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1077-2618/11/$26.00©2011 IEEE
BY RACHEL M . BUGAR I S & DAV ID T . ROLLAY
BECAUSE OF THE RECENT FOCUS ON ARC-
flash hazards, arc-resistant equipment is becoming
increasingly common in industrial facilities throughout
North America. This article addresses arc-resistant equip-
ment as it applies to low-voltage motor control centers (LVMCCs) [National
Electric Manufacturing Association (NEMA) 600V LVMCC/International
Electrotechnical Commission (IEC) 690V LV controlgear]. A brief overview and
history of applicable internal arc fault testing guides (IEEE C37.20.7 and IEC/TR
61641) are included. Test procedures and their considerations as well as arc-resistant
assessment criteria are reviewed. An examination of various methods to achieve an arc-
resistant rating (device-limited rating and duration rating) as well as considerations for
implementing arc-resistant equipment are addressed.
An Arc-Resistant LVMCC
This article deals with arc-resistant equipment as it applies to an LVMCC, i.e., NEMA 600V
class LVMCC and IEC 690V LV controlgear. An arc-resistant LVMCC can be thought of as a sub-
set of a standard Underwriters Laboratory (UL) 845 motor control center or IEC 61439 controlgear.
(In 2009, the IEC 60439 series of standards were revised and renumbered as the IEC 61439 series.)
In addition to complying with the requirements of one of these standards, the designation of arc resist-
ant is an indication that the LVMCC has satisfied certain performance criteria when subjected to internal
Digital Object Identifier 10.1109/MIAS.2010.939620
Date of publication: 17May 2011
For low-voltage motor control center applications
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arcing conditions. An arc-resistant LVMCC may con-tain additional features that are normally not presentin a standard LVMCC and may have additional installa-tion requirements.
While UL 845 [1] and IEC 61439 [2] outline a series ofrequired short-circuit type tests to ensure that the LVMCCwill be able to withstand the effects of bolted faults, over-load currents, etc., neither of these documents requirestesting or specifies a level of performance that the equip-ment must meet when subjected to specific arcing faultconditions. An arcing fault differs from a bolted fault inthat the circuit is completed through air (as a result of insu-lation breakdown, ionized gases, etc.) rather than a conduc-tor. The equations in IEEE 1584, which are based on testdata, predict that arcing currents in a typical LVMCC canbe as low as 40% of the rms-bolted fault current level [3].One of the reasons that arcing faults are typically moresevere than bolted faults is because the lower arcing currentmay not cause the short-circuit-protective device (SCPD),located in or upstream of the LVMCC, to trip in a timelymanner. Clearing time during an arcing event is critical;longer fault durations can result in increased equipmentdamage and incident energy exposure. Adding provisionsto the equipment that allow the enclosure to be rated as arcresistant will help limit and contain some of the undesir-able mechanical and thermal effects that result from aninternal arcing fault.
The arc-resistant LVMCC has been fairly common,particularly throughout Europe, for many years. To someextent, this is a result of the fact that low-voltage switch-gear and controlgear assemblies must comply with thesame standards and codes. When controlgear is used in adistribution-type installation, the potential for greaterenergies is more probable. In contrast, in North America,there is a distinction in product standards between con-trolgear, such as an LVMCC, and low-voltage electricalswitchgear that functions primarily for power distribu-tion. Additionally, lineups of IEC controlgear assembliestend to have a larger footprint than their NEMA counter-parts; this greater enclosure volume can be a valuable con-dition for improving the ability of the enclosure towithstand and contain the pressure effects resulting froman internal arcing fault.
With regard to electrical arc safety in countries thatadopt IEC standards and directives, the focus tends to beon arc containment or protection. In North America, how-ever, the focus has been on arc prevention, particularly bylowering potential incident energy levels. Recent guide-lines in North American standards for workplace electricalsafety (NFPA 70E [4] and CSA Z462 [5]) have led to aphilosophy of working to lower the potential incidentenergy level of a system, rather than reinforcing equipmentto make it arc proof or resistant.
Even with global application differences, there is still aneed in both the IEC and NEMA communities for thearc-resistant LVMCC. Ultimately, when safety is a con-cern, utilizing a combination of both strategies—usingpreventive features to reduce the likelihood of an arc flashoccurring as well as incorporating containment featuresto minimize the resulting hazards—presents a more com-plete solution to address the dangers associated with arc-flash hazards.
