What do National Electrical Code® (NEC®) articles 700, 701, and ...

KEEP THEPOWER ONFOR VITALLOADSby Evangelos Stoyas P.E.What do National Electrical Code ® (NEC ® ) Articles 700, 701, and 708have in common? They all focus on keeping certain critical loads up andrunning. In some cases, the loads only need to run until the building canbe safely evacuated. In others, the loads must remain energized until theemergency has passed, which could be hours or days.48 NeC DIGEST 2 DECEMBER 2007

It’s that common goal of continued operation ofcertain vital loads that has been driving some changesin the NEC. Perhaps the biggest change that occurredduring the 2008 revision cycle was the introductionof Article 708, Critical Operations Power Systems,designed to keep critical operations required forpublic health and safety running during emergencies.But minor changes have also garnered attention.One of those changes concerns selective coordination,which is the operation of protective devices to isolateproblem circuits from the remainder of the systemwithout unnecessarily taking out parts of the systemthat are not in trouble.The reliability of electrical systems supplying vitalloads must be greater than that of the systems supplyingpower to normal loads. People’s health and safety relyon the power to these critical loads, even under adverseconditions such as fires, earthquakes, hurricanes, andman-made catastrophes. Selective coordination of all theovercurrent protective devices for the circuits supplyingthese loads adds another assurance of reliability.The best way to understand the reason for addingselective coordination requirements to the NEC is toreview the code proposals, comment substantiations,and panel statements.New selective coordination requirements went intothe 2005 NEC as Sections 700.27 and 701.18. The substantiationfor the original proposal for Section 700.27provides the reasons:“This article specifically mandates that the emergencycircuits be separated from the normal circuitsas shown in [Section] 700.9(B) and that the wiringbe specifically located to minimize system hazards asshown in [Section] 700.9(C), all of which reduce theprobability of faults, or failures to the system so it willbe operational when called upon. With the interactionof this Article for emergency lighting for egress, it isimperative that the lighting system remain operationalin an emergency. Failure of one component must notresult in a condition where a means of egress will be intotal darkness as shown in [Section] 700.16….Selectivelycoordinated overcurrent protective devices willprovide a system that will support all these requirementsand principles. With properly selected overcurrentprotective devices, a fault in the emergencysystem will be localized to the overcurrent protectivedevice nearest the fault, allowing the remainder of thesystem to be functional…Due to the critical nature ofthe emergency system uptime, selective coordinationmust be mandated for emergency systems. This can beaccomplished by both fuses and circuit breakers basedon the system design and the selection of the appropriateovercurrent protective devices.”Similarly, the requirements of Article 708 werecreated to address national security and public safetyneeds arising from our dependence on certain vital electricalloads and the vulnerability of standard premiseselectrical systems. In summing up why selective coordinationwas included in this new Article, Code-MakingPanel 20 (CMP-20), which was responsible for it, notedthat “selective coordination is obviously essential forthe continuity of service required in critical operationspower systems. Selective coordination increases the reliabilityof the COPS [critical operations power systems].”Selective coordination requirements first appearedin the Code for circuits supplying elevators in 1993.The 2005 edition of the Code expanded the selectivecoordination requirements to circuits supplyingvital loads fed from an emergency system—Section700.27—or from a legally required standby system—Section 701.18. This requirement became effectivefor health-care circuits supplying loads required onessential electrical systems—Section 517.26.For the 2008 code cycle, CMP-20 was formedto develop requirements for COPS. The resultingArticle 708 requirements apply to systems supplyingloads that are vital for national security, public safety,or the economy. Selective coordination is requiredfor all overcurrent protective devices in the circuitssupplying COPS loads, according to Section 708.54.Whether the vital load is fed from the normal sourceor the alternate source doesn’t matter. The selectivecoordination requirement applies to all overcurrentprotective devices, whether they are in the normalsource path or the alternate source path circuits.What Is Selective Coordination?So what is selective coordination and how can itbe achieved?In simple terms, selective coordination meansthat, if an overcurrent causes an overcurrent protectivedevice to open, only the nearest upstream fuse orcircuit breaker will open. No other, larger upstreamfuse or circuit breaker should open. In this way, onlythe circuit with the overcurrent condition is interrupted.In a system in which all overcurrent protectivedevices are selectively coordinated, an overcurrentshould only affect the loads where the problem hasoccurred. The other loads still have power.According to Section 700.27, “Emergencysystem(s),” overcurrent devices shall be selectivelycoordinated with all supply side overcurrent protec-DECEMBER 2007 2 NeC DIGEST 49

