Exported PME

[Neil Holdsworth]

Went to look at a job today. I only went to look at an outside light that was faulty. It's turned into so much more.... Potentially! 

At the meter cupboard the tails go into Henley blocks where  they split to the house and to a small 2 way dB with a 30mA rcd up front. 16mm 3 core armoured is glanded into this but no banjo to earth the armour. 

That 3 core then goes underground to a external large garage with a flat above. The armoured hasn't  been glanded at this end at all. The DB is a 16th 10 way split board .

From this DB the flat above is fed with a 16mm twin and earth and another 16 mm swa goes to a indoor swimming pool which is also another separate building. The pool isn't heated but does have a sauna. 

I haven't fully checked the pool installation for compliance to the regs. 

The swa from the 16th board to the pool building was done in the last couple of years and signed off part p.

My first concern was the exported earth, and the armour from the meter cupboard not being earthed at one end and the other end having no gland at all.

My proposal is to disconnect the earth out of the swa that comes from the meter box and put an earth spike protecting the garage DB which in turn then puts the pool building protected by the spike too. Gland the armoured at the garage end and earth the armour from there. 

Any further pearls of wisdom for  me please?  Or perhaps I'm over doing it?

 

[Tony S]

Look on it as extending the equipotential zone and have a rethink.

 

[ProDave]

And while you are thinking, earth that bloody banjo
 

 

[phil d]

Straighten out the issues that you have got, you say it's a 3 core swa and one of the cores is used as earth, gland the armoured off correctly at both ends, fit banjo's and earth them and don't worry about the mythical exporting of pme! If you have a nice low stable loop value at source, which you should have unless it's a TT system, you are not going to achieve anything better by isolating the earth at the far end and driving a rod in, indeed you'll probably make things worse. Yes there are situations where extending an earth system is not advisory, I use the term extending because 9 times out of 10 exporting isn't strictly the correct term to use, however in my opinion this isn't one of them.

If you had a house on a TT with a single rod and in the back garden you had a large metal building with steelwork going into the ground then the odds are the earth value on the building would be lower than the house, (think multiple earth points) this would definitely not be a case for utilising the house earth, however when you are talking about a perfectly good earth in the house and a building with a pool in it, it would be sheer madness to abandon this earth in favour of a rod. 

 

[Sidewinder]

The issue that you have with TN-C-S earths (I'm not saying PME for a reason for a moment), is that the potential of the neutral, and thus suppliers earth can rise above that of true earth not only in the event of an "upstream" fault, but, also due to the current being carried by the N conductors in the upstream supplier network, which depends on the design of that network, which you have no control over.

The armour of an SWA, to protect in the event of ADS being required, must, ALWAYS be solidly earthed at supply end.

Think about it and do a design FMEA and you'll see why.

You need to analyse if the remote buildings are suitable for extending or exporting the TN-C-S earth to, and go from there.

Some will be, some won't be.

Also you need to ensure that you have adequate protective conductor sizing to comply with the requirements of table 54.7/54.8.

TN-C-S/PME can be safely exported/extended anywhere, as long as it complies with the requirements of BS7671, ESQCR & the local DNO rules, which, may in the first instance when asked be, NO, you can't do it, but, once further questions are asked, and further measures instigated, may, well be, "ok, no worries".