UL 1008 Withstand and Close On Ratings

Transcription

1 Power topic # Technical information from Cummins UL 1008 Withstand and Close On Ratings White Paper By Rich Scroggins, Technical Advisor Electrical distribution system design requires sizing equipment so that it can safely withstand any level of fault current to which it may be exposed. In the case of transfer switches this involves an evaluation of the transfer switch s short circuit withstand and close on rating (WCR), the available fault current at the line terminals of the transfer switch and the fault clearing time of the overcurrent protection device. This creates a challenge for engineers when specifying transfer switches as the available fault current and the overcurrent protection device are usually not known at the time specifications are written. Engineers often take a conservative approach and specify high fault clearing time based ratings however this can result in oversizing and limiting choices of transfer switches. Not specifying fault duration times and specifying only that transfer switch and overcurrent protection devices be coordinated at the available fault current allows contractors and transfer switch and circuit breaker manufacturers the flexibility to design a cost effective system that meets fault withstand requirements.

2 UL 1008 Requirements UL 1008 requires that all transfer switches have a short circuit withstand and close on rating (WCR). This refers to a level of fault current which the transfer switch can withstand and close on to without creating a hazardous condition. For a safe installation the WCR of the transfer switch must be higher than the available fault current at the point in the circuit where the transfer switch is installed. UL requires that all WCRs and their associated durations and/or overcurrent devices (breakers or fuses) be listed on the product. A proper installation requires either that the transfer switch be protected by an overcurrent device which is listed on the ATS label or by a device that clears the fault in less time than the duration associated with the switch s WCR. Figure 1 is an example of a transfer switch label listing WCR and their associated circuit breakers for a particular switch. The WCR presumes that the transfer switch is protected by an overcurrent device so that the fault current persists for a limited period of time. The WCR includes either a specific overcurrent device or a duration of the fault and it is common for a transfer switch to have several different WCRs for different overcurrent devices or fault durations. 02 Power Topic # Figure 1 Transfer Switch WCR with specified circuit breakers

3 Using a time based rating (sometimes known as 3 cycle or any breaker ratings) has the advantage that it doesn t limit a contractor on their choice of circuit breakers. The disadvantage to the time based rating is that the consultant or contractor is required to show that the breaker clears the fault in less time than the transfer switch s WCR duration at the fault current listed on the product. With a specific breaker based rating all the contractor has to do is confirm that the selected breaker is listed for the transfer switch. Also, because most breakers actually clear faster than 50 msec (3 cycles) as is specified in most time based ratings, most breaker based ratings are much higher than time based ratings. UL th Edition Effective in November of 2014 UL changed the requirements for adding circuit breakers to transfer switch specific breaker lists without running additional short circuit tests. In earlier versions of UL 1008 a new breaker could be added by comparing the maximum published trip time with the maximum published trip time of a previously tested breaker at the rated short circuit current. If the trip time of the new breaker was less than or equal to that of the previously tested breaker the new breaker could be added to the list. With the introduction of the UL th edition adding a new breaker now requires comparing the published trip time of the new breaker with the actual trip time from a previous short circuit test. For example if a tested breaker has a maximum published trip time 0.05 seconds then under the earlier versions of UL 1008 any breaker that has a maximum published trip time of 0.05 seconds or less could be added to the list regardless of how long it took that breaker to trip during the test. Under the 7th edition of UL 1008 if the tested breaker actually tripped in 0.03 seconds during the test only circuit breakers that have a maximum published trip time of 0.03 seconds or less can be added to the list without an additional test. This change resulted in some breakers that had been listed being removed from lists and has made it more difficult to add new breakers to lists without performing additional short circuit tests which are very expensive and are difficult to complete because there are very few facilities capable of performing them. The impact of this code change is significant but has been overstated by some in the industry. While all transfer switch manufacturers were forced to remove some breakers from their listing many of the most commonly used breakers by major circuit breaker manufacturers remain. For example Table 1 lists specific breaker WCR ratings for Cummins transfer switches with four of commonly used breaker product lines in North America. Squared D PowerPact GE Spectra Siemens Sentron Eaton C Series Amps ATS Bypass Breaker WCR@Volts (000s) Breaker WCR@Volts (000s) Breaker WCR@Volts (000s) Breaker WCR@Volts (000s) OTPC, OTEC HL HED HFD OTPC, OTEC HL HFD HFD OTPC, OTEC HL HFD HFD OTPC, OTEC BTPC HL SGL HFD HKD OTPC, OTEC BTPC JL SGL HJD HKD OTPC, OTEC BTPC LL SGL HJD HKD OTPC, OTEC BTPC LL SGL CLD HKD OTPC, OTEC BTPC LL SGL CLD HKD OTPC, OTEC BTPC LL SGL CLD HLD OTPC, OTEC BTPC PJ SKP HMD HLD OTPC, OTEC BTPC PJ SKP HMD HLD OTPC, OTEC BTPC PJ SKP HMD HLD Table 1 WCR of Cummins transfer switches with commonly used circuit breakers 03 Power Topic #

