Change from TT to TN-S earthing

[Ken Sparky]

At the moment, I have two consumer units - one for the usual domestic supply and the other for off-peak heating with each having its own ELCB and earth rod.

About ten years ago, the electricity board fitted a new off-peak timer and unwrapped a perforated strip which was connected [and still is] to the sheathing on the incoming cable, screwed an earthing block to it, and earthed the new timer to that block. I'm a bit hazy on this, but I seem to recall the fitter saying at the time that it was a PME system, but it's connected as TN-S.

I'd like to fit new CUs - one split load with a 30mA RCD, and the other without RCD for the off-peak heating - junk the ELCBs and TT system, and earth to the block that the fitter put in.

A couple of years ago, my neighbour had storage heaters put in by the board and they installed a new CU for him. A couple of weeks later, a different crew turned up to remove his ELCB and re-earth to the incoming cable sheathing.

Is the process as straightforward as that or is there some work that the board needs to do behind the scenes?

[usenet]

Changing the actual earth connection from your local earth rod to the company supplied earth is probably "as straightforward as that". However there *may* be issues with the sizing of your earth bonding connections as for a PME system the earth bonding requirements are more demanding.

By the way PME is TN-C-S if I remember correctly, not TN-S. It depends whether the earth that the 'electricity company' have supplied is actually their neutral or separate.

TN-C-S 'Common' company earth, i.e. the earth block is connected to their neutral, 'separate' house earth wiring. Supply feed wire will be a single conductor with screen/armour used as neutral/earth.

TN-S 'Separate' company earth and house earth wiring. Supply feed wire will be two core (live and neutral) with screen/armour being just the earth.

[Mike]

And if it's TN-C-S do you really want to change to it. Ours runs at up to

40volts away from the potential of the ground outside, which I found to my shock unfortunately includes the soil below the stone slabs in much of my house.

[Ken Sparky]

snipped

Any response from the supplier about this?

I'd be happy with TN-S because I was trying to avoid having to install a 100mA time-delayed RCD which is required with TT earthing.

[Ken Sparky]

Agreed. What I was trying to say was that although the fitter said it was PME, I think it is connected as TN-S. I thought it was TN-S because the earth block is wired to the sheath [but maybe it's not TN-S in view of your definition of TN-C-S using the sheath as the neutral conductor].

Would the sheathing also be likely to be the neutral conductor in a supply cable installed 45 years ago?

How do I find out if the sheath is earth only or earth/neutral combined?

[Lurch]

On Wed, 15 Dec 2004 13:20:43 GMT, sparky [e-mail] (Ken Sparky) strung together this:

Yes and no. If the supplier alters things on the network then a previously TT only supply can become PME.

Ask the supplier. Some TT systems can look as if they might be PME from tests carried out, but actually aren't.