Emergency system overcurrent devicestive devices.” The requirements in Sections 701.18 and708.54 are identical, except for the type of systems towhich they apply. Sections 700.27 and 701.18 containtwo exceptions that clarify selective coordinationapplications.Todd Stafford, senior director with the NationalJoint Apprentice Training Center, is a member ofCMP-13, which is responsible for Articles 700 and701. In the 2005 and 2008 code cycles, CMP-13 discussedand debated selective coordination.“The committee discussed this topic thoroughly,” hesays. “The words were discussed and chosen to be specific,in that ‘Emergency system(s) overcurrent devicesshall be selectively coordinated with all supply sideovercurrent protective devices.’ This means the overcurrentprotective devices from the load branch circuit upthrough to the alternate source and from the load branchcircuit up through to the main of the normal source.Both paths are important for a more reliable delivery ofpower. If a fault on the load side of the transfer switchcaused the normal path feeder or main to unnecessarilyopen, we are looking at the alternate source and transferswitch needing to activate. There is some probability thatthe generator may not start or the transfer switch maynot transfer, which reduces the system reliability. Also,the analysis should be done for the fault 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 alternatepower source, and the emergency overcurrent protectivedevices on the load side of the transfer switchmust selectively coordinate with the main and feederovercurrent protective devices in the normal circuitpath (see Figure 1). However, there is a difference onthe minimum requirement for the overcurrent protectivedevices in the normal source path. Read the textadjacent to the one-line.Figure 1. Selective Coordination of Overcurrent DevicesNormal system overcurrent devices that aresupply side overcurrent devices for emergencysystem overcurrent devicesNormal system overcurrent devicesSelective Coordination Requirement700.27 “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, 62 must selectively coordinate with 3, 4, 5, 63 must selectively coordinate with 45 does not have to selectively coordinate with 6NormalSource6512NEAlternateSource34ATSHow to Achieve Selective CoordinationFor many years, selective coordination of overcurrentprotective devices has been a common system designpractice for circuits that supply critical business,government, and military loads. In the financial andcommunications industries, for instance, where thereare significant dollars at risk, many critical loads aredesigned with supply circuits that have selectivelycoordinated overcurrent protective devices. No lessshould be expected for the few important loads thatare critical for life safety.I have spent a career with the U.S. Army Corps ofEngineers specifying and designing high-reliabilityelectrical systems for mission-critical operations for allbranches of the military and many government agencies.One thing that my experience has taught me is thatselective coordination is important for powering criticalloads for as long as possible, even under adverse physicalcircumstances. This can be done by either circuit breakersor fuses. Selective coordination of overcurrent protectivedevices is one of the most important studies thatshould be completed when designing systems for emergencyor critical-operations power systems. Qualifiedelectrical power system engineers competent in selectivecoordination can get the task accomplished efficiently.This does not mean any circuit breaker or fuse canbe used indiscriminately. In some cases, specific typesof fuses or circuit breakers may be required. However,many have mischaracterized this as a fuse vs. circuitbreaker issue. That is not the case.As Alan Manche of Square D said on the floor of theNFPA’s 2004 World Safety Conference & Exposition®,“This is not a breaker/fuse issue. You can selectively coordinatewith breakers and fuses.” In the 2008 code cycle,experts from several circuit breaker and fuse manufacturerspresented to CMP-13 and each stated that circuitbreaker and fuse systems can both be designed to comply.A range of circuit breaker and fuse types andoptions can be used, depending on the specific applicationneeds, including molded-case circuit breakers,insulated-case circuit breakers, low-voltage-powercircuit breakers, and current-limiting fuses. In somesituations, relays may be a suitable option.The simplest circuit breaker solution is moldedcasecircuit breakers with instantaneous trips, if theavailable fault currents are sufficiently low. Circuitbreaker manufacturers publish selective coordinationtables for their molded-case circuit breakers that showselective coordination at higher values than obtainableby merely interpreting the time-current curves.New molded-case circuit breakers with fixed highmagneticinstantaneous trips have been introduced forthe very purpose of achieving selective coordinationwith downstream branch circuit breakers. These circuitbreakers can achieve selective coordination at a much50 NeC DIGEST 2 DECEMBER 2007

