Bonding

What is the impedance of the earth rod you already have ? It may be enough on its own. Also the wire to the electrodes can be quite a lot thinner than the typical TN-x woud require - the rod resistance to ground means that the current is limited and even 4mm would be ok for those connections. 

It is perfectly OK to have one large TT system with interconnected CPCs and several electrodes in many places, but equally islands each with an RCD and electrode fed by 2 core supply is also OK 

In general the armour of a 10mm SWA can be taken as comparable to the copper cross section you will need in most cases. 

There is somewhere in circulation a document that describes the cross sections better - if no one else posts it in a few minutes, I'll try and link a copy

Mike

It sounds as though your installation is TT, BUT you need to verify this to decide on the bonding requirement. You need to measure the Ze at the CU and see whether this is essentially zero or some figure probably above 50 Ohms. The low figure is TNC-S and 10mm equivalent bonding may be needed, the higher figure is TT, and bonding via the SWA is adequate in all cases.

It may be useful to look at the original Electrical Installation Certificate, which you should have, this has tick boxes for TNC-S and TT, and a Zs number saving you trying to measure it.

I hope that helps

David

There is somewhere in circulation a document that describes the cross sections better

From appendix B of (an old) GN 8 - 2.5mm2 3-core SWA has an armour of 19mm2 steel.

That would be a bit shy of what's required for a PME system where the demand is equivalent conductance of 10mm2 copper - which would be 80 or 90 mm2 steel. But if this really is a TT installation (with no equivalent conductance requirement), then it's likely satisfactory.

   - Andy.

To be absolutely clear a photo of the DNO intake and the earth terminal would help.

If the earth rod is connected to the DNO earth terminal on their intake it is almost certainly the property of the DNO and the supply is TN-C-S.

If there’s no earth connection to the DNO intake and cable at all with the rod being connected to a completely separate MET then it’s TT.

If it’s TT then the consumer unit probably should have a RCD main switch or there should be an upfront RCD, but the earthing will be okay.

If it is TN-C-S it is probably worth converting it to TT, but there’s then there is how to maintain separation from the DNO earth rod to consider.

Could this be a P.N.B. arrangement?

Z.

Thank you all for your advice.

These images show what things look like.  My supply is taken from the top and bottom conductors ~ L1 & L3?  ~ my neighbours variously connect (left) to L1 & L2 and (right) to L2 & L3 ~ so nicely balanced.  There is no connection between the earth conductor passing through my meter cupboard and the new DNO head.

I don’t know whether what I see as neutral in my CU is connected to the top wire, the one always closest to any cumulo-nimbus cloud, or whether it is the other way round. In the past lightning has ‘cooked' a modem, but that strike probably hit the telephone line.

There is no connection between the earth conductor passing through my meter cupboard and the new DNO head.

Looks like TT to me (especially if that G/Y goes just to a local rod and nothing else).

My supply is taken from the top and bottom conductors ~ L1 & L3?  ~ my neighbours variously connect (left) to L1 & L2 and (right) to L2 & L3 ~ so nicely balanced.  

It would be far more usual to connect each (single phase) customer between one L and N - and N is usually at the bottom (as it's the least hazardous and most likely to be touched by ladders, poles, etc) … although it does look like that might have been reversed for that 3-wire feed going off at an angle to the left.

    - Andy.

Indeed it does look like a TT installation now that we can see the images. I came across an installation in the middle of nowhere in the countryside that was TT earthed. The owner said that they could not have a P.M.E. earthing terminal because the overheads were too long and Ze was too high, so TT earthing had to be used. The image overheads are likely to be in an impoverished area or very rural neglected area, as they are bare and old. Upgrading required.

Z.

Your supply is TT, but I suspect the overheads may be TNS.

Are you in South Wales?

Sparkingchip: 
 

Your supply is TT, but I suspect the overheads may be TNS.

Are you in South Wales?

It says North Eastern on the meter, the whole setup hardly looks fit for purpose.