GFCI and an Open Ground
Given: A 120 VAC GFCI supplying power to a 30 AMP RV. The RV has one problem, the 3rd Prong Ground wire at the 30 AMP plug is not making connection. (The RV chassis is floating because there are no ot...
Forum Discussion
BurbMan wrote:BB_TX wrote:
OK, my head is starting to hurt. After all the reading I still cannot grasp how the circuit can detect a fault between the neutral and an open (not connected) ground. So I will just accept that it happens. I was a systems engineer with little experience at the circuit level, so that may be my weakness here.
You are thinking about this all wrong BB....
The GFCI is designed to detect an imbalance in the current from the hot to the neutral. Even without a ground connection, an imbalance is created when YOU become the ground, ie you are getting shocked!
In the example given by the OP, the lack of a ground connection to the trailer will not trip the GFCI since it is monitoring the hot/neutral balance. Because the trailer is not grounded, if any part of the skin becomes electrified, you will get a shock when you touch it, because YOU become the ground path from the skin to the ground. In that case the GFCI will see that current is leaking from hot to ground THROUGH YOU and trip the circuit.
Burbman, your description is completely correct, however BB_TX was asking about the other aspect of GFCI which can detect a short between downstream neutral and ground with no loads or humans involved in the circuit. It does that by exploiting the concept called a ground loop. With the downstream short and the neutral/ground bonding in the main service panel, there are now 2 places where neutral and ground are connected. This forms a (long) loop of wire. The GFCI wraps a primary coil around the neutral and essentially uses that loop as a secondary coil to form a transformer. If current flows in that loop, it is detected by the original detector circuit as current in the neutral that is not in the hot.
