Ground-Fault Relay

To be competitive, some electrical contractors often supply low-voltage (LV) service entrance switchboards and switchgear that meet the minimum safety requirements of the National Electrical Code (NEC). The Code stipulates that a ground-fault relay must be installed on the service entrance for services with a rating higher than 1,000A. This relay shall be set for no more than 1,200A, and it shall trip the service entrance breaker.
As related to safety, this is a great improvement over earlier requirements of the Code, which did not call for any ground-fault relay. However, it must be noted that this level of ground sensing and time delay protection arrangement provides limited coordination with any possible downstream protective devices. As a result, when the main service entrance device trips the entire building suffers a blackout.
So what can you do to improve the overall protection and coordination of the power delivery system? Let’s take a look at some other options you have to improve system performance and better serve your customer.
As per Code requirements, all 3-phase, 4-wire circuits, where the neutral is distributed and used for the load(s), must be solidly grounded. As a matter of habit, many designers and installers also solidly ground 3-phase, 3-wire systems. In such systems, the ground-fault sensing relays, which are installed at various levels throughout the distribution network, are time-current coordinated.
Where there are multiple levels (i.e., zones) in the power system, there is a need for coordination of the zones so that, whenever possible, the higher levels are unaffected by downstream faults. The branch circuits are like tree branches, and all of the relays will “see” the fault current in a particular branch when the fault is downstream.