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

"Ben Blaukopf" wrote in message
...
Hopefully this is the last set of questions!


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).

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

Ben Blaukopf wrote:

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!)

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

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'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.)

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.

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).

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?

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.

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.

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

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!

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.

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?

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

--
Andy