1077-2618/11/$26.00©2011 IEEE

BY RAJ IV KUMAR , DOUGLAS REED ,ROBERT MORR I S , & SAMUEL TERRY

THEUNDERWRITERS LABORATORIES

(UL) 845 standard for low-voltage (LV) motor

control centers (MCCs) requires a short-circuit

withstand rating on the bus for only three

cycles. The LV power-circuit breakers are typically rated for

up to 30 cycles short delay, which can be greater than the UL

845 short-circuit withstand test requirements. The trip set-

tings on these LV power-circuit breakers may include short-

time delay that can result in theMCC-enduring fault currents

for much more than the three cycles for which it is rated. In

this article, it is proposed that MCC bus designed and tested

for higher number of cycles of withstand can address the

above potential issue by offering better coordination and safer

power distribution design options.

Motor Control Centers

Typically, the MCC assemblies in the United States are UL

listed. UL requires the short-circuit withstand ratings for

these MCCs to meet the UL 845 standard that requires a

three-cycle, three-phase short-circuit test on a horizontal

and vertical bus for predetermined levels of short-circuit

current such as 42, 65, and 100 kA [1]. However, MCCs

Digital Object Identifier 10.1109/MIAS.2010.939638

Date of publication: 4 March 2011

Short-circuit higher-withstand requirement in MCCs

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can sometimes be fed from LV power-circuit breakerslocated in the LV switchgear ahead of or in an MCC line upand, sometimes, without a corresponding main circuitbreaker at the MCC. The short-time delay on the powerbreakers could result in the MCC bus enduring fault cur-rents greater than three cycles, which leads to the question:Is the MCC designed to meet the present UL 845 short-cir-cuit withstand requirement adequate for all the MCC powerdistribution applications today? It should be noted that, incomparison, the global International Electrotechnical Com-mission (IEC) standard for LV controlgear–MCC products(IEC 60439-1) states that the short-circuit withstand ratingis assumed to be as high as 60-cycle duration, unless other-wise stated by the manufacturer [2].

Traditional MCC configurations generally use moldedcase circuit breakers (MCCBs) with instantaneous trip thatare rated for a short-circuit withstand rating of three cycles(per UL Standard 489). Recent revisions to National Elec-tric Code (NEC) require that for certain critical systems allovercurrent protective devices are to be selectively coordi-nated with all supply-side overcurrent protective devices inthe system, which would be difficult to accomplish withMCCB as the associated time delays could exceed the lowwithstand ratings of MCCB [3]. The required time delaysfor selective coordination would also exceed the withstandratings of MCC buses that are designed just to meet the UL845 withstand requirement. This leads to the question:

What type of MCC construction andconfiguration is suitable for selectivecoordination needs of critical systems?

Limitations of the ExistingWithstand Rating

Protective DeviceCoordination ChallengesFigure 1 shows an MCC power distri-bution one line, where the low-voltagesubstation feeder (LVSUB-FDR) is an800-A LV power-circuit breaker feed-ing theMCC, andMCC-bus is a typicalthree-cycle withstand-rated bus. Cir-cuit breakers MCC-main, MCC-FDR,and panel board (PNLBD)-main are ofMCCB type, where MCC-main is the800-A MCC main circuit breaker;MCC-FDR is the 200-A MCC feedercircuit breaker feeding a downstreampanel board, and PNLBD-main is the200-A main circuit breaker of thepanel board. Figure 2 shows the cor-responding protective device curvesfor this configuration. LVSUB-FDRsettings include a short-time delayof 0.2 s and an instantaneous settingof eight times the plug setting. Itcan be seen from the coordinationcurves in Figure 2 that it can be chal-lenging for the fixed-curve MCCB tocoordinate with each other [4]. Also,MCCB can be difficult to coordinatewith LV power-circuit breakers due

LVSUB-Bus

LVSUB-FDR (Power CB)

CBL-01

MCC-Main (MCCB)

MCC-Bus

MCC-FDR (MCCB)

CBL-02

PNLBD-Main (MCCB)

PNLBD-Bus

Panel Board1

One line: MCCB configuration.

