Ground Fault Protection - engineering site - Schneider Electric

2.2.2. Discrimination between upstream GFPdevices and downstream SCPDsDiscrimination rules between GFP devices and downstream fusesBecause of threshold setting values of GFP devices (a few hundred amps),protection using GFP devices can interfer with protection using fuse devicesinstalled downstream in case of an earth fault.If downstream switchgear is not fitted out with a ground fault protection device, it isnecessary to verify that the upstream GFP device setting takes the downstream fuseblowing curve into account.A study concerning operating curves shows that total discrimination is ensured with:b a ratio in the realm of 10 to 15 between the upstream GFP setting threshold andthe rating of downstream fusesb an intentional delay of the upstream GFP device that is greater than the breakingtime of the downstream device.A function of the I²t = constant type on the GFP device setting allows thediscrimination ratio to be slightly improved.The ratio can be greatly reduced by using a circuit-breaker thanks to the possibility ofsetting the magnetic threshold or the short delay of the downstream circuit-breaker.DB117273DB117274Diagram 11 - coordination between upstream GFP device and downstream devicesDiscrimination rules between GFP devices and circuit-breakersb the above condition is equivelant to a GFP device setting at 1.5 times that ofmagnetic protection or time delay of the downstream circuit-breaker.b if this condition is not verified and so that it may be executed:v lower the magnetic setting threshold while being careful of nuisance tripping onthe downstream outgoer dealt with (especially on the motor feeder)v aise the GFP device threshold while being careful of keeping the installation’sprotection against stray currents because this solution allows the flow of strongercurrents.DB117275DB117276Diagram 12aDiagram 12b13