power distribution - Siemens

TechnicalGround Fault ProtectionTime/Current Selective Ground CoordinationSelectionFigure 17. Zone Interlocking SchemeFigure 16. Fully Coordinated Multizone GFP SystemThe zone selective or zone interlocksystem provides fast tripping of thenearest disconnect upstream of theground fault. The damage level is thelowest that is possible because theground fault is cleared as quickly as theprotective equipment can respond andthe disconnect can open. Additionalinterlocking wiring and circuity forsending and receiving the restrainingsignals are required.The zone selective or zone interlockscheme is for those few specialapplications where exceptionally fasttripping is necessary for all feedersthroughout the entire system to reducedamage. Note that although the relaytime can be reduced appreciably, thecircuit breaker mechanism and arcingtime (plus safety margin) will still bepresent.Zone Selective Operation (Figure 17):a) Relay-1 will sense a ground fault atA when it exceeds 10 amperes. Itwill instantly initiate tripping of theBranch breaker and send restrainingsignals (transfer from instantaneousoperation to time-delayed operation)to Relay-2 and Relay-3 (Relay-2 andRelay-3 will then back up Relay-1 ona time coordinated basis). Relay-4will be restrained by Relay-2 ifground fault exceeds 100 amperes.b) Relay-2 will sense a ground fault atB when it exceeds 100 amperes. Itwill instantly initiate tripping of theSub-Feeder breaker and sendrestraining signals to Relay-3 andRelay-4.c) Relay-3 will sense a ground fault atC when it exceeds 400 amperes. Itwill instantly initiate tripping of theFeeder breaker and send arestraining signal to Relay-4.d) Relay-4 will sense a ground fault atD when it exceeds 800 amperes. Itwill instantly initiate tripping of theMain breaker.Table 17.1Typical AmpereSetting80040010010RestrainedTime Delay0.4 SEC.0.3 SEC.0.2 SEC.0.1 SEC.18 TECHNICALSiemens Power Distribution & Control, SPEEDFAX 2007-2008 Product Catalog 18-9