GFCI strange behavior - Page 1
Forum section: electronics / projects

author: calzap
author_rank: Frequent Contributor
timestamp: January 02, 2025, 05:12:26 pm
content: We have a 120 VAC GFCI (aka RCD) outlet in our bathroom that’s used mostly for small space heaters. About every 10th time or so when a heater is switched off, the GFCI trips. It’s done this with 3 different heaters. All the heaters have simple rotary or pushbutton selector switches with 4 positions: off, fan only, low heat, high heat. Fan only position is never used. One has a 3-prong plug; the other two have polarized 2-prong plugs. All have coiled nichrome wire heating elements. The 3 heaters are of different makes.My theory is that the switch is single pole and controls only the hot line. So, when a heater is switched off, the magnetic fields around in the nichrome elements and fan motor field coil collapse and send a pulse down the neutral line. However, the size of the pulse depends on where the AC cycle is when the heater is switched off. Sometimes the pulse is big enough to trigger the GFCI into action. Sound feasible?Doesn’t happen often enough to be worth a fix. Maybe someday I’ll use my scope and probe isolators and try to capture the action. But it will have to be done on the bench, not in our bathroom.Mike

author: jmelson
author_rank: Super Contributor
timestamp: January 02, 2025, 05:59:51 pm
content: Yup, the coiled nichrome element is an inductor. When switched off, you get a voltage spike that makes an arc in the switch contacts. A crummy GFCI may detect this as a fault condition. I'd suggest replacing the GFCI with one, maybe of a better quality. OR, replace the heater with one that had a ribbon - zig-zag heating element. That will have less inductance.Jon

author: Konkedout
author_rank: Frequent Contributor
timestamp: January 02, 2025, 06:01:55 pm
content: Your question is one of the more interesting ones!! I can offer what I would call educated speculation.The key element in GFIs is a magnetic component which resembles a common mode choke. But it is designed to be very sensitive so that a small difference in current between the hot and neutral wires will trigger a fault.Anyway I am guessing that your switching off causes a significant common mode voltage spike. This is not completely surprising. But I suspect that your GFI may have a reaction time which is a bit too fast.One question: Do you always have your space heater resting on the same surface...maybe a tile floor? What happens if you locate it (and its cord) up on an elevated non-conductive surface such as a wood chair?Another idea (be really careful) try an external switch which is bypassed with a resistor and capacitor connected in series. I am thinking of a 100 nF X type (across the mains type) capacitor in series with 100 ohms, connected across your external switch.For both of these: BE CAREFUL!! Don't start any fires and don't give anyone a shock! Both of these are experiments only and are not proposed as solutions.As a solution, you may need to try a different GFI.

author: ajb
author_rank: Super Contributor
timestamp: January 02, 2025, 06:07:58 pm
content: As partly-mechanical devices, GFCIs are subject to wear that can make them trip more easily, and this gets worse over time. Frequent nuisance tripping is sort of an expected failure mode, so if you've already swapped the heater out it's probably time to just replace the receptacle.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 02, 2025, 07:36:05 pm
content: Yeah, nuisance tripping from whatever capacitive inrush current spike or inductive turn-off voltage spike is the usual failure mode, some poor quality products are broken from factory (and it can happen to any brand every now and then, just not that often), some start tripping when older, which would be then the end of their lifetime.Since you have pretty much ruled out the heater being defective by testing three different heaters, it's time to change the RCD/GFCI.One thing is worth checking, though: do you have leakage current behind the same GFCI? Because if you have leakage which makes it close to the nominal trip value, then tripping from whatever small event would not be malfunction. Measuring leakage current is a bit difficult, though; but you can try to remove all other circuits (e.g., by tripping their fuses) and see if that affects your problem at all. If it does, then go through the circuits one by one to find the one which has leakage.

author: calzap
author_rank: Frequent Contributor
timestamp: January 02, 2025, 08:00:21 pm
content: The heaters are located on dry linoleum and not touching anything else. Two have all plastic cases; one has a metal case. All have plastic knobs or buttons. We are standing on dry linoleum and not touching anything else when we turn them off. What's happening is occurring via the cord and not via the case or knob.Mike

author: Aninjaflamingo81
author_rank: Newbie
timestamp: January 02, 2025, 08:18:51 pm
content: Curious, you have anything else plugged into the same circut? It used to be common practice to run more outlets off of 1 gfci. If there are more outlets, is anything plugged into them? The leakage(?) Current from other items plugged in can make them act more sensitive.Replacing the outlet, and confirming good connections would be where we would normally start.

author: calzap
author_rank: Frequent Contributor
timestamp: January 02, 2025, 08:47:29 pm
content: There are other outlets on the same breaker, but they have their own GFCI protection. No outlets are downstream of the flakey one. None of the other GFCIs on the same circuit have unexpected tripping. Only things that are plugged-in continuously on the circuit are a clock radio, toothbrush chargers and WaterPik. Periodically, a hair drier and vacuum cleaner are plugged in (but not to the flakey outlet) and have no issues. Wall and ceiling lights are on the same circuit but w/o GFCI protection. Breaker is standard 20 A breaker; all wiring is 12 AWG NM cable.Mike