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I have Christmas lights (a mix of LED and small incandescent) on the outside of my house, along the eaves. I had them plugged into one of our exterior GFCI outlets via an outdoor-rated mechanical timer switch, and everything worked fine for a few weeks until the GFCI tripped. I figured the snow/ice on our roof must have infiltrated the lights, caused things to trip, and that we'd just be light-less until I'd inspected all the connections between the strands.

I reset the outlet, and on a whim I tried plugging in the lights directly, instead of via the timer--and they lit up fine! I added the timer back in, and pop.

I've since tried three different timers, mechanical and digital, on two different outlets, and it's always the same: Plugging the lights directly into the outlet works fine. Plugging them in via a timer trips the GFCI. The timers work fine when switching a different load, so they're good.

Can anyone explain what's going on here? Why would the presence of a timer cause a GFCI to trip?

(The only thought I have is that perhaps timer plugs power themselves by going directly from hot to ground, for some reason. Under normal circumstances, the current draw is so minimal it won't trigger a GFCI, but when combined with slightly-wet Christmas lights that have their own tiny bit of leakage, the total leakage current enough to trip the GFCI. This feels like a major stretch, though, since the manual for the timer specifically says to use a GFCI outlet.)

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  • You could easily test the theory of lights and timer both having leakage which combined trips the GFCI. Plug the lights and the timer both to an outlet strip and see what happens. Also, is the GFCI trip instant in of these circumstances, or is it always a delayed result? Are we working with a GFCI receptacle directly, or an ordinary receptacle with an upstream GFCI receptacle or circuit breaker?
    – Greg Hill
    Feb 7, 2022 at 23:23
  • Are the lights 2-prong or 3-prong plugs? Same question the timer. Feb 7, 2022 at 23:24
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    Oh, I like the power strip idea, @GregHill! To answer some questions: - The trip is instant. - The GFCI that's popping is in the receptacle. - Timer is 3-prong. Lights are polarized two-prong, plugged into a 3-prong extension cord. I did some other tests this evening, and noticed that the mechanical timers will always pop the GFCI, even if the lights are configured to be "off" while a digital one only pops when they're actually "on." Thought that was kinda neat.
    – mucow
    Feb 7, 2022 at 23:34
  • Try using an outlet strip that has RFI suppression (electronic noise suppression) and MOV over-voltage protection, if you have one on hand or can borrow one (they're US$20 or more, so hardly worth buying for that use), and place the strip in the GFCI outlet, then the time and lights, to see if a short transient from the sudden switching of the timer is the cause of the issue. Feb 8, 2022 at 0:53
  • What make and model is the timer plug in question? Can you get it to trip with a different load? Feb 8, 2022 at 2:33

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Well, I am not sure what causes the issue but have found an exterior timer/remote switch (has app for cell phone), that apparently has solved my problem.
The ENERGIZER SMART Wi Fi Outdoor plug. This plugs into your exterior outlet, and has two individually programable outlets(15 Amps per outlet).

I had tried several other timers with no success. Even contacted my local electrical company about the issue. They told me that they would research it but never got back to me. Every other timer I used tripped the GFI as soon as I plugged it in, but this one WORKS ! Hope this solves everyone else's problem like it has mine.

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