Hot tub electric shock

[Samhhh]

Hi I'm looking for some advice, yesterday when exiting my hot tub I felt a slight electric shock when my foot made contact with the ground around the hot tub. I work as a cable jointer so a tingle is second nature to me so I double checked when in contact with ground and water I can feel a slight tingle in my fingers when isolated from ground I feel nothing. Today I put my multimeter onto a nearby earth terminal and probed the water to get a reading of 0.2volts. Im on a pme supply and have a cable running from a rcd to a isolated and then a feed to the tub. Any idea where this voltage could be coming from? And if installing a seperate earthing point with rods would solve the issue? Thankyou

 

[marcuswareham]

Probably PME and true earth voltage difference, due to voltage drop on PEN conductor,

I for one would not use a PME earth for a hot tube surrounded by true earth, due to the shock risks that occur with a broken PEN / CNE conductor, although as a jointer you probably have an idea of how common broken CNE conductors are and if the DNO install enough PME earth electrodes to reduce voltages on CNE if broken

Having a separate TT earthing system for the hot tube in most cases would solve the issue

 

[pc1966]

Did you measure from the water to a spike in to the ground by the tub?

I doubt you would feel 0.2V even if wet, but that would be typical for volts around the CPC (earth) wiring in a home. However, you can easily get a few volts on PEN to true Earth just from normal currents and that would be noticeable if wet.

As above, easiest option is to make the tub supply TT and have the related earth rod in the general location of the tub.

 

[Samhhh]

Probably PME and true earth voltage difference, due to voltage drop on PEN conductor,

I for one would not use a PME earth for a hot tube surrounded by true earth, due to the shock risks that occur with a broken PEN / CNE conductor, although as a jointer you probably have an idea of how common broken CNE conductors are and if the DNO install enough PME earth electrodes to reduce voltages on CNE if broken

Having a separate TT earthing system for the hot tube in most cases would solve

Probably PME and true earth voltage difference, due to voltage drop on PEN conductor,

I for one would not use a PME earth for a hot tube surrounded by true earth, due to the shock risks that occur with a broken PEN / CNE conductor, although as a jointer you probably have an idea of how common broken CNE conductors are and if the DNO install enough PME earth electrodes to reduce voltages on CNE if broken

Having a separate TT earthing system for the hot tube in most cases would solve the issue

What is the Pen conductor? Could it be possible that a component of the hot tub is what's leaking voltage into the water? But if so wouldn't this trip the rcd or is the voltage not large enough to do so? Its really crazy I'm 100% getting and electric shock but I only feel in on the parts of my fingers where I have open wounds I'm assuming because the voltage is so low and my skin has less resistance due to the cuts. I've asked the hot tub installer to come and check that the hot tub doesn't have any component issues as it's still withing warranty. I feel like if this issue has always been related to being surrounded by true ground wouldn't I have had this issue since day one?

 

[pc1966]

PEN stands for "protective earth neutral" and it is the conductor on the supply (DNO) side carrying the return current and also serving as the protective earth. It is the 'C' in a TN-C-S supply.

Within your property that has been separated out at the supply cut-out (main fuse) in to both neutral and earth. So anything connected to your earth system is showing any voltage drop on the neutral system of the supply network. This is the 'S' in a TN-C-S supply.

Normally that is small, several volts at most, and normally folk are not in contact with the installation earth AND the true Earth outside. But a hot-tub is a case where there is such an issue, same as electric cars (and washing them while charging).

The real danger is not the few volts normally seen, but if there is a fault in the supply network causing an open circuit PEN then all neutral current on the faulted side will attempt to return via the Earth by any means possible, such as bonded metallic pipes, etc. You can end up with over 230V in the worst case and what is worse is the usual RCD protection will do nothing to protect you. Basically the RCD looks at the N & L in your installation, not at the current flowing in the E route.

So it sounds like either you have a faulty hot tub (e.g. 12V or 24V lights leaking to water) or the hot tub is class I and the earth for it is on a TN-C-S supply so you are seeing the PEN voltage.

Really you ought to get it checked professionally and look at putting it on a TT supply if it turns out you are seeing the concequences of a TN-C-S supply.

 

[timhoward]

I feel like if this issue has always been related to being surrounded by true ground wouldn't I have had this issue since day one?

I'd add that soil ground/conditions can make a difference, i.e. bone dry or soaking wet, and type of soil.

 

[marcuswareham]

What is the Pen conductor? Could it be possible that a component of the hot tub is what's leaking voltage into the water? But if so wouldn't this trip the rcd or is the voltage not large enough to do so? Its really crazy I'm 100% getting and electric shock but I only feel in on the parts of my fingers where I have open wounds I'm assuming because the voltage is so low and my skin has less resistance due to the cuts. I've asked the hot tub installer to come and check that the hot tub doesn't have any component issues as it's still withing warranty. I feel like if this issue has always been related to being surrounded by true ground wouldn't I have had this issue since day one?

