BUS Ele Keep the Power on for Vital Loads

7/27/2019 BUS Ele Keep the Power on for Vital Loads

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W do National Electrical Code ® (NEC ® ) arice 700, 701, nd 708

e in common? tey ocu on keeping cerin criic od up nd

running. In ome ce, e od ony need o run uni e buiding cn

be ey ecued. In oer, e od mu remin energized uni e

emergency ped, wic coud be our or dy.

KEEP thE

POWER ON fOR vItallOaDs

by evAngeLOS StOyAS p.e.


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It’s that common goal o continued operation o

certain vital loads that has been driving some changesin the NEC . Perhaps the biggest change that occurredduring the 2008 revision cycle was the introductiono Article 708, Critical Operations Power Systems, designed to keep critical operations required orpublic health and saety running during emergencies.

But minor changes have also garnered attention.One o those changes concerns selective coordination,which is the operation o protective devices to isolateproblem circuits rom the remainder o the systemwithout unnecessarily taking out parts o the systemthat are not in trouble.

Te reliability o electrical systems supplying vitalloads must be greater than that o the systems supplyingpower to normal loads. People’s health and saety rely on the power to these critical loads, even under adverseconditions such as res, earthquakes, hurricanes, andman-made catastrophes. Selective coordination o all theovercurrent protective devices or the circuits supplyingthese loads adds another assurance o reliability.

Te best way to understand the reason or addingselective coordination requirements to the NEC is toreview the code proposals, comment substantiations,and panel statements.

New selective coordination requirements went into

the 2005 NEC as Sections 700.27 and 701.18. Te sub-stantiation or the original proposal or Section 700.27provides the reasons:

“Tis article specically mandates that the emer-gency circuits be separated rom the normal circuitsas shown in [Section] 700.9(B) and that the wiringbe specically located to minimize system hazards asshown in [Section] 700.9(C), all o which reduce theprobability o aults, or ailures to the system so it willbe operational when called upon. With the interactiono this Article or emergency lighting or egress, it isimperative that the lighting system remain operational

in an emergency. Failure o one component must notresult in a condition where a means o egress will be intotal darkness as shown in [Section] 700.16….Selec-tively coordinated overcurrent protective devices willprovide a system that will support all these require-ments and principles. With properly selected over-current protective devices, a ault in the emergency system will be localized to the overcurrent protectivedevice nearest the ault, allowing the remainder o thesystem to be unctional…Due to the critical nature o the emergency system uptime, selective coordinationmust be mandated or emergency systems. Tis can be

accomplished by both uses and circuit breakers based

on the system design and the selection o the appropri-

ate overcurrent protective devices.”Similarly, the requirements o Article 708 werecreated to address national security and public saety needs arising rom our dependence on certain vital elec-trical loads and the vulnerability o standard premiseselectrical systems. In summing up why selective coordi-nation was included in this new Article, Code-MakingPanel 20 (CMP-20), which was responsible or it, notedthat “selective coordination is obviously essential orthe continuity o service required in critical operationspower systems. Selective coordination increases the reli-ability o the COPS [critical operations power systems].”

Selective coordination requirements rst appearedin the Code or circuits supplying elevators in 1993.Te 2005 edition o the Code expanded the selec-tive coordination requirements to circuits supplying

vital loads ed rom an emergency system—Section700.27—or rom a legally required standby system—Section 701.18. Tis requirement became eectiveor health-care circuits supplying loads required onessential electrical systems—Section 517.26.

For the 2008 code cycle, CMP-20 was ormedto develop requirements or COPS. Te resultingArticle 708 requirements apply to systems supplyingloads that are vital or national security, public saety,

or the economy. Selective coordination is requiredor all overcurrent protective devices in the circuitssupplying COPS loads, according to Section 708.54.Whether the vital load is ed rom the normal sourceor the alternate source doesn’t matter. Te selectivecoordination requirement applies to all overcurrentprotective devices, whether they are in the normalsource path or the alternate source path circuits.

Wat I selective Coordination?

So what is selective coordination and how can itbe achieved?

In simple terms, selective coordination meansthat, i an overcurrent causes an overcurrent protec-tive device to open, only the nearest upstream use orcircuit breaker will open. No other, larger upstreamuse or circuit breaker should open. In this way, only the circuit with the overcurrent condition is inter-rupted. In a system in which all overcurrent protectivedevices are selectively coordinated, an overcurrentshould only aect the loads where the problem hasoccurred. Te other loads still have power.

According to Section 700.27, “Emergency system(s),” overcurrent devices shall be selectively

coordinated with all supply side overcurrent protec-Reprinted with permission from necdigest December 2007

Copyright© 2007, National Fire Protection Association,

Quincy, MA. All rights reserved.


