600V solid ground fault protection

[SELEC]

I am doing a protective coordination for 600V system, which is the secondary side[Y] of 1500KVA Transformer. The sencondary side is 3W+[N+G]. N& G are in one wire. GF current calculated is 7.6KA, 3phase fault current is 10.1KA.
I am wondering if we need any G-F protection on MCC main breakers. If so, how can I do the settings?

I read IEEE Buff book. It mentions that G-F setting on the mains should be higher than the phase fault protection settings on the branches.Time delay should be longer than 0.1s. Is is right? But the problem is, it is hard to achieve such high G-F settings on some breakers. Who have a better solution for this?

Thank you in advance.

 

[davidbeach]

First, if you are only running 4 wires, you don't have a usable neutral, at least not per the NEC, nor per good safety practices; the ground should never be used for carrying any normal currents. Your last layer of ground fault devices will have many miscoordination problems with the downstream phase devices unless you can severely limit the size of those downstream phase devices. That is not often possible. So, the best bet is to get as many layers of ground fault as works with the breakers you are using, typically 3 layers, occasionally 4 layers, and deal with the miscoordination, but at least the miscoordination will cause smaller portions of the system to be impacted by tripping in response to a ground fault. But you have to look at each case, occasionally you will get better coordination by removing the last layer of ground fault protection.

If the transformer feeds one main, that main has motor breakers as the next layer, and you put ground fault trip on all of the motor breakers you need not have any coordination issues.

 

[SELEC]

Actually, I have 3 layers, first one is MCC-C for a group of motors, second layer is MCC-B to feed MCC-C and other loads on MCC-B, third layer is MCC-A to feed MCC-B and the loads on MCC-A. That's why I am running across miscoordination problems.In our system, N & G wires would have to be connected to Ground terminals on CBs, Which may cause some misopreations.I didn't agree with my lead to do so. but he insisted. I have to convince him not to do so.