Transfer Switches
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POWER SYSTEMS TOPICS 112 Understanding the Withstand and Close-On Ratings for TRANSFER SWITCHES AUTHOR INTRODUCTION MIKE LITTLE Principal Engineer Numerous short circuit current ratings and references exist for transfer Kohler Co. switches that are often confusing and seemingly contradictory. This paper Power Systems Division provides some explanation and clarification to help engineers specify the proper equipment to meet local and national regulations. For an electrical system to operate safely, the design should consider a variety of scenarios where things do not go as planned. One of those scenarios is when a short circuit occurs in the system and causes extremely high currents. An electrical system needs to be designed to safely react to these extreme conditions and, ideally, to continue to function afterwards. This article looks at automatic transfer switches (ATS) which are integral pieces of the power distribution system that help ensure power for home, office, factory or process, when served by an emergency or standby generator in addition to the local utility. This paper also aims to help engineers understand what the withstand and close-on ratings (WCR) means and provide background information to allow the proper sizing and selection of the transfer switch. Two key abilities of the transfer switch are tested under the WCR: the quantified ability to withstand fault currents for a specified period of time while maintaining functionality; and the ability to close into a fault current and continue to operate. Both abilities are critical to allow the electrical system to sustain a fault current with minimal impact. SELECTING AN ATS I. UNDERSTAND DAMAGES CAUSED BY HIGH II. STUDY THE POWER SYSTEM FAULT CURRENTS Detailed knowledge is required about the Prior to selecting an ATS, it is important to section of the power system where the ATS understand how high fault currents can will be connected. Modern power system damage an electrical system’s functionality. studies are performed by engineers who build High fault currents flowing through power models of power systems using computer system components can create significant software. The study covers all system heat over the duration of the fault, which parameters including the sizes, conductors’ can cause damage to the insulation on quantities and lengths, transformer ratings conductors and transformers, as well as and impedances, overcurrent protective to the overcurrent protective devices and devices and related Time-Current Curves switch contacts. (TCC), and other relevant data. Fault currents can also create significant Through a power system study, engineers mechanical stress, as they produce high compute fault currents at each system bus for magnetic forces that can bend bus structures, normal, contingency and future system separate switch contacts, or cause power configurations and evaluate capability of cables to pull out and energize surrounding upstream and downstream protective devices. structures or circuits. All these factors should Additionally, engineers provide computations be taken into consideration during the on subjects such as available arc-fault electrical system’s design phase in energies at each bus along with analysis and conjunction with the power system study recommendations on device coordination. and proper selective coordination. Based on the study results, the specifier can then make an intelligent ATS selection. Figure 1 III. APPLY SELECTIVE COORDINATION AND FAULT ISOLATION As part of the power system study, it is important to understand the concept of selective coordination with overcurrent protective devices. Proper selective coordination ensures that an electrical fault will be cleared as close to the point of occurrence as is practical. The National Electrical Code (NEC), in the Fault Fault 2017 standard, defines selective coordination as the following in Article 100: “Localization of an overcurrent condition to restrict outages to the circuit or equipment Without Selective Coordination With Selective Coordination affected, accomplished by the selection and installation of overcurrent protective devices Tripped Tripped and their ratings or settings for the full range Affected Outage Affected Outage of available overcurrents, from overload to the maximum available fault current, and for the full range of overcurrent protective device opening times associated with those overcurrents.” 