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Automatic Transfer Switch, Bypass Isolation, Drawout, Contactor Type

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Automatic Transfer Switch, Bypass Isolation, Drawout, Contactor Type Technical Paper Cutler-Hammer Effective: June 2001 Page 1 Supersedes SA 199 David D. Roybal, P.E. Automatic pages 1-12, dated January 1996 Cutler-Hammer Fellow Application Engineer Transfer Switches A Performance Comparison — Molded Case Switch Versus Contactor Type Table of Contents Page Part I: Abstract application for mechanically held About the Author . 1 devices is for lighting control, where Molded case switch type automatic using a latched device allows a user Part I: Abstract. 1 transfer switches with trip units and Part II: “Circuit Breaker” to avoid having to reset all contactors “contactor” type automatic transfer after a utility outage. Versus “Contactor” switches protected by circuit breakers Design Transfer Switches . 1 perform equally for various fault Transfer switches are also capable of Part III: Time-Current Coordination . 3 conditions. High withstand molded high endurance, but not nearly as great Part IV: Transfer Switch case switch type automatic transfer as that of a motor starting/lighting Performance During a Fault . 5 switches without trip units perform contactor. UL 1008 defines the mini- Part V: Conclusions . 8 exactly as “contactor” type automatic mum endurance of a transfer switch. Part VI: References . 8 transfer switches, except that “contac- This is detailed in Table 1. Both “con- tor” type designs have limited with- tactor” and “circuit breaker” designs stand capabilities. Such “contactor” meet these minimum UL 1008 require- About the Author type designs have potential problems ments as evidenced by their UL label. in applications with upstream power Some transfer switch manufacturers David D. Roybal received the Bachelor circuit breakers having short delay trip may publish endurance capabilities in of Science degree in electrical engi- characteristics. excess of these minimums, especially neering from Santa Clara University, for the larger ampacity units. Santa Clara, California, in 1969. Part II: “Circuit Breaker” Versus “Contactor” Design Transfer switches known as “contactor” He is a Fellow Application Engineer with Transfer Switches designs do not use motor starting/light- Cutler-Hammer in Lafayette, California. ing type contactors. Rather, “contactor” He previously was an engineer with A transfer switch is a device for trans- type transfer switches — along with Westinghouse for more than 24 years. ferring one or more load conductor “circuit breaker” type transfer switches connections from one power source — in fact use circuit breaker design Mr. Roybal is a Senior Member of the ➀ to another. Many engineers believe contacts, arc chutes, and arcing horns. Institute of Electrical and Electronics that there are two types of transfer Engineers (IEEE) and presently an Moreover, most “contactor” type switch designs, “contactor” designs transfer switch manufacturers pur- officer of the San Francisco Chapter and “circuit breaker” designs. There are of the Industry Applications Society. chase these parts from manufacturers indeed two different design concepts, of circuit breakers. Thus “contactor” He is a member of the National Fire but this terminology is misleading. type transfer switches actually owe their Protection Association (NFPA), the “Contactor” type transfer switches do design more to circuit breaker technol- National Society of Professional not use motor starting/lighting contac- ogy than to contactor technology. Engineers (NSPE), the International tors, and “circuit breaker” type transfer Association of Electrical Inspectors switches do not use circuit breakers. Transfer switches known as “circuit (IAEI), and the NEMA California Safety breaker” type use specially designed Contactors are NEMA devices designed switching devices that are typically Regulations Advisory Committee. for motor starting and lighting control He is a registered professional engi- molded case switches; circuit breakers which are capable of high endurance are an option. The contacts, arc chutes, neer in the State of California. The (long-life switching capabilities). Their Westinghouse Board of Directors and arcing horns are completely no-load and full-load mechanical opera- enclosed in an insulated housing, as awarded him the Westinghouse Order tions can number in the millions. They of Merit in 1993. He was a recipient they were originally designed to be. can be electrically held or mechanically The switching devices themselves of the IEEE Third Millennium Medal held devices. Electrically held devices in the year 2000. are further required to meet UL 1087 will drop out upon loss of control volt- (Molded Case Switches) or UL 489 age. A typical application is as part of (Molded Case Circuit Breakers) a motor starter. Mechanically held requirements, as well as UL 1008 devices have mechanical latches or (Automatic Transfer Switches) designs which inherently remain closed requirements. This means that “circuit upon loss of control voltage. A typical breaker” type transfer switches are Table 1 — ATS Endurance (UL 1008 Table 30.2) Switch Ampere Rate of Number of Cycles of Operations Rating Operation With Current Without Current Total 0–300 1 per minute 6000 – 6000 301–400 1 per minute 4000 – 4000 401–800 1 per minute 2000 1000 3000 801–1600 1 per 2 minutes 1500 1500 3000 ➀ NEMA ICS 10-1993, AC Automatic Transfer Switches. 