. %otice that between points I8 and I the line has only a modest slope. (tthe point I8, it changes radically. This is the impact of the change from the instantaneousregion to the thermal region. The point is that if the equipment is operating near thispoint I8, consideration should be given to performing the calculation with time as in thethermal region.

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0 . 0 0 1

0 . 0 1

0.1

1

10

100

100 0

10 0 0 0

1 10 10 0 10 0 0

Current in multiples of ratin

C l e a r i n t i m e & s e c o n d s '

Instantaneousregion

Thermal Region

T y p i c a l M C C B T i m e - C u r

r e n t

C h a r a c t e r i s t i c



&igure

(ocation in the circuit

( critical point regarding calculations relates to choosing which overcurrent protectivedevice=s clearing time to use. "onsider figure C. (ssume the 8** ( branch circuitbreaker is located in a panelboard and we are calculating for a fault within thepanelboard on the load side of the branch circuit breaker. When an arcing fault occurs,ioni/ed gas is produced by the arc, which could cause the fault to propagate to thesupply side of this branch circuit breaker. &or that reason, it is advisable to performcalculations with the understanding that the device feeding the equipment containingthe branch breaker will clear the fault. In the case of figure C, it would be the 9 (

circuit breaker.

&igure C G Typical system

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$)) M*A+ ,R.$/

$012 k* sec

$012 k*

$1/ M*A+

3)2 *

33/ A

$)) A

4+$5) AFault initiates

at 6ranch

Assume fault 7ill

propaate tosuppl! side

I8

Incident energy for long delay

extends to very high values

8traiht lineE.MI6f 9:

$13 Calcm3

/ Calcm3

2 Calcm3

(vailable &ault "urrent, Ibf

I n c i d e n t e

n e r g y ,

+

Flash Hazard Risk Cateor!

I



If the fault is assumed to be at equipment on the load side of the branch circuit breakerand is not located within the same enclosure or compartment with the circuit breaker, itis reasonable to assume that the branch circuit breaker will clear. In our example, if weassumed that the work were being performed at a motor located in a separateenclosure or compartment from the 8** ( circuit breaker and on its load side, it would

be reasonable to assume that the 8** ( circuit breaker would clear the fault.

IEEE $/24 shortcut calculations for circuit 6reakers

&or circuit breakers, it is possible to enter the I+++ spreadsheet using the time ofinterruption for the specific circuit breaker. This interruption time is based on the arcingcurrent. (n alternative is to enter the I+++ spreadsheet knowing only the circuit breakertype. This shortcut method is based on simplified equations as described below. Thegreat advantage of the shortcut method is that you do not need to obtain or interprettime-current characteristics for each circuit breaker.

The input information needed includes0• #olted fault current

• ystem voltage

• "urrent rating of the circuit breaker

• Type of trip unit thermal-magnetic, magnetic only, or electronic$

• Type of circuit breaker molded-case or low-voltage power$

• Tripping current setting for the circuit breaker or default value$

%otice that input information does not include arc current or time to clear. The shortcutmakes 1worst case2 assumptions for system grounding, class of equipment, conductorgap and enclosure. It assumes that all working distances are 8: inches, a normalworking distance. It requires an accurate determination of bolted fault current, as doesevery other method. This method is published in I+++ 89:;.

8hortcut e;uations

T(#H+ 8+quations for Incident +nergy and &lash protection boundary

$1$1$1$1$1$ 42) * and lo7er <)) *

Ratin ABreaker

T!peTrip =nit

T!pe >

Incidentener!

&calcm3'

Flash6oundar!

&mm'

Incidentener!

&calcm3'

Flash6oundar!

&mm'

8** G ;** !""# T! or ! *.*;9 Ib *.8C 7.8< Ib 87; *.*<9 Ib *.*; 88.: Ib 87<

<** G 8** !""# T! or ! *.*9C Ib *.C: :.;9 Ib C<; *.*:* Ib *.*7 88.; Ib C<7

<** G 8** !""# +, HI *.*7 Ib *.C; 8.9 Ib ;: *.88 Ib 88.* 8;.C Ib 9<:

8<** G <*** !""# orI""#

T! or +, HI *.8*) Ib *.) 88.8 Ib <7< *.8<; Ib *.*; 8<.) Ib <*<

:** G <C** HB'"# +, HI *.89 Ib *.:: 8;.9 Ib ):< *.C Ib *.*)* 87.8 Ib :<;

:** G <C** HB'"# +, H 8.*7 Ib <.98 ;). Ib <<* 8.<; Ib *.987 <.; Ib 7C*

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The circuit breaker types are0!""# G molded-case circuit breaker I""# G insulated-case circuit breaker HB'"# G low-voltage power circuit breakers

The various types of trip units are briefly defined below.

