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Andrew Gabriel
 
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In article ,
"andrewpreece" writes:
Hello Folks,
Looking at my earthing (TN-S) I see a 6mm2 earth wire come from the
service head thingy


That sounds more like TN-C-S.
For TN-S, the earth conductor would come from the cable sheath
under the service head.

to my CU, then another green 6mm2 earth wire exits the CU and dives below
the floorboards to clamp onto my lead water pipe.


Your installation isn't up to current regs.
If you are going to do anything to the installation, you need
to bring the earthing up to current regs, which includes replacing
the existing earth bonding, based on how you have described it.

Since I now have gas and central heating pipes which appear
unbonded ( except by secondary means ) I decided to bond them to the main
earth, but I have some questions that Googling didn't clear up.

(1) Can I use 6mm2 earth cable for the short runs from my gas and CH pipes
to the main earthing point?


Service bonding should normally be 10mm2.
Main earth conductor to supplier's earth connection should be 16mm2.

(2) I have some BS951 clamps which are a puzzle to me: I thought that one
threaded the strap through the clamp and tightened the screw, which would
tighten up the clamp on the pipe, but what I
took to be the clamp section ( it has the earth terminal attached to it )
does not move at all when the screw is tightened ( yes I backed the locknut


The strap goes round the pipe and then through the slots which raise
it off the pipework a second time.
Pull the strap tight with pliers and bend it sharply where the
tail exits the bracket to stop it slipping back in. Then the screw
clamps onto the strap, deforming in the bracket and pulling it
tight on the pipe. The screw should not touch the pipe itself, nor
deform the strap enough that it is pushed back onto the pipe under
the bracket -- if it does, you didn't pull it tight enough to start
with or it slipped back through the bracket before you got it tight.
Once you've used a clamp, the strap is deformed and cannot be reused.

(3) I believe that it is unnecessary to run an earth cable to my bathroom
( which has no bonding at all ), and that merely bonding all the exposed
metal bits to each other with 4mm or protected 2.5mm cable is OK??????


Yes. It should be connected to an earthing terminal in the bathroom
(ideally the earths of all circuits in the bathroom), but does not
need a dedicated run back to your main earthing terminal.

(4) I understand that suplementary bonding is not required in anyother
situation, i.e. kitchens and toilets????


Only in rooms containing a bath or shower.
Personally, I also do it within 2-3m of the kitchen sink too, but
this is not done by everyone, not a requirement, and not always held
to be a good idea either.

(5) I cannot fit all the new earth cables into the earthing block in my CU -
only one terminal is still available, so I bought a metal busblock (?) which
I intend to mount exposed on the board, alongside the CU, and I will bring
the main service head earth into that and then take all other earths out of
that block, i.e. to the CU, to the CH pipes, to the gas pipe etc.


That's fine. You need 16mm2 from the supplier's earth to this
terminal, and into the CU. The other bonding can normally be 10mm2.

--
Andrew Gabriel