1601 and above 1 per 4 minutes 1000 2000 3000 TP.15A.01.T.E Technical Paper Automatic Cutler-Hammer Page 2 Effective: June 2001 Transfer Switches held to a more rigorous testing stan- current protection and those using normally have much higher withstand dard than “contactor” type switches. molded case switches without over- and endurance ratings than the The main contact assemblies in “circuit current protection. Smaller ampacity “contactor” type transfer switches. breaker” type transfer switches are designs use molded case switches with subject to periodic UL 489 and UL 1087 fixed instantaneous trip units set high to “Contactor” type transfer switches and follow-up testing versus the one-time- allow the maximum withstand of the “circuit breaker” type transfer switches only UL 1008 design test for “contac- device. Other molded case switches, using molded case switches with tor” type automatic transfer switches. such as the Cutler-Hammer Type SPB instantaneous trip elements perform Typically, the switching devices in a Insulated Case Switch, can be provided identically when applied in systems “circuit breaker” type transfer switch without trip units. These are switches with molded case circuit breakers. are oversized for the ampacity of the that have extremely high withstand and When the upstream breaker is a power transfer switch. For example, an 800 endurance ratings, often greater than circuit breaker with no instantaneous ampere “circuit breaker” type transfer those of a comparable “contactor” type trip element, it may have a short delay switch uses 1200 ampere switching switch. Typical ratings are shown in time setting for coordination with other devices, as shown in Table 2. Thus Table 3. As an alternate design, the devices under high fault conditions. the contacts used in a “circuit breaker” transfer switches can be provided When a downstream fault occurs, the type transfer switch are likely to be with molded case switches having power circuit breaker can delay tripping larger than the contacts used in a overcurrent and ground fault trip units. in order to allow the downstream “contactor” type of equal rating These are used where integral over- device to selectively clear the fault. because of this oversizing. current protection is desired, such as in service entrance applications. If the power circuit breaker short delay There are actually two types of “circuit time setting is set high, fault current breaker” transfer switch designs: those Since “contactor” type transfer may flow in excess of the limited time using molded case switches with over- switches do not use contactors and and current withstand ratings of a both “contactor” and “circuit breaker” “contactor” type transfer switch. A Table 2 — Molded Case Switch ATS type automatic transfer switches use “circuit breaker” type transfer switch With Instantaneous Trip Elements circuit breaker parts, what is their using molded case switches without difference in performance? Under UL 1008 UL 1087/UL 489 UL 1087/UL trip units can be selected to withstand Switch Cutler-Hammer 489 Frame most downstream fault conditions, the short delay time, while a transfer Ampere Molded Case Frame Ampacity both designs will perform identically, switch design using molded case Rating i.e., the upstream breaker will trip and switches with instantaneous trip ele- the switch logic will initiate the trans- ments would merely trip and reset. In 100 F 150 fer sequence to the alternate source. 150 K 225 some situations the high-magnitude 150 K 400 “Contactor” type transfer switches fault may cause a drop in voltage which could initiate a transfer 225 K 400 and “circuit breaker” type transfer switches using molded case switches sequence for either the “contactor” 300 K 400 type transfer switch or the “circuit 400 L 600 without trip units perform identically under all circumstances. However, it breaker” type transfer switch. When 600 M 800 should be noted that these “circuit this occurs, the transfer switch will 800 N 1200 breaker” type transfer switch designs attempt to interrupt the fault current. 1000 N 1200 The engineer may find that the limited Table 3 — Transfer Switch Withstand Ratings (Time and Current) Transfer UL 1008 3 Cycle Rating Extended Rating Switch
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