2 OverviewRecent changes to the National Electrical Code (NEC) require the selective coordination of overcurrent protective devices at hospitals and other mission-critical facilities.Transfer switches with 30-cycle closing and withstand ratings dramatically simplify designing to that requirement.
3 Transfer Equipment in a Common Scenario CommercialUtility PowerUPSUPS BatteriesNetworkComputerLoadsTransfer SwitchgearEmergencyGeneratorsGenerator ParallelingControl SwitchgearAir conditioning, Lighting,Mechanical, Building Loads,etc.With regard to the emergency back-up and transfer scheme, it is incumbent upon engineers to select the proper equipment for the application. There are many considerations and they are becoming more with each decade.
4 What is Selective Coordination? Definition (Article 100 – NEC)Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings.Article 100 provides the Code definition. Here is another way to describe it:“For the full range of possible overcurrents, the act of isolating an overloaded or faulted circuit from the remainder of the electrical system, thereby eliminating unnecessary power outages.”The circuit causing the overcurrent is isolated by the selective operation of only that overcurrent protective device which is closest upstream to the overcurrent condition.
5 Selective Coordination, History & Requirements Selective coordination was first required by the NEC in 1993 for elevator circuits. Amendments to the Code in 2005 and 2008 strengthened the requirements and expanded them to include emergency and legally required standby systems, as well as critical operations power systems. Selective coordination, as defined in the 2008 NEC, is the (as in previous slide) “localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings.” It is a complicated process of coordinating the ratings and settings of overcurrent protective devices, such as circuit breakers, fuses, and ground fault protection relays, to limit overcurrent interruption (and the resultant power outages) to the affected circuit or equipment (the smallest possible section of a circuit). In other words, the only overcurrent protective device that should open is the device immediately “upstream” from the circuit/equipment experiencing an overcurrent condition.
6 Proper Selective Coordination is becoming more and more of an engineering consideration and is being enforced by inspectors more & more often…Refer to IAEI handout “Selective coordination restricts outages to the circuit or equipment affected,ensuring reliability of electrical power.”
7 NEC 2008 –Verbiage on Selective Coordination NEC(2008) Coordination: requires “Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.”NEC(2008) Coordination: requires “Legally required standby system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.”NEC(2008) Application of other articles: requires “The essential electrical system shall meet the requirements of Article 700.”The overcurrent protective devices may include the following:Molded Case Circuit BreakersFused devicesInsulated Case Circuit BreakersAir Power Circuit breakers
8 More on Selective Coordination RequirementsSelective coordination requirements for life safety are not a new concept for the Code. There has been a Code requirement to coordinate selectively the over-current protective devices for elevator circuits since 1993.Most engineers agree this is the simplest way to assure coordination, however…..BreakersInstantaneous circuit breakers will not coordinate properly because typically, they aren’t adjustable.Fuses
9 One-lineGUtility4000AAPCBAPCB's (Air Power Circuit Breaker) are typically 30 cycle withstand devices.1600AAPCB1600AAPCBICCB's (Insulated Case Circuit Breaker) are 30 cycle withstand or up to 4 Cycle Instantaneous.ATS800AICCBMCCB's (Molded Case Circuit Breaker) typically instantaneous or Current Limiting Devices.400AMCCBAn overcurrent event (overload, short circuit, or ground fault) here should trip the 400A MCCB
11 Selective Coordination GUtility4000AAPCB1600A800AICCB400AMCCBIn the absence of other means to satisfy selective coordination, the ATS must withstand a fault or even close on potential fault to be properly coordinated.ATSATSFault on load side of ATS could see up to 30 cycles of fault current -depending on the Air Power Circuit Breaker settings that is feeding it- and could travel through the ATS and the ATS contacts.If the 400A MCCB does not trip/clear…
12 Review of Code Requirements Article 517 Health Care FacilitiesApplication of Other ArticlesArticle 620 Elevators, etcSelective Coordination (2008)Article 700 Emergency Systems700.9 (B)(5)(b), ExceptionArticle 701 Legally Required Standby SystemsCoordinationArticle 708 Critical Operations Power SystemsSelective Coordination