Internal Arcing Test GuidesThere are no standards when it comes to arc-resistant equip-ment but rather internal arcing test guides. The differencebetween a standard and guide is that a standard specifiesmandatory requirements and performance levels that mustbe met and verified, while a guide provides information,including recommended procedures and explanations, whileleaving performance levels subject to an agreement betweenthe customer and manufacturer [6]. As a result, arc-resistantratings can vary by manufacturer, and users should ensurethat they understand not only how a specific manufacturerhas tested his/her arc-resistant equipment but also the levels(voltage, current, arcing duration, and accessibility type) towhich the equipment has been tested.
On a global level, there are two documents that are appli-cable to the testing and qualification of the arc-resistantLVMCC: IEC/TR 61641 and IEEE C37.20.7. Each testguide outlines the scope of equipment intended to be cov-ered by the particular document. For instance, IEC/TR61641 Enclosed Low-Voltage Switchgear and Controlgear Assem-blies—Guide for Testing Under Conditions of Arcing Due to Inter-nal Fault provides guidance for testing the IEC styleLVMCC that is manufactured according to IEC 61439. Bothdocuments are maintained by the IEC SC (subcommittee)17D—LV Switchgear and Controlgear Assemblies. Cur-rently, there are no internal arcing test guides that directlyinclude in their scope the NEMA style LVMCC that com-plies with UL 845. The most relevant document is IEEEC37.20.7 IEEE Guide for Testing Metal-Enclosed SwitchgearRated Up to 38 kV for Internal Arcing Faults. Originally, amedium-voltage test guide, the 2007 release of IEEEC37.20.7 incorporated some guidance for testing low-volt-age equipment. This article is maintained by the IEEEPower Engineering Society Switchgear Committee, specifi-cally the Switchgear Assemblies Subcommittee. Addition-ally, some countries have adopted or are developing theirown guides for testing electrical equipment, includingLVMCCs, for internal arcing faults. Furthermore, since theinternal arc testing documents are guides subject to agree-ment between the manufacturer and customer, customersmay occasionally have additional (or fewer) requirementsbased on a specific application.
These internal arc test guides are similar in that eachdocument typically includes guidelines for configuring testsamples, laboratory calibration requirements, test procedure,evaluation criteria, and information to be included in thetest report. However, these documents are not harmonizedand are not always in agreement. Typical differences includeanything from minor variations in terminology to moresignificant discrepancies within the test evaluation criteriaor test procedures. For example, the type and placement ofthermal indicators, shorting wire size and location, andrequirements for repetition of the test if the fault self-clearsall vary between these two documents. This is understand-able owing to the difference in the scope of the two guides.Specifically, the difference in voltage levels has an impactsince voltage affects the amount of incident energy presentduring an arc flash.
Arc-resistant guides typically evaluate the degree ofprotection provided by equipment against the thermal andmechanical effects of an arcing fault. Note that in the titleof each test guide, a distinction is made that the testing is 63
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for internal arcing faults; this indicates that during testingdoors and covers are secured and latched.
It is important to point out that even on equipmentrated as arc resistant, safe work practices are paramountand must always be followed. Arc-resistant equipmentalone is not enough to satisfy the requirements for safework practices outlined in NFPA 70E or CSA Z462. Arc-resistant equipment is not exempt from the arc-flash haz-ard analysis described in these documents, and it muststill be field marked with the available incident energy orthe required level of personal protective equipment.When doors and covers are open while the equipmentis energized, an arc-resistant LVMCC, like a standardLVMCC, has the potential to release energy should anarcing fault occur.
Equipment TestingArc-resistant equipment provides several benefits within thepreviously described framework. The extent of these benefitsdepends on the assessment criteria and the degree to whichthe test guide was followed when testing took place.
Assessment Criteria and Accessibility TypeAfter an internal arcing test is conducted, equipment istypically inspected for, at a minimum, the following crite-ria [7], [8]:
n doors and covers do not open (some distortionis allowed)
n parts are not ejected (i.e., the enclosure remainsintact and does not fragment)
n arcing does not cause holes to develop in externalparts of the enclosure (i.e., assessment of burn-through, a thermal effect)
n thermal indicators do not ignite (i.e., any gasesreleased are not hot enough to cause ignition ata specific distance from the enclosure) and
n the grounding or protective circuit remains effective.The above criteria are primarily focused on the protec-
tion of personnel who are in the vicinity of the LVMCCfrom the thermal and mechanical effects of an internal arc-ing fault. Some internal arc test guides, including IEC/TR61641, provide additional criteria that can be used to assessthe equipment. Optional criteria [8] that can also be usedas part of the assessment include the following:
n the arc is confined to the compartment in which itwas initiated (e.g., an arc initiated in a unit does notflash over and ignite in the bus compartment)
n emergency operation of equipment is possible(verified by a dielectric test).
