Hot Tub GFCI troubleshooting

[bmbouter]

I am the homeowner of a hot tub that's been working for well for 6 years. One day the water began to provide a shocking sensation which would prevent you from putting your finger fully into the water. I determined the shock was coming from a 12V light mounted in the tub whose water seal had broken. The light has not worked for a long time, but it was still powered from the hot tub's circuit board. A multimeter in the water and the other lead in the earth confirmed the voltage amount. I have disconnected the light (there was only 1) from the board, and the shocking stopped.

Why did my GFCI on the external breaker disconnect not prevent this from happening?

The most likely root cause is that the hot tub or its circuitry are not properly grounded, or that the GFCI is broken or not wired correctly. Here's the mystery though: upon inspection and continuity testing, everything does seem properly grounded and a GFCI breaker properly installed...

Let's start with the circuit panel which uses a 50A non-GFCI split phase circuit. The split phase circuit's ground is connected firmly to the ground of the circuit panel. I verified both with a visual inspection and a multimeter continuity test. For good measure, I ensured the screw holding it was tight, and it already was. I also verified the neutral and two hot lines similarly. Pics below.

The hot tub has an external service disconnect, and that's where the 50A GFCI breaker is installed. I verified similarly that the ground, neutral, and two hot lines are correctly wired to the GFCI. Same visual inspection; same continuity tests passing, and same loose screws check with none being loose. Pics below.

Then the wiring from the outdoor GFCI circuit to the tub was inspected at the tub itself. I visually inspected the neutral, two hot lines, and ground wires are wired to the board correctly and every looks right. I also ran continuity tests with a multimeter for the neutral and two hot lines, all checked out fine. For good measure I checked various metal points on the hot tubs metal internals (various pumps, circuit board box, etc) for continuity against the incoming ground wire and it also showed connectivity. All screws are tight. Pics below.

Both breakers provide power as expected in the ON position and disconnect power as expected in the OFF position. While both breakers are on the hot tub powers on. When I press the "test" button on the GFCI it trips the breaker and a manual reset is needed to power the GFCI breaker on again. This can be repeated over and over and it trips as expected with its self-test each time and powers on as expected each time after reset.

Question 1: So how could this happen? Here are my theories, and I'd like to hear other possible causes too.

a) my inspection reached an incorrect conclusion, something is miswired here and I did not find it. If so, what can I check to investigate if this is the case.

b) The GFCI self-test is working, but somehow the GFCI is actually broken. I don't have a proper GFCI tester, but I've read such things exist.

c) The 12V signal is too weak to trip the 50A GFCI. My multimeter showed 12V, but I didn't see if it was DC or VAC. I could safely measure this again if this is helpful.

d) Somehow the hot tub circuit board was to blame? Extra info: a spa company tested this board 4 months prior to this event due to a motor issue being investigated. Their statement was "it tested correctly on their test equiment". It's still a possibility though? How would a board be able to do something the GFCI wouldn't detect though?

e) your suggested root cause.


Question 2: Is possible a bad hot tub board could create an unsafe situation in a way the GFCI could not protect against?

Question 3: Is installing an additional 50A GFCI Square D breaker in the main breaker panel to provide a 2-layer GFCI protection sound useful or ridiculous?

Question 4: What would you do if you were me and your family went in this hot tub?

 

[bmbouter]

Well I got the testo and I tested the shorting underwater light what I believe is correctly this time, pic for amp measurement attached. It reads a stable 0.76 mA and 20V. Notice it also shows a nearly perfect 60Hz signal. Image upload seems to not be working on the forum so here's my pic link: New photo by Brian Bouterse - https://photos.app.goo.gl/hXwTB2GniHrXwvDf9

So that's the kind of power we're working with. It's less than 5mA which explains why the GFCI doesn't trip. Do you agree?

 

[DPG]

Well I got the testo and I tested the shorting underwater light what I believe is correctly this time, pic for amp measurement attached. It reads a stable 0.76 mA and 20V. Notice it also shows a nearly perfect 60Hz signal. Image upload seems to not be working on the forum so here's my pic link: New photo by Brian Bouterse - https://photos.app.goo.gl/hXwTB2GniHrXwvDf9

So that's the kind of power we're working with. It's less than 5mA which explains why the GFCI doesn't trip. Do you agree?

That looks like a more realistic figure.

 

[Lucien Nunes]

What is the meter actually measuring there? Pool water to true ground? Clearly that current wouldn't trip.a GFCI and isn't likely to be harmful, but I'm still not sure what it represents. Aggregate leakage from the whole electrical system? Leakage across the lighting transformer? Difference between supply ground and true ground? We're only seeing the symptom, not the cause. Insulation tests and differential leakage tests will locate its source...

 

[pc1966]

Certainly that reading looks more realistic, though as Lucian has said, it is not clear exactly where you are measuring this (i.e. what is each probe connected to)?

 

[bmbouter]

I should have given info about how the test was conducted in addition to the data observed. Thank you for teaching me this lesson also.

The test performed was with 1 probe in the water itself, and the other in the dirt/earth next to the tub. The dirt is very dense, so I believe it provides direct, low impedence conductivity to the earth.

 

[Lucien Nunes]

It depends what you are comparing it to. There could still be many kilohms or tens of kilohms between your probe and true earth, insignificant when taking a voltage reading, significant when measuring current and barely conductive at all compared to the main ground connection.

But the law of diminshing returns sets in about now. You have some measurements that imply there is some leakage that might be worth investigating, albeit that the leakage only occurs via the present path when the insulation of the underwater lights is compromised. If you want to trace its source, then tests of a more specific and targeted nature are required. I can come up with about half a dozen completely different explanations for your 20V and 0.76mA, but with a dozen other tests I could prove or disprove each theory, in a logical sequence. We don't have much info (not being there with you) and you don't have much test equipment, so I am not sure how far we can help with that. It might be the time to get a service agent or electrician skilled at fault-finding to give it a proper series of tests.

 

[pc1966]

The test performed was with 1 probe in the water itself, and the other in the dirt/earth next to the tub. The dirt is very dense, so I believe it provides direct, low impedance conductivity to the earth.

You would be surprised by how hard it is to get a genuinely low earth rod impedance!

Even so, the test you did implied an impedance of around 26k but a portion of that could be the test probe in to the ground. That would make little difference to the 20V you see, but might impact on the current you observe. Same goes for the size of the conductive object in the water, if you have a foot or two of bare wire in the water it would be better connected to the source.

I would be reluctant to suggest you drive in an earth rod for the sake of it as you might just hit a buried cable/pipe, but you could try using a long wire to an internal (power supply) earth, or something like a metal water pipe entering the building.

But as already said, you have shown readings that are more in keeping with the failed low voltage light than a leak from the AC power itself.

 

[bmbouter]

I had an electrician company that deals with hot tubs specifically come out, and they confirmed the tub, GFCI, and electrical disconnect are all wired correctly. Also the wires going through the hole now have a grommet.

Unfortunately this hot tub electrician could not perform the earth leakage current test, perform RCD trip time measurements, or perform the insulation resistance test. It's frustrating to understand these tests, yet when I asked about them they say things like "what is that" or "that sounds complicated".

Thanks for all the help from this forum!