2 / Withstand and Close on Ratings POWER SYSTEMS TOPICS 112 Figure 1 on the left shows an electrical 10 kA available fault current, therefore system without selective coordination. requiring a much smaller WCR which allows Without selective coordination, when a fault more flexibility regarding selection of occurs at a particular load, several levels of overcurrent devices and ATS model. overcurrent protective devices (even the main disconnect) may see the fault and react, or V. CONCLUSION trip. When this happens, it may potentially impact the entire facility. Engineers should conduct a comprehensive power system study prior to installing an ATS. Figure 1 on the right illustrates proper The study would help determine available fault selective coordination, which is equipped with currents from sources present or considered properly timed overcurrent protective devices. for future installation and recommend When a fault current happens, the upstream appropriate WCRs for the ATS in Figure 1. breakers from the fault "withstand" the current Only with an accurate understanding of the long enough that only the nearest overcurrent electrical system and its design implications, protective device will open and clear the fault. can a proper ATS be selected to provide In this way, most of the distribution circuit safe, robust and cost-effective solutions. This remains unaffected and operational. prestudy will also help avoid problems that can occur during the commissioning phase, IV. IDENTIFY THE REQUIRED WCR such as impermissible disconnect and ATS FOR THE ATS combinations, incorrect switch purchase or failure to satisfy a specification. Engineers need to identify the required WCR for the point of installation. To ensure safety, the ideal WCR should be high enough that the UNDERSTANDING THE FOUR transfer switch can handle available fault WCR TYPES currents. At the same time, the WCR should not be overrated, which may result in a higher WCR are fundamentally the short-circuit cost and larger switch, contributing to a lack ratings of a transfer switch. Various standards, of space in the electrical room. such as UL 1008, define the methods and criteria for testing and reporting the WCR For example, two identical 200 A-rated ATS levels. The four main types of WCR installed at different parts of a facility may (Figure 2) are specific breaker ratings, time- require different WCR levels. If a specifier based ratings, short-time ratings and fuses. installs a 200 A-rated ATS close to the utility The categorization is based on how the service entrance, it may need to withstand a overcurrent protection of the transfer switch 35-kA available fault current as determined by is accomplished and tested. the power system study. To achieve the required WCR, the ATS may be upsized or The WCR for any KOHLER® ATS is found in may require specific upstream overcurrent the model’s specification sheet and near the protection. Meanwhile, the same 200 A ATS contactor inside the enclosure. installed in a branch circuit on the other side of the facility may only need to withstand a Withstand and Close on Ratings / 3 Typical contactor rating nameplate: SUITABLE FOR CONTROL OF MOTORS, ELECTRIC DISCHARGE AND TUNGSTEN LAMPS, ELECTRIC HEATING EQUIP. WHERE THE SUM OF MOTOR FULL LOAD AMPS FigureAND A 2MPS OF OTHER LOADS DOES NOT EXCEED THE SWITCH AMP RATING AND THE TUNGSTEN LOAD DOES NOT EXCEED 30 PERCENT OF THE SWITCH RATING. Typical contactor rating nameplate: TSyUpITicaABlL cEon FOtacR CtorON rTaRtingOSL HnOOaFmR MTeO-pCTlIaORCUtRe:S,I TE LWEICTHTRSTICANDDIS/CHCLOARSIGNEG AND TUNGSTEN SLAUMITPSAB,L EEL FEOCRTR CICONHTERAOTILAN ONDGF E MSQHOUOTIPOR. TRW-STH,I MERELE CURRC TTHREIC ESNDUTIMS RCHOAFTA MIRNOGTESO ARN FDU TLUNL LGOSATDEANM PS LAANDMPS AM, PSELE OCFT ORITCHHEERA LTOINAGD SE QDUOIESP. WNOHTEREEX CTEEHED S TUHME O SFW MITOCHTO ARM FPU LRLALTOINAGD AANDMPS TH E WATUNNDHEG NAS MPTPSREON TO LEFOC AOTDTE HDE OBYRE SL AO N ACODIRTS CE DUXOICTESEE BR DNEAKE O3T0 PEERXRC,C TEEHEIDNS TT RHOAEFN TSSHWFEEIT RSCH WSWI TAICMTHCHP RR AIASTT ISINNUGGI.T AABNDLE T HE FTOUNRG USSETE INN LAOCAIDRCU DOIETS C NAPABOT ELXEC OEEF DD E30L IVEPERRCINEGN TH OEF STHHOER STW-CITIRCUCH RITACURRTING. ENT FOR THypEi caMAl XcIonMUtMac TtorIM EraDURtingA nTaIOmNe pAlNDate V: OLTAGE MARKED BELOW. SUITABLE FOR CONTROSL HOOFR MTO-CTIORCURS,I TE LWEICTHTRSTICANDDIS/CHCLOARSIGNEG AND TUNGSTEN LAMPS, ELECTRIC HEATSIANHNDGO ER SQTH-UCOIPIRRCU. TW-THITME REW ECURRIT THHSET EASNNDUTM / RCOALFTO MISNOIGNTSGO R FULL LOAD AMPS THE CIRCUIT BREAKER MUST INCLUDE AN INSTANTANEOUS TRIP RESPONSE UNLESS AND AMPS OF OTHER LOAANDDS SDHOOESRT N-TOIMTEE XCURRCEEDE NTHT ER SAWTINITGCHS AMP RATING AND THE THE AVAILABLE SHORT-CIRCUIT CURRENT IS LESS THAN OR EQUAL TO THE SHORT- WTUNHEGNS PTREONT LEOCATDE D OBYES A N COIRT CEUXICTEE BRDEAKE 30 PERRC, THEINST ROAFNTSHFEE RS WSWITICTHCH R AIST ISNUGI.TABLE WFTOIMHRE N UR SEPART IONTG AE OCCTFIRCU ETDH EBYITT CRAAPABA CNIRSFCLEUERIT OS BFWR DIEAKETECHLIVE ARNDR, TINH TGIHS TEHR CEAI RCUNSSHFOEIRTR TB -SRCWEAKEIRCUITCHIRT ISCURRINC SULIUDTEABNESTL E FA O R SHORT-TIME TRIP RESPONSE. FTOHER MUASEX IIMNU