14 State Adopted Unincorporated Areas 2008 Code AdoptionWAMEMTNDM IVTMNNHORNYMAWIIDSDCTRIM IWYNJPANBIAExpected July 2010OHMDDENVINILWVExpected July 10’UTCOVACAKSMOKYExpected January 2011NCTNState AdoptedARSCAZNMOKS. Carolina Code Council adopted 2009 IRC with 2008 NEC 3/22/10 with implementation effectiveTXMSALGALAState Adopted Unincorporated AreasAKAKFLHIHI, basically 2002 NEC but some islands back to 1993 NEC2008 NEC – 32 States2005 NEC – 8 StatesLocal Adoption – (10)Revised April 19, 2010Note: Some local adoption states have earlier than 2005 adoptions in some jurisdictions
15 Code Rulings Panel 13 Statement: Panel 20 Statement: In the 2008 Code Cycle there were challenges to the selective coordination requirement. Proposal proposed the elimination of the selective coordination requirement forThe proposal was to remove the selective coordination requirement from the mandatory text and places it in anon-mandatory in a FPN (fine print note).But Code Panel 13 rejected this proposal by a vote of 9-4. To follow is their statement:Panel 13 Statement:“This proposal removes the selective coordination requirement from the mandatory text and places it in a non-mandatory FPN (fine print note). The requirement for selective coordination for emergency system over-current devices should remain in the mandatory text. Selective coordination increases the reliability of the emergency system. The current working of the NEC is adequate. The instantaneous portion of the time-current curve is no less important than the long time portion. Selective coordination is achievable with the equipment available now”.Then, Code Panel 20, which was responsible for the new Article 708, summed up the need for selective coordination in their statement to Comment 20-13, (which was another proposal for the deletion of the selective coordination requirement).This comment was rejected The actual panel statement to Comment 20-13:Panel 20 Statement:“The overriding theme of Articles 585 (renumbered to 708) is to keep the power on for vital loads. Selective coordination is obviously essential for the continuity of service required in critical operations power systems. Selective coordination increases the reliability of the system.”
16 Exceptions to Code Rulings Refer to IEEE handout “Selective Coordination versus Arc Flash…” page 12 There are numerous proposals being adopted by States and/or City or local governmental bodies which modify the selective coordination requirements.The most commonly heard proposals fall into two categories:1. Allow the degree of selective coordination needed to be the responsibility of the qualified person responsible for the project.(The Commonwealth of Massachusetts was the first State to adopt such a proposal as an exception to the Articles in , and , which require selective coordination as follows:Exception No. 2: Where the system design is under the control of a licensed professional engineer engaged in the design or maintenance of electrical installations, the selection of overcurrent protective devices shall be permitted to coordinate to the extent practicable. The design shall be documented, stamped by the professional engineer, and made available for review by the authority having jurisdiction.
17 Exceptions to Code Rulings (cont.) 2. Proposals to modify the NEC requirement for selective to only be required for above a specific time. The leading proposal is 0.1 seconds (6 cycles) and above. The State of Oregon recently adopted a proposal submitted by the National Electrical Contractors Assoc., Oregon Pacific Cascade Chapter, as Statewide Alternate Method No. OESC applying to Articles in , and This states the following: “The requirements in NEC , and for selective coordination may be demonstrated by providing a selective coordination study utilizing trip-curve data in the range of 0.1 seconds or more.Substantiation for this proposal included:1). “…selective coordination is not always possible or practical for all fault current levels when protection is provided by MCCB’s. The requirement for “total” selective coordination means that over current protection devices must be coordinated for all faults, regardless of their magnitude or duration, including the most extreme case, the bolted fault. However, bolted three phase faults which rapidly generate extremely high current in the instantaneous range rarely occur in practice, except at start-up when interruption of power due to a lack of coordination is not likely to compromise safety...” “In order to achieve total short circuit selective coordination, the size of upstream overcurrent protective devices may need to be increased and/or time delay trip characteristics increased, thereby possibly increasing the arc flash hazard.”“Findings: By omitting the instantaneous range from the requirements for selective coordination, reasonable and affective safety can (still) be achieved. Signing supervisors and engineers can use readily available and published time current curves to determine if a system is selectively coordinated to a substantial degree without having to relay on unregulated manufacturer testing data and inconsistent engineering and design practices.”