T! G Thermal-magnetic trip units trip under short circuit conditions instantaneously, withno intentional delay. #elow the instantaneous trip current, they have a long-time delayestablished to protect conductors while allowing momentary current surges such as for motor starting and transformer inrush. In many cases they have ad3ustableinstantaneous trip current settings.

! G !agnetic instantaneous only$ trip units are used for short circuit protection only,usually in motor circuits. They have no long-time characteristic and will not trip belowthe instantaneous trip current, which is usually an ad3ustable setting.

+ G +lectronic trip units have three characteristics that may be used separately or incombination, H$ long-time, $ short-time and I$ instantaneous. ( trip unit may bedesignated HI when it has both long-time and instantaneous features. 5ther commondesignations are H and HI, indicating other combinations of these samecharacteristics.

• H G The long-time setting is for lower overcurrent conditions to allow for momentary

current surges. It usually has a current pick-up ad3ustment and a time-delayad3ustment.

•

G The short-time setting is for coordination purposes through the overload andshort circuit current levels. It usually has a current pick-up and a time-delayad3ustment.

• I G The instantaneous feature sets a current level above which tripping occurs with

no intentional delay. It is usually turned off or is absent when the short-time functionis used.

Rane of shortcut e;uations

The range of these equations is from *.< to 8*< k( and for the voltages mentioned in

the table. +ach equation is additionally limited to the range I8 J Ib J I.

I is the interrupting rating of the "# at the system voltage.I8 is the minimum arcing fault current at which this method can be applied. It is definedas the lowest bolted fault current level that generates arcing current great enough for instantaneous tripping to occur. ecall that the arc current flowing is lower than theavailable bolted fault current.$

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+q. 8$ I8 K 8*LM*.*:8 8.*7 Hog8.C I t$N at <** B and,

+q. $ I8 K 8*LM*.*;*) 8.8) Hog8.C I t$N at ;:* B and lower

and It is the tripping current setting for the circuit breaker, as illustrated in figure ;.

I t Default Value

When the tripping current, It, is not known, use a default value of 8* times thecontinuous current rating of the "#, except for "#s rated 8** ( and below. &or "#srated 8** ( and below, use a default value of I t K 8C** (. Where an H trip unit is used,It is the short-time pick-up current.

Fiure 4 ? It on Time-Current Characteristic

8hortcut charts

I+++ 89:; provides shortcut equations for fuses as well as circuit breakers. The valuesfor fuses are directly from tests while the circuit breaker values are worst casecalculations. The charts below review the incident energy values for a range of boltedfault current levels.

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T !pical time-curren t char acteristic

*.*8

*.8

8

8*

8**

8***

8****

8** 8*** 8**** 8*****

Current

T i m e & s e c '

It



"hart ( G * to ;** ( ratings

"hart # G 8** ( ratings

'age 8* of 8;

*

;

<

:

8*

8

* * ;* <* :* 8** 8*

Fault current &kA'

I n c i d e n t e n e r ! &

c a l c m - s ; '

*-**( D8

;**( D8

*-;**( "#

"at. 8 bdry

*

;

<

:

8*

8

* 9* 8** 89*

Fault current &kA'

I n c i d e

n t e n e r ! & c a l c m - s ; '

8**( H

8**( "#

"at. 8 bdry

"at. bdry



"hart " G *** ( ratings

"hart @ G *** ( ratings showing impact of instantaneous setting

There are several points to notice regarding "harts ( through @.

The * to ;** ( ratings shown in "hart ( are all within the two lowest ha/ard categories,categories * and 8. !inimum ''+ is required regardless of the device used. Theseratings are those most frequently found installed for branch circuit protection.

"hart # shows the impact of the thermal region versus the instantaneous region ofoperation for the device. &ault current levels are quite high before the lower incident

'age 88 of 8;

*

9

8*

89

*

9

C*

C9

* 9* 8** 89*

Fault current &kA'

I n c i d e n t e n e r ! & c

a l c m - s ; '

***( H

***( !""#"at. 8

"at.