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tive devices.” Te requirements in Sections 701.18 and708.54 are identical, except or the type o systems towhich they apply. Sections 700.27 and 701.18 containtwo exceptions that clariy selective coordinationapplications.

odd Staord, senior director with the NationalJoint Apprentice raining Center, is a member o CMP-13, which is responsible or Articles 700 and701. In the 2005 and 2008 code cycles, CMP-13 dis-cussed and debated selective coordination.

“Te committee discussed this topic thoroughly,” hesays. “Te words were discussed and chosen to be spe-cic, in that ‘Emergency system(s) overcurrent devicesshall be selectively coordinated with all supply sideovercurrent protective devices.’ Tis means the overcur-rent protective devices rom the load branch circuit upthrough to the alternate source and rom the load branchcircuit up through to the main o the normal source.Both paths are important or a more reliable delivery o power. I a ault on the load side o the transer switchcaused the normal path eeder or main to unnecessarily open, we are looking at the alternate source and transerswitch needing to activate. Tere is some probability thatthe generator may not start or the transer switch may not transer, which reduces the system reliability. Also,the analysis should be done or the ault current suppliedby the normal source and by the alternate source toensure the worst case is analyzed.”

All emergency overcurrent protective devices mustbe selectively coordinated through to the alternate

power source, and the emergency overcurrent protec-tive devices on the load side o the transer switchmust selectively coordinate with the main and eederovercurrent protective devices in the normal circuitpath (see Figure 1). However, there is a dierence onthe minimum requirement or the overcurrent protec-tive devices in the normal source path. Read the textadjacent to the one-line.

Figure 1. Selective Coorination of Overcurrent devices

how to Acieve selective Coordination

For many years, selective coordination o overcurrentprotective devices has been a common system designpractice or circuits that supply critical business,government, and military loads. In the nancial and

communications industries, or instance, where thereare signicant dollars at risk, many critical loads aredesigned with supply circuits that have selectively coordinated overcurrent protective devices. No lessshould be expected or the ew important loads thatare critical or lie saety.

I have spent a career with the U.S. Army Corps o Engineers speciying and designing high-reliability electrical systems or mission-critical operations or allbranches o the military and many government agen-cies. One thing that my experience has taught me is thatselective coordination is important or powering criticalloads or as long as possible, even under adverse physicalcircumstances. Tis can be done by either circuit break-ers or uses. Selective coordination o overcurrent protec-tive devices is one o the most important studies thatshould be completed when designing systems or emer-gency or critical-operations power systems. Qualiedelectrical power system engineers competent in selectivecoordination can get the task accomplished efciently.

Tis does not mean any circuit breaker or use canbe used indiscriminately. In some cases, specic typeso uses or circuit breakers may be required. However,many have mischaracterized this as a use vs. circuitbreaker issue. Tat is not the case.

As Alan Manche o Square D said on the oor o theNFPA’s 2004 World Saety Conerence & Exposition®,“Tis is not a breaker/use issue. You can selectively coor-dinate with breakers and uses.” In the 2008 code cycle,experts rom several circuit breaker and use manuac-turers presented to CMP-13 and each stated that circuitbreaker and use systems can both be designed to comply.

A range o circuit breaker and use types andoptions can be used, depending on the specic appli-cation needs, including molded-case circuit breakers,insulated-case circuit breakers, low-voltage-powercircuit breakers, and current-limiting uses. In some

situations, relays may be a suitable option.Te simplest circuit breaker solution is molded-

case circuit breakers with instantaneous trips, i theavailable ault currents are sufciently low. Circuitbreaker manuacturers publish selective coordinationtables or their molded-case circuit breakers that show selective coordination at higher values than obtainableby merely interpreting the time-current curves.

New molded-case circuit breakers with xed high-magnetic instantaneous trips have been introduced orthe very purpose o achieving selective coordinationwith downstream branch circuit breakers. Tese circuit

breakers can achieve selective coordination at a much

ATS

AlternateSource

N E

NormalSource

46

35

2

1

Emergency system overcurrent devices

Normal system overcurrent devices that aresupply side overcurrent devices for emergencysystem overcurrent devices

Normal system overcurrent devices

Selective Coordination Requirement

700.27 “Emergency system(s) overcurrent devices shall beselectively coordinated with all supply side overcurrentprotective devices”

This wording is inclusive of the alternate path and normalsource path overcurrent devices for each emergency load.

