Discuss Shocks From Hot Tub Water? in the Electrical Forum area at ElectriciansForums.net

Weezy

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Hey all!
I’m off to look at a job tomorrow, client says their kids have reported they got a shock from touching the water in the hot tub (customer hasn’t experienced this themselves) first thought was static discharge, but when asked if it happens continuously when touching the water customer said “no but will happen again within a short amount of time”

We install the hot tub feeds for a local company all the time (roughly one a week) and this is the first iv heard of this, just wondering if anyone has any idea or experiences with this? Apart from the usual basic inspections, polarity, insulation resistance and zs.

Normally we like to install a earth rod local to the tubs incase pme lost neutral occurs, i’m wondering if there’s not one this might be the solution if all other checks come back fine?
 
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littlespark

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Not one of these, is it?

 

Weezy

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Not one of these, is it?

Sorry no, should have made my self more clear it’s a proper fixed hot tub.
My guess is it’s a TNCS earth and will need converting to TT…
 

Weezy

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This pretty much backs up my thought on adding local earth rods to tubs on pme supply’s:

Regulation 702.410.3.4.3 therefore recommends: ‘Where a PME earthing facility is used as the means of earthing for the electrical installation of a swimming pool or other basin, it is recommended that an earth mat or earth electrode of suitably low resistance, e.g. 20 ohms or less, be installed and connected to the supplementary protective equipotential bonding’.
 

pc1966

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Check out both possibilities to be sure - both the hot tub IR and bonding/earthing of any metal parts, and see if TN-C-S you measure any sort of voltage from CPC to a small spike in the ground outside.

In the ideal world for TN-C-S an earthing mat would have been put in first, or at least some sort of buried earth ring around the tub, but failing that going TT would most likely be the answer providing hot tub faults have been ruled out.

As mentioned above, this sort of fault report has the potential to end very badly indeed if something is overlooked on the assumption it is PME issues again.
 

buzzlightyear

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Most hot hubs are plastic tub and the soundings either wood or plastic, either there this a fault on it causing a leakage current on it would cause a shock and kill some body,
https://dms.niceic.com › ...PDF
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Electrical design considerations for the electrical supply to hot tubs
 

Weezy

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Check out both possibilities to be sure - both the hot tub IR and bonding/earthing of any metal parts, and see if TN-C-S you measure any sort of voltage from CPC to a small spike in the ground outside.

In the ideal world for TN-C-S an earthing mat would have been put in first, or at least some sort of buried earth ring around the tub, but failing that going TT would most likely be the answer providing hot tub faults have been ruled out.

As mentioned above, this sort of fault report has the potential to end very badly indeed if something is overlooked on the assumption it is PME issues again.

Providing there’s no faults on the tub, the way i read the reg i linked (702.410.3.4.3) is that adding a local earth rod to the tub and connecting it to the tubs earthing terminal would be fine even if the supply is tncs, as it should ensure the earth potential is the same between the tub and the ground, adding earth rods to tncs supply’s is considered a good thing if i remember correctly they wanted to make it a mandatory reg in the 18th edition, i believe the tub is situated in a pretty big open garden as well so the stake should have no impact on any other surrounding neighbouring supply’s.
 

pc1966

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You absolutely have to rule out a hot-tub fault first. If you don't check that properly and the worst happens you will be up the proverbial without a paddle. If it were me looking at this, I would:
1) Get the hot tub make, model and serial number and check against any product recalls or advisories.

2) Get myself an earth spike, either a commercial one (e.g. the sort for MFTs to do fall-of-potential rod resistance measurements, etc) or failing that an old uninsulated screwdriver with around a 20cm blade. If the ground around the tub is really dry as no rain and nobody been splashing around then pour about a pint or two of water on to a spot and stick the probe in there, allow a minute or two for the water to soak down in to the ground first.

Make a "water probe" from around 3m of insulated copper wire. Strip around 1m of insulation from one end and form in to a loop just over 30cm diameter, tidy up the joint so there are no sharp edges from the end of the wire to either cut you or scratch the tub finish. So tape or better still heat-shrink it. For the other end either solder on a 4mm safety socket and heat-shrink it, or make a smaller loop with around 10cm of stripped wire (again tidy and covered so no sharp bits) so a croc clip can reliable attach to it while keeping your test probes out of the tub's water.

Bring a multimeter (you know the drill, as a sparky always CAT-III rated no mater what job...) so you can measure any AC and DC leakage (in case of low volt LED supply getting in to the water, etc).

Use your MTF or similar to IR test:
  • L+N to CPC
  • L+N to water probe in tub
  • CPC to water probe
Both L+N should be very high as for circuit testing.
The CPC to water might be low if really class I, or should also be high if class II.

Then use your multimeter to check for both the AC and the DC volts from:
  • CPC to earth spike
  • CPC to water probe
  • water probe to earth spike
If you see any significant voltage (more than 1-2V) between the water & earth then obviously it confirms the reported sensations. If you see much difference from CPC to earth it probably is the PME effects, but if CPC to earth is low-ish and water to CPC and/or water to earth higher then I would suspect leaking lights or similar in the tub.
 
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pc1966

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Providing there’s no faults on the tub, the way i read the reg i linked (702.410.3.4.3) is that adding a local earth rod to the tub and connecting it to the tubs earthing terminal would be fine even if the supply is tncs, as it should ensure the earth potential is the same between the tub and the ground, adding earth rods to tncs supply’s is considered a good thing if i remember correctly they wanted to make it a mandatory reg in the 18th edition, i believe the tub is situated in a pretty big open garden as well so the stake should have no impact on any other surrounding neighbouring supply’s.
Earth rods are generally a good thing.

The problem is it is very difficult to get down to the 20 ohm or similar that is mentioned in connection with this. If you have nice moist clay soil then yes, probably you will get that with a 2.4m rod, but often it is 5 times higher.

The earthing around a pool (or generally for protection) has two different but closely related functions:
  • To pull down the fault to true Earth potential, possibly triggering ADS.
  • To raise the local earth potential closer to the fault so anyone there has less voltage difference across them (bit like the Faraday cage principle)
An open PEN fault can dump many tens of amps easily, so to bring it down not only do you need a very low rod Ra, you also need the 10mm or so bond size due to the high and sustained fault current.

However, to reduce the voltage difference you can look at a higher Ra but it has to be around the area of interest, so even in poor conductivity soil a big enough earth mat, or a ring conductor buried 50cm or so below the soil just outside of the tub, can greatly reduce the step potential even if it is having a negligible effect on the absolute voltage of an open PEN fault (as it is only dumping a couple of amps).

But that is very hard to do after the install is done! Really it should be considered as part of installation which would be the case for a properly planned swimming pool, but is almost always neglected for hot tubs.

There is talk of mandatory earths being part of amendments to the 18th but that is only going to be very effective if part of foundations (where as low as a couple of homs is relaistic from buried rebar in concrete kept slightl damp from the ground), or if practically every property had a rod supplementing the PME system.
 
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pc1966

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If you see any significant voltage (more than 1-2V) between the water & earth then obviously it confirms the reported sensations.
Just to add - ideally you would put something like a 10k / 3W resistor in parallel with your multimeter to deal with very low leakage current ("phantom voltage") and simulate a poor body connection for get the sort of voltage that a person in contact would experience.

Or use your voltage tester to load the probes as well.

Or bring an AVO Mk8...
 

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