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I recently got some of these auto-dimming, light-sensitive, night light / emergency flash lights and plugged one in into a grounded (three-prong) GFI outlet in the bathroom. In the few days since I did that, I've found twice that the GFI had tripped. I have not had trouble with this outlet before, including when using it with a simpler kind of light-sensitive night light.

More details:

  • Neither the current night light nor the older one have a ground prong.
  • The outlet is dry and clear of obstructions.

Why would this kind of night light cause the GFI to trip? Should I simply not use this light in this outlet, or does this point to a possible fault in the circuit? How would I check?

(Context: USA, 1950s-built house)

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    Does this device have a ground pin (i.e. a 3-prong plug)? What happens if you move it to a GFCI on a different circuit (e.g. kitchen)? – Harper Dec 21 '16 at 14:50
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    Without seeing the guts of the nightlight, I don't think there's any way to know for sure. Looks like it only has two prongs, so it shouldn't be leaking current to ground. Doesn't look like there are any exposed metal parts, so it shouldn't be a shock hazard. Probably just some crap circuitry putting noise on the line, which is causing the GFCI to trip. – Tester101 Dec 21 '16 at 17:23
  • @Harper Yes, the outlet is grounded. I added that to the question. I forgot to test on a different circuit. I'll try that tonight. – Isaac Moses Dec 22 '16 at 20:07
  • Sorry, I don't mean if the receptacle has a female ground hole (void). Of course it does. I mean to ask Does the Night Light have a (male) ground prong? Is anything else conductive touching the night-light in any way whatsoever? Is it wet? I'm fishing for any conceivable way which current could escape the night-light, other than via the neutral. – Harper Dec 22 '16 at 20:16
  • @Harper Sorry; I didn't read carefully. Fixed. – Isaac Moses Dec 22 '16 at 20:18
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A GFCI device compares the current flowing on the "hot" to the current returning on the "neutral". In principle, the only way a GFCI can trip is if current finds some third path.

What about battery charging? Isn't the current going into the battery and not returning? NO. Think of a battery charger like an irrigation water pump driven by a water-wheel. All the current going into the water-wheel falls out the bottom. That turns a shaft. And on the other end of that shaft, the pump pushes a completely different stream of current. The GFCI is akin to having current meters on top and bottom of the water wheel proper, to make sure all the water that enters the wheel, exits. It has nothing to do with the pump current.

So, if your nightlight has only 2 connections, hot and neutral, then there is no third path - every electron which enters via hot must exit via neutral. A GFCI trip should be impossible. That would seem to leave 2 possibilities.

  • Something about the electronics of the night light is creating noise or signal delay in a way which is irritating the GFCI. This could be exacerbated by a defective or poor-quality GFCI.

  • There actually is a ground fault, but it's somewhere else and the problem is being triggered by the presence of the nightlight. Picture a broken receptacle where the neutral is very close to the ground, and plugging the nightlight in shorts neutral to ground.

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