Troubleshooting a ground fault circuit interrupt, or GFI, breaker is pretty straightforward. Troubleshooting the circuit itself can be quite time-consuming. The GFI breaker is designed with a test button incorporated into the breaker itself. Pushing the test button should trip the breaker. On GFI-style breakers the neutral wire going into the house's outlets is connected to the breaker's neutral connector, the white neutral that comes out of the breaker is connected to the neutral bus in the panel, isolating the neutral bus from the neutral wire going into the house. The test button actually shorts the neutral wire feeding the circuit to the neutral bus in the electrical panel creating a ground fault that should trip the breaker. It is considered a ground fault because the neutral bus in the main electrical panel is actually connected to the ground bus through the panel's metal casing.
What to do if the test button isn't tripping the breaker
Push the test button on the GFI breaker. The breaker should trip. If the breaker does not trip, then it may be that the breaker has already tripped and just looks like it's on. The position of the switch may only move slightly from the on position towards the off position when tripped.
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Push the switch on the GFI breaker all of the way toward the off position. It may take some force to get the breaker to reset. Turn the breaker back to the on position. When the breaker has been reset properly you should feel some resistance when pushing the switch back on.
Push the test button again and the breaker should trip. If the breaker still doesn't trip then you should test for power at the screw connections inside of the electrical panel. Remove the screw that holds the dead front covering the breaker's connections. Remove the dead front cover.
Test for power with your voltmeter set on AC volts on the highest scale. For a single pole GFI breaker, touch the black lead from the tester to the silver screw on the GFI breaker and touch the red lead from the tester to the brass screw on the GFI breaker. You should see 110 volts on the tester. If voltage is seen but the test button won't trip the breaker, then the breaker is bad and should be replaced.
Test for power on a two pole breaker by touching the red voltmeter lead to one of screws with a black or red wire connected to it. Touch the black lead to the other screw with a black or red wire connected to it. You should read 220 volts or close to it on your voltmeter. If you read voltage and the test button won't trip, the breaker is bad and needs to be replaced.
What to do if the breaker won't reset and keeps tripping when turned on
Unplug everything that is plugged into any of the outlets on the circuit in question. Try resetting the breaker again by pushing the switch all the way to the off position and then turning it back to the on position. If it won't reset and trips when the breaker's switch hits the on position, it could be a bad breaker or a problem in the circuit itself.
Use your straight-tipped screwdriver to loosen the brass connection screw or screws on the GFI breaker. Pull the black hot wire, or wires, out of the breaker's connectors. Loosen the silver screw the white wire is connected to and remove it from the GFI breaker.
Push the switch all the way to the off position. Turn the switch back to the on position. If the breaker still won't reset, then the problem is the breaker itself and it should be replaced with a new one of the same size, brand and model. If the breaker resets normally and the test button trips the breaker when pushed, the problem is in the circuit itself and an electrician should be called to find your ground fault.
Reconnect the black wire, or wires, to the brass screws on the GFI breaker. Reconnect the white wire to the silver screw on the GFI breaker.
Replace the dead front cover into the breaker panel. Install the screw or screws that hold the dead front in place.