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[SOLVED] GFCI tripping when transformer is plugged in.

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Junior Member level 1
Jun 9, 2022
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Hello I am attempting to fix my hot tub controller. The ancient stablok gfci dual pole breaker that was in there was broken on one pole so I replaced it with a square D spa box with a gfci dual pole breaker inside, leaving that box is 2x hots(black, red) 1x neutral(white) and 1x ground.(green)

inside the box:
L1 Breaker in: Black -> SpaBox
L2 Breaker in: Red -> Spabox
L1 Breaker out: Black -> Controller
L2 Breaker out: Red -> Controller
ground bar: In and out Ground Connected
Neutral bar: In, Out, And breaker Neutral pigtail connected.

On the controller is a transformer that I believe is a step down 120v-12v transformer (google "ASSY Transformer duplex 240 sv") note it says 240v on the website but you'll notice the label says black 120v white 0v. which is how it was connected.
this feeds a bridge rectifier and powers all the finer circuitry on the control board. if this transformer is plugged in, the GFCI breaker trips immediately and I cant for the life of me figure out why.

First of all I want to make sure i'm understanding the GFCI correctly, so this is how I understand it:
The GFCI will measure leakage current on each of the 2 lines (hot and hot) and if any is leaking more than 5-10ma to ground(earth?) then it will trip.
The GFCI does not protect against overcurrent at all, so a trip is for sure leakage current.

The thing is: Ground (earth) doesn't actually connect to the PCB at all, just to the metal box the pcb is in, which in turn connects to the grounds of the pumps and whatnot.

So in my circuit current goes from black(hot) to white(neutral) via the two pins on the primary coil of the transformer. nuetral is connected to ground at the service panel, so isn't current going between black and ground then? causing the gfci to trip..? how did it work before if this is the case.

The other thought I had is I know transformers have a leakage current in themselves, is it possible the leakage current on the transformer has increased causing it to trip the gfci? I believe the previous breaker(stablok) was tripping too when the hot tub first broke a few years ago, before the stablok breaker itself broke on one pole, so i dont think it's an issue of the square d breaker being more sensitive.

attached is a board that is nearly identical (the transformer I'm mentioning is not in the picture but it connects black and white on primary side and secondary side has 2 yellow lines that feed into the rectifier at the bottom left.)
balboa53210 2500esr2

any ideas on steps i could take to troubleshoot this?
EDIT: I tried connecting only the black and the white to the transformer, and nothing else, left the transformer secondary open and it still tripped the gfci.... sooo too much leakage current in the transformer or is there an issue with running the black-neutral (120v of the 240v into the transformer primary)?
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1) Ground fault is detected when the current between hot and neutral are different, not between two hots. Maybe this is what you meant, but itsnot clear.
2) A GFCI breaker may also break on overcurrent. You have given us no specific information about it.

As noted, the GFCI measures the difference in current is between hot and neutral.
The ground connection is not monitored directly.

With the primary open, check the resistance between one leg of the transformer primary and the transformer case.
It should look open to a multimeter resistance measurement on the highest scale.

So I found my issue: it was the fact that I connected load neutral to the neutral bar instead of the neutral output of the GFCI breaker, that I did not know existed.
I'm still confused about the dual pole GFCI however. I'll start by clarifying a bit:
I didn't realize there was any internal difference between different, identically rated GFCI breakers, so I didn't put up the model of the breaker. it is: Square D 40 Amp Spa Pack

I read that the GFCI has no overcurrent protection and that is why you always run them in series with a regular breaker of the same size. Judging by your responses this doesn't seem to be the case.

You guys are saying that GFCI measures current between hot and neutral, this doesn't make sense to me because in a 240v (NA) two pole breaker you could have current flowing on both hots (L1, L2) and no current at all flowing on the neutral? If it is measuring the difference between one hot and neutral it will see all the current on hot and none on neutral and trip the breaker?

It made so much more sense on a single pole GFCI to me, perhaps you guys can help me understand.


A GFCI measures the sum of all active lines. (But it nit includes the PE (protecting earth current)
If you have a GFCI with N and L it measures the sum of N and L.
If you have a GFCI with L1 and L2 (unusual in Europe) it will measure the sum of L1 and L2.
If you have a GFCI with N, L1, L2, L3 it measures the sum of all these 4 lines.

While one may imagine it as "difference" ... indeed it's the sum.
(In a proper N, L1, L2, L3 system the instantaneous currents may be: +0.1A, -3.3A, -3.1A, +6.3A ... the sum is 0A). Due to phase shift you can not calculate with RMS currents.
In a proper system the sum of all currents is zero at every time.
If the sum is not zero, then there must be a faulty current either to protecting earth, water pipes, concrete ...


Think of it this way:

The two wires carry alternating current, no matter which is hot or otherwise, the current has to flow up one and down the other to complete the circuit.

The sum of the currents if you break the circuit should be zero because they should be equal but flowing in opposite directions.

If you pass the currents through two windings on a magnetic latch or a transformer, and you ensure the windings are identical and in the correct phase relative to each other, the magnetic fields created by the currents will cancel out, there will be no overall magnetic field.

Now imagine you use that magnetic field to operate the trip mechanism, it will not trip if the currents are equal but it will trip if there is a difference because a magnetic field has been created by the imbalance.

So no earth connection is needed at the GFCI but it can still sense if current flows through a different path than along the N and L wires. Any current flowing in or out of the load through a different path, for example ground leakage, will unbalance the currents and operate the trip mechanism.


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