The basic monthly test (30 seconds)
- Push TEST. Button in the center of the GFCI outlet. You should hear/feel a click. The RESET button pops out.
- Verify power is off. Plug in a lamp or a plug-in tester at the GFCI and at any downstream outlet on the same circuit (GFCI protects downstream too). Confirm no power.
- Push RESET. Button back in flush. Click. Power restored.
If the test button doesn't trip the outlet — replace the GFCI. If RESET won't hold — see "Why won't my GFCI reset" below.
The trip-time test (annual, with a plug-in tester)
A plug-in GFCI tester (Klein RT250 or Fluke ST120+, $24-$35) does what the on-outlet button can't: measures the trip time. Plug it in, push the tester's trip button, and the LCD displays the actual trip time in milliseconds.
- Under 25 ms: within UL 943 spec. You're protected.
- 25-50 ms: trips, but slow. Replace soon.
- 50 ms+ or won't trip: replace now. Personnel protection is compromised.
What is the GFCI trip time standard, in milliseconds?
The number everyone quotes is 25 milliseconds — and a healthy GFCI beats it. Here's what UL 943 (the GFCI standard) actually says: a Class A GFCI must trip when leakage current hits the 4–6 mA band — it must NOT trip below 4 mA (that's how it avoids nuisance trips) and MUST trip above 6 mA. The allowed trip time follows an inverse-time curve — the bigger the fault, the faster it has to open. At heavy fault current (250 mA and up), the maximum is 25 ms. That's where the famous number comes from.
In practice, a healthy modern GFCI trips well under 25 ms even at threshold-level faults — which is why 25 ms is the pass/fail line pros use on a plug-in tester, and why the readings table above treats anything slower as a replacement flag. The physiology behind the spec: ventricular fibrillation risk rises with both current and time, and the UL 943 curve is drawn to open the circuit before a 4–6 mA-and-up fault can hold long enough to hurt you.
Why won't my GFCI reset?
Three causes, in order of likelihood:
- Actual ground fault on the circuit. Unplug everything downstream and try RESET. If it holds, plug devices back in one at a time until you find the one that trips it. That's your faulty appliance or extension cord.
- GFCI is at end-of-life. Sealed-lithium-class GFCIs commonly fail at 10-15 years (less outdoor). UL 943 added end-of-life self-test requirements in 2015 — newer units have an indicator LED that blinks when they fail. If the LED blinks, replace.
- Wiring backwards. LINE and LOAD terminals reversed during install. Newer GFCIs auto-detect reversed wiring and refuse to reset. Power off at breaker, swap LINE and LOAD wires, restore power, RESET.
Where GFCI is required (NEC 210.8)
- All bathroom receptacles
- Kitchen receptacles serving counter surfaces (extended in NEC 2020 to all kitchen receptacles)
- Laundry / utility sinks
- All outdoor receptacles
- Garage receptacles
- Unfinished basement receptacles
- Within 6 ft of any sink, tub, or shower (NEC 2017+)
- Dishwashers (NEC 2014+)
- Bathtub + shower walls (NEC 2020+ — heated towel racks, etc.)
What GFCI does NOT protect against
- Short circuits. That's the breaker's job.
- Overloads. Also breaker.
- Arc faults. That's AFCI's job (NEC 210.12). Many modern circuits need BOTH GFCI and AFCI — DUAL-FUNCTION breakers solve this in the panel.
- Surges. See surge protectors.
- Standard 120V line-to-neutral hand-to-hand contact. GFCI protects against ground faults (line-to-ground through your body). It won't trip on a perfect line-to-neutral fault where you're touching both conductors at once. Rare scenario, but worth knowing.
FAQ
How often should I test my GFCI outlets?
Monthly per NEC and manufacturer recommendations. UL 943 requires trip in <25 ms at 6 mA.
Is the TEST button enough?
For mechanism check — yes. For trip-time verification — no. Use plug-in tester (Klein RT250, Fluke ST120+).
What is the GFCI trip time standard?
UL 943: a Class A GFCI trips at 4–6 mA leakage, on an inverse-time curve — max 25 ms at heavy fault current. A healthy unit trips under 25 ms even at threshold, which is the pass/fail line on a plug-in tester.
Why won't my GFCI reset?
(1) Real ground fault downstream — unplug things. (2) End-of-life — sealed-lithium GFCIs commonly fail at 10-15 yrs. (3) Wiring reversed.
How long do GFCI outlets last?
10-15 yrs indoor; 5-10 outdoor. UL 943 added self-test requirements in 2015 — newer units have indicator LEDs that flash at end of life.
Related guides
- GFCI keeps tripping for no reason — when the trip is real, not random
- Best home electrical tester kits 2026 — Klein RT250, Fluke ST120+
- Lights flickering — adjacent electrical diagnostic
- Best screwdriver set — the #1 driver you need to pull the cover plate
- Best needle-nose pliers — for forming the wire loops onto the terminal screws
Editorial standards: Cited authorities include UL 943 (GFCI standard) and NEC 210.8 (GFCI required locations) + NEC 210.12 (AFCI required). Reviewed by Al, Building Doctor — 18 years Class A commercial electrical Chief Engineer.