Yet another GFI question

[tung tied]

I have a dedicated 15 amp circuit to a GFI outlet in the garage.
I thought I had a bad GFI outlet, and replaced it, twice.
It still doesn't work. I have 120V to the box (tested by multimeter). There is no load side wiring. I have it wired to the line side. I can't reset the GFI (after circuit breaker is reset). The button won't go in. So either I have a terrible supplier of GFI outlets, or something else is wrong. Testing the voltage by touching the screws after all is connected (hanging out of the box) shows the wires are connected. The circuit breaker is 15 amp, and it used to work. Can it be the breaker?

[Dragster]

GFI Question

Sometimes when you measure voltage without a load it will give you a false reading, and tell you have voltage when in fact you don't. Try to make the plug work without the GFI to be sure you do in fact have a complete circuit. If you do then the device is faulty.

[LCHIEN]

You should definately try it without a GFI, just a regular plu and put a load on it just to make sure all your wiring is working OK.

With the GFI, of course, you can't reset it until you trip it first... is it tripped? You have to push the red button and it should give a little click.

So if its tripped and won't reset then you A- Have a bad GFI or B- have a ground fault.

If its a ground fault then by definition you have some current leaking back from the hot side (Black wire) to ground and not returning thru neutral (white wire). Which is prettyy unlikely if the GFI is at the end of the line with no load.

You can certainly test the GFI outlet in another location. You can even wire it with a 3-wire grounded cheater cord and plug directly into an outlet, just be real careful.

[rnelson0]

What happens if you replace it with a regular receptacle?

[tung tied]

Thanks all. I hadn't considered trying a normal outlet. I don't understand how you can get a false reading, which illustrates the level of my competence I guess. I'll give it a shot (probably Saturday morning) and respond then

[rnelson0]

I don't understand how you can get a false reading, which illustrates the level of my competence I guess.

The other reason to try a regular receptacle is that you don't know what's going on inside the GFI. You could try a continuity tester on the poles and see if you get a pass. If you don't, it's either tripped or broken.

[eccentrictinkerer]

Try disconnecting the ground wire. If you have a floating ground, it can mess up the voltage comparator that makes the GFI operate.

BTW, code does allow the use of GFI's without a ground wire. This is handy where you need a three-wire outlet on an old un-grounded circuit.

[tung tied]

Sorry for the delayed reply. Kudos to Dragster and rnelson. I hooked up a regular outlet and had no function. So the reading of 120V between black and ground was misleading.

Why would I get this 120V reading? Something else wrong with other house wiring?

Anyway, I'm off to get a new circuit breaker.

My delay to getting back to this thread is because I'm installing ceramic tile in the master bath (unrelated). But that is for another thread sometime.

[LCHIEN]

Voltmeters are very high impdeacne loads, which i why I suggested you try it with a low impedance load on a regular socket.

Votmeters usually are 10,000,000 ohm loads, a 100W light bulb is a 140 ohm load.
If your breaker is good it will be around an ohm. if bad it might be anywhere from a few thousand to several hundred million ohms.
The voltage will divide itself proportionally to the higher impedance of the breaker and the load.

Assuming the breaker is 10,000 ohms (bad) then

in the case of the
bad breaker + 10,000,000 ohm voltmeter, the voltage will appear at the voltmeter hence reading 120V

In the case of the bad breaker and the 140 ohm bulb, then the voltage will be held by the breaker and the light won't light.

When things are working as they should, e.g. breakers either under 1 ohm (closed) or over 100,000,000 (open) then the voltmeter will indicate properly. When you have a marginal component, the voltmeter will read the voltage through the bad component - it's not wrong, you have to understand how the components interact.
That's why a load test is always the sure answer.

[tung tied]

Very nice answer and description LCHIEN. I hope this post is of value to others. I think I've got it now.