The criteria for personnel and equipment protectionmay apply to one or more external surfaces described byaccessibility type. Two accessibility types are defined inIEEE C37.20.7: Type 1—front only and Type 2—front,sides, and back [7]. (IEEE C37.20.7 also defines suffixesthat can be applied, but these are typically used only formedium- or high-voltage equipment and do not apply toLVMCCs.) Distinction is not made between different levelsof accessibility types of IEC/TR 61641; the documentstates that all sides of the equipment readily accessible bypersonnel must be evaluated [8]. This is essentially equiva-lent to IEEE Accessibility Type 2 assessment. Note thatalthough the accessibility-type assessments are similar, the
actual arc-resistant ratings may not be equivalent since theguides are not harmonized.
Selection of Compartments for TestingWhen qualifying configurable equipment such as anLVMCC for an arc-resistant rating, a series of internal arc-ing tests may be necessary. Because the nature of an arcingfault can be unpredictable, it is often difficult to determinethe fault location that will lead to a worst-case conditionwithout testing. Therefore, a thorough test program is nec-essary to assess the effects of an internal arcing fault inseveral locations including bus compartments, outgoingunits and feeders, and incoming units (mains).
1) Bus compartments: Although energy levels will vary bymanufacturers and designs, in the case of an LVMCC,the highest energy levels generally are expected tooccur in the bus compartment. This is due to the factthat there is less impedance at this point in the cir-cuit, and unless the bus is insulated, an arc that devel-ops in the bus compartment is not likely to self-extinguish. Both the main power and distributionbus should be considered for testing.
2) Outgoing compartments (units and feeders): When anarc fault occurs, high temperatures cause materials tovaporize, resulting in rapidly expanding gases thatproduce a pressure wave. In an LVMCC unit, orbucket, the volume is typically much smaller thanthe volume of the bus compartment. Smaller volumesleave less space for gases to expand, which could leadto higher pressures and more stress on the equipment.
Since the effects of an arcing fault are mitigatedby distance, barriers and proximity to areas of theequipment that are accessible by personnel are thefactors that need to be considered when developingan arc-resistant test strategy. When units areinstalled in an LVMCC, they are typically locatedtoward the front of the enclosure that is usuallymore accessible than the bus compartment.
Conversely, depending on the location in the unitwhere the fault occurs, the arcing current and clearingtime might be considerably less than a similar fault ina bus compartment, leading to lower energy levels.There may be a protective device within the unitupstream of the fault, or the unit may be designedwith careful attention to spacing and insulation thatwould make an arcing fault likely to self-extinguisheven before an upstream protective device would open.
3) Incoming compartments (mains): The line side of anLVMCC incoming device or main lug compartmenthas the potential for the highest arc-flash energy,because if a fault were to occur at this location, theupstream device may be slow to operate. The volumeof the compartment, spacing and arrangement of con-ductors and components, and speed of the upstreamclearing device all factor into evaluating this com-partment for testing.
Test SampleThe test sample(s) should be constructed so that the worst-case scenarios can be evaluated. Since an LVMCC is typi-cally configurable with an almost infinite combination ofoptions, bus sizes, and unit layouts, it would not be64
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realistic to test every possible arrangement. Thus, engi-neering judgment must be used in establishing a testprogram. An example of the worst-case constructioncould be a bus compartment with the smallest spacingbetween phases or a unit compartment with the highestrated device packaged in the smallest possible volume.Devices that allow the largest current let-through (I2t)should also be considered. When designing a test sam-ple, externally mounted options should be taken intoaccount (e.g., pilot lights and buttons, viewing win-dows, meters). Enclosure strength is critical, and anyoptions that may weaken the integrity of the LVMCCshould be evaluated.
Conducting the TestOnce the test sample is constructed, testing can begin. Themanufacturer must determine the particular test parame-ters that, upon the completion of successful testing, willbecome the arc-resistant rating of the equipment. Proce-dures for calibrating the test circuit are provided in the testguide documents. Low-voltage equipment tests can usuallybe performed at full voltage (this is not always the case whentesting medium- or high-voltage equipment).
Arc-resistant equipment is tested by applying a short-ing wire and then subjecting the equipment to a specificcurrent and voltage for a specific duration, as selected bythe manufacturer. The size of the shorting wire, specifiedin the test guide, is typically sized so that the wire willquickly ionize. Before the circuit is energized, all doors andcovers are closed and secured, and thermal indicators arearranged at a specific distance around the enclosure surfacesthat will be assessed (Figures 1 and 2).
