The button test
- Plug the cord into a live outlet. Plug a lamp or a small tool into the cord so you can see the power.
- Press RESET if the device is not already on. The lamp lights.
- Press TEST. The lamp should go out at once, with a click from the device.
- Press RESET. The lamp comes back.
That is a pass. The TEST button pushes a few milliamps around the sensing coil through a resistor, which is a real imbalance as far as the electronics are concerned, so a pass proves the sensor, the trip circuit and the solenoid all work. It does not prove the trip level is exactly 4 to 6 mA; that needs a calibrated tester and a lab, and it is not something a user does.
Why the built-in button beats a plug-in tester
The little three-light plug-in testers with a GFCI button work by leaking current from the hot pin to the ground pin. On a grounded outlet that works and the GFCI trips. On a two-wire circuit with no ground, or on a cord whose ground is broken, the tester has nowhere to send the current and the GFCI never sees an imbalance. The tester then reports a failure that is not there, or, worse, a technician concludes the GFCI is fine because the lights look right. The button on the device does not use the ground conductor at all. Use the button. Use the plug-in tester as a second opinion on outlets you know are grounded.
How often
Manufacturers' instructions say monthly, and UL 943 has required that instruction on the device or its packaging for years. On construction sites, OSHA's assured equipment grounding conductor program (29 CFR 1926.404(b)(1)(iii)) sets the cadence for cord sets that are not GFCI-protected: before first use, after any repair, after any incident that could have damaged them, and at least every three months. GFCI cords are the alternative to that program, not exempt from good sense, so most contractors test them on the same three-month cycle and log it, plus the monthly button press by whoever is using them.
Test outside the schedule whenever the cord has been:
- dropped from height, driven over, or pinched in a door;
- left in water, rained on, or pressure-washed;
- stored through a winter in an unheated trailer;
- tripping for no reason you could find.
Three ways a test fails, and what each means
It will not trip on TEST. The device has failed. Take the cord out of service and replace the device (or the cord, if the device is moulded in). Do not keep using it because the tool still runs; the tool running is the problem.
It trips on TEST but will not RESET. First check there is power on the line side: is the outlet live, is the breaker on, is the generator running? A GFCI cannot reset without line voltage. If there is power, something downstream is leaking right now: unplug the load and try again. If it resets with nothing plugged in, the load is faulty. If it still will not reset with nothing plugged in, the device is faulty.
It resets and then trips again on its own. Either a leaking load or a wet cord. Dry the connectors, look for a cracked housing or a cut jacket, and try a different load. Guide 7 takes this apart in order.
Self-test devices
Since 2015, UL 943 GFCI receptacles have tested themselves automatically and either lock out or show a warning when the test fails. Some cord-mounted devices now do the same and carry an indicator light. A self-test does not replace the button: it checks the electronics on a schedule, while the button test checks the whole device, including the mechanical trip, in front of you. If the indicator says fault, believe it.
Keeping a record
A date on a piece of tape around the cord, or a line in the site log, is enough: cord ID, date, pass or fail, initials. When the safety inspector asks how you know the GFCI cords work, "we press the button on the first of the month and write it down" is the answer that closes the conversation.
Next: Nuisance tripping.