this post was submitted on 18 Jun 2024
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GFCI tripped, sourced to basement outlet, help (lemmy.world)
submitted 5 months ago by dodeca@lemmy.world to c/electricians@lemmy.world
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I have a GFCI in the garage that tripped and keeps tripping. I traced it to an outlet in the basement but there is something weird going on. I'm not an electrician, but I've done a bit of wiring, but I don't know how to interpret this.

In the picture is the basement outlet pulled apart and the power is on and the GFCI is reset and working. This basement outlet has 3 14/2 cables coming in. I think one is power from the garage, and the other two lead to outlets outside the house. I checked the wires until I found 120V and then marked them with yellow tape, which is what is shown in the image.

However, if I connect the multimeter to the black/yellow and one of the other whites I get a reading of 101.8, and the other white reads 0.7. This shouldn't be happening, right?

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[–] litchralee@sh.itjust.works 29 points 5 months ago (3 children)

From your detailed description, it sounds as though one of the downstream outlets outside the house has a fault between neutral and ground. Follow the white wire which was reading 101.8, and that should get you closer to where the fault is. It might be anything such as: the receptacle got wet, a nail went through the NM-B cable, or something of that nature.

Separately, the white wire that reads 0.7 is fairly normal for a wire segment that should be dead. So no cause for alarm there.

Now to explain how I reached these conclusions. Firstly, a GFCI operates by detecting any difference in current between the live and neutral wire, downstream of the GFCI. If this difference exceeds ~6 mA, then the GFCI will trip.

Secondly, North American grounding systems will bond the ground rod and the neutral wire at only one point, at the service entrance where the main disconnect switch is located. Nowhere else should ground and neutral be bonded except here. In the IEC nomenclature, this is called a TN-C grounding system.

Thirdly, if a circuit has daisy-chained outlets, then any current drawn by upstream outlets will induce a small voltage onto the neutral wire, detectable at downstream outlets, compared to the ground wire and ground rod.

Let me explain a bit clearer: suppose a circuit has outlets A, B, and C, where C is the farthest from the breaker box. Plugging in a space heater into outlet A will cause current to flow to the space heater and back to the breaker. Because current is flowing on both the live and neutral wires, and because house wiring has a small, non-zero resistance, Ohm's law dictates that there must be a voltage difference across the wire, however small. That is, if the breaker box would measure the live wire at 120.0v and the neutral wire at 0.0v, then the voltage at outlet A might measure 119.4v and 0.6v, respectively, when the space heater is powered.

Because outlets A, B, and C are in parallel, we would also see this hypothetical 0.6v at outlets B and C as well, measuring between neutral and ground. This is normally not a problem.... except if outlet B is a GFCI and outlet C has a neutral-to-ground fault.

In this new scenario, the voltage induced on the neutral by outlet A will cause current to flow through outlet C's neutral-to-ground fault. After all, that 0.6v would prefer to flow to the ground wire, which remained at 0.0v (since the ground wire didn't have current until now).

The amount of current through the fault could be sizable, and almost certainly above 6 mA. This current flows through outlet B's neutral but not its live wire. Accordingly, the outlet B GFCI detects the difference and trips off.

This situation is only possible precisely when: a) there is current on the neutral wire somewhere, and b) there is a neutral-to-ground fault. The fault may be new, or perhaps you haven't used this circuit in a while.

In any case, the course of action is to identify which of the downstream 14 gauge cables leads to the fault. Since you measured 101.8 v through one cable's neutral, that is probably the direction of the fault, since the fault means you're measuring from live to ground, which should be somewhere in the ballpark of 120v.

The usual course of action to diagnose GFCI issues is to slowly reattach downsteam circuits until the GFCI starts tripping again. That gets you closer to the true issue that's causing the GFCI to trip.

Good luck and stay safe!

[–] Tylerdurdon@lemmy.world 9 points 5 months ago

I really have to go to bed so I only made it halfway, but I appreciate you taking the time out to explain your craft and standards in your field. I will make a strong effort to finish in the morning, but thank you either way, stranger.

[–] notfromhere@lemmy.ml 4 points 5 months ago

Very thorough explanation, definitely helps make sense for my own issues I’ve experienced. Much obliged!

[–] dodeca@lemmy.world 1 points 5 months ago (1 children)

Thanks for the reply.

Yesterday I was able to determine which outside outlet was causing the GFCI to trip and I connected everything else up on the basement outlet and it didn't trip. I went back to it again tonight, pulled apart the basement outlet again and did the same tests with the multimeter I did yesterday and I didn't get the weird 101, just another 0.7.

So I wired it all together again and it's working, not tripping now.

