TT to TNCS conversion or regs fror earth rod

[Invisible Man]

Got a TT installation but the earth rod is in a flower bed that will shortly be a step to the moved front door surrounded by a block paved drive.

System at present is TT via 2 overhead cables. No obvious earths down any of the supply posts.

I have asked my supplier whether they will convert to TNC-S and am awaiting a reply. Can anyone comment on my chances of getting the supply changed over? EDF Energy area but my supplier is Atlantic Electricity and Gas.

DIY bit if I have to stick with TT. The solid porch floor needs re-tiling with another layer of ceramic tiles on top of existing. To get an earth to an earth rod in a suitable area it would need to cross the porch. Can I cut a channel for the earth wire in the floor and lay conduit for an electrician to use when the rod is moved? If so how deep does it need to be and does it need to be covered by anything special? Total run would be about 6 metres. How thick would the cable need to be so I can get the right conduit.

Thanks as ever for any replies.

[Andy Wade]

Fairly likely, unless you're in the back of beyond. It might help to ask for 'PME conversion' rather than TNC-S, although it's exactly the same thing of course. You may have to pay. With an older TT installation you'll probably need to add or upgrade the main bonding, otherwise connection of your earthing conductor to the PME terminal will be refused.

Sounds OK. The earthing conductor to the rod will need to be 16 mm^2 if it will be buried in the ground once outside. This will easily go through 20 mm PVC conduit for a a straight run, but you'll need a larger duct if there are many bends. There's no specific rule for depth in a solid floor, other than "sufficient to avoid damage due to any foreseeable disturbance".

[Rob G]

Why on the earth does a TT earth require 16mm^2 cable ? Our own Wiki, backed up by various other sources of information, indicates that the ground resistance is in the order of 100 ohms, which gives maximum fault current of 2.4 amps. Even if the ground resistance is half this that is still only 5 A, so why the dirty great cable ?

I've found a reference that says that 2.5mm^2 cable has a resistance of 7mOhms/m, so you would need to have an awful lot of metres of even that before it has any influence on the total resistance.

Rob

[Invisible Man]

Many thanks Andy. We had a new consumer unit and new bonding to the water and gas earlier in the year.

We are in a village just outside Chelmsford. Most of the village has underground electricity supplies but our 1960s box was fitted in to the older area of the village which has overhead cables.

[Tim S]

Rob G coughed up some electrons that declared:

I haven't got my books to hand (down at Bungalow) but you need a certain minimum size for mechanical robustness, unless the wire is otherwise protected.

You can't go sticking a bit of 2.5mm2 dangling outside, over the ground and down a hole to a rod - it's too weedy and liable to get damaged.

The same logic is applied to supplimentary bonding conductors dangling around pipes, but as my memory is s**te, you'll have to wait until I get my books or someone else with books/better memory can comment.

From memory 4mm2 or 6mm2 is likely to be the minimum acceptable size depending, purely from a mechanical POV.

But on the argument regarding resistances I agree completely.

Cheers

Tim

[BruceB]

On earthing conductor size for TT.... See table 4.2, page 29 in the latest OSG (Cu and Zs not

[Rob G]

Thanks Bruce and Tim - I need to put a secondary TT earth at my garage/ workshop and was a bit gobsmacked about the cable size Andy Wade mentioned. My house TT earth was installed by the local board aver 30 years and in all likelihoods needs inspection and probable upgrading - note to self must do this !!

Rob

[Tim S]

Rob G coughed up some electrons that declared:

Aye - 16mm2 isn't that bad, being the standard size you'd need for any TN-x main earthing. If you went for 25mm conduit where necessary (common and available) and use gentle bends and not many of them, I don't think you'll have much trouble pulling it through 2-3m worth.

Shorter lengths, 20mm ought to be OK, though I've only pulled 10mm2 thought that, but no trouble.

Cheers

Tim