Thank you in advance for help with this mystery.

We detected a voltage in our hot tub, my wife detected a small shock.

My multimeter detects between 9-11v. The unit has a 10ma rcd which doesn’t trip. Nothing else on the circuit trips either. Even switched off it still reads the same voltage. Only if we remove the plug from the socket it returns to 0v.

An electrician has checked the circuit to the plug, changed the plug socket etc. He can find no fault.

The hot tub plugs into a plug that is a spur from a garage circuit.

He is stumped and so am I. I cannot think what else to check. Also just to add if we plug the unit into the main house circuit, the unit reads 0v.

Don’t really know what else to look at.



  • 2
    the plug will not be the issue, instead you want to check the tub and plumbing of the tub for leaks where this may contact the electrical bits. Jun 25, 2021 at 10:38
  • 2
    Where actually was the voltage and what did you use to measure it ?
    – JACK
    Jun 25, 2021 at 11:21
  • Was the small shock you wife detected just momentary as she entered the tub or was it continuous? Jun 25, 2021 at 12:52
  • The voltage was between the water and the ground. We put the live tip of the multimeter in the water and the neutral in the ground. That gave us the voltage reading. I understand that it could be the unit causing the issue, however if it is plugged into the house circuit the voltage reads 0v. If it was the unit would it not cause the issue in every socket.
    – user137734
    Jun 25, 2021 at 13:40
  • 2
    I don’t know how your system is wired but a parallel path on the ground can cause this. In the US we call them boot leg.boot legs when neutral and ground are connected in the field (the only place they are allowed to be connected is in the main panel). A difference in grounding methods at the garage and the home can also cause this and the reason US code requires 4 wire connections to a sub and if detached from the main structure additional grounding rods. We can feel a shock in water at about 8 volts but the current is usually only a couple of milliamperes (not lethal) voltage +15 gets closer
    – Ed Beal
    Jun 25, 2021 at 15:19

2 Answers 2


Based on your diagnostics, you are plainly running into hot-earth leakage from the garage circuit. It does not have a (dedicated, safety rated) RCD.

The problem is that the in-appliance RCD is inadequate. It does nothing to protect the electrical socket and the wiring up to that electrical socket, i.e. things on the "Line" side of the RCD.

So my advice is go "American style" and fit a safety rated RCD (5ma, or if it's the fashion over there, 10ma) at the head of the garage circuit - e.g. a receptacle based device at the first receptacle, or an RCBO circuit breaker at the consumer unit.

That way, all the wiring anywhere near the hot tub is also protected.


9 to 11V is unlikely to cause 10mA to trip the RCD. But it mught give you an alarming tingle.

This is one of those cases where you really need to know what voltages you are measuring. A multimeter has two probes, so the 9 to 11V is between what and what?

What did the electrician check? Don't assume that the Earth pin on the socket is at 0V when compared to the soil around the hot tub. It should be, but that doesn't mean it is. Don't assume that the neutral is at 0V either, without measuring it.

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