4 In most cases the WCR rating of the transfer switches based on a specific breaker is substantially higher than the time based ratings of the same transfer switch. Table 2 displays WCR ratings for the Cummins ATS product line, including time based ratings, ratings for the commonly used breakers listed in table 1, current limiting breakers and fuses. As an example, the appendix displays the circuit breaker list for Cummins 300 and 400 amp transfer switches at 480 volts. A complete circuit breaker list for all of Cummins automatic transfer switches can be found at From this table it is clear that the ratings for the commonly used breakers are higher than the time based ratings, and the ratings based on current limiting breakers and fuses are even higher. For this reason specifying time based WCR before any analysis has been done to evaluate the available fault current could unnecessarily force a transfer switch manufacturer to quote an oversized transfer switch resulting in a larger, more expensive product than is really needed. Specific Breaker Specific Breaker Fuse Protection Time Based Ratings Bypass Protection (Common) Protection (General) Amps ATS Max Fuse, Size & Type WCR@Volts (000s) WCR@Volts (000s) WCR@Volts (000s) Time WCR@Volts (000s) (sec) OTEC, OTPC 200 A Class, J, RK1, RK OTEC, OTPC 200 A Class, J, RK1, RK OTEC, OTPC 200 A Class, J, RK1, RK Power Topic # OHPC CHPC A Class J, RK1, RK 5 or BTPC 600 A Class J, RK1, RK5, 1200 A Class T, or 2000 A Class L OTEC, OTPC 2000 A Class L or 1200 A Class T or 600 A Class J RK1 or RK A Class L or T BTPC 600 A Class J, RK1, RK5, 1200 A Class T, or 2000 A Class L OTEC, OTPC 2000 A Class L or 1200 A Class T or 600 A Class J RK1 or RK OTPC* BTPC 2000 Amp Class L OTEC, OTPC** 3000 Amp Class L OTPC BTPC 2500 A Class L OTPC BTPC 2500 A Class L OTPC BTPC 4000 A Class L OTPC BTPC 6000 A Class L Table 2 Cummins ATS WCR ratings (*Closed Transition **Open Transition)

5 The AIA transfer switch spec uses the following language to begin the transfer switch WCR requirement. Tested Fault-Current Closing and Short-Circuit Ratings: Adequate for duty imposed by protective devices at installation locations in Project under the fault conditions indicated, based on testing according to UL This language is sufficient when writing a specification when the available fault current is not known. A transfer switch can comply with this specification with either a specific breaker based or a time based rating. When the available fault current is known at the time of specification it is added to the spec, often in tabular form for applications requiring multiple transfer switches. At the time the spec is written circuit breakers typically have not been selected for the project so the spec may call out only Molded Case Circuit Breaker as the overcurrent protection. Most transfer switch manufacturers will quote based on a time based WCR in that case because they can t be sure that the breakers ultimately selected will be listed for their transfer switches. Once the breakers are selected the ATS supplier may be able to provide a smaller, less expensive switch than what was originally quoted while still complying with the original specification. Conclusion Most transfer switches have several short circuit Withstand and Close On ratings based on fault duration times and on specific overcurrent protection devices. Specific overcurrent device ratings are almost always higher than time based ratings and will usually allow for the smallest most cost effective transfer switch to be selected when these ratings can be applied. For this reason specifications should only require that the transfer switch short circuit ratings be coordinated with the overcurrent protective devices at the fault current available on the line side of the transfer switch. This allows for flexibility in selecting transfer switches once breakers are chosen for the project and minimizes the possibility of having to oversize the transfer switch to meet the short circuit WCR requirements. 05 Power Topic #

