Swimming pool light transformer replacement.

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Swimming pool renovation has included replacing the deck box that connects to a (new) LED light in the pool. The light is - as before - 12v, fed from a transformer in the plant shed.

As you'd expect, the cable from the light to the deck box is two core, ie no earth. Surprisingly (to me) the armoured cable coming to the deck box from the transformer was 3 core (ie with earth). I think that's because the existing (original) transformer is not double insulated and has 3 cores in and 3 cores out, the earth on the lamp side presumably being terminated at the deck box (had been removed and disposed of, along with the old lamp, so can't be sure).

So - I've bought a new transformer recommended for the new lamp and deck box. It's double insulated and so has just two connections (no earth) in and out. Presumably the existing 3 core armoured cable can stay in situ (I certainly hope so, it's now buried under concrete and expensively completed new paving) and the earth core not connected (or capped safely) at either end.

On the supply side to the transformer, there's currently a 3 core 240v supply from a fused spur into the existing transformer. The new transformer is not expecting an earth wire and has nowhere to connect the earth. Is the solution to a) find a new fused spur that has a termination for the earth and only two cores out to the transformer or b) take the output from the spur to a new JB inside which I cap off the earth into a block or wago and run two cores from that to the transformer?
 
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I would want to use class II equipment around an out door swimming pool, would not want any equipment earthed unless on a TT supply. However I note the old 2008 regulations seems to say all must be bonded, reading section 702 it is quite complex, and has likely changed with more modern editions of BS 7671. I would say leave it to your electrician to sort out.

The problem is with a TN-C-S supply the supply earth may not be same voltage as real earth, this is causing problems with EV charging points, keeping the difference within 70 volts, with a swimming pool you want it within a lot lower voltage, but location can make a huge difference. It is possible when the pool was made the reinforcement has been linked and bonded to ensure a equipotential area, this can't be assassinated remotely, so you need to ask the electrician on site.
 
Joined
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Location
London
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I would want to use class II equipment around an out door swimming pool, would not want any equipment earthed unless on a TT supply. However I note the old 2008 regulations seems to say all must be bonded, reading section 702 it is quite complex, and has likely changed with more modern editions of BS 7671. I would say leave it to your electrician to sort out.

The problem is with a TN-C-S supply the supply earth may not be same voltage as real earth, this is causing problems with EV charging points, keeping the difference within 70 volts, with a swimming pool you want it within a lot lower voltage, but location can make a huge difference. It is possible when the pool was made the reinforcement has been linked and bonded to ensure a equipotential area, this can't be assassinated remotely, so you need to ask the electrician on site.

Yep, I believe in the US the norm is to bond to the rebar but this pool is typical for the UK at the time which is to say the construction is block without rebar so nothing to bond to anyway. Even the ring beam has no steel reinforcing. I believe the lamp was always 12v no earth and the earth from the transformer was terminated in the old deck box.

The new (double insulated, that is class II) transformer requires no earth on either supply or load side, so my plan is to run 2 core from the supply spur to the new transformer, and cap off the now redundant earth wire at either end of the buried armoured cable from new transformer to deck box. That would seem safe to me, but I'm open to correction - is the perceived problem a lack of earth continuity if the earth from the mains supply is simply terminated at the spur?

To be clear, I will be asking an electrician to sign it all off before use, as soon as I can find one prepared to turn up (not an easy thing to secure - at any price - in this part of glorious post-Brexit England). I'd nonetheless prefer to fully understand what's being done and why.
 
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