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: “” 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

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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 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 Cutler-Hammer Cutler-Hammer Manufacturer Manufacturer Manufacturer Cutler-Hammer Manufacturer Manufacturer Ampere Transfer Switch Transfer Switch “A” “R” “Z” Transfer Switch “R” “A” Rating with Molded with Type SPB Transfer Transfer Transfer with Type SPB Transfer Transfer Case Switches➀ Switches➁ Switch➀ Switch➀ Switch➀ Switches➁ Switch➀ Switch➀ 30/40 65 kA N/A 10 kA➂ N/A 10 kA➂ N/A N/A N/A 70/80 65 kA N/A 10 kA➂ N/A 10 kA➂ N/A N/A N/A 100 65 kA N/A 10 kA➂ 42 kA 10 kA➂ N/A N/A N/A 150 65 kA N/A 10 kA➂ 42 kA 10 kA➂ N/A N/A N/A 225/260 65 kA N/A 35 kA 42 kA 35 kA N/A 40 kA 10 cycles N/A 300 65 kA N/A N/A N/A 35 kA N/A N/A N/A 400 65 kA N/A 35 kA 42 kA 35 kA 35 kA➃ 60 cycles 40 kA 10 cycles N/A 600 50 kA➅ 100 kA 50 kA 65 kA 50 kA 35 kA➃ 60 cycles 40 kA 10 cycles N/A 800 50 kA➅ 100 kA 50 kA 65 kA 50 kA 35 kA➃ 60 cycles 40 kA 10 cycles N/A 1000 50 kA➅ 100 kA 85 kA 85 kA 50 kA 35 kA➃ 60 cycles 50 kA 10 cycles 65 kA 30 cycles 1200 N/A 100 kA 85 kA 85 kA 50 kA 35 kA➃ 60 cycles 50 kA 10 cycles 65 kA 30 cycles 1600 N/A 100 kA 100 kA 100 kA 100 kA 51 kA 60 cycles 50 kA 10 cycles 65 kA 30 cycles 2000 N/A 100 kA 100 kA 100 kA 100 kA 51 kA 60 cycles N/A 65 kA 30 cycles 3000 N/A 100 kA 100 kA 100 kA 100 kA 51 kA 60 cycles N/A 65 kA 30 cycles 4000 N/A 100 kA 100 kA 100 kA 100 kA 85 kA 60 cycles N/A 65 kA 30 cycles

➀ Symmetrical rms amperes at 480 volts and breakers are tested at a more severe 15% ➃ Also available with a rating of 51 kA at 20% short circuit power factor (X/R ratio of 4.9). short circuit power factor (X/R ratio of 6.6) 60 cycles. ➁ Symmetrical rms amperes at 480 volts and which results in a higher fault duty. ➄ Contact Cutler-Hammer for availability. 15% short circuit power factor. Power circuit ➂ 1.5 cycles only. ➅ 4-pole units rated 35 kA.

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interrupting rating of “contactor” type transfer switches is not adequate for 10000 the system design and may select the 5000 self-protecting “circuit breaker” type design instead. The use of “contactor”

type transfer switches in these cases 1000 would be a misapplication. 500 The electrical engineer must deter- mine which design characteristics MAXIMUM