18 Arc Flash Considerations Refer to IEEE handout “Selective Coordination versus Arc Flash…” page 10 This is the other side of the argument regarding the subject of Selective Coordination VS Arc Flash Considerations.The presenter will not delve into this side of the argument, as he is in the business of providing emergency powerto critical facilities and therefore is in the camp of having a non-sensitive, robust type system, selectively coordinated,that facility managers want to perform well when called upon. In cases of catastrophic outages, Arc flashconsiderations might take a back seat to keeping as much of the facility up and running as possible and onlyTripping CB’s closest to the fault.For more details on the ARC Flash concerns, and that whole side of the argument, please refer to your handout.
19 UL 1008 Withstand Test34.1 When tested under the conditions described in 34.2 – 34.15, a transfer switch shall withstand the designated levels of current until the over-current protective devices open or for a time as designated in At the conclusion of the test:The switch shall be capable of being operated by its intended means;The fuse mentioned in shall not open,There shall be no breakage of the switch base to the extent that the integrity of the mounting of live parts is impaired,The door shall be prevented by its latch, without bolt or lock installed therein, from being blown open, and deformation of the door alone is not determined to be unacceptable;No conductor shall have pulled out of a terminal connector and there is no damage to the conductor insulation or the conductor (see 41.56); andFor a plug in or draw out unit, the point of contact is to be the same both mechanically and electrically as before the test.
20 UL 1008 Closing Test36.1 When tested in accordance with 36.2, a transfer switch shall comply with the requirements in 34.1(a) –(f).36.1 Revised September 18, 199636.2 The sample for this test is to be that used for the withstand test. Test procedures and conditions for the closing test are to be as described in 34.3 – The switch is to be closed on the circuit.36.3. The test (for close on) current shall be the same as that used in the withstandtest.
21 UL 1008 Short Circuit Test History Around 1989 UL introduced an optional 3 cycle test for any over-current protection device.Prior to this, manufactures could test with any over-current device.If a manufacturer didn’t test to 3 cycles, they would be required provide a label that lists all breakers that the switch was “coordinated with”.This requirement did not take into consideration air power circuit breakers APCB’s. Some of these breakers were 4-5 cycle devices (GE AKR and Westinghouse DS)January 9th, 2002 UL introduced an optional short time currentrating test.A withstand and a close and withstand test is required to get a UL short time rating.This requirement did not take into consideration air power circuit breakers APCB’s. Some of these breakers were 4-5 cycle devices (GE AKR and Westinghouse DS)
22 UL 1008 Short Time Current Test 36A.1 A switch marked with a short-time current rating in accordance with shall be tested under the conditions described in 36A.2 -36A.12 and shall withstand the short-time current for the period specified. At the conclusion of the test:The transfer switch shall be capable of being operated by its intended means,The fuse mentioned in 36A.7 shall not open,There shall not be any damage to the switch base to the extent that the integrity of the mounting of live parts is impaired,The door shall be restricted by its latch, without bolt or lock installed therein, from being blown open. Deformation of the door itself is not reason for rejection,No conductor shall have pulled out of a terminal connector and there shall not be any damage to the conductor insulation or the conductor (see 41.56),For a plug-in or draw-out unit, the point of contact shall be the same both mechanically and electrically as before the test,The Temperature Test, Section 29, shall be performed on the transfer switch at the completion of the tests described in 36A.8 and 36A.9, without maintenance, and the temperature rise shall not exceed the values given in Table 29.1, increased by 10° C or 18° F, andThe Dielectric Voltage-Withstand Test (Repeated), Section 36B, shall be performed on the transfer switch at the completion of the tests described in 36A.8 and 36A.9.
