Block heaters tripping GFCI

[sw_ross]

What is it about block heaters tripping gfci receptacles!?!

I've been called out more than once because a gfci was tripped by a block heater.

The call I got this evening was from a customer I've done a fair amount of work for that has a ranch with various outbuildings.

Last year i installed a receptacle outside an old garage that is mostly used for storage. I ran the circuit complete to a dedicated gfci. He said the gfci tripped after his tractor was plugged into it for about 10 minutes. I gave him "the talk" about how GFCI's work and maybe something is wrong with his block heater and that the gfci is doing what it was designed to do...

He said the block heater was brand new and doesn't have more than an hour on it yet. I told him I'd come out and take a look.

If the heating element was/is shorting out to ground (tripping the gfci)it must be taking a few minutes to happen according to the guy. I will ohm out to ground on the power cord, also probably ohm out ground to chassis.
Is this one of those situations where a megger would help out in testing the element.
Not totally sure why the heater took 10 minutes to trip the gfci?

Any suggestions for more ways to check out the block heater?
Of course I'll also check out the gfci and circuit as well.

Thanks!

 

[zbang]

Element heats, expands, touches case. You could let it get hot, trip the GFCI, and immediately meter either heater lead to the EGC.

 

[kwired]

What is it about block heaters tripping gfci receptacles!?!

I've been called out more than once because a gfci was tripped by a block heater.

The call I got this evening was from a customer I've done a fair amount of work for that has a ranch with various outbuildings.

Last year i installed a receptacle outside an old garage that is mostly used for storage. I ran the circuit complete to a dedicated gfci. He said the gfci tripped after his tractor was plugged into it for about 10 minutes. I gave him "the talk" about how GFCI's work and maybe something is wrong with his block heater and that the gfci is doing what it was designed to do...

He said the block heater was brand new and doesn't have more than an hour on it yet. I told him I'd come out and take a look.

If the heating element was/is shorting out to ground (tripping the gfci)it must be taking a few minutes to happen according to the guy. I will ohm out to ground on the power cord, also probably ohm out ground to chassis.
Is this one of those situations where a megger would help out in testing the element.
Not totally sure why the heater took 10 minutes to trip the gfci?

Any suggestions for more ways to check out the block heater?
Of course I'll also check out the gfci and circuit as well.

Thanks!

Megger is the best choice for this testing. The higher test voltage of the megger should remove all doubts you may have if testing without a megger.

Make sure there any extension cords or other items that may be in use are in good condition also. They had me check out why a school bus was tripping when plugged in one time. Discovered ground fault immediately, but something told me look a little more as it was some sort of inlet assembly on the front bumper that you plug the extension cord into. Been long enough I don't remember exactly what was there but do remember behind the bumper was something that wasn't all that well sealed and water with road deicing salt in it is about guaranteed to allow enough fault current to flow to trip GFCI's even after mostly dried out.