Practical Application Requirement Example:

1 must selectively coordinate with 2, 3, 4, 5, 6

2 must selectively coordinate with 3, 4, 5, 6

3 must selectively coordinate with 4

5 does not have to selectively coordinate with 6

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higher ault current than can be achieved by using thestandard adjustable instantaneous trip circuit breakers.Other alternatives include molded-case circuit breakers

and insulated-case circuit breakers with a short-time delay option. Tese circuit breakers typically have an instanta-neous override to protect the circuit breaker. With thisoption, the designer simply ensures that the ault currentat the downstream circuit breaker does not exceed theinstantaneous override on the upstream circuit breaker.I ault current does exceed this value, the designer mightuse low-voltage-power circuit breakers or insulated-casecircuit breakers that have short-time delay settings withoutan instantaneous override. With these circuit breakers, it isrelatively easy to achieve selective coordination.

In most cases, a study to determine the availableshort-circuit currents throughout the system will berequired when designing with circuit breakers, as willplotting the circuit breakers’ time-current curves oreach circuit path and interpreting the results. I selec-tive coordination is not achieved, adjustments will berequired in the design or type o circuit breakers.

Some people have observed that using short-timedelay settings on circuit breakers increases the arc-ash incident energy and resulting equipment damage.However, the industry has remedies, such as arc-ash

reduction maintenance switches. When a workerneeds to work on or near a circuit that is protected by a circuit breaker with a short-time delay, the arc-ash-reduction maintenance switch can be thrown to theposition that puts the circuit breaker into an instanta-neous trip setting. Tis lowers the clearing time, andincident energy, should an arc ault occur.

Another solution is zone-selective interlocking, inwhich the circuit breakers communicate with eachother and use short-time delays to achieve selectivecoordination. Te other benet: i a ault occurs ina given circuit breaker’s protection zone, the circuit

breaker can open as ast as possible since the instan-taneous setting overrides the short-time delay. Somedesigners include power inductors and isolationtransormers in the system to choke the current downbelow the circuit breakers’ instantaneous settings.

Designing selectively coordinated use systems isalso relatively simple. Each low-voltage use manu-acturer publishes selectivity ratio guides or its uses.Tese ratios are based on the use type and the ampererating o the uses. Divide the ampere rating o theupstream use by the ampere rating o the downstreamuse. I this ampere rating ratio is greater than the ratio

published by the use manuacturer, selective coordina-

tion is achieved up to the use’s interrupting ratings.ypically, a short-circuit current analysis is not neces-sary, nor is plotting the time-current curves.

I selectivity ratios are not available, a coordinationstudy is needed. And i the use types and ampereratings do not have sufcient ratios adhering to theminimum published selectivity ratios, dierent typeso uses must be used or design changes implemented.

Wat About te Cot?

Many engineers and contractors routinely provideselectively coordinated systems or mission-criticalbusiness loads, and many already comply with the new requirements. As with any signicant Code change, therewere objections along the way, among them that they result in more work or the engineer and a higher priceor the system. But all the requirements in Articles 700,701, and 708 result in extra work and cost. An alternatepower source with additional electrical distribution gear,oen with sophisticated sensors, monitoring, and control,is warranted. It takes extra time and money to test,maintain, and retain records or these systems to providea reliable system that can provide electrical power todesignated vital loads. Te extra cost is expected.

When selective coordination results in extra cost,

the engineers who conduct a selective coordinationstudy have to account or the extra time in theirbids and negotiations with owners or architects. I the equipment an engineer chooses adds to the costo the project, this money must be accounted or inthe equipment bids. As the changes work their way through the industry, however, the playing eld willlevel out as people become willing to pay or morereliable systems. Reliability and availability o systemsand equipment will become a deciding actor in theuture. With the use o availability data currently ound in NFPA 70B, reliability-centered maintenance

will be able to predict and determine maintenancerequirements and cost. Tis, in turn, will be used to

justiy the need or selective coordination.For lie saety reasons, selective coordination require-

ments have expanded to increase the reliability o supplying power to a ew critical loads. For the circuitssupplying these loads, all overcurrent protective devicesmust be selectively coordinated. Both usible systems andcircuit breaker systems can be designed to comply. 0

evAngeLOS StOyAS, P.E., is chie o the Power Reliability Enhancement Program in the Special Missions Ofce at Fort Belvoir, Virginia. He is a member o CMP-20.

AS THE cHANgES WoRk THEIR WAy THRougH THE INDuSTRy,

THE PLAyINg fIELD WILL LEvEL ouT AS PEoPLE BEcoME

WILLINg To PAy foR MoRE RELIABLE SySTEMS.

Reprinted with permission from necdigest December 2007



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BUS Ele Keep the Power on for Vital Loads