The thermal indicators are squares of fabric, untreatedwith flame retardants that are intended to approximate typi-cal industrial clothing. The test guides specify the type,color, and density of the indicator material. Although sometest guides require vertical and horizontal indicators, othersonly require that vertical indicators be present for low-volt-age equipment tests. The thermal indicators are used to eval-uate any materials, including hot gases, which may bereleased during an internal arcing event. Horizontal indica-tors are used to evaluate the arc shower and are often used inconjunction with a ceiling above the test cell. An arc showeris the result of gases or molten material being ejected andreflecting off nearby surfaces such as a ceiling or walls. If theequipment is designed to exhaust out the top of the enclo-sure, the horizontal indicators will help ensure that any gasesor material ejected in the arc shower will not be hot enoughto cause serious burns.
Test conditions should be as similar as possible to nor-mal operating conditions. Although replications of fullinstallations are generally not required, manufacturers maybe asked to provide installation guidance. Considerationssuch as the ceiling height, distance between the LVMCCand adjacent walls, and floor mounting are important.Testing should simulate these types of conditions so thatpractical guidance can be provided.
Methods to Achieve Arc-Resistant RatingsThe internal arcing test guides define two methods toachieve arc-resistant ratings. The two methods are device-limited ratings and duration ratings (Table 1).
Device-Limited RatingsA device-limited arc-resistant rating is based on the inter-nal arcing fault being limited by a specific protectivedevice. The device may be located in the LVMCC orupstream in the distribution system feeding the LVMCC.This protective device will limit the peak current and/ortime duration of the arcing fault. (If an arc-resistant ratingis achieved as a result of using a protective device, the loca-tion of that device is important. For instance, when using acurrent-limiting device, the arc-resistant rating appliesonly downstream of the current-limiting device and not tothe supply side.)
When an arc-resistant rating utilizes current- or dura-tion-limiting devices, the protection ultimately relies onthe function of the specific device. When using device-limited arc-resistant equipment, care should be taken toensure that the device that will provide the protection isreliable and robust and complies with any applicableindustry standards. Additionally, if a manufacturer specifiesthat certain device settings are required, the user must fol-low these instructions (as well as all other manufacturer
1Arc-resistant test setup with horizontal and vertical thermalindicators arranged around an LVMCC. A simulated ceilingis also constructed above the test cell. (Photo courtesy ofRockwell Automation.)
2An LVMCC with horizontal thermal indicators. (Photocourtesy of Rockwell Automation.)
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instructions) for the device-limited arc-resistant rating toremain valid.
Device-limited ratings are a simple method of applyingarc-resistant equipment since any application that uses thedefined protective device(s) will be protected as long asthe system voltage and maximum available fault currentare not exceeded for which the manufacturer conductedthe arc-resistant testing. However, the requirement to
utilize specific protective device(s) may limit the flexibilityof the user in selecting a device that will achieve thesystem coordination and performance desired in a spe-cific application.
1) Current-limiting devices: These devices are most fre-quently used, and they can be either current-limitingfuses or fast-acting circuit breakers. These deviceswill limit the peak current and time duration of the
TABLE 1. COMPARISON OF ARC-RESISTANT RATINGS.
Device-Limited Arc-Resistant Ratings
Methods A specific protective device is used to achieve the arc-resistant rating. The devicehelps reduce the severity of the fault by limiting the fault current and/or duration(e.g., current-limiting fuses, fast acting circuit breakers, crowbar).
Testing Current-limiting devices Duration-limiting devices
Two tests are required:Devices intended to automatically trip anupstream device (e.g., protective relay).Test 1
The SCPD is part of the circuit and isinstalled in the equipment under test.
The device is made inactive.
Current: Rated arcing short-circuit current
Voltage: Rated voltage*Current: Rated arcing short-circuit current
Duration: The maximum clearing time ofthe device and circuit interrupter (thisbecomes the rated arcing duration)
Voltage: Rated voltage*
Duration: Rated arcing duration (theobserved arcing duration will be deter-mined by the performance of the SCPD;this is typically much shorter than therated arcing duration) Devices intended to limit the duration by
other means (e.g., crowbar).
The device is active and part of the testcircuit.
Current: Rated arcing short circuit current
Test 2
Voltage: Rated voltage*
The SCPD is not part of the circuit.
Duration: Rated arcing duration** (IEEEC37.20.7 requires the preferred arcingduration of 0.5 s to be used)
Current: The time–current curve of theSCPD is used to determine the currentthat will be used in the test circuit.