23 UL 1008 Overload Test Table 28.1 Overload Test Transfer switch equipment shall perform in an acceptable manner, as intended by the manufacturer, when subjected to an overload test consisting of the number of operations specified in Table 28.1, controlling a test current as described in Table 28.2.Table 28.1 Overload TestSwitch rating, amperesNumber of cycles of operationRate of Operation*0-3002501 and above502531 per minute1 per 2 minutes1 per 3 minutes1 per 4 minutes1 per 5 minutesTable 28.2 Method of determining test current for overload tests on transfer switchesDevice used forDevice rated in amperesPower test currentFactorMotor loads or total systema-c6 times rated current28.4 A cycle is defined as making and breaking the required test current on both the normal and alternate contacts. During the test, the alternate source shall be displaced 120 electrical degrees from the normal source for a 3 phase supply or 180 electrical degrees for a single phase supply.28.6 The minimum on time in each contact position is to be 1/6 second (ten electrical cycles based on a 60Hz source), unless automatic tripping of the over-current device occurs.
24 Method of determining test current for endurance tests UL 1008 Endurance Test30.1 A transfer switch shall perform as intended when subjected to an endurance test controlling a test current as described in Table 30.1 and at a rate and number of cycles described in Tables 30.2 and 30.3.Table 30.1Method of determining test current for endurance testsThe test cycle is to be 1 second “on” and 59 seconds “off”. A controller may be operated at a rate of more than 1 cycle per minute if synthetic loads are used or if a sufficient number of banks of lamps controlled by a each bank will cool for at least 59 seconds between successive applications of current.Table 30.2Endurance test cycles for emergency system switches including legally required stand-by systems.
25 UL 1008 Temperature Test29.1 Transfer switches when tested under the conditions described in 29.2 – shall not attain a temperature at any point high enough to constitute a risk of fire or to damage any materials employed in the device, and shall not show temperature rises at specific points greater than those indicated in Table 29.129.2 For the temperature test the transfer switch is to be operated under intended use conditions and is to carry its test current continuously at the test potential specified in Table 18.104.22.168 The test current shall be 100 percent of the rated current.
26 Overcurrent Protective Devices Molded Case Circuit Breakers –MCCB (UL489)May be Current Limiting to 200KALong Time OvercurrentInstantaneous Interruption is less than 3 cyclesFuses and Fused DevicesCurrent LimitingMostly used on 200KA circuitsInsulated Case Circuit Breakers -ICCB (UL489)Instantaneous Interruption is typically less than 4 cyclesShort Time delay available (30 cycles) with Instantaneous over-rideLow Voltage Air Power Circuit Breakers -APCB (UL1066)Short Time delay available (30 cycles) without Instantaneous
27 Low Voltage Air Power Circuit Breakers APCB’s are ideal protective devices for the application of selective tripping.Short Circuit Duty Cycle: Oc,15 s – CO (applying fault currentto a closed CB for ½ second [30 cycles] separated by 15 secondsof zero current flow, then close on fault current for another ½second [30 cycles] ).This test may be performed with or without an instantaneous override on the closing cycle.The GE AKR was tested without the instantaneous. Note some breakers now have a “Trip Free” feature in which the breaker will still clear a fault without instantaneous trip. Opinions vary on whether this is desirable or not in emergency power systems.Short Time CurrentANSI C (2)-1990Short-Time Current Duty Cycle Application. The applicable short-time current duty cycle for unfused circuit breakers consists of two periods of 1/2s current flow, separated by a 15 s interval of zero current.
28 Selective Coordination - Good No overlapping fault current of individual devices.This is coordinated properly.In a perfect world this is great.
29 Selective Coordination - BAD ATS Feeder BreakerIn this case, since it takes 8 cycles for the upstream breaker to clear the fault, a 3 cycle rated transfer switch is inadequate.8 cycles to clear
30 Complete Coordination A 30 cycle UL rated Transfer Switch truly gives you complete coordination with any over-current protective device.
