In article ,
"andrewpreece" writes:

"Andrew Gabriel" wrote in message
...
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.

Thanks Andy. I understood that TN-S is most common for urban houses, and
that there would
most probably be a sign saying so if it was PME (TN-C-S ). I can't
immediately see why if the sheathed cable enters the service head/cutout
that means it's TN-C-S. Is there a foolproof way to tell?

Look at the FAQ: http://www.diyfaq.org.uk/electrical/...al.html#system

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

I've read conflicting stuff about this - some say the size of service
bonding depends on the earthing type in question.

It does. I gave you the figures which cover most residential
installations. In theory, your supplier can ask for larger
conductors in some cases too.

16mm2 is out of the
question - the CU won't take 16mm2 nor will the brass earth terminal on the
service head: you're going to say now I should get the 'lectric company in
to replace my service head........ :-(

I don't think I've seen such a service head, except possibly a 5A
one in the base of a streetlamp column. Does this or the CU have
multiple terminals -- if so you can split the conductor strands
into two terminals. Otherwise, I guess you're stuck with using
10mm2, which is probably OK if you only have a 60A cutout.

(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.

I figured pulling the strap tight with pliers might work. However, the screw
does not deform the strap
pulling it tight. The strap sits in the U portion of a metal stamping shaped
thus:- U¬ , which is held in position by the rest of the clamp. All that

Sorry, but I can't picture what you're doing wrong.
Can't find any web pictures to point you at either.

(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.

What passes for an earthing terminal in a bathroom? I have no electrics in
my bathroom.

In that case, just bond.

--
Andrew Gabriel