Hi,

I've been discussing this on the IET forum and offline too. This is
complicated - if anyone has a thought but doesn't want to commit to it in
public, feel free to email me (address is valid).



I'm partly posting this here as the engineers over at the IET have been very
helpful, but are good at pointing out all sorts of interesting snags. They
did produce some interesting guidance but not really a definative answer.

So I'm here for a fresh dose of common sense and perhaps someone's done this
before...

As previously mentioned, I need my consumer unit 14m away from the
supply+meter. Moving the meter is out of the question.

------
So, my proposal has always been, which seems pretty standard is to:

Run 25mm2 3 core armoured cable from near the meter to new CU. The third
core will be parallelled with the armour to function as the main earth
(circuit CPC *and* earthing conductor). My calculations for disconnection
times, R1+R2 and volt drops look perfectly OK, relative to the worst case
final circuits.


In line with EDFs rules, I must provide overcurrent protection and means of
isolation with 3m of the meter, so I was going to use a 100A DP switch
fuse. Seems normal so far.

-----

However, some of the folk at the IET pointed out some theoretical snags,
including:

Where does the MET (main earth terminal) and main bonding go?: next to the
meter or next to the CU at the other end. Conclusion was the MET is better
next to the CU, as during a fault on the distribution circuit, voltages on
the "earth" conductors all round the house may easily hit 100+V for upto 5
seconds, so it's better the water pipes etc come up with them in the spirit
of the equipotential zone that the house is supposed to be.


However, that leaves the reverse problem at the other end, eg big fault,
pipes go up to 100+V, exposed conductive parts like the switch-fuse case
don't. Ditto pipes outside and in shower see the 100+V potential difference
with respect to the floor slab.


Having read the whole of chapter 54 in the IEE Wiring Regs (17th) (twice!)
and 2 guide books (ECA guide and NICEIC Earthing Snags) and been all over
google, it appears that there's nothing wrong with my cable - it's CPC
oversized if anything.

But there's nothing prescriptive about multiple main bonding.

*** So should I bond gas and water to the means of earth back at the meter
as well, directly to EDF's earth terminal? BTW - this is a TN-S setup, Ze
of 0.19 Ohms

I came up with another way round: put a Type S 100mA DP RCCD in place of the
main switch in the switch fuse enclosure, and thus reduce the disconnection
time of the distribution circuit to 0.15s which avoids all these issues.
Unconventional with a TN-S system, but harmless at worst...

*** Overkill? Or not?

Random thoughts gratefully received...

Cheers

Tim