Strange pool pump breaker tripping

[Siracusa]

I have a 20 amp gfci pool pump breaker that trips only when the pump is off during the night. Pool pump runs fine all day. I turn pump off manually at night by using the manual switch on a Intermatic pool timer. Watch it for 5 minutes and note that breaker remains in on position. Go out in morning and breaker is tripped. Breaker is in a protected box with no water intrusion, no rain on these days etc. The only item on the circuit is the 220 pump.

Pool timer pins have been removed temporarily so we dont worry about timer clicking on.

Since timer is after the breaker and is set to off, doesnt seem the timer or the pump itself should have anything to do with the tripping.

Any ideas? Many thanks

 

[Lucien Nunes]

As it is a 240V pump, if the Intermatic timer contacts are only disconnecting one hot leg of the power feed, the motor windings can remain live via the other leg when switched off and therefore can still trip the GFCI if there is a problem. The ground leakage from the windings could be increasing as the motor cools down, either because a small amount of moisture is then able to build up, or just because something contracts as it cools.

The logical next step is an insulation resistance test of the motor windings both hot and cold.

 

[Siracusa]

As it is a 240V pump, if the Intermatic timer contacts are only disconnecting one hot leg of the power feed, the motor windings can remain live via the other leg when switched off and therefore can still trip the GFCI if there is a problem. The ground leakage from the windings could be increasing as the motor cools down, either because a small amount of moisture is then able to build up, or just because something contracts as it cools.

The logical next step is an insulation resistance test of the motor windings both hot and cold.

Super- very helpful.. thanks lucien