1,000

100

200-A TripMCCB

200-A TripMCCB

PNLBD-Main (MCCB)

600-A TripMCCB

800 A Sensor800 A PlugLTPU 1×, LTD 7STPU 4×, STD 0.2INST 8×

Tim

e (s)

10

1

0.10

0.010.5 1 10 100 1,000 10,000 100,000

Current in Amperes

MCC-Main (MCCB)

LVSUB-FDR (Power CB)MCC-FDR (MCCB)

2Device curves: MCCB configuration.

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toMCCB low-withstand ratings, thus requiring an instanta-neous dial or an instantaneous override. Typically, theMCCB-adjustable instantaneous setting has a maximumsetting of ten times trip rating or is equipped with instan-taneous overrides that are approximately 10–15 times theframe ampere rating. This limitation, in combination withthree-cycle withstand rating of a traditional MCC bus,makes it difficult to achieve selective coordination for thisMCC power-distribution scenario.

It should be noted that, from a selective coordinationstandpoint, it is not required by 2008 NEC articles 700.27701.18 for breakers that are in series with each other in abranch circuit to coordinate, as the load outage impact isthe same regardless of which of the breakers in series tripsfirst [3]. So, LVSUB-FDR and MCC-main are not requiredto coordinate as well as MCC-FDR and PNLBD-main.However, as can be seen from Figure 2, it is difficult toselectively coordinate MCC-FDR and PNLBD-main withMCC-main and LVSUB-FDR due to the nature of MCCBcurve characteristics.

NEC-Selective Coordination RequirementsArticles 620.62, 700.27, 701.18, and 517.26 of NEC 2008require the system overcurrent devices for elevators, emergencysystems, legally required standby systems, and health-carefacilities to be selectively coordinatedwith the supply side overcurrent pro-tective devices [3]. In addition, thenew section 708.54 for critical opera-tions power systems requires selectivecoordination. The objective of thisNEC requirement is to ensure betterpower system reliability for criticalsystems, where the safety of humanlife can be compromised upon a poweroutage. Based on the discussions inthe previous sections of this article, weknow that it can be difficult to achieveselective coordination for these criticalsystems with a traditional MCC powerdistribution consisting of three-cyclewithstand rated bus and MCCBs con-nected in series. It should be notedthat the NEC-mandated selective co-ordination over the entire protectivedevice range, in many cases, can sig-nificantly increase the level of avail-able arc-flash energy, with a resultingincrease in potential for safety to per-sonnel and damage to equipment.

Benefits of HigherWithstand Ratings

Improved Selectivity Using LVPower-Circuit Breakers in MCCFigure 3 shows an MCC power distri-bution one line that is similar tothe one-line configuration shown inFigure 1, with the difference beingthat the MCC-bus is assumed to berated for 30-cycle withstand, and circuit

LVSUB-Bus

LVSUB-FDR (Power CB)

CBL-01

MCC-Main (MCCB)

MCC-Bus

MCC-FDR (Power CB)

CBL-02

PNLBD-Main (MCCB)

PNLBD-Bus

Panel Board3

One line: power breakers configuration.

1,000

100

10

Sensor 600 A, Plug 600 ALTPU 0.8, LTD 7STPU 8, STD 0.4INST Disabled

Sensor 200 A, Plug 200 ALTPU 1, LTD 24STPU 14, STD 0.3INST Disabled

Sensor 600 A, Plug 600 ALTPU 1, LTD 7STPU 8, STD 0.5INST Disabled

1

0.10

0.010.5 1 10 100 1,000 10,000 100,000

Current in Amperes

Tim

e (s)

LVSUB-FDR (Power CB)

MCC Main (Power CB)

MCC FDR (Power CB)

PNLBD_Main (MCCB)

