I just bought a pop-up pool for my kids, got it up, running, and all the chemical levels correct.... now i just have one problem: When the timer I bought to regulate when the filter runs turns off, it also trips the breaker/GFCI (not really sure what its called, but there is a red reset and a yellow test button) on the end of the pool filter power cable. Why is this happening?
Switching GFCIs line-side tends to make them a bit cross
Plug-type GFCI devices revert to the "tripped" state when unpowered, unlike receptacle-type designs, which mechanically latch in both "tripped" and "untripped" states. This was done to allow them to protect against an open neutral, which'd otherwise defeat the GFCI protection and make it impossible for it to trip.
However, this makes them incompatible with being switched on the line side, as you see here. The solution is to move the GFCI protection to a location upstream of the timer -- GFCI breakers are ideal here, but a receptacle or deadfront GFCI can be used upstream of wherever the switch or timer is. Once you've done this, you'll have to remove the GFCI plug from the end of the plug-in pool's cord and replace it with a regular plug, with a big note on it saying "FOR USE IN GFCI PROTECTED OUTLETS ONLY".
Either that, or you can remove the plug and wire it into a timer that's then wired in series with another length of appropriate cordage that is terminated in the existing GFCI plug, effectively moving the existing GFCI upstream of the timer...
I had the same issue, even though I'm a general contractor it still took a pause to figure out. If you can picture the issue as a "ground sandwich" then it would make more sense. The timer acts as a ground once chosen to not operate or provide electricity, as such the pump tail end circuit trips on this ground. I solved this issue by replacing the GFCI tail end of my pump cord with a standard male connector. This should not be done if the actual outlet of which the timer is plugged is not GFCI.
That being said, the existing tail end GFCI plug to my pump simply unscrewed, not drastic cuts necessary. The white and black (neutral/hot) end of wiring plugged right into a male adapter I picked up at Home Depot. I wrapped the ground (green) wire in electrical tape and tucked it into the new assembly. Plugged it in, tested several times and KABOOM! it made sense.