I believe the DNO call it a CNE cable / conductor

yes the main risk is a break in this conductor which could make 230v be present in the water or any exsposed metal work

Why you have not has issues before I dont know, maybe a joint on the DNO network is higher impedance than it was before causing more volt drop, or perhaps a problem with the hot tub as you say.

 

[pc1966]

I believe the DNO call it a CNE cable / conductor

Yes, a number of related acronyms:
PEN = protective earth neutral
CEN = combined neutral earth (usually a DNO term)
PME = protective multiple earthing
PNB = protective neutral bond (not usually domestic stuff)
MEN = multiple earthed neutral (Australia & NZ)
MGN = multi-grounded neutral (USA)
TN-C-S = Terre Neutral – Combined – Separate (terre = French for earth)

 

[Samhhh]

@pc1966 hi so I've just got home from work and I've done the same test again, now this time I ran a remote earth rod into ground and then measured from the water to the rod, same reading a 0.2v. I also did the same wet foot test (I know I shouldn't) but I wanted to be sure I'm not just going crazy, same result if I am ground and wet feet and touch the water I feel the discomfort of the shock. Now for the fun part.. I switched isolator off and re did the same test and still recieved 0.2v and a tingle when I touch the water and I'm grounded. How?!

 

[stevethesparks]

Its got to be the potential difference between natural earth and the pme earth supplied at the income.
I think the only way around this is to change the hot tub earthing system to a TT arrangement and make sure the RCD/rcbo is double pole.

 

[timhoward]

@pc1966 hi so I've just got home from work and I've done the same test again, now this time I ran a remote earth rod into ground and then measured from the water to the rod, same reading a 0.2v. I also did the same wet foot test (I know I shouldn't) but I wanted to be sure I'm not just going crazy, same result if I am ground and wet feet and touch the water I feel the discomfort of the shock. Now for the fun part.. I switched isolator off and re did the same test and still recieved 0.2v and a tingle when I touch the water and I'm grounded. How?!

The isolator doesn't interrupt the earth that's coming from the house. Nor should it. And on your setup the Neutral's of all circuits are connected to the earth in the house, which is why there is a small voltage on the earth from the house. You are probably measuring and feeling the small difference between earth from house and real earth, exactly as @stevethesparks said.
While not dangerous it's disconcerting. I've seen some put rubber matting around the edge of the hot tub to solve this. The proper solution is what @stevethesparks said too.

 

[Samhhh]

Its got to be the potential difference between natural earth and the pme earth supplied at the income.
I think the only way around this is to change the hot tub earthing system to a TT arrangement and make sure the RCD/rcbo is double pole.

I've just removed my main fuse at the cut out and ran all my usual tests, 246v 0.22ze so it's good coming in. Like you say I think it's has to be potential difference especially if the hot tub is still recieving a voltage when the isolater is switched off. Could this voltage increase and be dangerous or will it always stay the same dependant on conditions, i.e if the ground is dry I do not feel it but if the ground is wet I do, will it still only be 0.2volts or could it get worse/be life threatening thankyou

 

[timhoward]

I've just removed my main fuse at the cut out and ran all my usual tests, 246v 0.22ze

I don't see why you needed to pull the main fuse.

Could this voltage increase and be dangerous or will it always stay the same dependant on conditions, i.e if the ground is dry I do not feel it but if the ground is wet I do, will it still only be 0.2volts or could it get worse/be life threatening thankyou

As long as the main supply cable to the property remains in good condition (which is likely), this particular issue should never be life threatening.

 

[pc1966]

@pc1966 hi so I've just got home from work and I've done the same test again, now this time I ran a remote earth rod into ground and then measured from the water to the rod, same reading a 0.2v. I also did the same wet foot test (I know I shouldn't) but I wanted to be sure I'm not just going crazy, same result if I am ground and wet feet and touch the water I feel the discomfort of the shock. Now for the fun part.. I switched isolator off and re did the same test and still recieved 0.2v and a tingle when I touch the water and I'm grounded. How?!

Did you check for both AC and DC volts?

Just wondering if there is anything odd with the hot tub like a DC control panel or DC ELV lights leak or similar.

 

[Samhhh]

Did you check for both AC and DC volts?

Just wondering if there is anything odd with the hot tub like a DC control panel or DC ELV lights leak or similar.

Yes I did mate, nothing in DC. I've just spoken to my brother and he thinks it's potential difference too. He's said one way to prove it completely would be to turn off and open up the rotary isolator and remove the earth connection then test again if the voltage has gone then its potential difference as the tub Is no longer grounded itself, if the tub still has volts it could be that a component of the hot tub is leaking voltage into the water and not potential difference?
I'm coming to terms with it being the potential what's the cause and my mind is slowly starting to ease ? thankyou for your help