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I have noticed that when I touch the water of my Hot Tub when I am grounded (barefoot standing outside of tub) I am getting a very small electric shock. Using a multimeter it is reading 2Vac between the water and the ground outside.

I have an RCD on both my ring main and a dedicated RCD on the Hot Tub, none of which trip.

Now the odd thing is that even with the socket switched off (but still plugged in) it still reads 2Vac. If I unplug the socket it reads 0Vac.

How is it reading 2Vac with only neural and earth wires connected?

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    If you google "hot tub electrical shock deaths" there are lots of articles. You should stop trying to troubleshoot this and go hire a proper electrician. – scorpdaddy Jun 28 at 15:37
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    Basically neutral is not zero... So, you need to get this checked, and sorted, quickly. Waiting for you to find it eventually may injure someone... – Solar Mike Jun 28 at 16:08
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    Look closely at the RCDs. Do they have a trip rating? Typical numbers are 6ma, 8ma or 30ma. Also, RCDs are not the be-all-end-all fixer of all defects electrical. – Harper Jun 28 at 16:12
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    US GFCI normally have a TEST button. No idea if your RCD does. But if it does, does the TEST button work as expected? – manassehkatz Jun 28 at 16:20
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    This question could do with a country tag, because earthing arrangements are different in different countries. – Andrew Leach Jun 29 at 6:43
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Edit: This is sounding more and more like a problem with either a neighbor's electrical or the utility's supply. See last entry.

DON'T get in that hot tub again!

The "very small" shock was due to high impedance between you and the current. Impedance is extremely luck-based. Someone getting out of the pool might splash water where it had not been before, now the impedance is much lower and that "small" shock puts you in a wheelchair.

Hot tub miswiring is so common it's legendary. RCDs do not protect you from all wiring errors.

Don't get in the hot tub again until you positively find the cause and remove it.

8ma vs 30ma protection

RCDs also do not protect you from shock, necessarily. In Europe, RCDs have been used for a long time, and for a different reason - protecting houses from fires, not protecting humans from shock. Also, since they use whole-house RCDs, they must use a higher (more permissive) threshold, otherwise small leaks from many appliances will "stack" to cause nuisance trips. So whole-house protection is typically 30ma threshold.

Whereas in the US, GFCI is specifically for human safety, and it is 6ma or 8ma. This is too sensitive for a whole house, so it is applied circuit by circuit on circuits where it is helpful. (near sinks).

Your whole-house RCD would need to be 30ma, which is not enough for life-safety protection. It's less likely to outright kill you, but it's plenty enough to stun you - and when you are in water, being stunned means drowning.

Your hot tub should indeed have its own RCD, at the 6-8ma sensitivity level. However if you walk into a store and buy any random RCD, it's quite possible you'll find yourself holding a 30ma one. Perhaps your hot tub installer did exactly that.

RCDs do fail

I.E. they stop properly detecting ground faults. That's why the device has a "TEST" button. Use it.

It may be miswired to bypass the RCD

In fact, this is likely if the hot tub installer hit a situation where the RCD tripped when he hooked it up correctly. The RCD would trip when hooked up correctly, if the hot tub had a ground fault. Typical installer move is to jury-rig it any method that will work, then get paid.

This can also happen with amateur installers. They typically refuse to learn how to do the job properly (because that takes time learning about subjects they just don't care about), and simply "try random combinations" until one works. Many will work and also kill you. This might be one of those.

If the hot tub has a ground fault, fix it - seriously

Sometimes RCDs trip because the downline device is connected correctly, but actually does have a ground fault. This tends to surprise the heck out of people, they are in full disbelief -- "MY appliance has a ground fault? This cannot be!" (even they know nothing about ground faults).

I've seen people replace RCDs twice simply because they cannot believe they actually have a ground fault. And of course they feel totally justified in bypassing safety protection altogether.

So don't be in denial about that. There is a real possibility that the hot tub has a ground fault. If so, fix it.

