Generator Ground Fault Alarm

Backup Generators have been installed at a great many commercial and industrial sites in the past decade.    One of the NEC requirements that is often overlooked for standby generators is the requirement for ground fault sensing and alarm.  Engineers, consultants, contractors and inspectors have consistently missed this obscure code requirement and as result a hazard exists for standby generator owners and operating personnel.

Ground-fault protection of equipment is not permitted on the output of the generator, but ground-fault sensing is required on the emergency system. Therefore, the ground-fault sensor should be located at, or ahead of, the circuit breaker. Instructions have to be posted at the sensor location that explain what should be done if the alarm activates because of a ground fault. This requirement appears in 700.7(D). There is no requirement for ground-fault protection for legally required systems.

In order to mee the requirement of 700.7 (d) a grounding resistor should be in place along with sensing and alarm relays as depicted below.

The requirements in 215.10 apply to a feeder rated over 1,000 amperes on a solidly grounded wye system where the voltage is over 150 to ground but not over 600. Exception No. 2 to 215.10 eliminates the requirement for ground-fault protection of equipment where the load is a fire pump, and Exception No. 3 does not require GFP where this protection is provided on the feeder’s supply side.

However, this protection does not protect the generator feeder when the normal source is off and the transfer switches have connected the load to the generator