View Single Post
  #3   Report Post  
andrewpreece
 
Posts: n/a
Default


"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?

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.


I had a feeling someone would say that :-(

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.


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

(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
happens when you tighten the screw is that the strap is clamped to the 'U'
and prevented from moving - it does not tighten at all, hence my
headscratching. All the tightening has to be provided by the initial pulling
the strap tight, and neither with fingers nor pliers does this result in a
good connection.

Am I to understand that the clamp as purchased has to be disassembled ( with
a bit of grunting I can pull out the U¬ shaped bit and reassemble the clamp
so the U¬ shaped bit is free to move down under the action of the screw ( by
reinserting it in the slotted bit below the metal dimples ). I can see in
that way that the strap will be dragged down under the action of the screw
to eventually clamp the strapagainst the U¬ shaped bit and the pipe. Why no
clues if that is the way it is supposed to work? It's like one of those Xmas
cracker puzzles. Will try it anyway.

(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.

(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.


OK

(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.


Hmm, as befroe, not physically possible with my main earth terminal and CU
accepting 6mm2
maximum AFAICS.

cheers,

Andy.