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I have a home lab that I try to keep some level of static protection on, so I have a cheap anti-static mat that is grounded to a normal outlet in my lab. I recently installed a AGFCI circuit protector on that lab space (extra bedroom) because I'm sometimes tired working with high voltage electronics and do stupid stuff. I now have had a few nuisance trips usually at night and when it is cold, when I flip the breaker back on it usually says "ground to neutral" or "arc fault to ground", It's very strange to me that this happens most when it's cold, does anyone have any thoughts

Sam
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    I’d suggest double checking the wiring to your outlet to make sure your mat is connected to earth (vs neutral) and that the earth has a low impedance. As well, the mat should have a 1M resistor in series to the earth. – Kartman May 26 '22 at 00:19

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I can't tell if you have an AFCI or GFCI/AFCI dual-mode device.

AFCIs have two operating modes of interest.

  • A digital signal processor that "listens" for the "crinkle-crunch" "sound" (electronic signature) of arcing. If you ever hooked up stereo speakers with the power on, you know the sound.
  • A "GFCI-like" device measuring the difference between live/hot current and neutral current. An AFCI detects at the 30mA threshold, but AFCI/GFCI dual-mode breakers protect at 5mA threshold as do GFCI-only breakers.

Why are AFCIs interested in GFCI-like detection? It's a cheap/simple/reliable way to detect "hot-ground" and neutral-ground" parallel arc faults, since those are also ground faults.

Note that a "combination AFCI" is not AFCI/GFCI. It is a full-featured AFCI, detecting series arc faults (arcing along hot or neutral) AND parallel H-G N-G and H-N shorting arc faults.

Here's the important bit.

Neither AFCIs nor GFCIs have any connection whatsoever to earth.

That's most obvious on circuit breakers which have no contact with safety ground. Yes, xFCI receptacles take a safety ground wire, but that's only for the socket and yoke (frame). It does not connect to the xFCI mechanism at all.

So, whatever is causing your trip is unrelated to tying your anti-static mat to safety ground. It is a red herring. Your problem lies elsewhere.

We see a lot of people pathologically replace their GFCI or AFCI device, starting at the assumption that it's faulty. But that's only because they don't know what else to do. Failed devices do happen but don't be distracted - it's not that likely.

Usually a trip of a GFCI or AFCI is exactly what it says on the tin.

Review your breaker's instructions to make sure you are understanding the diagnostic info in full detail.

Since it is reporting a neutral-ground fault, I would work the trouble tree assuming that's exactly what it is.

Harper - Reinstate Monica
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  • It's a dual mode breaker in the panel, thanks for the info. It's a very strange problem, I have unplugged the mat for now to try to diagnose, but it's really difficult to debug when the "trigger" is weather. I almost wonder if the breaker is getting backfeed from the power company and tripping? the cold weather often corresponds with snow and power outagages? – Sam May 27 '22 at 03:46
  • @Sam Correlation is not causality :) You installed the dual-mode breaker because of the static mat, but that doesn't mean the static mat has anythig to do with the problem :) Have you identified ALL the receptacles and loads powered by that circuit? (you need to identify receptacles in order to comply with Code: NEC 110.3 "follow instructions", since instructions 8(C) require labeling all outlets as "GFCI protected" and "No Equipment Ground" where the latter is applicable). – Harper - Reinstate Monica May 27 '22 at 17:18
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The matt must not be less than 1 Mohm to PE gnd to limit current and risetime of discharges. That should fix it by adding 1M in series with the wire to gnd.

eh it's a GFCI unless you have a British accent ;) or it's a Class A type.

Tony Stewart EE75
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