First of all, a ground fault circuit interrupter does not need a ground to work. A ground fault circuit interrupter compares the amount of supply current running through the black circuit wire with the amount of return current running through the white circuit wire. As long as the amount of current is the same, the electrical outlet will not trip. If a ground fault occurs and part of the current goes through your body instead of the white circuit wire, the “do-hickey” inside the outlet will trip the outlet in 1/40^th of a second. A ground fault occurs when the current running through the black wire goes into the ground or earth instead of back to the electrical panel box through the white wire.

For example: If you are washing dishes in the kitchen sink with your hands in the water and someone knocks an electric can opener into the sink, a ground fault will develop through the water, through the water piping finally into the ground. This will be detected by the GFCI and the outlet will trip.