Hot tub electric shock

[LastManOnline]

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

You have raised a very interesting question there. Gonna ponder that one...

 

[UNG]

You have raised a very interesting question there. Gonna ponder that one...

It is one of the things that sticks in my mind, about 20 years ago I was asked about putting lightning protection on a steel shelter out on a golf course I had some very interesting conversations with some very knowledgeable lightning protection people about earth rods, earth mats, voltage gradients and step potentials as the shelter was already part built and as it was too late to put an earth mat under the shelter and around it the optimum solution was do nothing as even earthing the structure could actually cause more problems than it would have solved, I realise we are talking a lot more voltage with lightning but I'm sure a low voltage high current fault could have similar consequences

 

[telectrix]

It is one of the things that sticks in my mind, about 20 years ago I was asked about putting lightning protection on a steel shelter out on a golf course I had some very interesting conversations with some very knowledgeable lightning protection people about earth rods, earth mats, voltage gradients and step potentials as the shelter was already part built and as it was too late to put an earth mat under the shelter and around it the optimum solution was do nothing as even earthing the structure could actually cause more problems than it would have solved, I realise we are talking a lot more voltage with lightning but I'm sure a low voltage high current fault could have similar consequences

all golfers should wear lightning protection. big aerial onto top of golf cap, wired down golfing jacket to testicles.

 

[telectrix]

whole new take on the "lost balls" situation.

 

[UNG]

all golfers should wear lightning protection. big aerial onto top of golf cap, wired down golfing jacket to testicles.

Some of lightning protection people did mention something about rearranging golfers balls if the step potential was to high

 

[marcuswareham]

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.

It is one of the things that sticks in my mind, about 20 years ago I was asked about putting lightning protection on a steel shelter out on a golf course I had some very interesting conversations with some very knowledgeable lightning protection people about earth rods, earth mats, voltage gradients and step potentials as the shelter was already part built and as it was too late to put an earth mat under the shelter and around it the optimum solution was do nothing as even earthing the structure could actually cause more problems than it would have solved, I realise we are talking a lot more voltage with lightning but I'm sure a low voltage high current fault could have similar consequences

Isnt the idea that ADS reduces this risk due to rapid disconnection, and the risk could be the same from being in contact with exsposed conductive parts at the time of a fault?

 

[LastManOnline]

You have raised a very interesting question there. Gonna ponder that one...

Your point about step voltage is valid. In fact it's inevitable for any earth rod/mat with a res. of 20 ohm(or1 10 ore even 5.) So while a 20 ohm earth mat would alleviate the "tingle" felt by the OP while stepping out of his hot tub, the same mat could become a serious liability under an open PEN fault. It could well be at mains potential and there would be a significant step voltage so you would not want to be in bare (and wet) feet while walking away from the tub.
So, we are back to the same issues once again with TNC-S. Yes, it's getting a bit repetitive, is, nt it!!

 

[timhoward]

So, we are back to the same issues once again with TNC-S. Yes, it's getting a bit repetitive, is, nt it!!

Anyone fancy forming a political party called the Tea Tea Party with the sole manifesto promise of converting the entire country to TT?

 

[brianmoooore]

Count me out, I'm sticking with my TNS!
May need to have a larger Tx installed in a couple of years though (possibility of two new dwellings), so who knows what I'll end up with then.

 

[pc1966]

the same mat could become a serious liability under an open PEN fault. It could well be at mains potential and there would be a significant step voltage so you would not want to be in bare (and wet) feet while walking away from the tub.

That also depends on how deep the mat is as that impacts on the voltage gradient seen at the soil surface.

 

[pc1966]

You can get software to compute step & touch voltages for any sort of earthing structure, but (a) it is complex to use, and (b) it costs in the region of £5k per year or more to license (SafeGrid tell you, some others like XGSLab are a bit less forthcoming). So as you can guess it only gets used for major projects like the design of HV substation safety, etc!

It is a shame that someone with access to it did not run a few examples for typical hot tub sizes and so on to get the overall size & depth needed to ensure no more than 50V surface step even for a full 230V PEN on the buried mat.