It might not even be the hot tub

There's an outside chance this shock is entirely unrelated to the hot tub. It's possible the dangerous voltage is being sourced from some other thing in your yard, and the hot tub is correctly providing grounding, thus completing the circuit. If so, the cure is a thorough search of your house's electrical system for any defects. And if neighbors are close, you may need to bring out someone to test for problems coming from them.

In any case, this is nothing to trifle with.

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    @Lee Oh dear. Then it's possible the shock has nothing to do with the hot tub, and you could sell the hot tub, and be standing in your backyard and get shocked. – Harper Jun 28 at 17:36
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    They are, either through ignorance or lawyers, mixing up grounding and ground fault protection. Bottom line is that when water is involved (a) a little problem can become deadly and (b) double insulation only helps until a nick in the insulation let's water get to where it shouldn't. Listen to Harper. – manassehkatz Jun 28 at 17:39
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    if I test the voltage between the Earth coming into my house (the supplier side of the meter outside) and ground (actually sticking the probe into the soil), it is reading 2Vac as well. Not sure that is the right way to test...... – Lee Jun 28 at 18:11
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    @trognanders I'd be more inclined to suspect that the utility has a stray current issue if it's not an issue in the premises wiring. a floating neutral would cause all sorts of other havoc besides this, anyway. – ThreePhaseEel Jun 29 at 5:04
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    I was electric shocked by touching a non-electric shower. Turned out the shower was well earthed. The floor was live because water leaked through to inadequately earthed trunking underneath. GET A PROFESSIONAL TO FIND AND FIX THE FAULT! – user20637 Jun 29 at 20:07
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I had an electrician to check out the problem and:

  1. I am in the UK and there is only live and neutral at the meter (earth and neutral are the same wire at that point)
  2. The Hot Tub water is directly conncted to Earth
  3. The mains electrics in my property all checked out fine, incoming supply fine.
  4. No faults with the Hob Tub
  5. RCD's tested and working
  6. He is surpirsed I can even feel 2Vac, he could not feel it.

So it's still a bit of a mystery, but if I stand near the Hot Tub (off and unpluggedd) and measure voltage between the ground (the actual soil) and the Earth wire, it reads 5Vac, no current. If I switch off ALL power (break live and neutral) at the meter I still get the 5Vac Anyway, the Hot Tub is safe to use.

  • This sounds like an issue with faulty utility wiring introducing a stray current into the earth... – ThreePhaseEel Jun 29 at 17:14
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    this is what I am thinking: en.wikipedia.org/wiki/Stray_voltage#Persons – Lee Jun 29 at 17:59
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    It's not surprising to have a potential difference between true (literal) ground and the neutral wire of a circuit with a high current flowing through it. The reason is that the neutral wire's resistance is non-negligible with high currents, so a potential difference between different locations on the same "neutral" wire (and between those locations and "true" ground) develops. That's one reason houses have several distinct circuits and care is taken to balance loads among those circuits instead of putting all lights on one and all heaters on the other. – Peter A. Schneider Jun 30 at 13:48
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    Another tale from the UK. One day my father tried to turn on the tap in his bathroom, and got a bad shock. He called the power company, who could find nothing wrong. Sure enough, he could now touch the tap without getting shocked. Then it happened again days later - and again the power company could find no fault. It turned out his neighbour 100m away had hung a painting on a wall; the nail had hit the wire to a light-switch, and connected the live wire to the earth wire, which meant that the earth wire and everything connected to it was now live - but only when that light was turned on! – Gary McGill Jul 1 at 10:42
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    @GaryMcGill How does the breaker not trip when earth is connected to live, forming a short circuit? – clemisch Jul 1 at 13:30
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Now the odd thing is that even with the socket switched off (but still plugged in) it still reads 2Vac. If I unplug the socket it reads 0Vac.

How is it reading 2Vac with only neural and earth wires connected?

By ground wire not being at 0Vac.

Typically this would be caused by problems elsewhere in your home, such as an appliance drawing high amount of current and your ground rod making poor contact with the ground. It could also be poor connections in the ground wires themselves.

If you want to test it yourself, you can do the ground voltage measurements from various outlets and also turn appliances on/off. If your house has all the grounds connected together well, they'll probably all read the same voltage. In that case, the problem is likely with the central grounding rod.

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