Voltage: Rated voltage*
Duration: Rated arcing duration**
Duration Arc-Resistant Ratings
Methods Arc-resistant equipment with a duration rating typically has structural modifications thatenable it to withstand an internal arcing fault and contain certain effects (mechanicaland thermal).
This type of arc-resistant rating does not require the use of a specific protective device.
Testing One type of test is required; this test may be repeated in various compartments or config-urations. Protective devices are not used during this test.
Current: Rated arcing short-circuit current
Voltage: Rated voltage*
Duration: Rated arcing duration**
*Voltage is applied for the entire rated arcing duration.** The observed arcing duration may vary from the rated arcing duration due to equipment designs (e.g., spacing and insulationbetween conductors and ground), which may cause the arc to self-extinguish.66
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arcing fault. Table 2 lists how some common protec-tive devices performed under arcing conditions in atypical LVMCC. With either of these protectivedevices, the user should consider both the boltedfault and reduced arcing fault currents. Both fusesand circuit breakers can be effectively used whentheir current-limiting characteristics are understood.Typically larger protective devices are not as effec-tive at lower arcing currents.a) Current-limiting fuses: These fuses are very effec-
tive, particularly at lower currents, but may havemore limitations as fuse ampere ratings increase.Typical arcing currents seen in LVMCC applica-tions may not put larger current-limiting fuses(fuses rated above 1,200 A) into the current-limiting range. This results in longer clearingtimes and reduced protection (Figure 3). How-ever, current-limiting fuses may offer advantagesin achieving coordination with downstream pro-tective devices.
b) Fast-acting circuit breakers: These breakers thatprovide current-limiting or current-reduction char-acteristics may also be used. Circuit breakers mayprovide better protection as part of a device-limited rating package than the time–currentcurve of the circuit breaker would indicate. Underarcing conditions, current-limiting circuit break-ers reduce both the fault current and duration [9].Note that circuit breaker (and fuse) curves repre-sent the worst-case conditions and what is shownon the curve is often conservative.
c) Additional testing for current-limiting devices: Sincedevice-limited ratings are dependant on the per-formance of a specific device and the performanceof current-limiting devices can vary under differ-ent circuit conditions, the guides require that themanufacturer successfully complete two tests toclassify the equipment as arc resistant. These testsare conducted as described in the section“Equipment Testing.” One test is conductedat the maximum available prospective short-circuit current with the device installed andactive. Then the second test is conducted, with-out the protective device, at a lower current for alonger arcing duration. The second test repre-sents applications where the prospective short-circuit current may be low, resulting in lowerarcing currents and long device clearing times.This test verifies that the LVMCC will still com-ply with the assessment criteria under these con-ditions (Table 1).
2) Duration-limiting devices: These devices can also beutilized in arc-resistant equipment with a device-limited rating. (Provisions for testing duration-lim-iting devices are not mentioned in IEC/TR 61641.)These devices operate by utilizing various methodsto sense the arcing fault and then use this informa-tion to trigger the operation of a circuit openingdevice. Although the sensing method may be veryfast, the opening and clearing time of the circuitopening device must be included when evaluatingthe performance of the duration-limiting device.
TABLE 2. TYPICAL DEVICE PERFORMANCE UNDERINTERNAL ARCING CONDITIONS IN AN LVMCC.
Device Rating (A)Actual ArcingDuration (ms)
Test per IEC/TR 61641 at 415 V and 65 kA
MCCB 25 1.7
1.7
225 3.7
6.2
4.2
7.2
600 7.1
10.4
Test per IEEE C37.20.7 at 480 V and 65 kA
Class L fuse 1,200 9.7
12.9
8.5
9.9
9.6
9.6
10.0
7.6
MCCB 1,200 13.5
14.0
18.7
Other Optical andcurrent sensorwith crowbar
2.5
15
10
5
00 10 20 30 40 50 60 70 80 90 100
Available Current (kA)
Inci
dent
Ene
rgy
(cal
/cm
2 )
800 A Fuse800 A Circuit Breaker1,200 A Fuse1,200 A Circuit Breaker2,000 A Fuse2,000 A Circuit Breaker
3Comparison of typical LVMCC main protective devices(energy calculations based on IEEE 1584 equations).
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Methods to limit the duration of an arcing fault includediverting the arcing current to a low-impedance short-circuitdevice (e.g., metallic short circuit or crowbar) or transfer-ring the arc to an isolated arc chamber to reduce arc-inci-dent energy exposure. These methods can be quite fast(Table 2) but requires the user to consider any stresses thatthis may cause to the electrical system.
