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I'd like to test if my hot tub was connected to ground.

The hot tub is underground and its electrical panel is not easily accessible. I'm trying to figure things out without access to the hot tub's panel.

The hot tub is 240V single-phase (TT earthing system) and is connected to a 10mA GFCI in my house's main panel. I have verified this connection and it's definitely taking current from the GFCI.

What I don't know is if the guy that installed it wired the ground properly. I tried taking a multimeter to test for continuity between the house's ground and the hot tub water, but it didn't show anything. If I test for voltage between a phase and the hot tub water, I get 240V.

Is the hot tub water itself not supposed to be grounded?

Any other test I could perform or that I could ask a licensed electrician to perform?

The GFCI never tripped so far, so maybe no valid reason for concern. Just trying to make sure as there are 2-year-olds using the hot tub.

  • Hello, and welcome to Home Improvement. Interesting question; let's see if you get a good answer. And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Oct 3 at 0:22
  • Where are you (nation will suffice) and what kind of power do you have? – Harper - Reinstate Monica Oct 3 at 16:36
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I tried taking a multimeter to test for continuity between the house's ground and the hot tub water, but it didn't show anything.

You should not see anything if the ground were properly connected. Most of the time, the hot tub is fiberglass and the piping is plastic, so there should be no connection to ground, EXCEPT via the pump housing, assuming a metal volute (the chamber part of the pump) and a proper grounding conductor on that pump housing. So showing no voltage between ground and the water means they are are the same potential, which is what you want.

If I test for voltage between a phase and the hot tub water, I get 240V.

As you should in your system, because the neutral of your 240V TT system is connected to ground somewhere and so is your water (again, through the pump volute and the pump body being properly grounded), so measuring between those two points should show as the full 240V.

  • One misunderstanding: I checked for continuity between ground and water, not voltage. My multimeter says there's no continuity between house grounding and the water. – rwilsder Oct 3 at 1:32
  • Hot tub has a heater and 3 pumps. It's a Caldera Martinique. – rwilsder Oct 3 at 1:40
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    Water is a poor conductor (but not "zero") so a simple continuity tester won't necessarily tell you much one way or the other. – manassehkatz-Reinstate Monica Oct 3 at 1:57
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If I test for voltage between a phase and the hot tub water, I get 240V.

I'm a bit concerned about that. The big question is how much current is flowing during the test. If it is > 10mA then it should trip your GFCI, as you are leaking current through the hot tub water, which is exactly the hazard that a GFCI is supposed to prevent.

Is there anything else on this GFCI-protected circuit? If the GFCI circuit goes to a subpanel and there is a convenience outlet there then you can test it with a GFCI tester. If not, a little more work may be involved. It also seems odd that The hot tub is underground and its electrical panel is not easily accessible. See if you can trace where the wires go from the circuit breaker to the hot tub. There should be a relatively accessible junction box at the hot tub, even if the wires are coming through an underground conduit. Essentially you want to place a small load between a hot wire and ground that will draw a little more than 10mA. 240V x 10mA = 2.4W. So, in theory, any load > 3W to ground should trip the GFCI. Be very careful doing this, as a problem (fumbling with the wires) combined with the GFCI not working properly, could result in a very shocking experience.

  • There's only the hot tub on the GFCI circuit and the wire goes directly from the GFCI to the hot tub, no junction box or anything. While testing voltage I used the phase taken from a nearby socket which is connected to a different 30mA GFCI. No GFCIs tripped while testing. I can test the GFCI using the test button or at the electrical panel. I don't know how to test if the hot tub water is grounded or not. – rwilsder Oct 3 at 1:33
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    So the testing the water with a meter was a different circuit? That tells you (almost) nothing, and certainly doesn't validate or invalidate the functioning of the hot tub circuit GFCI. – manassehkatz-Reinstate Monica Oct 3 at 1:40
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    Yes, different circuit, but it's connected to the same ground. I'm not trying to test the GFCI. I'm trying to test if the hot tub is grounded. – rwilsder Oct 3 at 1:41
  • Arguably, GFCI may be more important than traditional grounding. Traditional grounding works very fast in an extreme overload. But in a small overload it is much slower (by design). And there can be life safety situations that will not trip at all (or not for a long time - and even a few seconds can be "too long") that will trip instantly with a properly installed GFCI. There are even situations (though not hot tubs) where GFCI is "good enough" that you can skip even trying to hook up a regular ground and still be able to install 3-prong receptacles. GFCI FIRST. – manassehkatz-Reinstate Monica Oct 3 at 1:55
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    I couldn't agree more; GFCI is often more important than traditional grounding. – Harper - Reinstate Monica Oct 3 at 5:46
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If you're in North America or Japan, I am very concerned about this 240V reading between a phase and the hot tub water.

It's possible that your grounding is messed up, and your house is accidentally grounding to a phase. Generally, that happens with a combination of two things:

  • A lost grounding electrode, so the system voltage ends up "floating" relative to ground.
  • A hot-ground fault somewhere, which is biasing the system so one leg is now near ground, instead of netural being near ground as intended.
  • You are then reading 240V from the other leg to ground.

So I would give your grounding electrode system a very thorough once-over, to make sure everything is in order. Don't be surprised if fixing the ground causes a circuit to malfunction; that would be the aforementioned hot-ground fault. Even though it seems like fixing ground broke the circuit, the circuit was already broken and you just improved the fault detection.

With that in good order, I would expect to measure 120V between either pole and the electrical wired ground at its location, and anything else associated with the hot tub.

Hot tub wiring is not to be trifled with - it can kill.

  • TT earthing is used in neither North America nor Japan.... – ThreePhaseEel Oct 3 at 11:42
  • @ThreePhaseEel But OP is pinging hard as being US/Canada. The "TT" is the only thing that's off-beat. I would attribute using that term to us not having a term for that, other than "nothingburger". – Harper - Reinstate Monica Oct 3 at 16:39
  • I'm in Europe. I said the tub is 240V single-phase. Isn't 240V three-phase in USA? – rwilsder Oct 3 at 21:02
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    Our 240V is either split-phase (neutral/ground in middle, giving two 120V legs), or 240V "delta" with three sides of 240V. I assumed you have the split-phase system, ergo my answer is wrong. 240V from hot to ground is not weird. – Harper - Reinstate Monica Oct 3 at 23:14

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