At home: monthly
Every manufacturer's instruction sheet and UL's guidance say the same: press TEST once a month. Power should drop (a lamp in the outlet goes dark, the indicator changes), RESET should restore it. Self-test devices (post-2015) check their own electronics at least every three hours, but the button also proves the mechanical latch and is still the instruction on the label (self-test).
On a job site: before use
OSHA does not put a number on GFCI testing, but its assured-grounding alternative requires tests before first use, after repair and every three months, and the industry practice for portable GFCIs is the button test before each day's use. A device that fails comes off the site the same day. Log it; the inspector asks.
How
- Plug in a lamp or a tester downstream so you can see power drop.
- Press TEST. Power off, indicator changes, the button clicks out.
- Press RESET. Power back.
A plug-in three-light tester's GFCI button works only on a grounded circuit; on an ungrounded one use the device's own button (testing in detail).
What a failed test means
Power does not drop: the sensing circuit or the latch has failed — replace the device. Power drops but will not come back: see won't reset. On a portable manual-reset device, "will not come back until I press RESET" is the design.
After a storm
Lightning within a mile is a good reason to test every GFCI in the house that week; surges are the leading cause of the silent failures the 2015 self-test rule was written for.