are preferable based on the system T 100 requirements and the performance of I M 50 the transfer switch to be applied. This E MINIMUM paper provides information that can be I used in making that choice. N 10 Part III: Time-Current Coordination S E 5 C To better understand the operation of O molded case switches used in transfer N D NOMINAL switch applications, it is necessary S 1 MAXIMUM to review their time-current characteris- INSTANTANEOUS .5 NOMINAL SETTING tics and their coordination with other MINIMUM devices in a power distribution system. INSTAN- TANEOUS Molded case circuit breakers are avail- SETTING able with both thermal-magnetic and .1 solid state trip units. Typically, smaller .05 molded case circuit breaker frame sizes are provided with thermal-magnetic trip units and larger frame sizes (approxi- .01 mately 400 amperes and above) are .5 1 5 10 50 100 500 1000 CURRENT IN MULTIPLES OF CIRCUIT BREAKER RATING provided with solid state trip units. Examining a typical circuit breaker time- Figure 1 — Typical Time-Current Curve for Adjustable Instantaneous Thermal-Magnetic current curve, we can discuss the pro- Molded Case Circuit Breakers (NEMA AB3) tection provided by the various portions of the curve. Figure 1 shows a time- current curve for an adjustable instanta- 10000 neous thermal-magnetic molded case 5000 AMPERE RATING OR circuit breaker. The long time portion of LONG TIME PICKUP the curve provides overload protection for the circuit, while the instantaneous 1000 TYPICAL TIME-CURRENT CURVE portion provides short circuit protection. ADJUSTMENTS FOR SOLID STATE TRIP The long time response is usually pro- 500 UNIT WITH ADJUSTABLE GROUND FAULT PICKUP AND DELAY SETTINGS. vided by a thermal element where the delay is in seconds, with shorter time LONG TIME DELAY delays as the current increases. This T 100 10 I 8 6 “inverse-time” characteristic is typical M 50 T E I 4 GROUND FAULT of thermal-magnetic breakers. The 3 M PICKUP instantaneous pickup is normally adjust- I E 2 N able for frames rated 225 amperes and 2 10 I 1 I T RAMP higher. Since instantaneous elements .8 S N .6 E 5 operate with no intentional time delay, SHORT TIME .4 GROUND FAULT C PICKUP S .3 TIME DELAY only the magnitude of the fault current O E determines which circuit protective N C .2 D O devices in series will trip. S 1 N .1 .08 SHORT TIME D .06 .5 S Figure 2 shows the time-current curve DELAY .04 .03 adjustments for a solid state trip unit .2 .3 .4 .6 .8 1 2346810 with adjustable phase current settings. GROUND FAULT PICKUP .1 CURRENT IN MULTIPLES OF The long time pickup and delay provide FRAME OR SENSOR RATING INSTANTANEOUS overload protection. The short time .05 pickup and delay are adjusted to coor- PICKUP dinate with other circuit breakers and to allow acceptable overloads, such as .01 motor or inrushes, to .5 1 510 50 100 500 1000 5000 10000 occur without tripping the circuit CURRENT IN MULTIPLES OF RATING PLUG OR CURRENT SENSOR breaker. The instantaneous portion of the curve protects against high- Figure 2 — Typical Time-Current Curve for Solid State Trip Unit with Adjustable Phase magnitude faults such as short circuits. Current Settings (NEMA AB3)

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Technical Paper Automatic Cutler-Hammer Page 4 Effective: June 2001 Transfer Switches