32 ASCO Non UL 100-400a 30k@30cycles 600-800a 42k@30cycles Short Time Close & Withstandaaaa4000a** ATS only68. With fuses only
33 News regarding Cummins Power Generation 2009 News ReleasesJune 12, 2009Cummins Power Generation Transfer Switches First With UL-Listed 30Cycle RatingsMINNEAPOLIS, MINNESOTA – Series OHPC and CHPC PowerCommand® automatic transfer switches from Cummins Power Generation Inc. are the industry's first transfer switches to achieve UL-listed 10-cycle and 30-cycle withstand and closing (short-time) ratings.The OHPC open transition and CHPC closed transition switches demonstrated unprecedented short-time ratings of 25 kA at 10 cycles for 125 to 260-amp switches, 30 kA at 30 cycles for 300 to 600 amps, and 50 kA at 30 cycles for 800-amp switches in UL tests. The switches continued to operate safely at full load, even after testing.The UL listing provides consulting and specifying engineers independent assurance that the OHPC and CHPC transfer switches offer the industry's highest performance level, particularly for standby power systems requiring selective coordination.As defined by the 2008 edition of the National Electrical Code (NEC), selective coordination is mandatory for emergency and legally required electrical systems in buildings where life safety is paramount, including hospitals, health care facilities, emergency shelters and high-rise buildings with multiple elevators. NEC 2008 also requires selective coordination for critical operation power systems (COPS) in secure buildings such as banks, data centers, embassies and government offices.Selective coordination localizes an overcurrent condition to restrict outages to the circuit or equipment being affected. It is achieved by selecting circuit breakers and transfer switches with timing and withstand characteristics that delay or prevent faults from tripping upstream overcurrent protection devices. Selective coordination requirements are more easily satisfied by specifying OHPC or CHPC switches rather than products without UL-listed short-time ratings.The ability of CHPC and OHPC transfer switches to withstand fault current for up to 30 electric cycles (one-half second) gives the consultant the flexibility to adjust the instantaneous time delay on the ATS overcurrent protection device to prevent upstream 2breakers from tripping unnecessarily. Both transfer switches feature Cummins Power Generation's innovative High Endurance8/12/2009Page 2Mechanism (HEM), designed to ride through a fault condition undamaged and retain itscapability to carry 100 percent of the rated load. Magnetic forces developed during a faultcause a typical transfer switch's contacts to blow open, producing an electrical explosion thatoften results in extensive internal damage to the switch, requiring replacement of contacts,arc chutes and, in some cases, the controller.In contrast, the HEM uses that same magnetic energy to hold the contacts closed during afault, virtually eliminating arcing, contact damage and performance degradation. It cansurvive multiple faults of the specified magnitude - listed on the nameplate as the withstandand closing current ratings (WCR) - and continue to carry the rated current withoutoverheating. This proprietary Cummins Power Generation technology means that there willbe no costly repairs or inconvenient downtime after a fault."Cummins Power Generation is proud to be the first manufacturer of transfer switchesproven to survive a fault condition and to continue to operate at full load without repair," saidRich Scroggins, ATS product manager of Cummins Power Generation. "This ability givesconsulting engineers more design flexibility in addressing requirements, and potentially tolower costs by using fewer transfer switches.“Underwriters Laboratories (UL) is a leading safety testing and certification organization thathas conducted product safety testing for nearly 115 years. On electrical products, the ULmark designates products that have been certified for safety regarding foreseeable hazardsthat include electric shock, fire and mechanical hazards.Although Cummins Power was first to the market with some sizes, this is all they’ve published so far.
34 Switch Current Ratings So we’ve distilled the info and put the values into a table.PowerGenerationThe OHPC open transition and CHPC closed transition switches demonstrated unprecedented short-time ratings of 25 kA at 10 cycles for 125 to 260-amp switches, 30 kA at 30 cycles for 300 to 600 amps, and 50 kA at 30 cycles for 800-amp switches in UL tests. The switches continued to operate safely at full load, even after testing.Switch Current RatingsShort Time RatingsTransfer SwitchesBypass SwitchesDurationCyclesa25 ka10a30 ka30800a50 kaaShort Timeaaaa4000a** ATS onlyOnly the 800awe don’t matchNo Mention in the press release of bypass switches
36 Contactor based transfer switches. Same type we make EATON CUTLER HAMMERContactor based transfer switches.Same type we makeNo withstand values based on how many cycles??While inconclusive as to whether these type switches carry a UL 30 cycle close and withstand rating, it doesn’t seem as if they do.
37 Circuit Breaker based transfer switches. EATON CUTLER HAMMERCircuit Breaker based transfer switches.First mention of 3 cycle rating but where is the 30 cycle listing ??While inconclusive as to whether these type switches carry a UL 30 cycle close and withstand rating, it doesn’t seem as if they do.
