Consumer unit change - earth bonding

[Ben Blaukopf]

Hopefully this is the last set of questions!

1) My current installation is PME with the following differences from the diagram in the OSG:

All the conductors (supply and main equipotential) are 6mm2 (diameter is

4.4mm so it's either 6mm with .8mm insulation, or 10mm with .4mm - 6mm sounds more likely!)

As far as I can tell, the gas pipe is not directly bonded to the main earthing terminal. The water pipe is, and then there is a second wire running away from the water pipe, which I believe connects to the gas pipe.

There is no separate main earthing terminal - it's more of an integrated one. The CU earth connects to a terminal at the side of the supply fuse, as does the water bond. I don't see any more terminals for adding a third (gas) bond there.

meter tails are 16mm2 (as are the fuse-meter conductors)

When I put the new CU in, I will provide 25mm tails and a 16mm CU earth for the supplier to reconnect (I assume I'm not allowed to touch anything on their board so they will want to do this bit). Is there any chance that they will refuse to reconnect me because my main bonding conductors aren't up to scratch? As I understand it 6mm2 is OK, just doesn't comply - there's even an example PIR in Appendix 7 illustrating this precise case - TN-C-S with 6mm earth. However, I don't (yet) understand why TN systems require a higher main bonding csa than TT, and things I don't understand bother me!

2) Is there anyway of testing the continuity of the main bonding without using a long test lead? There is no neutral to use on a gas pipe.... Would it be acceptable to temporarily run a second lead, calibrate its resistance and deduce R2?

Thanks!

Ben

[Martin]

Make sure they do a good job! I inspected mine after they had left to find exposed live copper so I re terminated it. Lucky for me they had not tagged the main fuse so I could remove it (and still can).

Martin

[Andy Wade]

Yes that sounds like 6 mm^2. 10 mm^2 has an o/d of about 6.1 mm.

There's nothing wrong with that (apart from the wire size). Look at Fig. 4(b) in the OSG which shows daisy-chained main bonding via a gas pipe to structural steelwork. (Although not a requirement, it's conventional practice when daisy-chaining not to cut the wire at intermediate connection points - just remove insulation, form a U-loop and hook same under the terminal screw & washer of the earth clamp.)

In that case the earth bar in the CU is the main earth terminal and you can connect any additional main bonding there. If there's no room or insufficient terminal capacity then you can interpose a multi-way earth terminal block in the earthing conductor (that's the one between the CU earth bar and the supplier's PME terminal).

Yes, every chance. You will need to upgrade the main bonding to at least 10 mm^2. (I usually use 16, which you have to use for the earthing conductor in any case.) Technically BS 7671 requires you to check the required main bonding size with the DNO (see note above Table 54H: "local [...] supply conditions may require a larger conductor"). I suggest you call EDF networks and ask, but I very much doubt that they'd accept old 6 mm^2 bonding on what amounts to a new installation.

On an old installation presumably installed when those sizes were OK?

It's because of the risk of much greater current flowing under fault conditions. In a TT system earth currents are limited by the relatively high resistance of the earth electrode and the sizes are mainly determined from considerations of mechanical strength. With PME there's also the possibility of circulating currents in the bonding conductors - your main bonding might be connected to your neighbour's via (e.g.) all-metal water mains and this forms a parallel path for neutral mains current.

That is a long test lead, surely? See OSG 10.3.1(ii).