In many power system designs where and personnel, while minimizing the to the fault is the first to clear it. Each devices with high withstand ratings outage of the remainder of the system. upstream circuit protective device has are available, the instantaneous trip is the backup capability to isolate and clear deleted in order to achieve improved During the design of the power distribu- the fault in the event of the misoperation coordination. Instantaneous trips set tion system, the various protective or nonoperation of the downstream below the available fault current can- devices are evaluated in a series circuit device, so that the damage to the not be fully coordinated because the from the power source to the load. faulted circuit and the effect of the magnitude of the fault current cannot The object is to localize the power disturbance on the rest of the power be controlled. All breakers in series interruption so that the device closest distribution system are minimized. with instantaneous trips respond to the fault current and try to clear the Table 4 — Electrical Characteristics of Magnum DS Power Circuit Breakers fault with no intentional time delay. Magnum DS Frame Trip UL Listed Interrupting Capacity at 480 Volts Typically, time delay is used to allow Model Size Ampere rms Symmetrical Amperes (kA) devices to selectively coordinate. Range With Instantaneous Trip Without Instantaneous Trip All molded case circuit breakers are (30 cycle maximum delay) designed to NEMA standards and MDS-408 800 80 – 800 42 42 have instantaneous trips. Typically, MDS-608 800 80 – 800 65 65 the maximum setting is 10 to 13 times MDS-808 800 80 – 800 85 85 the frame rating. Special designs — MDS-C08 800 80 – 800 100 85 called insulated case circuit breakers MDS-616 1600 80 – 1600 65 65 by the industry — such as the Cutler- MDS-816 1600 80 – 1600 85 85 Hammer Type SPB Insulated Case MDS-C16 1600 80 – 1600 100 85 Breaker, may have higher than normal MDS-620 2000 80 – 2000 65 65 instantaneous settings. These designs MDS-820 2000 80 – 2000 85 85 allow use of the short time pickup and MDS-C20 2000 80 – 2000 100 85 delay to achieve improved coordina- tion and selectivity. MDS-632 3200 1000 – 3200 65 65 MDS-832 3200 1000 – 3200 85 85 Power circuit breakers are designed to MDS-C32 3200 1000 – 3200 100 85 ANSI standards, and their interrupting MDS-840 4000 1280 – 4000 85 85 rating equals their short time rating. MDS-C40 4000 1280 – 4000 100 100 This means that they can be applied MDS-850 5000 1280 – 5000 85 85 without instantaneous trip elements MDS-C50 5000 1280 – 5000 100 100 to achieve true coordination between devices. A typical power circuit breaker is the Cutler-Hammer Magnum DS. Its interrupting and short time ratings are 1000 shown in Table 4. Power circuit break- ers allow the engineer the most flexibil- 500 M 1600A 100 MVA B AMP SETTING 1.0 ity in designing a truly coordinated 100A TM LONG DELAY 4160V system. Both the power circuit breaker NON- 2.0 SEC. @ 6X ADJUSTABLE SHORT TIME P.U. and the that incorporates it 100 3X SHORT DELAY A are designed and tested to withstand, 0.2 SEC. without damage, the high magnitude 50 T 139A 1000kVA fault currents that flow through the I F 1203A 5.75% Z equipment for up to 30 cycles. M 600A AMP SETTING E 1.0 17,800 SCA Current limiting breakers as well as LONG DELAY 10 M switches with current limiting fuses I 2.0 SEC. @6X SHORT TIME 480V can substantially reduce the available N 5 P.U. 22.6kA fault current. Fuses selectively 4X S SHORT DELAY coordinate when devices in series 0.1 SEC. F are selected in ratios that allow the E A C 200A 17.5kA downstream fuse to clear for a given 1 FG CURRENT O GFPU LIMITING current before the melting level of the N 400A FUSE .5 GFTD upstream fuse is reached. D 0.1 SEC. B S An overcurrent in a power system can MG LTG. occur as a result of normal conditions GFPU LOAD such as motor inrush during starting or .1 800A GFTD transformer inrush upon energization. 0.3 SEC. It can also occur as a result of abnormal .05 conditions such as overloads, short MG ZONE INTERLOCK circuits, or ground faults. The circuit NO DELAY protective devices in a system sense .01 abnormal conditions and protect .5 151050 100 500 1000 against them by opening the power cir- cuit. In a properly designed system, they CURRENT IN AMPERES X 100 AT 480 VOLTS will have been chosen to operate selec- tively to protect equipment, property, Figure 3 — Typical System — Selectively Coordinated (NEMA AB3)

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Cutler-Hammer Automatic Technical Paper Transfer Switches Effective: June 2001 Page 5

