Hot tub electric shock

[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

 

[Samhhh]

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

Not very common, the issue generally revolves around the cable I mentioned before (consac) it was a very badly designed and cheap cable that was laid from the 70's right through to the early 90's. There's cable thats been in the ground 100 years and is still in perfect condition but sections of it through other utilities and over load are starting to suffer but with that cable it tends just to blow a phase and become open circuit usually down to the lead plumbs failing and moisture tracking in

 

[Samhhh]

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 !)

TN-S or as us commoner cable jointers call it "split" is 100% still manufactured we actually stock it but for some reason the majority of the company will opt for its 3 core version due to it already being PME'd at the substation. Its actually easier to joint on the 4 core version than it is the 3 core so I have no idea why we do it the way we do but it's been the norm all the time I've been in the industry. As for customers or the electricians they would have absolutely no idea as to what the feed really is because if there has ever been a fault repair prior to there connection it's more than likely been combined on that section making their service pme

 

[marcuswareham]

TN-S or as us commoner cable jointers call it "split" is 100% still manufactured we actually stock it but for some reason the majority of the company will opt for its 3 core version due to it already being PME'd at the substation. Its actually easier to joint on the 4 core version than it is the 3 core so I have no idea why we do it the way we do but it's been the norm all the time I've been in the industry. As for customers or the electricians they would have absolutely no idea as to what the feed really is because if there has ever been a fault repair prior to there connection it's more than likely been combined on that section making their service pme

Maybe the 3 core is cheaper, that's very interesting though, thanks for that, iv heard alot of people say the 4 core is not made, good to know it is

what is the common practice for petrol stations surrounded by PME, as far as i know PME is not allowed for petrol station forecourt, dedicated TNS supply or TT? just out of curiosity

 

[Samhhh]

Maybe the 3 core is cheaper, that's very interesting though, thanks for that, iv heard alot of people say the 4 core is not made, good to know it is

what is the common practice for petrol stations surrounded by PME, as far as i know PME is not allowed for petrol station forecourt, dedicated TNS supply or TT? just out of curiosity

Yes it will definitely be cheaper but I don't think it would be by much to be honest, the only difference between the cables is 1 less aluminium core so surely it can't add that much to the cost. As for petrol stations I've no idea, thinking back all the ones I've ever worked on through faults have been TNS supplies but whether they are true TNS I would highly doubt

 

[LastManOnline]

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..... In 12 years of being a jointer I have never worked on a neutral fault that didn't revolve around a consac main.

Your insight into this area is much appreciated. I think much of the concern around TNC-S has resulted from the general lack of information within the electrical industry about it. The number of posts on this forum alone is a reflection of that.
As electricians we need to know what we are dealing with the pro, s and the con, s. To scaremonger is to do the industry and the public a disservice. To ignore the potential risks is equally wrong.
I would suggest the "crossing the road analogy" as mentioned by "Tim Howard" is the way to go. Educate people about the potential risks.
Similar to a TT system there are potential risks. It's essential that rcd, s are regularly tested to ensure safety.

 

[buzzlightyear]

IMO i think making a house a pme is most dangerous especially a pen fault.
Me thinks a system has a TNS is better for faults has it has a true earth not a one that being tagged to neutral, but you may think it all goes back to the grid.
You has well make all property's TT system in which you know will go earth
Ask the Spanish they been doing it for year, or any Eastern countrys are we behind or what.

 

[Samhhh]

Your insight into this area is much appreciated. I think much of the concern around TNC-S has resulted from the general lack of information within the electrical industry about it. The number of posts on this forum alone is a reflection of that.
As electricians we need to know what we are dealing with the pro, s and the con, s. To scaremonger is to do the industry and the public a disservice. To ignore the potential risks is equally wrong.
I would suggest the "crossing

I wonder what the odds on are on this.. Im on call today and about to make my way to a loss of neutral. The funny thing is the cable feeding these property's is a a split main and not PME ?

 

[buzzlightyear]

wonder what the odds on are on this.. Im on call today and about to make my way to a loss of neutral. The funny thing is the cable feeding these property's is a a split main and not PME

I wonder if the the houses have got hot
Tubes in the garden, on foot in the grave. Lol

 

[HappyHippyDad]

What an interesting thread. I think @Samhhh may have answered more questions than had answers given ?.

 

[pc1966]

Ask the Spanish they been doing it for year, or any Eastern countrys are we behind or what.

There are trade-offs both ways. On TN systems you should be able to disconnect on the OCPD under earth fault conditions, so in that sense it is safer than relying on the RCD electronics. Of course you still want the RCD as most of the time it would disconnect on an electric shock type situation.

Down-side is the open PEN risk for outdoor equipment. In that past that was very little, probably a few floodlamps and often they are out of reach (OK, on fence another issue...)

TT avoids the issue of open PEN faults but then you need an RCD to achieve disconnection with any realistic earth rod installation. And you also have the issues of installing and maintaining rods in built up areas, etc. Of course you can reduce the impact of RCD reliability by having both a 100mA delay incomer and your usual 30mA instant "additional protection" RCDs/RCBOs so you don't have any single point of failure, but I would guess many TT installs just have the one RCD in any given circuit's path.

Any idea of how TT supplies are done in blocks of flats? Do they have a communal earth system?

 

[timhoward]

What an interesting thread. I think @Samhhh may have answered more questions than had answers given ?.

It has been most informative and helpful to hear the other side of the coin on this from a cable jointers perspective. I hope he continues to linger around this forum!

 

[brianmoooore]

Time to bring back the old voltage operated ELCB?
That's as well as, not instead of, RCCDs

 

[pc1966]

Time to bring back the old voltage operated ELCB?
That's as well as, not instead of, RCCDs

They would need to disconnect the CPC as well, much like the Zapi chargers, etc, do.

It is a solvable problem, but who should carry the cost of solving it is another matter altogether!

 

[brianmoooore]

Yes, disconnect live, neutral and earth.

 

[UNG]

Excuse quick diversion, then back to interesting TNS question!

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.

Using an earth mat is a little bit different to a single earth rod
How much thought does anyone give as to where they decide to knock an earth rod into the ground, step potential under fault conditions could be interesting especially around a hot tub

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.

Which DNO do you work for?