As the changes work their way through the industry,the playing field will level out as people becomewilling to pay for more reliable systems.higher fault current than can be achieved by using thestandard adjustable instantaneous trip circuit breakers.Other alternatives include molded-case circuit breakersand insulated-case circuit breakers with a short-time delayoption. These circuit breakers typically have an instantaneousoverride to protect the circuit breaker. With thisoption, the designer simply ensures that the fault currentat the downstream circuit breaker does not exceed theinstantaneous override on the upstream circuit breaker.If fault 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 foreach circuit path and interpreting the results. If selectivecoordination is not achieved, adjustments will berequired in the design or type of circuit breakers.Some people have observed that using short-timedelay settings on circuit breakers increases the arcflashincident energy and resulting equipment damage.However, the industry has remedies, such as arc-flashreduction maintenance switches. When a workerneeds to work on or near a circuit that is protected bya circuit breaker with a short-time delay, the arc-flashreductionmaintenance switch can be thrown to theposition that puts the circuit breaker into an instantaneoustrip setting. This lowers the clearing time, andincident energy, should an arc fault occur.Another solution is zone-selective interlocking, inwhich the circuit breakers communicate with eachother and use short-time delays to achieve selectivecoordination. The other benefit: if a fault occurs ina given circuit breaker’s protection zone, the circuitbreaker can open as fast as possible since the instantaneoussetting overrides the short-time delay. Somedesigners include power inductors and isolationtransformers in the system to choke the current downbelow the circuit breakers’ instantaneous settings.Designing selectively coordinated fuse systems isalso relatively simple. Each low-voltage fuse manufacturerpublishes selectivity ratio guides for its fuses.These ratios are based on the fuse type and the ampererating of the fuses. Divide the ampere rating of theupstream fuse by the ampere rating of the downstreamfuse. If this ampere rating ratio is greater than the ratiopublished by the fuse manufacturer, selective coordinationis achieved up to the fuse’s interrupting ratings.Typically, a short-circuit current analysis is not necessary,nor is plotting the time-current curves.If selectivity ratios are not available, a coordinationstudy is needed. And if the fuse types and ampereratings do not have sufficient ratios adhering to theminimum published selectivity ratios, different typesof fuses must be used or design changes implemented.What About the Cost?Many engineers and contractors routinely provideselectively coordinated systems for mission-criticalbusiness loads, and many already comply with the newrequirements. As with any significant Code change, therewere objections along the way, among them that theyresult in more work for the engineer and a higher pricefor 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,often with sophisticated sensors, monitoring, and control,is warranted. It takes extra time and money to test,maintain, and retain records for these systems to providea reliable system that can provide electrical power todesignated vital loads. The extra cost is expected.When selective coordination results in extra cost,the engineers who conduct a selective coordinationstudy have to account for the extra time in theirbids and negotiations with owners or architects. Ifthe equipment an engineer chooses adds to the costof the project, this money must be accounted for inthe equipment bids. As the changes work their waythrough the industry, however, the playing field willlevel out as people become willing to pay for morereliable systems. Reliability and availability of systemsand equipment will become a deciding factor in thefuture. With the use of availability data currentlyfound in NFPA 70B, reliability-centered maintenancewill be able to predict and determine maintenancerequirements and cost. This, in turn, will be used tojustify the need for selective coordination.For life safety reasons, selective coordination requirementshave expanded to increase the reliability ofsupplying power to a few critical loads. For the circuitssupplying these loads, all overcurrent protective devicesmust be selectively coordinated. Both fusible systems andcircuit breaker systems can be designed to comply. 0Evangelos Stoyas, P.E., is chief of the Power ReliabilityEnhancement Program in the Special Missions Office at FortBelvoir, Virginia. He is a member of CMP-20.DECEMBER 2007 2 NeC DIGEST 51