The time-current coordination of a rated operating current. Overloads are transfer switch is a “circuit breaker” power system is normally evaluated usually low level and cause thermal type or a “contactor” type, with either on a log-log plot of time versus current. damage. Short circuits are usually high design yielding the same end result. Manufacturers provide time-current level, with the only limiting factors curves for their circuit breakers, and the being the source kVA and the circuit Faulted Source engineer plots the appropriate device, impedance. The damage due to short As shown in Figure 4, with either with the chosen adjustments and indi- circuits can be catastrophic. Ground design of transfer switch when the cated operating limits, to reflect its oper- faults are currents that flow through an fault (F1) is on the normal source (line) ation in the power system. Also included unintended path (ground). They require side of the switch, the upstream circuit special consideration because they are breaker (CB1) will trip. If this occurs, on the time-current coordination curve the most common type of fault and there is a loss of the voltage source to are a one-line diagram showing all the may be below the operating level of the the switch and the transfer sequence is protective devices, their interconnection, protective device ampacity or may initiated. If the alternate source is a and the major distribution or utilization quickly increase to a higher level. All generator, the generator will be given apparatus (motors, generators, trans- three types of faults can cause damage, a start signal. When the voltage and formers, etc.); indication of the available but each has unique characteristics that frequency of that source are within an fault current at important points in the must be addressed. acceptable range, the switch will trans- power system; inrush and damage char- fer the load. In this case there is no acteristics of motors and ; “Contactor” type transfer switches sub- difference in performance between the and other appropriate information. jected to a fault are designed to remain “contactor” and “circuit breaker” type in the position they were in prior to the transfer switches. A typical example of time-current fault unless the fault conditions cause coordination is shown in Figure 3. the voltage-sensing relays to operate. Overload The 100 ampere circuit breaker B pro- “Circuit breaker” type transfer switches When the system problem is an over- tects the load and the load conductors using high withstand molded case load, the performance of both designs of and selectively coordinates with switches without instantaneous trip transfer switch is again identical. Neither upstream devices for overload and elements, such as the Type SPB Insu- transfer switch design inherently has short circuit conditions. Selective coor- lated Case Switch, perform identically overload protection, although this fea- dination for low level ground faults on to “contactor” type transfer switches. ture can be provided as a design option branch circuit B is not possible since They also remain in the position they in a “circuit breaker” type transfer circuit breaker B is not provided with were in prior to a fault. An alternate switch. For an overload of the transfer ground protection. Circuit breaker F “circuit breaker” type transfer switch switch circuit (F2), the upstream circuit is rated 600 amperes and selectively design, using molded case switches protective device (CB1) will trip and the coordinates with upstream and down- that contain high instantaneous trip switch will respond to the power outage stream devices. Circuit breaker M is elements, will trip if the fault is down- by transferring to an alternate source the 1600 ampere main breaker and stream and of sufficient magnitude. At when available. If the overload is down- coordinates with the downstream cir- this point the transfer switch will seek stream on one of the emergency feeders cuit breaker F as well as with the 200 the next available power source, with (F3), the corresponding feeder breaker ampere primary fuse A for all faults the normal source being the preferred (CB2) will trip and the load will be shed. on the load side of the circuit breaker. source. Note, however, that during Typically, no transfer of the switch The primary fuse provides short circuit such a high-magnitude fault the occurs with either design. protection for the transformer, while upstream protective device will also the secondary main breaker provides trip, causing any type of transfer switch Ground Fault the overload protection. to operate. This occurs whether the When the system problem is a ground Part IV: Transfer Switch fault, the performance of both designs Performance During a Fault of transfer switch is once again identi- UTILITY GENERATOR cal. The only exception is when a high- Proper power system design requires magnitude ground fault (F3) occurs consideration of both normal and downstream of a “circuit breaker” type abnormal operating conditions. transfer switch with instantaneous Normal operating conditions require CB1 trip elements. In this case, the system consideration of system voltage, load disturbance is similar to that of a short flow, effects of motor starting, service F1 circuit (described below), yielding the continuity, and reliability. Abnormal ATS same end result for either design of conditions require consideration of switch. But, in general, ground faults system and apparatus protection and are low-level faults because of the high minimization of service interruption. impedance of the ground return circuit. This is why special relaying is provided Several types of faults are possible: F2 to sense and protect against ground 3-phase bolted faults, phase-to-phase faults. If an upstream device (CB1) faults, double line-to-ground faults, trips due to a ground fault (F1 or F2), single line-to-ground faults, line-to- the switch will respond to the power neutral faults, and so on. outage by transferring to the alternate CB2 Three types of operating conditions source when available. If the ground are typically evaluated for purposes fault is downstream (F3) and one of F3 of equipment protection and system the emergency feeders (CB2) trips, the operation: overloads, short circuits, and load will be shed. Typically, no transfer ground faults. Both overloads and short of the switch occurs with either design. circuits involve currents in excess of Figure 4 — ATS Response to Faults

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Technical Paper Automatic Cutler-Hammer Page 6 Effective: June 2001 Transfer Switches

