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I have an outside shed with a non-GFCI outlet and a light fixture. Only the light is controlled by a switch. The shed is wired off of an outside GCFI outlet. Everything, including the outlet, was installed about 3 years ago.

In the last few weeks, turning on the light switch in the shed gives light for about a second, then both the light and the outlet inside the shed are dead. The switch and light are both downstream from the outlet, which also loses power. However, the GFCI is not tripped. Anything plugged into it continues to receive power and testing/reseting it works normally, except the shed often remains without power. Sometimes waiting a while and then testing & reseting the GFCI outlet will return power to the shed. When the GFCI is tripped, there is never power to the shed, so I think it is wired correctly.

Is it likely the GFCI that is to blame (and should I try replacing it?) or some other issue with the wiring?

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Does the switch control the light and the receptacle in the shed, or just the light? –  Tester101 Jul 23 '12 at 13:17
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When you manually trip the GFCI, does the light stay on? This usually indicates that the line and load inputs were reversed –  Steven Jul 23 '12 at 14:21
    
The switch is just for the light and tripping the GFCI has always killed power to the shed. It's just recently that it started behaving oddly. –  noah Jul 23 '12 at 22:34
    
A crude wiring diagram of your setup would be helpful. Don't forget to include any and all devices on the circuit. –  Tester101 Jul 24 '12 at 12:37
    
In my case, it seems that it was just sketchy connections in a junction box. Unscrewed the wire nuts, straightened out the connections and screwed the nuts back on and haven't had a problem since. –  noah Aug 3 '12 at 22:31
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4 Answers

up vote 5 down vote accepted

You're going to have to inspect each junction and device in the circuit, starting with the first know bad device. It sounds like you have a GFCI in the house, which feeds a receptacle in the shed, which in turn feeds a switched light in the shed. Since the receptacle in the shed is the first know bad device, you'll want to start there.

Turn off the power at the circuit breaker. Pull the receptacle out of the box, leaving the wires connected. Inspect the wiring for loose connections, and fix any found. If there where no loose connections, you'll want to determine which wires entering the box feed the receptacle. If this is not obvious, you'll have to do some testing. For this you'll need a multimeter with a continuity function. Disconnect one of the neutral (White) wires from the receptacle, set the multimeter to test continuity, touch one probe to the equipment ground, touch the other to the disconnected neutral, then to the still connected neutral. If one tests positive (audible tone, or reading on display), that is the feed line. If neither or both test positive, you'll have to do testing with the power on (which I won't cover here since it is unsafe, and you should call an Electrician at this point).

Now that you've found your line wires, reconnect the neutral to the receptacle. Leave the receptacle out of the box; making sure nothing is in contact with the edge of the box, then turn the power back on. Turn the light on in the shed. If it goes out, use a non-contact voltage tester to test for voltage. If you have voltage on the line but not the load, replace the receptacle. If you have voltage at neither point, run similar tests on the GFCI receptacle.

If you have voltage on the load side of the GFCI, but no voltage on the line side of the receptacle in the shed. You'll have to inspect the wires connecting the two devices. Start by shutting off the power, at the breaker. Next disconnect the load wires from the GFCI, and the line wires at the receptacle. Run a continuity test between all combinations of the wires, none should test positive. Finally connect one pair of wires at one end (so in the shed, twist the black and white wires together), do a continuity test on the other end to make sure the circuit is not open.

If all of this has been done, and the problem persists, you'll want to contact an Electrician.

As always, if you don't have the proper tools, or you don't feel comfortable doing any of the work mentioned. Do not hesitate to contact a local licensed Electrician.

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You have a fine sense of what folks should do themselves and what they might want to leave to an electrician. Thanks for providing such a good example. –  Jeremy W. Sherman Jul 24 '12 at 15:16
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My best guess is there's a physical problem with the shed wiring connection to to the GFCI outlet. Maybe it's arcing, causing heating, which opens the circuit. Once things cool, the circuit closes enough to power the light for a bit until the faulty joint heats up again.

If the GFCI wiring is connected via the push in spring connectors often found on the back of outlets, move the wiring to the binding screws. In any case, loosen the shed wiring from the GFCI, ensure all contact surfaces are clean, then reconnect. If that doesn't help, are there any other connections between the GFCI and the shed light switch and outlet? Check any such connections as well.

You could then temporarily take the GFCI out of the circuit, wire the line side directly to the load side to see if the problem disappears. It's possible the GFCI's internal load side wiring is defective and this final test will confirm this.

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I checked the junction box off the GFCI and the connections were a bit sketchy. I redid the connections and didn't have a problem this morning, so I'm hopeful that was it. –  noah Jul 24 '12 at 13:10
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I have a temperamental switch and light as well. It'll run for a bit, then the light turns off. Waiting a while and flipping it on, it sometimes works, sometimes doesn't. My guess is that the switch is broken, possibly with some internal component heating and bending away from whatever it needs to be touching to complete the circuit. I haven't bothered fixing it because I'll be replacing fixture, switch, and switch box soon.

Switches are cheap, so I'd replace the switch - or, if you don't have or want to get a switch, then just bypass it - and see if that fixes everything. If not, come back and comment, and we'll see what we can do from there.

While you have the box open, take a photo and note what's wired and to what in there. If it doesn't fix things, open up the receptacle and take a look there, too, then diagram them. Understanding the details of the circuit you're working with helps tremendously in fixing it.

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The switch and light are both downstream from the outlet, which also loses power. –  noah Jul 24 '12 at 2:24
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@noah If the switched light is downstream from the receptacle in the shed, I'd suspect a bad connection in the receptacle. Open up the box and have a look (make sure power is off first), if you don't notice anything obvious try replacing the receptacle. You're going to have to inspect every junction and device in the circuit, until you find the fault. –  Tester101 Jul 24 '12 at 11:56
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What makes this interesting is what you say that waiting a while restores power.

That makes me wonder if the light fixture has a thermal sensor in it. This is common in many IC cans and designed to prevent fires. The heat will break the circuit. Perhaps your receptacle needs to be replaced.

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The outlet also loses power. Are you saying that it could break the full circuit, not just the power to the light? –  noah Jul 23 '12 at 23:06
    
@noah that's interesting... a thermal sensor in the light won't impact the outlet unless the outlet runs through the light. –  Matthew Jul 23 '12 at 23:08
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