# Strange GFI behavior.
**URL:** https://boards.straightdope.com/t/strange-gfi-behavior/849539
**Category:** Factual Questions
**Created:** [March 15, 2020, 5:01pm UTC](https://boards.straightdope.com/t/strange-gfi-behavior/849539 "2020-03-15T17:01:31Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@dauerbach](https://boards.straightdope.com/u/dauerbach)
#### Post date: [March 15, 2020, 5:01pm UTC](https://boards.straightdope.com/t/strange-gfi-behavior/849539/1 "2020-03-15T17:01:31Z")
I have an outdoor GFCI outlet which powers a pond pump. The pump stopped working over the winter and testing parts seems to indicate the pump itself is the problem. When I reset the GFCI the pump works for about 90 seconds and then the GFCI trips. This seems really strange to me. Why would it work for a little while and then trip?
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### Author:  [@beowulff](https://boards.straightdope.com/u/beowulff)
#### Post date: [March 15, 2020, 5:46pm UTC](https://boards.straightdope.com/t/strange-gfi-behavior/849539/2 "2020-03-15T17:46:06Z")
There’s probably a filter in the GFCI that takes awhile to integrate the current imbalance. I would guess you have a very small leakage, and it takes that long for the GFCI to react to it.
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### Author:  [@dauerbach](https://boards.straightdope.com/u/dauerbach)
#### Post date: [March 15, 2020, 7:00pm UTC](https://boards.straightdope.com/t/strange-gfi-behavior/849539/3 "2020-03-15T19:00:02Z")
Thanks Beowulf. I do not understand how they work and never thought they were complicated. Replaced the pump and it works just fine now.
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### Author:  [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)
#### Post date: [March 15, 2020, 7:48pm UTC](https://boards.straightdope.com/t/strange-gfi-behavior/849539/4 "2020-03-15T19:48:39Z")
> [@beowulff](#):
>
> There’s probably a filter in the GFCI that takes awhile to integrate the current imbalance. I would guess you have a very small leakage, and it takes that long for the GFCI to react to it.
I did a little research on this. Many GFCIs will simply trip within 25 ms when it senses a leakage current between 4 mA and 6 mA (or greater). But UL943 allows longer trip times for low levels of leakage current. The formula is
t = (20/I)^1.43
where I is the leakage current (mA) and t is the maximum allowable trip time (s). So according to UL943, a GFCI receptacle must trip in less than 5.6 seconds when it senses a leakage current of 6 mA. I find that interesting.
So… I guess it’s possible the OP’s GFCI is a “smart” unit that allows longer trip times for low levels of leakage current. According to the equation above, a trip time of 90 seconds would indicate a leakage current of 0.86 mA.
Another possibility is that the pump only develops a leakage current a certain amount of time after power is applied to the pump, perhaps due to a temperature or vibration issue.
Personally I would just replace the GFCI. They’re cheap. If the new GFCI trips then you know there is a problem with the pump or cable.
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### Author:  [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)
#### Post date: [March 15, 2020, 7:51pm UTC](https://boards.straightdope.com/t/strange-gfi-behavior/849539/5 "2020-03-15T19:51:49Z")
> [@dauerbach](#):
>
> Replaced the pump and it works just fine now.
Oops, just saw this. Good deal. 🙂
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### Author:  [@Marvin\_the\_Martian](https://boards.straightdope.com/u/Marvin_the_Martian)
#### Post date: [March 16, 2020, 4:37pm UTC](https://boards.straightdope.com/t/strange-gfi-behavior/849539/6 "2020-03-16T16:37:43Z")
It’s also possible that the pump must be generating pressure for a while before before water is forced into wherever the ground fault is occurring.
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### Author:  [@jjakucyk](https://boards.straightdope.com/u/jjakucyk)
#### Post date: [March 17, 2020, 1:26pm UTC](https://boards.straightdope.com/t/strange-gfi-behavior/849539/7 "2020-03-17T13:26:23Z")
> [@Marvin\_the\_Martian](#):
>
> It’s also possible that the pump must be generating pressure for a while before before water is forced into wherever the ground fault is occurring.
Or it took 90-some seconds for the pump to become sufficiently primed that the load on the pump motor increased enough to cause the fault.