Short Circuit switch senses a loss of voltage and instantaneous trip setting is much The only system disturbance in which initiates a transfer to the alternate higher than that of the upstream “contactor” and “circuit breaker” type source. Because the downstream breaker (CB1) protecting the transfer transfer switches may differ in response breaker has cleared the fault, a normal switch, as seen in Figure 5. If the mag- is a high-magnitude downstream fault transfer occurs. When the normal nitude of the downstream fault current (short circuit) when the “circuit breaker” supply is restored, the transfer switch (F3) is above the instantaneous setting type switch contains instantaneous trip returns to the normal supply. The key of the upstream breaker (CB1) and elements. If the “circuit breaker” type point to note is that the transfer switch below the instantaneous setting of transfer switch uses molded case operates only for a downstream fault the molded case switch in the transfer switches without instantaneous trip in excess of about 10 times the switch, the “circuit breaker” type elements (for example, the Type SPB switch rating. transfer switch will operate in the Insulated Case Switch), there is no same manner as described above for Let us examine a “circuit breaker” type the “contactor” type transfer switch. difference in performance from that transfer switch under such a distur- of a “contactor” type transfer switch. bance (high-magnitude downstream If the magnitude of the downstream For a high-magnitude downstream fault) and follow its performance. As fault current (F3) is above the instanta- fault, a “contactor” design transfer mentioned previously, there are two neous trip setting of both the upstream switch does not change operating designs of “circuit breaker” type trans- breaker (CB1) and the molded case position unless signaled by the under- fer switches. The first are devices with switch in the “circuit breaker” type voltage relays. Upstream of the trans- no instantaneous trip elements, such transfer switch, both devices will open, fer switch is a power circuit breaker, as the type SPB design. These are along with the downstream breaker molded case circuit breaker (CB1), or switches with extremely high with- (CB2). The transfer switch will be in fused switch protecting the transfer stand ratings, and they perform exactly a position with both switches open. switch and downstream circuit. The the same as the “contactor” type There is no power source available on upstream device has an overload trip switch for all operating conditions. the normal side, since the upstream rating sized to protect the ampacity The second type uses circuit breakers normal source breaker (CB1) has of the transfer switch. If the upstream classified as molded case switches. tripped. At this point both a “contac- device opens, the transfer switch These have nonadjustable instanta- tor” type transfer switch and a “circuit will sense a loss of voltage and neous trip elements set at 10 to 13 breaker” type transfer switch sense initiate a transfer. times the frame rating of the device. a loss of voltage, start the generator, To meet the UL requirements for and initiate a transfer sequence to Short Circuit: Upstream Breaker (CB1) transfer switches, the molded case the alternate source. When a “circuit With Instantaneous Trip Elements switches are sized approximately one breaker” type automatic transfer Let us first consider the situation size larger than expected. For instance, switch transfers to the alternate where the upstream power circuit an 800 ampere transfer switch uses source, it automatically resets the breaker or molded case circuit breaker 1200 ampere molded case switches, as normal switch from the tripped-open feeding the transfer switch has an shown in Table 2. This means that the position to the normal-open position. instantaneous trip. In general, circuit breaker instantaneous trip elements that are adjustable have an adjustment 1000 range of five to 10 times the overload UTILITY GENERATOR 500 trip rating. Molded case switches 800A with nonadjustable instantaneous CB1 800A CB1 trip elements are designed to trip at F1 approximately 10 to 13 times the frame 100 800A 200A ATS rating. Lower-magnitude overcurrent CB2 T 50 conditions are protected by other trip I elements (long time, short time, and M F2 480 VOLTS ground fault) of the upstream or E 10 downstream circuit breaker after a I 200A CB2 time delay. The instantaneous trip N 5 elements act with no intentional time F3 S delay and cause an immediate outage E of the circuit when the device opens. C 1 This means that if the short circuit fault O N current magnitude is below the instan- D .5 taneous trip setting of the device (CB1) S feeding the transfer switch, the down- stream circuit breaker (CB2) should .1 clear the fault and no loss of power to .05 the switch should occur. If the magni- 800A ATS tude of the fault current is above the INST. TRIP instantaneous trip setting of the device (CB1) feeding the transfer switch, .01 .5 1 5 10 50 100 500 1000 both the downstream circuit breaker (CB2) and the upstream circuit breaker CURRENT IN AMPERES X 100 AT 480 VOLTS (CB1) will trip. At this point the transfer Figure 5 — System Response to Downstream Overcurrent at F3

