power distribution - Siemens

18TECHNICALTechnicalGround Fault ProtectionNEC Requirements for Ground FaultProtection230.95 Ground-Fault Protection ofEquipment. Ground-fault protection ofequipment shall be provided for solidlygrounded wye electrical services ofmore than 150 volts to ground but notexceeding 600 volts phase-to-phase foreach service disconnect rated 1000amperes or more. The groundedconductor for the solidly grounded wyesystem shall be connected directly toground without inserting any resistor orimpedance device.The rating of the service disconnect shallbe considered to be the rating of thelargest fuse that can be installed or thehighest continuous current trip settingfor which the actual overcurrent deviceinstalled in a circuit breaker is rated orcan be adjusted.Exception No. 1: The ground-faultprotection provisions of this section shallnot apply to a service disconnect for acontinuous industrial process where anonorderly shutdown will introduceadditional or increased hazards.Exception No. 2: The ground-faultprotection provisions of this section shallnot apply to fire pumps.(A) Setting. The ground-fault protectionsystem shall operate to cause theservice disconnect to open allungrounded conductors of the faultedcircuit. The maximum setting of theground-fault protection shall be 1200amperes, and the maximum time delayshall be one second for ground-faultcurrents equal to or greater than 3000amperes.(B) Fuses. If a switch and fusecombination is used, the fuses employedshall be capable of interrupting anycurrent higher than the interruptingcapacity of the switch during a time thatthe ground-fault protective system willnot cause the switch to open.(C) Performance Testing. The groundfaultprotection system shall beperformance tested when first installedon site. The test shall be conducted inaccordance with instructions that shallbe provided with the equipment. Awritten record of this test shall be madeand shall be available to the authorityhaving jurisdiction.(FPN No. 1): Ground-fault protection thatfunctions to open the service disconnectaffords no protection from faults on theline side of the protective element. Itserves only to limit damage toconductors and equipment on the loadside in the event of an arcing groundfault on the load side of the protectiveelement.(FPN No. 2): This added protectiveequipment at the service equipment maymake it necessary to review the overallwiring system for proper selectiveovercurrent protection coordination.Additional installations of ground-faultprotective equipment may be needed onfeeders and branch circuits wheremaximum continuity of electrical serviceis necessary.(FPN No. 3): Where ground-faultprotection is provided for the servicedisconnect and interconnection is madewith another supply system by a transferdevice, means or devices may beneeded to ensure proper ground-faultsensing by the ground-fault protectionequipment.(FPN No. 4): See 517.17(A) forinformation on where an additional stepof ground fault protection is required forhospitals and other buildings with criticalareas or life support equipment.517.17 Ground-Fault Protection.(A) Applicability. The requirements of517.17 shall apply to hospitals and otherbuildings (including multiple occupancybuildings) with critical care areas orutilizing electrical life support equipment,and buildings that provide the requiredessential utilities or services for theoperation of critial care areas or electricallife support equipment.Selection(B) Feeders. Where ground-faultprotection is provided for operation ofthe service disconnecting means orfeeder disconnecting means as specifiedby 230.95 or 215.10, an additional stepof ground-fault protection shall beprovided in all next level feederdisconnecting means downstreamtoward the load. Such protection shallconsist of overcurrent devices andcurrent transformers or other equivalentprotective equipment that shall causethe feeder disconnecting means to open.The additional levels of ground-faultprotection shall not be installed asfollows:(1) On the load side of an essentialelectrical system transfer switch(2) Between the on-site generatingunit(s) described in 517.35(B) andthe essential electrical systemtransfer switch(es)(3) On electrical systems that are notsolidly grounded wye systems withgreater than 150 volts to groundbut not exceeding 600 voltsphase-to-phase.(C) Selectivity. Ground-fault protectionfor operation of the service and feederdisconnecting means shall be fullyselective such that the feeder device,but not the service device, shall open onground faults on the load side of thefeeder device. A six-cycle minimumseparation between the service andfeeder ground-fault tripping bands shallbe provided. Operating time of thedisconnecting devices shall beconsidered in selecting the time spreadbetween these two bands to achieve100 percent selectivity.(FPN): See 230.95, fine print note, fortransfer of alternate source whereground-fault protection is applied.(D) Testing. When equipment groundfaultprotection is first installed, eachlevel shall be performance tested toensure compliance with 517.17(C).18-12Siemens Power Distribution & Control, SPEEDFAX 2007-2008 Product Catalog