I don't think David read the question properly. The earthing transformer draws a very small magnetising current in normal operation, until called upon to pass earth fault current. It needs to be the last thing to trip on your network under earth fault conditions otherwise the other earth fault protection relays on the network will effectively be disabled, and you will end up with an unearthed system. Other than that, it needs to trip before it gets damaged by the fault current.
Regards
Marmite
No, I read the question just fine. Of course the relay should trip something other than the transformer for neutral/residual overcurrents, maybe/maybe not trip a lockout; no idea what's being grounded. The phase overcurrents on the transformer, set above the phase current associated with the maximum ground current, should trip a lockout that removes the whole circuit from service until the transformer damage gets repaired.
All that said, my original answer still stands. There is not enough information in the original question to actually recommend settings.
I don't think you want to trip out the zig-zag transformer.
The supply transformer should be tripped out before the I2T crosses the the zig-zag transformer's damage curve.
An alternative is to insert a series resistor in the ground path to limit the fault current and alarm the fault. This depends a lot on the application, and the protection philosophy of the protection group.
Bill
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"Why not the best?"
Jimmy Carter
A good method of protecting a zig-zag transformer is to
connect the overcurrent CTs in delta. The overcurrent
relay will then ignore the zero sequence current which
must flow for an external fault, and can be set low and
fast.