"at. C

*

9

8*

89

*

9

C*

C9

* 9* 8** 89*

Fault current &kA'

I n c i d e n t e

n e r ! & c a l c m

- s ; '

***( H

***( !""#

"at. 8

"at.

"at. C

Impact of lo7er

instantaneous

settin1



energy associated with instantaneous operation are seen. atings similar to this 8** (rating are normally seen at feeder locations.

"hart " shows an even greater impact of the thermal region. @evices of *** ( arefrequently seen in main device applications.

"hart @ shows a means of obtaining better arc flash protection by ad3usting theinstantaneous trip setting of the circuit breaker to a low value when coordination willpermit the lower setting.

Current limitation and current-limitin circuit 6reakers

The shortcut calculation methods presented here and in I+++ 89:; all assume thatarcing short circuit current will flow for the entire duration shown of the maximum timeon the trip curve of the circuit breaker. That is the best assumption to be made whenmore definite test information is not available. 6owever, there are two considerations

that impact the incident energy that deserve review.

The first is that the time-current characteristic for an !""# with current limitingcharacteristics can be expected to show much faster clearing times than for a standard!""#. This point is bypassed in the shortcut method.

The second point is that a second arc is generated between the circuit breaker contactswhile it is clearing. This second arc significantly reduces the magnitude of currentflowing. This point is presently bypassed by both of the circuit breaker calculationmethods in I+++ 89:;.

The first point is illustrated by the comparison of incident energy for a standard !""#compared to that for a current limiting !""# in figure 9.

&igure 9 G Incident energy for standard and current-limiting !""#s

'age 8 of 8;

4)) A 8tandard and Current (imitin

*

8

C

;

9

<

* * ;* <* :* 8** 8*

Fault Current &kA'

I n c i d e n

t E n e r ! & C a l c m - s ; '

"H

td



Technoloies for Reducin Arc Flash Risk

When arc flash considerations are a significant factor in the selection of electricaldistribution equipment, the following existing technologies should be considered0

• Oone elective Interlocking OI$0 OI deactivates the preset delay on the circuit

breaker closest to the fault, which then trips with no intentional delay.&aster tripping reduces the amount of time that current flows during a fault condition.Thus, /one-selective interlocking reduces the amount of arc flash and stress It energy$that the system encounters during fault conditions, resulting in improved personalprotection and prolonged equipment life.

• 4round &ault @etection0 trips the circuit breaker during the lower current stages of

fault development and prior to Pbolted faultP conditions.

• Ese of finger-safe electrical components as much as possible. This can reduce the

chance that an arcing fault will occur.

• Ese of current-limiting overcurrent protective devices. "urrent Himiting !""#s limit

the faulted circuit before that current reaches potential maximum value. The current-limiting action limits thermal and mechanical stress created by the fault currents.

'eak let-through current Ip$ and IT are two measures of the degree of currentlimitation.

• i/ing the current-limiting branch circuit overcurrent protective devices as low as

possible. Typically, the lower the ampere rating, the greater degree of current-limitation.

• Himiting the ampere rating si/e of main and feeders where possible. &or example by

splitting large feeders into two feeders.

• etting the instantaneous setting for circuit breakers as low as possible to maintain

desired selective coordination.

8ummar!

(rc flash is of significant concern as evidenced by requirements in the %ational+lectrical "ode, and %&'( )*+, as well as through enforcement by 56(. Thus the%+" requires certain equipment to be labeled to warn of possible flash ha/ards and%&'( )*+ requires a flash ha/ard analysis to determine the degree of exposure aworker may have to ha/ardous energy. I+++ 89:; provides spreadsheets that can beused to determine the arc flash ha/ard level in circuits protected by !""#s. These

spreadsheets take into account !""# current interruption times, which significantlyreduce the impact of arc flash ha/ards. The spreadsheets can be entered using eitherthe known interruption time, or the generic breaker rating, and the backgroundequations relative to the generic rating method have been explained. &uturecalculations will need to be refined in order to understand the full benefit of current-limiting !""#s, which reduce the current magnitude, and consequently further reducethe arc flash ha/ard. It is also noted that the focus of present calculations has been on

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the high arc fault levels, whereas it is critical to understand that higher incident energycan result from lower fault current.

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Molded-Case Circuit Breakers Reduce Arc Flash Hazard Impact