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What are typical causes of nuisance trips with two pole GFCI breakers with neutral (Not 240V)? What techniques can I use to diagnose them?

I put a two pole GFCI breaker on an MWBC circuit, as discussed here.

Currently, one of the legs "A" is connected to some existing room wiring. A few lights and outlets. The other leg "B" just has one 4W light bulb nutted directly to it for testing. (It's all temporary, the cables will be rerouted and each used for a single hard wired bathroom fixture).

In the current configuration, the breaker pops sometimes. It never pops randomly on its own. It popped several times when I turn on a light on leg A. For a while it would not reset, but when I removed the 4W bulb on leg B, it did reset .... then I replaced the bulb and it stayed on. Then I turned various lights on and off and it still stayed on and I removed and replaced the 4W bulb ... everything good.

I cannot identify a specific set of circumstances that reliably make it pop.

I'd like help with how to diagnose this. I'm concerned about the things I don't know about two pole GFCI breakers.

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  • I'm not ready to answer my own question yet but I'll start collecting thoughts here. One possibility is an actual wiring problem in the existing room wiring connected to Leg A. I can diagnose that by disconnecting it and connecting test loads to both legs. This article describes some diagnostic techniques.
    – jay613
    May 12, 2022 at 16:09
  • Are any of your switches anything other than "simple mechanical" - e.g., dimmers, smart switches, motion detectors, lighted switches, etc.? Some of those types of switches will use ground instead of neutral (legitimately) and a cumulative amount of current going back on ground instead of neutral will be enough to trip a GFCI. That's one reason why GFCI in Europe (RCD), which covers a large part of a house on each protected circuit, has a higher trip current than US GFCI. May 12, 2022 at 17:39
  • Not really. One light is a ceiling fan with its own remote-control dimmer, but the trips are occurring even when the wall switch to that light is off, ie all power is removed from it. There is a 19V DC PSU plugged in somewhere on the circuit. That's it.
    – jay613
    May 12, 2022 at 17:58
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    @manassehkatz it would take +5 of the switches that use ground if they are UL listed and for this reason there is supposed to be a limit of those devices per branch circuit.
    – Ed Beal
    May 12, 2022 at 19:38
  • Or 1 if someone didn't have neutral and used ground instead (against the UL listed installation instructions). But then it would likely trip very frequently. May 12, 2022 at 19:42

2 Answers 2

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I found the actual problem, and I'm extrapolating here to what I think is one good general answer to this question. Don't do what I did here: Don't assume a GFCI trip is a "nuisance" one while dismissing the most obvious cause: The GFI was tripping because there was a ground fault.

And what's more, the breaker's instructions, which are only about a page long, provide a clear way to determine that. So with egg on my face, one answer is:

  1. Read the instructions.
  2. Find the ground fault.

I temporarily connected the new cable from the new breaker to existing house wiring. In the box where I made the connection there is some old BX cable.

Usually when I touch old BX I tape the brittle wires as best as I can. I did not do that because it looked good enough and only needs to last a couple of weeks. But, something in that box was causing the trip. I can't be sure what it was, but must assume a neutral with cracked insulation was shorting to ground. If it was anything else the old breaker would have tripped. (I hope !)

The clamp meter techniques from the Fluke articles that I linked in a comment were not useful. As predicted by those articles themselves, the meter sensitivity is 10mA and the breaker trip current is 6mA. Even after experiencing a break, the meter's max reading showed zero.

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    This is true in your case and some others but I can take 2 ungrounded toasters on isolation pads (no fault current possible and make a double pole GFCI trip in a few try’s, this was the control for a college project.
    – Ed Beal
    May 12, 2022 at 19:32
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This is the reason MWBC have gone out of fashion. The current can be drawn across the 2 hot legs with another load or through the grounded neutral depending on the impedance and the path can change while in use this confuses the breaker and GFCI is fail safe so they trip, there are ways to make a MWBC work but it normally takes multiple GFCI receptacles not using the load terminals on each branch of the MWBC.

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  • Makes sense but why then do they put neutrals on two-pole GFCI breakers? What is the intended use for that?
    – jay613
    May 12, 2022 at 17:52
  • If you have a 240V-only load, the neutral isn't needed. If you have a mixed load then you do. For example, a typical clothes dryer will use mostly 240V for the heating element but controls, lights and possibly motor are 120V using just one leg. So the load is not balanced between the hots. GFCI across the two hots would trip every time you turned the dryer on. By including neutral, it can check for net of the two hots against neutral and only trip when there is a real problem. May 12, 2022 at 18:57
  • One of these days they will probably get good enough to work but now they only occasionally work they have to detect power leg to leg and then to the neutral this is doable but not with the simple circuit that is standard in GFCI’s., I actually built a circuit that did work in college (electronics program) I do have equipment if sufficient sensitivity to detect ground faults at much lower levels micro amps no faults and they still trip.
    – Ed Beal
    May 12, 2022 at 19:25
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    @manassehkatz-Moving2Codidact my question was not about theory but to say there must be some situations where these things work reliably? Not just dryers? In my case, I'm using a two-pole partly because I chose to run /3 cable, partly because they were in stock and I couldn't find one-pole anywhere .... but this is two circuits to a bathtub so honestly, I WANT common trip. That's a good thing isn't it? Regardless of my cable choices and the state of supply chain ... Common trip to a bathtub with a fault is GOOD right? So still I ask, what are these for if they never work?
    – jay613
    May 12, 2022 at 20:00
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    “they have to detect power leg to leg and then to the neutral this is doable but not with the simple circuit that is standard in GFCI” This does not accurately describe how GFCIs work. They run all the monitored wires through the core of a current transformer. If there is an imbalance, it will be detected. The transformer works exactly the same whether it’s monitoring hot/neutral, hot/hot, or hot/hot/neutral.
    – nobody
    May 13, 2022 at 3:21

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