Ground Fault protection

[Humble2000]

In a 3 wire grounded system, if the main breaker has a ground fault function, the CT's common will need to be connected to ground terminal of the relay.
I know that in this case the sum of three currents will pass to ground and create a signal to activate the relay.

How the setting and coordination would be done in this case?
The other question is if the arc fault current is calculated from %38 of 3phase bolted current or LG bolted current?

Thanks

 

[alehman]

The relay should be set so that it will activate for likely ground fault current. In NEC areas for low voltage systems exceeding 150V to ground, the pickup setting must not exceed 1200A and time delay must not exceed 1 second for ground current in excess of 3000A.

If possible, the ground relay should be set to coordinate (grade) with upstream/downstream ground protection (or, depending on grounding arrangement, phase overcurrent protection if no other ground overcurrent protection exists).

 

[waross]

The lower the setting of the ground fault trip, the less damage you will have in the event of a ground fault.
The issues are co-ordination with other ground fault devices upstrem and down stream, and nuisance tripping.
As an example, a system may have more than 20,000 amps available short circuit current, a main breaker of 1200 amps and a ground fault setting of 5 or 10 amps.
Within the limits of co-ordination and nuisance tripping, lower is better.
respectfully