Ground fault protection device

Wiring diagram of a stand alone PV system with a charge controller with DC lighting control and an inverter for AC loads:
(1) Ground fault protection device (GFPD)

A ground fault protection device (GFPD) uses a low current breaker (.5-1 A) to create a connection between a conductor (wire) and ground on the DC side of a PV system, which creates a grounded conductor. This breaker is ganged (connected) to another breaker through which the positive PV source wire passes. In the event of a DC ground fault, the low current breaker will trip removing the connection between the grounded conductor and the ground, but will alsoopen (disconnect) the ungrounded conductor of the PV source circuit. A GFPD is recommended for systems with a DC system ground to improve safety, although they may be difficult to find in many locations.

A GFPD plays an important role in a PV system with a DC system ground by helping to prevent fires from ground faults that go undetected by overcurrent protection devices (OCPDs). A ground fault can go undetected because the OCPDs for the PV source are sized to the maximum current of the PV source, which means that OCPDs are often ineffective against DC ground faults occuring at the PV source as it doesn't exceed their current rating. This can lead to a fire because PV modules will continue sending current into the fault as long as:

1. There is a ground fault on the ungrounded conductor.
2. A DC system ground that allows the equipment grounding conductor (EGC) to complete the circuit between positive and negative.
3. Sunlight hitting the PV source.

A GFPD works by removing the second of these requirements - the DC system ground. This has the benefit of reducing the potential of a fire resulting from the ground fault, but also creates a new hazard. A grounded conductor, like the black DC wire in the diagram, will have a voltage of 0 V to ground - grounded conductors are generally safe to touch. An ungrounded conductor, like the red DC wire in the diagram, will have the voltage of the PV source relative to ground and is not safe to touch. The new hazard is created because of two things that happen when a GFPD activates during a ground fault:

1. The previously ungrounded (red) conductor with the PV source votlage relative to ground effectively becomes a grounded conductor with 0 V to ground.
2. The previously safe grounded (black) conductor with 0 V is turned into an ungrounded conductor by removing its ground, which means that it may have the full voltage of the PV source relative to ground.

This creates a hazard for system users or anyone troubleshooting the problem as the issue may not be readily apparent and safe/unsafe conductors of a system have switched roles due to the GFPD. Ground faults should be taken very seriously and any troubleshooting should be done by someone with experience dealing with ground faults.