6 Appendix Cummins Amp : OTEC/OTPC Frame (Max Amp): C (600) Specific Breaker Voltage 480 WCR (ka) Eaton GHB, GD JGE KD, CKD 1, CMDL 1, CHKD 1, CHLD1, EGC, KDC, JGU, LGU DSL, LA, NB, GHC CLD 1, JD, LGS, MDL 1, CHMDL 1, CLDC 1, HJD, LDC, JGC, LGC JGX, LCL, JDB, JGS, HKD, HLD 1, HMDL 1, LGX KDB, LD 1, JGH, LGH LGE GE SEL, SFL, SGL SEP, SFP, SGP FE, FG AKRU, AKU, THLC4 Siemens ED4 ED6 HED6 FD6, FD6A, FXD6, FXD6A HED4 BQCH, CPD6, HFD6, HFXD6, HHFD6, HHFXD6 CJD6, CLD6, SCJC6, SCLD6 CED6, CFD6 Schneider HD, DG, HG, CK1000L, DJ, HJ, JJ, LJ, CD106L, DL, CF250L, CJ400L FI, KI, LI, HR, JD, JG, LG, PA, PAF, NSF150H, NSF250H, HL, JL, LL, JR, LR LD NSF150N, PAL NSJ400H, NSJ600H, NSJ400L, NSF250N, PH, PHF, PHL NSJ600L NSJ400N, NSJ600N : CHPC/OHPC Frame (Max Amp): C (600) Specific Breaker Voltage 480 WCR (ka) Eaton JDC, JGC, KDC, LDC, LGC, LA500 JGU, LGU NB800 DSL, JGX, LA400, LCL, LCLA, LCLG, LCLGA, LGX, NB500 GE SFL, TFL, SGL SFP, SGP AKRU, AKU, THCL2, THCL4 Siemens CJD6, CLD6, CED6, CFD6 SCJD6, SCLD6 06 Power Topic # Schneider HD, JD, LD HG, JG, LG, NSF150N, NSF250N, NSJ400N, NSF600N HJ, JJ, LJ, NSF150H, NSF250H, NSJ400H, NSJ600H HL, JL, LL, NSJ400L, NSJ600L CF250L, CJ400L DSL, HR, JR, KI, LI, LR

7 Cummins Amp : BTPC Frame (Max Amp): C (600) Specific Breaker Voltage 480 WCR (ka) Eaton EHD, FDB, EGB, GD EGE, JGE KD, CKD 1, CLD 1, EGS, FD, JD, CMDL 1, MDL 1 CHKD1, CHLD1, CHMDL1, CLDC 1, EGH, HFD, HJD, HKD, EGC, KDC, LDC, JGC, JGU, LGU DSL, LA, NB, JGX, GHB, JDB, JGS, KDB, HLD 1, HMDL 1, JGH LGC LCL, LGX GHC LD 1 GE TEYD TEYH SEL, SFL, SGL, TEYL SEP, SFP, SGP FE, FG AKRU, AKU, THLC4 Siemens BQCH, ED4 ED6, HED6 FD6, FD6A, CPD6, HDGA, HFD6, HFGA, HFXD6, CMD6, CJD6, CED6, CFD6 BQD, NGB, FXD6, FXD6A, HED4 HHFD6, HHFXD6, HJD6, HJGA, CND6, CLD6, CQD NGG JD6, JXD6, HJXD6, HHJD6, HHJXD6, SCMD6, SCJC6, NDGA, NFGA, HLGA, HLGB, HMG, LDGA, SCND6 SCLD6 NJGA, NLGA, LFGA, LJGA, LLGA, LLGB, LMG NLGB, NMG Schneider HD, DG, HG, JG, CK1000L, DJ, HJ, JJ, LJ, NSF150H, CE106L, DL, CF250L, FI, KI, LI, JD, LG, NSF150N, PA, PAF, NSF250H, NSJ400H, NSJ600H, HL, JL, LL, CJ400L HR, JR, LR LD NSF250N, PAL PH, PHF, PHL NSJ400L, NSJ400N, NSJ600L NSJ600N 07 Power Topic #

8 About the author Rich Scroggins is a Technical Advisor in the Application Engineering group at Cummins. Rich has been with Cummins for 18 years in a variety of engineering and product management roles. Rich has led product development and application work with transfer switches, switchgear controls and networking and remote monitoring products and has developed and conducted seminars and sales and service training internationally on several products. Rich received his bachelors degree in electrical engineering from the University of Minnesota and an MBA from the University of St. Thomas. Copyright 2017 Cummins Inc. All rights reserved. Cummins is a registered trademark of Cummins Inc. power.cummins.com Bulletin (6/17) 08 Power Topic #