4

200-A TripINST Fixed

Device curves: LV power-circuit breakers configuration.53

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breakers MCC-main and MCC-FDR are LV power-circuitbreakers, and not MCCB. LVSUB-FDR is a power-circuitbreaker feeding MCC-bus, and PNLBD-main is a MCCB,similar to the previous MCCB one-line configuration.Figure 4 shows the corresponding device curves. With ahigher withstand bus, LV power-circuit breakers can nowbe set with higher short time pick up (STPU) and highershort time delay (STD) settings, and the instantaneoussettings can be disabled to incorporate the necessary timedelays required to achieve better selective coordination.For this power distribution scenario, the STPU and STDfor LVSUB-FDR, MCC-main, and MCC-FDR have beenset at 83 0.2 s, 83 0.4 s, and 143 0.3 s, respectively,

and the instantaneous setting has been turned off for eachof these three LV power-circuit breakers, to achieve thedesired selective coordination. As can be seen from Figure 4,MCC-FDR and PNLBD_MAIN are selectively coordinatedwith LVSUB-FDR and MCC-MAIN, and the time delaysintroduced in the power-circuit breaker settings are withinthe 30-cycle MCC-bus withstand ratings.

It should be noted that MCC-FDR and PNLBD-maindo not selectively coordinate, but this is acceptable byNEC as they are in series in the same branch circuit [3].

Safer Power Distribution Configurationsfor Emergency Systems, Standby Systems,and Health-Care FacilitiesBased on the discussions in the previous paragraph, anexcellent level of selective coordination can be accomplishedfor emergency systems, standby systems and health-carefacilities by using a MCC power distribution configurationthat incorporates MCC buses with 30-cycle withstand ratingand LV power-circuit breakers.

Improved System Design FlexibilityWith the never-ending need to reduce the space require-ments for electrical equipment, a power systems designerseeks configurations that minimize equipment footprint.When faced with systems that have enough large or criticalloads to make LV power-circuit breakers a benefit, LVmetal-enclosed switchgear is utilized. Rather than designaround completely separate enclosure systems, integratingthese assemblies is a natural desire. Under these configura-tions, the main bus within the MCC may be the path ofcurrent flow between two LV switchgear assemblies. Toutilize the full withstand capability of the LV power-circuitbreakers and the LV switchgear, the MCC must be able tosafely carry this 30 cycles of fault current that could poten-tially flow through it. This is definitely a need when main-tie-main configurations are utilized, where the substationtransformers are close coupled to the LV switchgear; thisplaces the MCC in the middle of the switchgear betweenthe main and tie. An MCC equipped with a 30-cycle with-stand rated bus would meet this need.

Test Results

Eighty-Five-Kiloampere 30-Cycle Withstand TestA two-section MCC assembly was used for testing the hori-zontal and vertical bus system for 85-kA, 30-cycle three-phase short-circuit withstand rating. The horizontal andvertical bus system assembly for this test was equippedwith standard 100-kA bracing, modified by addition ofextra bracing as detailed below:

1) Horizontal Bus: Standard 100-kA horizontal bus brac-ing was modified by adding four braces with onebrace added at each end and a pair of braces locatedequidistant from the centerline of the bus. ReferenceFigure 5 for horizontal bus bracing modifications.

2) Vertical Bus: A 100-kA vertical bus was providedwith two braces every 6 in. Reference Figure 6 forvertical bus bracing modifications.

Both the horizontal and vertical bus assemblies success-fully passed the 85-kA 30 cycle three phase short-circuitwithstand tests. These tests were UL witnessed.

5Horizontal bus bracing modifications. (Photo courtesy ofEaton Corporation.)

6Vertical bus bracing modifications. (Photo courtesy of EatonCorporation.)

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Eighty-Five-Kiloampere Ten Cycleand 100-kA Six Cycle Withstand TestA two-section MCC assembly was used to test the horizon-tal and vertical bus system with just the standard 100-kAbracing to verify if the withstand ratings greater than theUL 845’s three-cycle requirement can be met. The horizon-tal and vertical bus assemblies were successfully tested for85-kA ten cycles and 100-kA six cycles. These tests wereUL witnessed.