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Since the downstream breaker has Table 5 — ATS Response to Faults cleared the fault, a normal transfer Location of Type of CB1 CB2 Instantaneous SPB Contactor occurs. When the normal supply is Disturbance Disturbance Response Response Trip Molded ATS Type ATS restored, both designs of transfer Case Switch Response Response switch return to the normal supply. ATS Response Note that under the above circum- stances the same end result occurs F1 Overload Trip No Start Start Start with a “contactor” type switch and Change Generator Generator Generator a “circuit breaker” type switch. The Transfer Transfer Transfer transfer switch operated only for Short Trip No Start Start Start a downstream fault in excess of about Circuit Change Generator Generator Generator 10 times the switch rating. Both the Transfer Transfer Transfer upstream and downstream devices Ground Trip No Start Start Start opened, causing the transfer switch Fault Change Generator Generator Generator to initiate a transfer sequence. A sum- Transfer Transfer Transfer mary of this discussion is shown in F2 Overload Trip No Start Start Start Table 5 and Table 6. Change Generator Generator Generator Short Circuit: Upstream Breaker (CB1) Transfer Transfer Transfer Without Instantaneous Trip Elements Short Trip No Start Start Start and With Short Time Delay Circuit Change Generator Generator Generator Let us now consider the situation where Transfer Transfer Transfer the upstream device is a power circuit Ground Trip No Start Start Start breaker with no instantaneous trip ele- Fault Change Generator Generator Generator ment or an insulated case circuit breaker Transfer Transfer Transfer with a high fixed instantaneous trip set- F3 Overload No Trip No No No ting above the available fault current. Change Transfer Transfer Transfer These devices have exceptionally high withstand ratings with short delay time Short Circuit No Trip No No No capabilities of up to 30 cycles. (Low Magnitude) Change Transfer Transfer Transfer Short Circuit Trip➀ Trip Start Start Start Withstand ratings indicate the ability of (High Magnitude) Generator Generator Generator a device to withstand the available fault Transfer Transfer Transfer current without damage. They are nor- mally expressed as a function of time, Ground Fault No Trip No No No because the device must withstand the (Low Magnitude) Change Transfer Transfer Transfer fault current until the fault is cleared. For Ground Fault Trip➀➁ Trip Start Start Start a passive device, such as cable, the fault (High Magnitude) Generator Generator Generator is cleared by an external operation. This Transfer Transfer Transfer may be the opening of an upstream or ➀ Does not trip if it is a power circuit breaker with no instantaneous trip. downstream circuit breaker or fuse. The ➁ Operates instantaneous pickup of molded case circuit breaker CB1. fault may also be cleared by the failure of the power source or by some other Table 6 — Transfer Switch Response to Downstream Overcurrent at F3 (Figure 5) means. For an active device, such as an instantaneous trip circuit breaker, the Current CB2 CB1 Molded Case Switch Contactor withstand rating is the same as the inter- Amperes Response Response Type ATS Response Type ATS Response rupting rating since the circuit breaker is 100 No Change No Change No Transfer No Transfer intended to clear the fault with no inten- tional delay. Some circuit breakers, such 500 Trip No Change No Transfer No Transfer as ANSI rated power circuit breakers 1,000 2,500 and some insulated case circuit break- 5,000 ers, have extended time withstand rat- ings. This allows them to selectively 10,000 Trip Trip Transfer Transfer coordinate while other devices clear 15,000 the fault. Products applied in electrical systems have withstand ratings associ- Note: CB1 — 800 ampere molded case breaker feeding the transfer switch CB2 — 200 ampere molded case breaker fed by the transfer switch ated with their design or the standards ATS — 800 ampere transfer switch applicable to the product. Withstand F3 — Fault on load side of CB2 ratings for each product are expressed in time and current. expected to interrupt fault current and the fault. The overload relay provides Interrupting ratings indicate the abil- generally have low interrupting rat- overload protection by opening the ity of a device to safely interrupt fault ings. Contactors are designed to inter- contactor. The contactor normally current. Some devices, such as circuit rupt load current and locked rotor interrupts only motor load current. breakers, are inherently designed to current. For example, in a combina- Its interrupting rating, based on its interrupt current and thus have high tion motor starter, the motor circuit ability to interrupt fault current, is interrupting ratings. Other devices, protector or fused switch provides the extremely low, typically 5000 such as contactors, are not normally short circuit protection by interrupting amperes for a smaller contactor.

TP.15A.01.T.E

Technical Paper Automatic Cutler-Hammer Page 8 Effective: June 2001 Transfer Switches