38 Magnum Circuit Breaker based transfer switches. EATON CUTLER HAMMERMagnum Circuit Breaker based transfer switches.First mention of 30 cycle rating but only when used with upstream fuses ??While inconclusive as to whether these type switches carry a UL 30 cycle close and withstand rating, it doesn’t seem as if they do without the inclusion of upstream fuses.
39 To Our Knowledge, they presently publish nothing other G.E. ZenithTo Our Knowledge, they presently publish nothing otherthan 3 cycle ratings.
40 New Russelectric 30 Cycle Automatic Transfer Switches and Bypass Isolation Switches
42 BACKPLATE: SIDE BARRIER: -increased thickness to 1.25” Back Plate AssemblyBACKPLATE:-increased thickness to 1.25”-added strength and stabilitySIDE BARRIER:5/8” thk glass polyesterGreater arc & track resistanceExcellent flame resistanceMovable contact support
43 Contact Design: Blow-off vs. Blow-on Blow-Off Contact Design:- Used for Interrupting High Fault Currents- Magnetic forces push contacts open- Used for 3 cycle devices- Contact springs - only force to withstand faultCurrent PathMovable Main ContactStationary ContactMovable Arcing ContactBlow Open ForceMovable and Arcing Contact Springs
44 Contact Design: Blow-on - Used for withstanding High Fault Currents- Magnetic forces from fault increase pressure on contactsOffset hinge point allows for rotation toward contactsfor blow on effectContact SpringsHinge PointForceMain ContactCurrent PathStationary Contact
45 Contact Comparison: 3 Cycle vs. 30 Cycle Russelectric 3 Cycle DesignBlow-Off DesignSingle Arcing ContactMultiple Contact Fingers depending onamperageMain Contact Pad material:AgWC50 – Silver Tungsten CarbideTungsten to reduce erosionArcing Pad Material:AgW73 – Silver TungstenStationary Contact Pad material:Contacts Rotate on Copper HingeBlock and Pin AssemblyMetal Contact Holder
46 30 Cycle Design Blow-On Design Arcing Contact Designed into each main contact – CopperMultiple Contact Fingers depending onamperageMain Contact Pad material:AgWC40 – Higher Silver Contact toprevent overheatingStationary Contact Pad material:AgC4 – 96% Silver, graphite toprevent welding during withstandFlexible Braided connectors –prevents overheating and hot spotsBrush Movement in Main Contacts –Cleans contact pad every operationMolded Contact Holder- Contains arc- BMC thermoset material- Withstands heat- Great arc and track resistance
47 CROSSARM MECHANISM Made from 1.125 Square Steel Stock Overcenter Spring Mechanism to Latch Contacts Closed and OpenUtilizes same mechanics as the 3 cycle switch – Heavier spring
48 OPERATORS Open Transition Switches with EMO Reliability of Motor Operators
49 30 CYCLE BYPASS SWITCHES Major Design Changes Elimination of Isolation HandleGearbox, Rack-in Mechanism to engage switchBottom and Side Guiderails to align and Contain SwitchSecondary Disconnects accessible on Left side of cabinetOptional Shutter Design800A Cradle is ‘On the Ground’ Rollout DesignComplete Finger Cluster Redesign for all Sizes
50 Removal of Isolation Handle Single Handle is for Bypass Operation800A Rollout SwitchCradle rolls out on Ground – not on rails
51 Gearbox Rack-in Mechanism Access Rack-in Shaft through door - only in Bypass ModePosition Indicator Window- Connected- Connected – Bypassed- Test- IsolatedGearbox needed for Increased Spring Pressure- Must Pass the “Liz Test”
52 Secondary Disconnect Located on left Side of Cubicle for accessibility Allows for Test PositionIncorporated into Side Guide-Rail
53 Guide Plates Used for Left to Right Alignment Prevents Rollout Switch from jumping or shifting during fault
54 Shutter Design (Optional) Shutter Closed (switch in test position or isolated)Shutter Open (switch racked-in)
55 Finger Clusters Added Spring Pressure for Clamping Increased Contact Surface AreaWithstood 100KA for 3 Cycles and 85KA for 30 Cycles - without a scratch2500A Cluster800A Cluster