Electrical – Why is the GFCI recpeptacle slow to trip

As well it should.

Normally, loads are connected between hot and neutral. Appliances are not supposed to connect hot or neutral to ground; ground is only a shield.

The GFCI compares current on the hot and neutral wires. They should be the same. If they are not, current has found another route, possibly through the grounding system (which isn't supposed to happen) and potentially through some poor human.

Circuit testers are trying to test whether ground is connected... cheaply. They mis-use "hot" as a power source, by connecting a light bulb between hot and ground. If ground is connected correctly, this will light.

In other words, it intentionally creates a hot-ground fault (by sticking a light bulb there). This is exactly the condition GFCIs are designed to detect.

I'm not talking about any ground-fault-test the tester may also have.

So why do testers often work? With a perfect GFCI, they wouldn't work. I suspect it is because GFCI's have detection thresholds above zero, and that is often enough for these testers to "get away with it". I even think there may be a tacit agreement among manufacturers for this, but obviously, lower sensitivity impinges safety. Remember a shock which only stuns you can kill you with secondary effects like falling or drowning.

So either your GFCI is pretty good, or your tester is pretty bad.

Grounded circuit (green/bare ground wire wired properly):

• Push "Test" on the GFCI protective device, and it trips -> PASS
• Push "Test" on the GFCI protective device, and it does not trip -> FAIL
• Push "Test" on a plug-in GFCI tester, and it trips -> PASS
• Push "Test" on a plug-in GFCI tester, and it does not trip -> FAIL

A fail here indicates the GFCI unit is probably defective.

Ungrounded circuit (green/bare absent or defective):

• Push "Test" on the GFCI protective device, and it trips -> PASS
• Push "Test" on the GFCI protective device, and it does not trip -> FAIL
• Push "Test" on a plug-in GFCI tester, and it trips -> FAIL FAIL FAIL
• Push "Test" on a plug-in GFCI tester at the GFCI receptacle, and it does not trip -> PASS
• Push "Test" on a plug-in GFCI tester at a downstream protected outlet, and it does not trip -> MEANINGLESS

Fail at the GFCI device probably indicates it is defective.

Fail on the plug-in GFCI tester (i.e. it trips!) indicates they have bootlegged ground at the GFCI receptacle- attached the neutral wire to both neutral and ground. It will seem to work at the GFCI, but is still dangerous.

Bootlegging ground at a downstream GFCI receptacle is a mistake, because one of several electrical faults could put 120V on the the grounds, e.g. the cover plate screws or a machine chassis. However this is difficult to detect, since a properly wired downstream receptacle will behave exactly the same way. This means for ungrounded downstream receptacles, plug-in tester testing is completely meaningless.

Once you have settled the question of bootlegged grounds, here's how you test an ungrounded GFCI. Plug your GFCI tester into your handy dandy 2-3 prong "cheater" - the kind with a short green wire as a pigtail. Extend that green wire all the way to a reliable ground source, e.g. the panel in the basement. Now, the GFCI tester should work normally, since you have rigged a proper ground to it.

GFCI protection is pretty effective, and I would be confident in an ungrounded circuit if it has GFCI protection. However if you are unable to get the external device to trip, you'll need to pop the cover off and see if the ground is present, missing or bootlegged.