Swimming Pool Electrics - PME or TT?

[Kes]

I really should stop reading articles on the internet.

We have an in-ground liner swimming pool installed around 1980, which has a pump and 12v underwater light, no other electrics. In 1997 we had a new house consumer unit fitted and at that time the installation was changed from TT to PME. The consumer unit is split power/lights, protected by RCCB's. The pool electrics are fed off a 20 amp 2.5mm radial circuit, the RCCD is 80 amp/30 ma.

Now, after swimming in the pool for some 25 years (without anyone ever being shocked), I read this:

https://electrical.theiet.org/media/1471/swimming-pools-bonding.pdf

The implication is (I am not an electrician so I don't understand it fully) that in some very extreme circumstances bathers could receive an electrical shock, albeit mild. It says that to alleviate this there should be a separate TT circuit feeding the pool only. Is this normal practice, does anyone know? Although it doesn't look like a too onerous job, is it something I should worry about?

PS I have just seen this thread about Isolating PME from TT addition which seems to be similar to my question - I shall plough through it

https://www.diynot.com/diy/threads/isolating-pme-from-tt-addition.573356/

 

[EFLImpudence]

I don't think the article in your first link will help you much as it seems to concentrate on bonding, not the reasons for different or separate earthing, and it doesn't seem to be very well written.

Not least:

 

[EFLImpudence]

Did no one raise the issue when the house had the new consumer unit?

 

[bernardgreen]

The article also has this recommendation '( my underlining )

Secondly, under normal operating conditions, it is possible, due to small differences in potential between the earth of the electrical system and true Earth, for a small voltage to be present. This is usually due to the voltage drop in the PEN conductor creating the difference in potential. This difference is detectable, for instance, by a wet person touching a handrail and coming into contact with true Earth. This perceived electric shock is minimised by installing the additional earth mat or earth electrodes as recommended by BS 7671, however, the most effective method of removing this risk is to provide a TT earthing arrangement and completely isolate the pool’s metalwork and any pipework from the PME supply

 

[Kes]

Yes, that's the part that concerns me. I'm not sure though what circumstances would cause a person to be shocked. Would they have to be touching a metal part of the system, which is only the pump? How can someone receive a shock by touching a handrail as in the example above? The handrail is fixed to the ground and goes into the water - neither of which has any connection to the electrical supply. Wouldn't they have to be touching the electrical system earth and the real Earth underfoot?

(It would be possible for a swimmer to leave the pool and go to the pool box and touch the pump when still wet, I have done this a few times but it is extremely unlikely anyone else would do it.)

The electricity service didn't raise this when the cu was changed, they wouldn't have had to go to the rear of the house to see the pool and of course we were unaware of this. Also when we had the pool relined and new light and pump refurb in 2009 nothing was mentioned. I have been buying pool equipment, following pool forums, and reading suppliers websites for over 20 years, but the article as in post 1 is the first time I have seen the PME/TT problem mentioned, which I think is a little peculiar.

 

[ericmark]

You said 12 volt which would not be a problem, in the main class II equipment used out doors is not a problem, this is why lawn mowers, hot tubs etc are class II, i.e. no earth.

The problem is not the line and neutral (live wires) it is the earth, and we talk about a gradient. So the likes of caravan hook ups are not TN-C-S even when the club house is, but fire regulations mean at least 2.6 meters between the two, it is not the volt drop and the PEN (this is combined neutral and earth) not being at true earth voltage, this is not going to be far from true earth, the problem arises if the PEN is lost, i.e. you have three phases but no neutral, now the voltage can get very much over true earth, and a real danger. However this is rare.

So with electric car charging points they are often not far enough from the house to have a different earthing system, and use TT, so they have a detection system so if the voltage goes out of the range 207 to 253 volt it will first trip the supply and then the earth. However for electric car charging points they are permitted 70 volt between the two earthing systems, it traditionally was 50 volt, and for swimming pools according to zones either 25 volt or 12 volt.

Water is not that good of a conductor, one foot of plastic pipe and unlikely the voltage would be felt, so it all depends on where the pump is, and what the pump earth is connected to. With plastic pipes and the only item earthed is the pump, it is unlikely to be a problem. I have been an electrician now for some 55 years, and in that time only twice found loss of PEN, so not very common.