Ground Fault Protection - Cooper Industries

1 106 2005 Cooper BussmannGround Fault Protection is equipment Protection from the effects of groundfaults. The National Electrical Code (NEC ) has specific Ground Fault equipment Protection requirements in , , and Fault relays (or sensors) are used to sense low magnitude groundfaults. When the Ground Fault current magnitude and time reach the relaypick up setting, the control scheme signals the circuit disconnect to Fault relays can onlyoffer Protection for equipment from the effects oflowmagnitude Ground faults. Equipment Protection against the effects of higher magnitude Ground faults is dependent on the speed of response of theconventional overcurrent protective devices (fuses or circuit breakers.)What It Is NotGround Fault Protection IS NOT: People Protection .

2 It will not prevent shock Ground Fault prevention Protection from 3-phase, phase-phase, or phase-neutral faults Protection from high level Ground faults A guarantee of a selectively coordinated system. In fact, coordinationmay be Fault relays are not simple and the ultimate reliability depends on thereliability of each element such as solid state sensor, monitor, control wiring,control power source, shunt trip, and circuit disconnecting means. If one element is incorrectly wired, inoperative, miscalibrated, or damaged, the lowlevel Ground Fault Protection may be negated. If the system neutral is incorrectlyor accidentally grounded on the load side of the sensor, a Ground Fault canhave a return path over the neutral and never trip the relay. Unfortunately, anuisance outage often encourages the building owner or maintenance crew todisconnect the Ground Fault relay so that the power stays on.

3 Ground Fault relays are not maintenance free devices. Ground Fault relayequipment relies on sensing equipment, shunt trips, switching devices, controlcircuits, etc. Complete periodic maintenance and electrical testing of theequipment by qualified personnel is necessary since it has components andmechanisms that can fail, malfunction, and/or lose Section Ground Fault Protection of EquipmentThis Section means that 480Y/277V, solidly grounded wye only connectedservice disconnects, 1000A and larger, must have Ground Fault Protection inaddition to conventional overcurrent Protection . Ground Fault Protection , how-ever, is not required on a service disconnect for a continuous process whereits opening will increase hazards. All delta connected services are not requiredto have Ground Fault Protection .

4 The maximum setting for the Ground Fault relay(or sensor) can be set to pick up Ground faults at a maximum of 1200A andactuate the main switch or circuit breaker to disconnect all phase Ground Fault relay with a deliberate time-delay characteristic of up to 1 second, may be specified, for currents greater than or equal to 3000A. (Theuse of such a relay greatly enhances system coordination and minimizespower outages). Ground Fault Protection in itself will not limit the line-to- Ground or phase-to-phase short-circuit current. When non-current-limiting mechanical protectivedevices such as conventional circuit breakers are used with GFP, all of theavailable short-circuit current will flow to the point of Fault , limited only by circuit impedance. Therefore, it is recommended that current-limiting overcur-rent protective devices be used in conjunction with GFP system offers:1.

5 Some degree of arcing and low magnitude Ground Fault Protection bythe Ground Fault relay operating Current-limitation for high magnitudeground faults and short circuits by current-limiting fuses, which providescomponent Protection for system offers:1. Some degree of arcing and low magnitude Ground Fault Protection bythe Ground Fault relay operating the circuit :This system DOES NOT providecurrent-limitation for high magnitudeground faults and short Fault ProtectionRequirements480V/ 277 VSix Service Disconnects800 Amps or Less480V3 3 WDeltaAny SizeService DisconnectService Disconnectless than 1000 Amps480Y/277V208Y/120 VAny SizeServiceDisconnect480 Amp orLarger SwitchCurrent-Limiting FusesSWBDSWBD480 AmpCircuit Breakeror LargerWhere Ground Fault Relays are NOT RequiredThere are many services and feeders where , , and donot require Ground Fault Protection including:1.