 

[LastManOnline]

to ensure no more than 50V surface step even for a full 230V PEN

Searched in "Special locations" in Irish regs regarding "hot tubs". Not a mention!!. The more you think about it, a hot tub is similar to a wet room, but with an extra layer of risk. It's a "Special, special location". We use bonding in bathrooms for protection under fault conditions. But we assume the bathroom floor is either non-conductive or not a significant factor. However with a hot tub, we stand on a surface which may be very conductive

 

[timhoward]

It's a bit late for creative thinking and there's probably at least one flaw in this...
But would it work if hot tubs had a triple pole voltage sensitive relay on the supply held closed by it's own supply, and special dispensation to switch the earth, then in broken PEN situation hopefully the volt drop caused by higher resistance alternative path via CPC's, bonding etc. to real earth would open the relay and complete disconnection would occur?

 

[marcuswareham]

It's a bit late for creative thinking and there's probably at least one flaw in this...
But would it work if hot tubs had a triple pole voltage sensitive relay on the supply held closed by it's own supply, and special dispensation to switch the earth, then in broken PEN situation hopefully the volt drop caused by higher resistance alternative path via CPC's, bonding etc. to real earth would open the relay and complete disconnection would occur?

Is this not effectively what the matt:e and similar devices do by closing a N.O 3 pole relay/contactor if the voltage is >207v & <253v. ( 722.411.4.1(iv) )

This doesn't work in a lot of situations as it is uncommon for each of the 3 phases on the DNO network to be balanced in such a way to ensure this on each phase, during a PEN fault 1 or more of the phases can remain within the voltage range but there can still be a very high voltage on the PEN conductor

Also bonded extraneous conductive parts can provide a high resistance path for the current to return, but if the load current is higher than what the bonded pipes can sink and by around the right percent it can mean that the voltage is still close to the expected 230v but you have a high voltage on the PEN conductor

If you wanted to keep the PME earth and could still sink a rod (can probably be a pretty small rod as doesn't need a low resistance) A way could be too use a voltage sensing relay measuring the voltage between the PME earth and the "test" rod, and if the voltage goes above 50v then it would open a 3 pole relay/contractor isolating L, N and PE

 

[brianmoooore]

A way could be too use a voltage sensing relay measuring the voltage between the PME earth and the "test" rod, and if the voltage goes above 50v then it would open a 3 pole relay/contractor isolating L, N and PE

A voltage operated ELCB, as we used years ago, but breaking the supply earth as well.

 

[marcuswareham]

A voltage operated ELCB, as we used years ago, but breaking the supply earth as well.

thats the one

 

[Mark Wright]

what was the outcome of this? I couldn't be bothered to read all 7 pages.


In America where they have more swimming pools they make more fuss about stray voltages

"In our case, stray voltage is coming from the grounded conductors of an electrical system nearby and the surrounding “earth” ground. When you are dealing with a pool, stray voltages usually feel like a small tingling sensation when you touch a puddle of water on the pool deck or a handrail/ladder attached to the pool. In the most extreme of cases, electricity in pools can cause paralysis and ultimately lead to drowning."

The Shocking Truth

Electricity in Swimming Pools

us.megger.com

 

[LastManOnline]

A voltage operated ELCB, as we used years ago, but breaking the supply earth as well.

I have long advocated a similar type solution. I don't understand why this option has not being explored more

 

[UNG]

what was the outcome of this? I couldn't be bothered to read all 7 pages.


In America where they have more swimming pools they make more fuss about stray voltages

"In our case, stray voltage is coming from the grounded conductors of an electrical system nearby and the surrounding “earth” ground. When you are dealing with a pool, stray voltages usually feel like a small tingling sensation when you touch a puddle of water on the pool deck or a handrail/ladder attached to the pool. In the most extreme of cases, electricity in pools can cause paralysis and ultimately lead to drowning."

The Shocking Truth

Electricity in Swimming Pools

us.megger.com

I would suggest you read the whole thread rather ask for a synopsis

I would also suggest you read the link you posted which clearly highlights the differences between the UK and US electrical systems and the differences in test equipment used and obviously the installation tests that are carried out

 

[pc1966]

I have long advocated a similar type solution. I don't understand why this option has not being explored more

Until EV came along in large numbers no one really bothered much about the open PEN problem.

But the flaw with the idea of opening the supply CPC is many houses will end up connected to the neighbour's PEN by virtue of bonded pipes, etc. Unless everyone on your "extraneous conductive neighbourhood" uses such a device it might well fail in practice to avoid high metalwork potentials.

Really the most practical routes are either very good additional earths, such as building foundations or a decent sized mat, or putting the tubs on a TT supply. Of the two, the local good earth is better as it ought to save you from over-voltage damage to equipment, but the TT option is likely to be easier to implement in practice.