I now suspect that the outside outlet got wet, since it rained yesterday. Does what I observed make sense if there was water inside the outlet? I pulled both outside outlets apart yesterday in my testing, and put them back together better. But it also rained today, though an outside deck light was plugged into it yesterday so maybe that allowed water in. I'm going to replacing the box cover (looks like this) with a plastic flip cover one this weekend.

FWIW, this GFCI has been tripping on the regular every few months. We has suspected the old refrigerator in the garage was the problem, because if we unplugged it the GFCI wouldn't trip. Once we simply replaced the extension chord to the fridge with a better one and it stopped tripping. Once we blamed a kid for running a space heater. Often we'd plug the fridge into a different outlet with an extension chord into the house and wait a couple hours before plugging it back into the garage outlet and it would be fine again. I thought maybe something in the basement could start running at the same time, like maybe the water softener and water heater and the fridge all kicked in at the same moment, but yesterday it was tripping with everything unplugged. So I finally got around to checking the whole breaker and finding every outlet on it. This confirmed that the other appliances are not on the same breaker, which I expected. It's garage lights and 5 garage outlets (3 wall, 2 ceiling for door openers), one outlet in the basement and two outside outlets. (Annoyingly the previous owners or the builders labeled the circuit "garage lights and laundry" but none of it is laundry.) The GFCI was tripping but only killing the 3 garage wall outlets, basement outlet and the two outside outlets.

Anyway, thanks again for the detailed response. Much appreciated!

[–] litchralee@sh.itjust.works 2 points 5 months ago* (last edited 5 months ago)

The electric gremlins and the electronic gremlins have a nasty habit of striking once and then crawling back to their lair to strike another day lol. It makes debugging problems much harder if the problem "went away" on its own.

But in any case, your description of a wet outlet box could be consistent with an intermittent fault. If your outlet was oriented sideways with the neutral (larger pin) facing down, it would only take maybe 1/4 inch of water trapped inside the box to short the neutral screw to the grounded case.

It's also worth noting that GFCI receptacles don't last forever. The nicer ones trip themselves off for the last time when they fail their own internal self-test, never to power back on again. The cheaper ones might become oversensitive with age, tripping for really tiny, harmless current diversions. But 8 or 9 out of ten times -- I'm spit-balling -- recurring trips are due to a real issue in the circuit, rather than GFCI.

Old equipment with motors, such as a fridge, are good candidates for leaking small currents to ground. But it's usually maybe 2 mA or something small. You'd need other appliances that are also leaking small amounts to reach the 6 mA to trip the GFCI.

All in all, since you observed tripping with no appliances plugged in, and I assume all the wiring is readily visible and you've already inspected it for any physical damage, that leaves the outdoor receptacle as the most probable cause.

You might consider looking at how the outdoor outlet box would drain water if it did accumulate inside. Most of those boxes are meant to be water tight, but that also means if water enters, it has no way to exit.

[–] designatedhacker@lemm.ee 6 points 5 months ago (2 children)

Not an electrician, but I have a theory. Both wires should be close to 0 since they aren't connected to the white neutral wire that goes back to the breaker box.

You know this, but don't do any of this without shutting off the breaker between changes.

Often the white neutral wire is tied to earth ground at the panel and you've left the ground wire connected to the other outlet. Something at the other end (or along the way) to the other outlet is partially tying ground back to neutral.

The white wire that you tested at 0.7 while disconnected is probably fine it doesn't have a path back to the panel. The other one is suspect. You can reconnect them one at a time and I bet the 101V run is the one that blows GFCI.

GFCI trips if the current going down the black wire isn't coming back on the white wire. That missing current is probably going to ground and not in a good way.

Water in an outdoor outlet box? Irrigation system gone bad? Bad outdoor light? Extension cord in a puddle? Staple through a wire? Etc.

[–] dodeca@lemmy.world 3 points 5 months ago

Thanks for the reply.

You are right, the 101V one would trip the GFCI. I traced it to the backyard outlet. I connected everything else yesterday and left it like that and it was fine today.

I went back to it again this evening, pulled apart the basement outlet and tested them all again. It was giving 122, 0.7 and 0.7, so no weird 101 today. I wired it all back together tonight and it's not tripping. IDK what happened, but I think the outlet might have gotten wet. I pulled apart each outlet yesterday and put them back together better, but I can't remember what order I did what. I'm going to leave it like this with a lamp for the next few days on so I'll know if it trips again.

[–] notfromhere@lemmy.ml 1 points 5 months ago

I had this happen to me when I installed a new outlet in a bedroom. I ended up rewiring every outlet and finally figured out it was when I buttoned up one of the outlets the ground was barely shorting to one of the blacks. Drove me nuts for a couple days…

[–] dodeca@lemmy.world 5 points 5 months ago

Here are pictures of the other black to white multimeter readings.

The 01.9 will reduce to 0.7 if I hold it there a while.