6 Continuous industrial process where a non-orderly shut down would increase All services or feeders where the disconnect is less than 1000 All 208Y/120 Volt, 3 , 4W (wye) services or All single-phase services or feeders including 240/120 High or medium voltage services or feeders. (See NEC and forfeeder requirements.)6. All services or feeders on delta systems (grounded or ungrounded) such as 240 Volt, 3 , 3W delta, or 240 Volt, 3 , 4W delta with midpoint Service with six disconnects or less ( ) where each disconnect is less than1000 amps. A 4000A service could be split into 5 - 800A Resistance or impedance grounded Fire Pumps10. For feeders where Ground Fault Protection is provided on the service (except forHealth Care Facilities. See )For instance, Ground Fault relays are not required on these 2005 Cooper Ground Fault Protection of EquipmentEquipment classified as a feeder disconnect must have Ground Fault protectionas specified in Fault ProtectionRequirementsCOMPLIANCE480Y/277 VFeeder of any ratingno Required(Except Per Article 517)

7 VoltageServiceBuilding A Service800A480Y/277 VBuilding B Service1000A or Greater480 RequiredVIOLATIONCOMPLIANCEHigh VoltageService 4160V480Y/277 GreaterHigh VoltageService 4160V480Y/277 VFeeder GreaterTIME IN SECONDS1, IN AMPS100,00080,00060,00010,00040,00030,00 020,0008,0006,0004,0003,0002,0001,000800 600400300200100 MAIN GFR1200 Amp12 CyclesMinimum6 Cycle SeparationFEEDER GFR800 Amp2 CyclesMAIN GFRFDR GFR480 will not be required on feeder equipment when it is provided on the supply side of the feeder (except for certain Health Care facilities requirements, Article 517).This requirement for GFP on feeders may subject the system to blackouts dueto downstream Ground faults as discussed previously. A selective coordinationanalysis is required to assure that needless blackouts do not Ground Fault Protection of EquipmentEquipment Ground Fault Protection of the type required in is nowrequired for each disconnect rated 1000A or more, 480Y/277V systems, thatwill serve as a main disconnect for a separate building or structure.

8 Refer and :GFP without current-limitation may not protect system components. and (FPN).This requirement for GFP on equipment may subject the system to blackoutsdue to downstream Ground faults, as discussed previously. A selective coordination analysis is required to assure that needless blackouts do Ground Fault Protection For Healthcare FacilitiesIf Ground Fault Protection is placed on the main service of a health care facility, Ground Fault relays must also be placed on the next level of feeders. The separation between Ground Fault relay time bands for any feeder and mainground Fault relay must be at least six cycles in order to achieve coordinationbetween these two Ground Fault relays. In health care facilities where noground Fault relay is placed on the main, no Ground Fault relays are required onthe feeders.

9 Therefore, if the requirements of do not require a groundfault relay and no Ground Fault relay is placed on the main service disconnect,then no Ground Fault relays are required on the feeders either (unless requiredby Sections and ).A Ground Fault relay time band includes the disconnect operating time and anytolerances in the relay control Facility1. When a Ground Fault relay is placed on the main service of a health care facilitythen,2. Ground Fault relays must also be placed on the feeders, and the feeder groundfault relay time band must have a 6 cycle separation from the main Ground :Merely providing coordinated Ground Fault relays does not prevent amain service blackout caused by feeder Ground faults. The overcurrent protective devices must also be selectively coordinated.

10 The intent of isto achieve 100 percent selectivity for all magnitudes of Ground Fault currentand overcurrents. 100% selectivity requires that the overcurrent protectivedevices also be selectively coordinated for medium and high magnitudeground Fault currents because the conventional overcurrent devices may operate at these 2005 Cooper BussmannAnalysis of Ground Fault Relay Curves and Overcurrent Device CurvesTo a fuse or circuit breaker, Ground Fault current is sensed just as any othercurrent. If the Ground Fault current is high enough, the fuse or circuit breakerresponds before the Ground Fault relay (this depends on the Ground Fault relaysetting, overcurrent device characteristics, speed of response of the overcurrent device, and Ground Fault current magnitude).