Automatic transfer switches have the fault current is within the published UL 1087 published withstand and interrupting rating of the transfer switch or (b) the Molded Case Switches ratings that must be complied with short delay time of the upstream power Underwriters Laboratories, Inc. for safe application. Their application circuit breaker can be set low enough to 333 Pfingsten Road requires an understanding of the protect the “contactor” design transfer Northbrook, IL 60062 operation of other devices in the switch within its withstand rating. system that can affect their ability ANSI/IEEE C37.16 to safely withstand or interrupt faults. Part V: Conclusions Low Voltage Power Circuit Breakers and AC Power Circuit Protectors — For instance, if an upstream power In all the scenarios described above — circuit breaker is applied with a setting Preferred Ratings, Related overloads, ground faults, and short Requirements, and Application utilizing the withstand capabilities of the circuits — “circuit breaker” type trans- Recommendations device (short delay time with no instan- fer switches and “contactor” type Institute of Electrical and taneous trip elements), the transfer transfer switches perform equally. Electronics Engineers switch must be rated for the fault current However, the performance of “circuit 445 Hoes Lane that the device will allow to flow. This breaker” type transfer switches is P.O. Box 1331 application of power circuit breakers typically superior for applications with Piscataway, NJ 08855-1331 is common in health care facilities and upstream power circuit breakers. data centers, where continuity of service Systems Pow-R-Breaker with “Circuit breaker” type transfer is paramount. Circuit protective devices Digitrip RMS Trip Unit

switches can be provided with over- TP15A01TE without instantaneous trip elements are Cutler-Hammer Descriptive current and ground fault trip elements applied to achieve coordination. The Bulletin 29-850 ANSI standards for the power circuit in a variety of configurations. They can be service entrance labeled and Cutler-Hammer breakers and switchgear ensure that this 1000 Cherrington Parkway is a safe and appropriate application. are available in fixed mounted or drawout construction. When the Moon Township, PA 15108-4312 In these cases, the withstand rating power switching devices are provided Transfer Switch Equipment of transfer switches must be carefully with integral overcurrent protection, Cutler-Hammer Technical Data 29-925 considered. In fact, the 3-cycle minimum they can be used in the power system Cutler-Hammer withstand required by the UL 1008 stan- for feeder protection, often eliminating 1000 Cherrington Parkway dard may not be adequate for a given the need for upstream breakers feed- Moon Township, PA 15108-4312 application. In such cases, “circuit ing the switch. For safety purposes, breaker” type transfer switch designs this option is provided with a lock- Zenith Bulletin O-5064-1,1993 offer some unique advantages because out function that prevents further Zenith Controls, Inc. of their inherent interrupting capability automatic transfer operation until the 830 West 40th Street and consequent withstand rating. appropriate source is manually reset. Chicago, IL 60609 Some circuit breaker designs, such Part VI: References Withstand and Closing Ratings for as the Cutler-Hammer Type SPB Insu- Transfer Switch Equipment lated Case Breaker, are available as NEMA ICS 10 ASCO Engineering Application molded case switches without trip Industrial Control and Systems Information units (nonautomatic switches) and have AC Transfer Switch Equipment Automatic Switch Company extremely high withstand ratings. The National Electrical Manufacturers 50-60 Hanover Road SPB Insulated Case Switch is available Association Florham Park, NJ 07932 with the ratings shown in Table 3 and 1300 North 17th Street, Suite 1847 Russelectric Automatic Transfer is applied in transfer switch designs Rosslyn, VA 22209 Switches having those same ratings. These 60 NEMA AB3 Type RMT, 1993 cycle withstand ratings allow proper Molded Case Circuit Breakers and Russelectric Inc. application of the transfer switch in Their Application South Shore Industrial Park high-magnitude fault applications National Electrical Manufacturers Hingham, MA 02043 with upstream power circuit breakers. Association Russelectric Engineering Newsletter 478 Other “circuit breaker” type transfer 1300 North 17th Street, Suite 1847 Russelectric Inc. switch designs have molded case Rosslyn, VA 22209 South Shore Industrial Park switches with instantaneous trip UL 489 Hingham, MA 02043 elements and are inherently self- Molded Case Circuit Breakers and protecting. They can also be safely Circuit Breaker Enclosures applied in systems with upstream Underwriters Laboratories, Inc. © 2001 Eaton Corporation, power circuit breakers that have 333 Pfingsten Road All Rights Reserved extended short time delays. Northbrook, IL 60062 “Contactor” type transfer switch UL 1008 designs must therefore be carefully Automatic Transfer Switches applied in systems with upstream Underwriters Laboratories, Inc. power circuit breakers. Their use 333 Pfingsten Road should be considered only when either Northbrook, IL 60062 (a) the upstream power circuit breaker has instantaneous trip elements and

Cutler-Hammer TP.15A.01.T.E Printed in U.S.A. / Z01807