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

 

[timhoward]

Excuse quick diversion, then back to interesting TNS question!

More and more I am thinking all hot tubs should be on a TT supply, as even if your supply appears to be TN-S there is a chance that elsewhere repairs or cable replacement has essentially converted your supply to TN-C-S (just a rather long 'S' there).

In addition, one could argue that:
-GN7 says hot tubs outside should have section 702 applied.
-702.140.3.4.3 (ii) NOTE says:
"Where a PME earthing facility is used as the means of earthing [.....] it is recommended that an earth mat [...] of suitably low resistance e.g. 20 ohms or less , be installed and connected to the supplementary bonding."

Ok, I accept that's a recommendation based on a guidance note but had it been followed the (likely) problem in this thread wouldn't have happened.

 

[brianmoooore]

Changed from TNC-S about ten years ago, when the single service head (at the time) was relocated, and the 3 separate overhead service wires (Ph, Ph, and N+E) were deemed to contravene some regulation, and was replaced by what looks like 4 wires loosely twisted together, but are actually joined, and reinstated as TNS.
There are three earth electrodes. One, on the Tx pole, earths the TX metalwork and the neutral, a second on the Tx pole is for the supply, and there is third at the base of the service pole.

 

[LastManOnline]

Changed from TNC-S about ten years ago, when the single service head (at the time) was relocated, and the 3 separate overhead service wires (Ph, Ph, and N+E) were deemed to contravene some regulation, and was replaced by what looks like 4 wires loosely twisted together, but are actually joined, and reinstated as TNS.
There are three earth electrodes. One, on the Tx pole, earths the TX metalwork and the neutral, a second on the Tx pole is for the supply, and there is third at the base of the service pole.

That's almost historical. A TNC-S supply being converted to a TNS
supply. I suspect the earth rods are a leftover from the original TNC-S.Which is a bonus. I think if I was selling up I would advertise "house with genuine TNS". It looks like there could be a growing market for such dwellings (especially if you like "tingle" free hot tubs)

 

[LastManOnline]

Ok, I accept that's a recommendation based on a guidance note but had it been followed the (likely) problem in this thread wouldn't have happened.

No question. Body resistance would be virtually shorted out.

 

[pc1966]

That is a fairly specialised form of advertising!

 

[LastManOnline]

That is a fairly specialised form of advertising!

Agreed. But by the time I, m finished with moaning about the shortcomings of TNC-S on this forum (which I do religiously) I, m hoping it all becomes a, bit more "mainstream"

 

[brianmoooore]

That's almost historical. A TNC-S supply being converted to a TNS
supply. I suspect the earth rods are a leftover from the original TNC-S.Which is a bonus. I think if I was selling up I would advertise "house with genuine TNS". It looks like there could be a growing market for such dwellings (especially if you like "tingle" free hot tubs)

Nothing extra was added at the Tx pole when the change was made, but the service pole and its earth rod were new. Old service pole (now a bird table) did not feature an earth rod.

 

[marcuswareham]

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

If your supply Neutral breaks in it in the future from some ground works or something like that then the voltage in the tub could be life threatening,

The joys of money saving PME

 

[timhoward]

If your supply Neutral breaks in it in the future from some ground works or something like that then the voltage in the tub could be life threatening,

The joys of money saving PME

I genuinely don't know how to quantify this one. It comes up fairly often on here and I've raised it before too. If it happens, it's certainly a nightmare scenario.
But stats seem to suggest we are taking around 500 occurrences in about 25 million properties in a year. So chances of any given property having this issue are mathematically miniscule. As new builds are almost all PME and more supplies are converted the number of incidents are bound to marginally increase as there are simply more out there.
Thing is, we don't tell people they might die crossing the road to go and buy the beer to have in the hot tub even though it is far more likely. Thoughts welcome!

 

[Samhhh]

More and more I am thinking all hot tubs should be on a TT supply, as even if your supply appears to be TN-S there is a chance that elsewhere repairs or cable replacement has essentially converted your supply to TN-C-S (just a rather long 'S' there).

There arent many true TN-S systems anymore. As I jointer everytime we drop a section of cable in especially within our area the cable is always PME'd in the joint. Everytime a new substation is installed or a board change is done the legs in the substation out of the new board are pme even if they are being jointed onto a split supply.

