Ground Fault Protection - engineering site - Schneider Electric

Implementation examplesExample 2:The discrimination rules applied to Masterpact NW32 and Compact NS800 result in the settings describedin the figure below. The indicated setting allows totaldiscrimination between the 2 circuit-breakers.Note: time discrimination upstream does not present problems.On the other hand, downstream time discrimination is onlypossible with short-circuit protection devices with a ratingy than 40 A.Use of the “l²t on” function improves this limit for the gl fusesplaced downstream (see example 1a)DB117322Example 1b: optimised discriminationGoing back to example 1, on a fault downstream ofcircuit-breaker , the ground fault protections , and are in series. Installation of a VigicompactNSX160 allows total discrimination of the groundfault protections as standards whatever the setting lr ofthe Vigicompact NSX160.Note: although thresholds may be very different (lg = 400 A forNSX400, lg = 30 A for NSX160), it is necessary to comply withtime delay rules between protection devices (index 0.2 forNSX400, index 0.1 for NSX160).DB117323Special use of ground faultprotectionDB117335Generator protectionb The principeThe Vigirex RH328 AP is installed as generatorprotection. The principle of this protection is as followsv tripping in event of a ground fault upstream(protected area),v non tripping in event of a ground fault downstream(unprotected area),b The constraintsThe protection functions must:v be very fast to avoid deterioration of the generator(and must control stopping and placing out of operationof the GE),v be very fast to maintain continuity of supply (andmust control the coupling device of the GE). Thisfunction is important in event of parallel-connectedgenerators,v have an average tripping threshold: normally from30 A to 250 A. On the other hand, discrimination withthe ground fault protections of the installation is“naturally” provided (no effect on the “unprotectedarea”),b Setting of the protection deviceDue to the above constraints, setting can be:v threshold l∆n: from 30 A to 250 A,v time delay: instantaneous.DB11732549