Eaton Magnum DS Technical Data Manual

Also see for Magnum DS: Instruction leaflet Instruction booklet Instructions Instructional Leaflet

Technical Data Effective: May 1998Page 16TD.44A.01.T.ECutler-HammerMagnum DSMetal-EnclosedLow-Voltage SwitchgearSince ungrounded and resistancegrounded systems produce minimalground current, no damage occurs tothe grounded equipment. Theseground currents are also too low fordetection by integral trip unit groundelements, therefore serve no groundfault tripping function if applied onthese systems. Ground fault elementson these types of systems can, how-ever, provide supplemental protec-tion. If a second ground occurs onanother phase, and exceeds theground element pickup setting, theground element can serve as a moresensitive short delay trip.Ungrounded or resistance groundedsystems can not be applied as 4-wirenetworks. Even if supplied from a 4-wire source, no line-to-neutral loadsmay be served. These applications arelimited to 3-wire distribution systemsonly.Need For Ground Fault ProtectionIf the magnitude of all ground currentswould be large enough to operate theshort delay or instantaneous elementsof the phase overcurrent trip devices,there would be no need for separateground fault protection on solidlygrounded systems. Unfortunately,because low magnitude ground cur-rents are quite common, this is not thecase. Low level ground currents canexist if the ground is in the winding ofa motor or a transformer, or if it is ahigh impedance ground. Low levelground currents may also be due to anarcing type ground. The arcing typegrounds are the source of the mostsevere damages to electrical equip-ment. The lower limit of the arcingground currents is unpredictable andthe magnitude may be considerablybelow the setting of the breaker phaseovercurrent trip devices. It is for thisreason that the National Electric Code,and UL, require ground fault protectionfor all service disconnect breakersrated 1000 amperes and greater,applied on systems with greater than150 volts line-to-ground.Since the breaker phase overcurrenttrip devices cannot provide sensitiveenough protection against low magni-tude ground faults, there is a need foran additional protective device. Thisadditional device is not to operate onnormal overloads and it is to be sensi-tive and fast enough to protect againstlow magnitude grounds. It is alsoimportant that this additional groundprotecting device be simple and reli-able. If the Magnum DS breaker solid-state tripping system including anoptional “ground element” is selected,good ground fault protection will beassured.The Ground ElementThe ground element of the solid-statetrip unit is in addition to the usualphase protection. The ground elementhas adjustable pickup with calibratedmarks as shown in Tables R8 and R9and adjustable time delay. The inputcurrent to the trip unit can be providedby:(a)Residual connection of phase sen-sors, with the residual circuit con-nected to the ground elementterminals. This is the Magnum DSLow-Voltage Switchgear standardground protection system for 3-wiresystems. On 4-wire systems, stan-dard ground fault protectionincludes a fourth “neutral sensor.” Itis connected to vectorally subtractfrom the residual current of thephase sensors. Its only function is tosense neutral currents. It does notsense ground current. These sys-tems produce pickup values asshown in Table R8.(b)External ground sensing currenttransformers connected to theground element terminals. Thismeans that this external ground sen-sor will trip the breaker whenever itssecondary output current exceedsthe values shown in Table R8.Tripping is independent of phasecurrents. The lower the CT ratio,the more sensitive the ground faultprotection.Ground Fault Protection Applicationand CoordinationIn all power systems, continuity of ser-vice is very important. For reliable ser-vice continuity, selective tripping isapplied between main, tie, and feederbreakers, and downstream protectingdevices, for phase-to-phase faults.Similar selective tripping is desirablewhen breakers trip on grounds. Theapplication of ground protection onlyto main breakers may assure goodground protection. However, it will notprovide good service continuitybecause the main breaker will trip ongrounds that should have been clearedby feeder breakers. For proper protec-tion and for good service continuity,main, tie and feeder breakers allshould be equipped with ground faultprotection.In view of the above, it is evident thatproperly applied ground protectionrequires ground elements as far downthe system to the loads as practical. Forbest results, downstream molded casebreakers should have individual groundprotection. This would result in excel-lent ground protection because groundelements of Magnum DS and down-stream breakers having similar trippingcharacteristics can be coordinated.Depending on the sensitivity of theground fault protection methodapplied, coordination between Mag-num DS Breaker ground elements anddownstream branch circuit fuses issometimes impractical. This is due tothe basic fact that the blowing of onephase fuse will not clear a ground on athree-phase system. The other two-phase fuses will let the load “single-phase,” and also continue to feed theground through the load, as shown inFigure A5.Applicationwww . ElectricalPartManuals . com