Duration RatingsDuration arc-resistant ratings are based on rating theequipment to withstand an internal arcing fault for themaximum time duration. The maximum system voltageand maximum available prospective short-circuit currentare also part of the duration rating, similar to a device-limited rating.
This rating method provides the user with more flexi-bility in selecting a main protective device to achievedesired system performance and coordination. Nonethe-less, the selection of the upstream short-circuit protec-tion is still critical. When using this method, an arc-flash hazard analysis should be performed to determinethe potential arcing current levels at the LVMCC. Thisinformation is then used to select an appropriate pro-tective device that will clear a potential arcing faultin a time that is less than the rated arcing duration ofthe equipment.
Identifying Arc-Resistant Equipment(Rating Nameplates)Arc-resistant equipment should be clearly marked with anameplate, or rating label, located on the exterior of theenclosure, where the level to which the equipment hasbeen tested is indicated (Figure 4). The internal arcing testguides indicate that the arc-resistant rating label shouldspecify attributes such as the maximum prospective short-circuit current, maximum nominal operational voltage,maximum arcing duration, and accessibility type. Ifthe arc-resistant rating is a device-limited rating, then the
label must identify the equipment as device limited andlist the specific devices that must be utilized. There are noguidelines or calculations for extrapolating test data forlonger arcing durations at lower currents or voltages, so therequirements on the rating label should not be exceeded.
It is possible that different compartments may carry dif-ferent arc-resistant ratings. If this is the case, the rating foreach compartment should be clearly marked.
Considerations for Applying Arc-ResistantEquipment to LVMCC Systems
Considerations for Electrical System DesignMany of the same techniques that help reduce arc-flashenergy levels at the LVMCCwill also reduce the arc-resistantrating requirements of the LVMCC equipment. Some tech-niques that will improve performance include the following:
1) Reduce the ampacity of the LVMCC bus system: This willreduce the size of the main SCPD. Smaller devicesclear faster under the arcing currents that are typicalin LVMCC applications. The typical arcing currentsallow lower rated protective devices (typically devicesup to 1,200 A) to operate in their current-limitingrange, leading to faster fault clearing times (Figure 3).
2) Select high-performance current-limiting protective devices:The selection of the main SCPD for applicationsusing arc-resistant LVMCC equipment is an impor-tant activity. If a device-limited rating is used, aspecific main SCPD may be required. Even for arc-resistant equipment with a duration rating, it isadvantageous to select a device that will clear anyfaults as quickly as possible to limit personnel expo-sure and equipment damage. Current-limiting pro-tective devices are designed to reduce the amount ofenergy released during a fault. At the same time,these devices can provide other desirable electricalsystem performance, such as system coordination.Another benefit of using a high-speed current-lim-iting device is that it will help reduce potentialincident energy levels when an arc-flash hazardanalysis is conducted.a) Use high-performance current-limiting fuses: Some
fuses provide more current-limiting performancethan others. Evaluate both the clearing time andlet-through characteristics when selecting current-limiting fuses.
b) Use high-speed current-limiting or -reduction moldedcase circuit breakers: Molded case circuit breakersmay provide better performance in protectingequipment against internal arcing faults thantheir clearing time curves indicate because oftheir current-reduction characteristics. When acircuit breaker clears a fault, an arc developsbetween its contacts. The impedance of the arcbetween the contacts reduces the fault current,resulting in lower incident energies [9]. Thisadvantage, combined with the fact that the cir-cuit breaker will often clear faster than thetime–current curve would indicate, can be use-ful when specific breakers are tested in conjunc-tion with a device-limited arc-resistant rating.However, in cases where a duration rating is
Arc-Resistant EquipmentPer IEEE C37.20.7
Accessibility: Type XArc Short-Circuit Current: XX kAArc Duration: Device LimitedProtective Device: Manufacturer and Part Number(s)Rated Max Clearing TimeProtective Device: XX ms
Arc-Resistant EquipmentPer IEEE C37.20.7
Accessibility: Type XArc Short-Circuit Current: XX kAArc Duration: XXX ms
(a)
(b)4
Typical arc-resistant nameplate: (a) duration rating and(b) device limited.
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used, the circuit breaker must be selected basedon the trip curves that may not show this im-proved performance.
3) Keep the system in the current-limiting range of the protec-tive device: When designing the system and layingout the floor plan, be aware of factors that mightlead to increased impedance in the system. Forexample, the impedance in long runs of cable canlead to much lower available current levels at theincoming terminals of the LVMCC than the avail-able current level found upstream before the cablerun (Figure 5). Remember that arcing current isonly a fraction of the available current. When usingarc-resistant equipment, it is critical that the protec-tive device will clear the arcing fault in a time thatis less than the rated arcing duration.