Summary of Test ResultsThe test results are summarized in Table 1.

Impact on Arc-Flash ProtectionArc-flash protection requires fast fault-clearing times fromthe protective devices to minimize the magnitude and timeduration of the arc current. This objective does not oftenalign with the selective coordination requirements of theprotective devices as selective coordination requires signifi-cant trip delays in circuit breakers located upstream of thefeeder breaker feeding the faulted branch circuit, to ensurethat the load power outage is localized to the faultedbranch circuit.

There are, however, several solutions available that canenable arc-flash safety in a selectively coordinated powerdistribution. One manufacturer offers an MCC that includesfeatures such as 1) the ability to connect and disconnectmotor starters using a remote racking mechanism with theunit door closed, 2) insulated horizontal and vertical bus,and 3) automatic shutters that isolate the bus when themotor starter’s stabs disengage from the bus. These safetyfeatures can significantly reduce the possibility of an arc-flash event occurring in theMCC.

Another solution can be to utilize zone-selective inter-locking schemes between the LV power-circuit breakerstrip units. For example, in the power distribution one lineshown in Figure 4, the LV power-circuit breakers are selec-tively coordinated; however, by employing zone-selectiveinterlocking between the LV power-circuit breakers, theMCC main breaker MCC-main can be interlocked to clearan internal MCC fault almost instantaneously.

Other solutions available in the market include1) using power-circuit breaker trip units equipped with

a switch, which when engaged during maintenance

of the equipment will change the breaker trip set-tings to settings that are favorable to fast fault-clearing times

2) using an optical relay detection system that cansense the intense light given off by an arc flashand trip the main breaker instantaneously; a per-missive current-detection interlock in the systemeliminates nuisance trips.

ConclusionsSelective coordination can be of high priority in some powerdistribution configurations and especially for critical systemsas required per NEC articles 700, 701, and 517. A higherwithstand MCC can enable 30-cycle LV power-circuit break-ers to be used in an MCC configuration to achieve selectivecoordination. A higher withstandMCC also allows the systemdesigner the flexibility of integrating MCC assemblies with30-cycle rated LV switchgear. While the design objectives forarc-flash safety may appear to be in conflict with selectivecoordination, there are several solutions available in the mar-ket that can enable arc-flash safety in a selectively coordinatedpower distribution system.

AcknowledgmentsThe authors gratefully acknowledge the valuable inputprovided by Tom Courtney, Chuck Nochumson, Bob Yan-niello, Ed Yee, and Neal Rowe in developing the valueproposition of higher withstand MCCs.

References[1] Underwriters Laboratories 845 Standard for Motor Control Centers, 2005.[2] IEC MCC Standard, IEC 60439-1, 1999.[3] 2008 National Electric Code, NFPA 70.[4] IEEE Recommended Practice for Protection and Coordination of Industrial and

Commercial Power Systems, ANSI/IEEE Standard 242–1986.

Rajiv Kumar (rajivkumar@eaton.com) and Robert Morris arewith Eaton Corporation in Fayetteville, North Carolina. Doug-las Reed is with MWH Americas in Cleveland, Ohio. SamuelTerry is with Eaton Corporation in Houston, Texas. Kumar,Reed, and Morris are Members of the IEEE. Terry is a SeniorMember of the IEEE. This article first appeared as “HigherWithstand MCC for Better Selective Coordination” at the 2009Petroleum and Chemical Industry Conference.

TABLE 1. THREE-PHASE SHORT-CIRCUIT WITHSTAND TEST SUMMARY.

Applied Short Circuitin KiloampereInterrupting Current

Duration(in cycles)

Bracing Assembly

Horizontal Bus Vertical Bus

85 30 Standard 100 kA horizontalbus bracing with fouradditional braces

100 kA vertical bus with twobraces every 6 in

85 10 Standard 100 kA horizontalbus bracing

Standard 100 kA vertical busbracing

100 6 Standard 100 kA horizontalbus bracing

Standard 100 kA vertical busbracing

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