Rule 19 of 40 · Chapter III — Grounding, Bonding, and Protection
Test GFCIs and AFCIs on schedule
Why this rule exists
Protective devices are electronics, and electronics fail, often silently, still passing power while no longer able to protect. A GFCI can feed a receptacle perfectly while its sensing circuit is dead, an outlet that looks fine and will not trip when you need it. AFCIs, which catch the arcing-fault signatures that start fires, degrade the same way. Manufacturers and the NEC call for periodic testing because a protective device you never test is one you are merely hoping works. The test button exists so you do not find out during a fault.
In practice
Press the test button on GFCIs and AFCIs monthly, or on a schedule you will actually keep. On a GFCI, the button should cut power; the reset restores it. Back it up occasionally with a plug-in GFCI tester on the receptacle. If a device will not trip on test, will not reset, or trips immediately, replace it, since it has done its last useful job. Note install dates; these devices have finite lifespans, and one over a decade old deserves suspicion even if it still tests good.
When it doesn't apply
A plug-in tester's trip button may not work on a legitimately ungrounded GFCI-protected circuit, because it trips by diverting current to ground; use the device's own test button there. Nuisance trips from long circuits or shared neutrals indicate a wiring issue, not a faulty device. If testing keeps failing and the wiring looks correct, stop guessing and have it diagnosed.