4) Use intelligent devices: Incorporating intelligent devi-ces and technologies that allow remote monitoringand control is beneficial when using arc-resistantLVMCCs. Remembering that the arc-resistant ratingonly applies when doors and covers are closed andsecured, it is particularly helpful to be able to re-ceive real-time feedback from devices and have thecapability to adjust device parameters via a networkconnection in an office environment, rather than hav-ing personnel enter the arc-flash boundary to open adoor or cover on live equipment.
In critical process applications, it is desirable to providesystem coordination so that a downstream SCPD will clearfaults under short-circuit conditions without causing themain SCPD feeding the LVMCC to open. This can mini-mize unnecessary outages and improve reliability for criti-cal processes and loads. Selecting high-speed protectivedevices for LVMCC mains to minimize arc-resistant ratingrequirements may make it more difficult to achieve thecoordination. The following are some considerations thatwill help a user achieve system coordination when using anarc-resistant LVMCC.
1) Sacrifice or compromise some system coordination: Since itis so critical to achieve low energy levels during anarc-flash hazard analysis and it is also desirable toachieve performance that will allow the use of arc-resistant rated equipment, the user may elect toutilize the best and fastest current-limiting protec-tive device upstream even when some system coor-dination may be compromised. When using thismethod, it is important that any potential arcingcurrents fall within the current-limiting range ofthe selected protective device.
2) Achieve system coordination for only low- and medium-level short circuits: It may be easier to achieve coordi-nation only for lower level fault currents than forthe entire range of possible fault currents. Since thechance of a bolted fault occurring at very high cur-rent levels is so unlikely, users may elect to compro-mise coordination for this rare condition (Figure 6).
3) Select the type of protective device based on applicationrequirements: Many a times, a facility may prefer tostandardize by using only fuses or only circuitbreakers for LVMCC protection. Users should con-sider the advantages and disadvantages of usingeach type of device for a particular application.
4) Utilize protective devices with dual settings (e.g., normaloperation and maintenance): Devices with dual set-tings allow coordination to be maintained at theexpense of higher potential incident energies dur-ing normal operation. The alternate settings,which may sacrifice some coordination but reduceincident energy levels, can be activated when sen-sors indicate an arcing fault has occurred. Anotherway to maximize coordination by using deviceswith dual settings is to activate the alternate set-tings when maintenance activities are scheduled.Technology now allows these alternate settings tobe automatically triggered by a presence sensingdevice that can determine when personnel are pre-sent in a predefined perimeter. This same technol-ogy will return the device to normal operatingsettings when personnel are no longer detected inthe vicinity.
5) Utilize other coordination methods: It is sometimespossible to achieve faster protective device clearing
100
10
1
.1
Tim
e (s
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.01
.1 1 10 100 1,000Current (kA) at 480 V
800 AMCCB
150 AMCP
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Coordination for low- and medium-level fault currents.Faults at high currents are unlikely.
05
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VM
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5Effects of cable length on available current at incomingLVMCC terminals (based on 800A LVMCC with three350 kcmil cables per phase at 480 V).
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times for upstream devices while still maintainingselective coordination. Consider an approach suchas zone-selective interlocking or bus differentialprotections. The use of these methods can providea broad range of selective protection operating inminimum time [10].
Considerations for Installing EquipmentTo meet the criteria established in arc-resistant test guides,an arc-resistant LVMCC may have some additional fea-tures that are not provided in a standard LVMCC. When adevice is used to limit the fault (device-limited arc-resist-ant rating), the arc-resistant LVMCC may be constructedvery similar to the standard LVMCC equipment. This isbecause the device will typically limit the peak currentand clear the fault in a relatively short time before exces-sive pressures and thermal energies build up. Similarly,with short duration arc-resistant ratings, the equipment istypically not subjected to extreme thermal stresses. Arc-resistant equipment ratings that do not require a specificprotective device may contain additional features such as apressure relief system, additional insulation, thicker orreinforced sheet metal, as well as require the use of largerenclosures. If additional features or requirements are in-cluded in an arc-resistant-rated LVMCC, the user must beaware of any special installation considerations. For exam-ple, if a pressure relief feature is part of the arc-resistantLVMCC design, it is critical that when the LVMCC isinstalled there is adequate free space around the feature toallow for proper operation without interfering, impeding,or blocking the operation of the system or the controlledrelease of gases that build up during an arcing fault. Theenergy experienced during an arcing fault in low-voltageequipment is typically lower than that associated withmedium-voltage equipment. Although medium-voltagearc-resistant equipment often requires some type of ple-num, ductwork, or chimney to channel hot gases and/ormolten metal and particles to a specific location, this addi-tional feature will not necessarily be a requirement forLVMCCs (although such a system could be used if a useropted to have one installed).