 

[Samhhh]

I genuinely don't know how to quantify this one. It comes up fairly often on here and I've raised it before too. If it happens, it's certainly a nightmare scenario.
But stats seem to suggest we are taking around 500 occurrences in about 25 million properties in a year. So chances of any given property having this issue are mathematically miniscule. As new builds are almost all PME and more supplies are converted the number of incidents are bound to marginally increase as there are simply more out there.
Thing is, we don't tell people they might die crossing the road to go and buy the beer to have in the hot tub even though it is far more likely. Thoughts welcome!

The only time we get loss of neutral on our network in the North West is on certain type of mains cable (consac) its renown for it. The reason being that under the pvc the neutral/earth is full aluminium and as soon as water tracks in the aliminium oxidises over time and the neutral breaks down. The newer cables we install although they are cheap the neutral/earth's are split strands of copper and the odds over half of those strands corroding is very very low. In 12 years of being a jointer I have never worked on a neutral fault that didn't revolve around a consac main. We are however looking at a huge issue over the next few years with cables running above there capacity with the introduction of electric charging points and the amount of appliances in every home these days. The network is old and no one is willing to pay the money to update it. Everything is reactive maintenance in my industry not preventative.

 

[brianmoooore]

But stats seem to suggest we are taking around 500 occurrences in about 25 million properties in a year. So chances of any given property having this issue are mathematically miniscule.

In my whole career, I've attended two such incidents, live (pun intended), and also found historical evidence on third place (took off a bath panel, and found a very charred 6mm G/Y running under the full length of the bath. Initially cursed wetpants and his blowlamp, but odd because the whole six foot length I could see was charred and bubbled up. Further investigation identified the wire as the bond to the water main from the MET, avoiding a few joists by passing under the bath). Only one house further up the service, so this can only have been caused by a broken neutral to next door.

 

[marcuswareham]

There arent many true TN-S systems anymore. As I jointer everytime we drop a section of cable in especially within our area the cable is always PME'd in the joint. Everytime a new substation is installed or a board change is done the legs in the substation out of the new board are pme even if they are being jointed onto a split supply.

Shame as i personally feel TN-S is a much better system (despite the cost of an extra conductor)

The typical reasoning is the DNOs can no longer get TN-S cable as its not made anymore (although perhaps because the DNO stopped buying) if cable manufacturers can make 3 core copper concentric cable, i would presume they can make 4 core

I have also seen evidence of UKPN combining the E and N (PME) to repair sections of the old TN-S lead covered cable (I wonder if the tell the customers !)

 

[marcuswareham]

The only time we get loss of neutral on our network in the North West is on certain type of mains cable (consac) its renown for it. The reason being that under the pvc the neutral/earth is full aluminium and as soon as water tracks in the aliminium oxidises over time and the neutral breaks down. The newer cables we install although they are cheap the neutral/earth's are split strands of copper and the odds over half of those strands corroding is very very low. In 12 years of being a jointer I have never worked on a neutral fault that didn't revolve around a consac main. We are however looking at a huge issue over the next few years with cables running above there capacity with the introduction of electric charging points and the amount of appliances in every home these days. The network is old and no one is willing to pay the money to update it. Everything is reactive maintenance in my industry not preventative.

How common are lost neutrals in your experience?

It is easy to slag of the network saying things should be better, but i bet you guys are working hard fire fighting most of the time as you say, leaving little resources left to upgrade

 

[marcuswareham]

I genuinely don't know how to quantify this one. It comes up fairly often on here and I've raised it before too. If it happens, it's certainly a nightmare scenario.
But stats seem to suggest we are taking around 500 occurrences in about 25 million properties in a year. So chances of any given property having this issue are mathematically miniscule. As new builds are almost all PME and more supplies are converted the number of incidents are bound to marginally increase as there are simply more out there.
Thing is, we don't tell people they might die crossing the road to go and buy the beer to have in the hot tub even though it is far more likely. Thoughts welcome!

Likewise, very hard to put a figure on it and I wonder how many are not reported, most of the reported figures are found in publications from people selling pen protection EV products , so that does make you wonder

I suppose with a new part of the network with new concentric cable and the least amount of joints correctly done the risk is probably low, however on the other end of the scale is my windy village with seperated overhead cables with the PEN on the bottom slung over some farm entrances in which tall vehicles go in and out off

I have never has an exspirence of a PEN fault however it must be a big enough risk, otherwise why all the new regulations for EV charging and why is PME not allowed to be used in alot of situations. I wonder with more EVs and people using power outside (lockdown hot tubs) we will see a rise in PEN fault incidents

I might die crossing the road, I best stay in