Once the initial pressure is relieved, as the arc continuesto burn, the effects on the LVMCC enclosure are mostlythermal. A manufacturer can employ several strategies todesign the equipment to sustain these thermal effects.Examples of features or techniques used to mitigate thesethermal stresses include the use of strategically placed insu-lation or barriers, careful design and placement of compo-nents with respect to electrical spacing, and the use ofthicker external sheet metal. It is important that the userfollows all instructions from the manufacturer to ensurethat these types of features are correctly installed when theequipment is energized and operational.
ConclusionsWhen arc-flash safety is a concern, using equipment thatoffers both arc prevention features and arc containment fea-tures is a good strategy for developing a comprehensivesolution to dealing with the many dangers associated witharc-flash hazards. Arc-resistant equipment offers enhancedprotection against the thermal and mechanical effects of aninternal arcing fault, but an arc-resistant rating alone is not
enough to ensure safety or compliance with NFPA 70E orCSA Z462.
Arc-resistant equipment is governed by test guides,not by standards. For this reason, arc-resistant ratings canvary by manufacturer. Users should ensure that theyunderstand the level and extent to which the manufac-turer has tested the equipment. Additionally, usersshould be aware of the type of arc-resistant rating theequipment carries (device limited or duration rating)and understand any limitations or restrictions that rat-ing might include.
Careful consideration and planning when designingan electrical system will help reduce both incidentenergy levels when an arc-flash hazard analysis is con-ducted and the necessary rating for arc-resistant equip-ment. The balance between arc-flash safety, systemperformance, and coordination may present difficulties,but there are many techniques that can be used to helpovercome these challenges.
While the manufacturer is responsible for the designand testing of arc-resistant equipment, it is the responsibil-ity of the user to ensure that the equipment is suitable forthe application, installed as intended, and properly usedand maintained. Safe work practices (training, equipment,etc.) must always be observed around arc-resistant equip-ment, especially when doors and covers are open. This istrue for any electrical equipment, not just arc-resistantdesigns. If possible, work on a deenergized system isalways preferred.
The use of arc-resistant equipment is a good way to helpcontain the mechanical and thermal effects associated withan arc flash as well as limit personnel exposure should aninternal arcing fault occur. In many instances, the use ofarc-resistant LVMCC equipment could be yet anotheraspect of a comprehensive arc-flash safety strategy.
References[1] The UL Standard for Safety for Motor Control Centers, UL 845 Fifth
Edition, UL 845-2005.[2] Low-Voltage Switchgear and Controlgear Assemblies—Part 1: General
Rules, IEC 61439-1:2009.[3] IEEE Guide for Performing Arc Flash Hazard Calculations, IEEE 1584-
2002.[4] NFPA 70E Standard for Electrical Safety in the Workplace 2009 Edition,
NFPA 70E-2009.[5] Workplace Electrical Safety, CSA Z462-08.[6] 2009 IEEE Standards Style Manual, New York, NY, IEEE.[7] IEEE Guide for Testing Metal-Enclosed Switchgear Rated Up to 38 kV for
Internal Arcing Faults, IEEE C37.20.7-2007.[8] Enclosed Low-Voltage Switchgear and Controlgear Assemblies—Guide for
Testing Under Conditions or Arcing Due to Internal Fault, IEC/TR61641:2008.
[9] G. Gregory and K. J. Lippert, “Applying low-voltage circuit breakersto limit arc flash energy,” IEEE PCIC Conf. Rec., 2006.
[10] M. Valdes, P. Hamer, T. Papallo, R. Narel, and B. Premerlani, “Zonebased protection for low voltage systems; Zone selective interlocking,bus differential and the single processor concept,” IEEE PCIC Conf.Rec., 2007.
Rachel M. Bugaris ([email protected]) and DavidT. Rollay are with Rockwell Automation in Milwaukee, Wiscon-sin. Bugaris is a Member of the IEEE. Rollay is a Senior Mem-ber of the IEEE. This article first appeared as “Arc-ResistantEquipment for Low-Voltage Motor Control Center Applications”at the 2010 IEEE Pulp and Paper Industry Conference.70
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