Looking at my earthing (TN-S) I see a 6mm2 earth wire come from the
service head thingy
to my CU, then another green 6mm2 earth wire exits the CU and dives below
the floorboards to clamp onto my lead water pipe.
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?
(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
off ). Two metal dimples in the main clamp section prevent it from
tightening on the pipe. I figured I was wrong about how they worked so just
pulled the strap tight and screwed the screw down to lock it, but it is well
nigh impossible to get the strap tight in this way - the clamp rotates and
so can't be giving a good connection. What am I doing wrong?
(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??????
(4) I understand that suplementary bonding is not required in anyother
situation, i.e. kitchens and toilets????
(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.
thanks for any answers or observations,
That sounds more like TN-C-S.
For TN-S, the earth conductor would come from the cable sheath
under the service head.
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.
Service bonding should normally be 10mm2.
Main earth conductor to supplier's earth connection should be 16mm2.
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.
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.
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.
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.
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?
I had a feeling someone would say that :-(
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........ :-(
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:- UŽ , 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
Am I to understand that the clamp as purchased has to be disassembled ( with
a bit of grunting I can pull out the UŽ shaped bit and reassemble the clamp
so the UŽ 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 UŽ 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.
What passes for an earthing terminal in a bathroom? I have no electrics in
Hmm, as befroe, not physically possible with my main earth terminal and CU
Correction, I have determined my main earthing cable is 10mm2, not 6mm2 as I
said, and the main earth terminal could take a 16mm2, but the CU cannot. I
have plans to upgrade the CU so there is hope I could have 16mm2 main
earthing cables eventually, only I have to stick with 10mm2 for now, unless
I parallel two up.
I have just unscrewed the Bakelite cover where the main cable terminates,
and emerging from the main cable there is a sheathed red core, probably
35mm2 or some such, a skein of smaller twisted black insulated conductors,
which I take to be the neutrals, and 4 bare copper conductors, adding up to
16mm2 probably, going tothe main earth terminal.
I suspect this proves it to be a TN-S system then, as I appear to have a
discrete earth return in the cable.
Look at the FAQ: http://www.diyfaq.org.uk/electrical/electrical.html#system
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.
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.
Sorry, but I can't picture what you're doing wrong.
Can't find any web pictures to point you at either.
Alas, having looked inside the service head, I can identify the live
insulated and central to the cable. Surrounding the red core are then some
smaller black insulated wires that are stuffed into a terminal to make
There are then four bare copper conductors of the same size that go to the
Thinking about it, I have no idea whether these are dedicated earths or
four neutral wires with their black insulation stripped back out of sight in
and co-opted into being earths, in which case I have TN-C-S. Why is nothing
I think you're right about the 16mm2 - I 've just found a website that gives
16mm2 as the size of the main earth conductor for TN-S. And TN-C-S.
I was wrong, the service head will take 16mm2, it was difficult to see, all
covered in dust
at at a bad angle.
See my earlier post - think they need disassembly then reassembling
differently. I think they may be assembled that way to keep them all in one
Back in the good old (pre-Part-P) days I deliberately omitted bonding
from the kitchen last time I did a rewire, having read up on this issue
here; and for my trouble I failed the Periodic Inspection carried out by
a sparks after I'd finished!
Yeah - there's a significantly misleading bit in Whitfield's usually
reliable guide-to-the-Regs, which more or less claims that Kitchenz Need
Bondage... and since Whitfield's a lot more readable and
practically-oriented than the Regs, it takes precedence in many a
real-life sparky's mind.
Talking of Whitfield's and bondage...
I'm quite confused by the explanation of bonding the incoming gas and
water supplies to te main earthing terminal.
Figure 5.13 (on p96 in the 6th edn) it shows the connection being made
to the gas supply on the house side of the gas meter - fine. But the
connection to the rising main is shown on the street side of the
stopcock, albeit with the proviso that it should go to the house side of
the stopcock if the pipe is insulated on the street side.
Now why is that? Why doesn't Whitfield show the default connection to
the rising main being on the house side of the stopcock? (This in fact
was my other 'failure' by the same electrician - I had my bonding cable
on the street side (all metal pipework)!
Careful! - you're only supposed to bond the consumer's installation
pipework. On metal pipe gas supplies Transco often install an
insulating insert on their side of the meter, to prevent (or at least
reduce the risk of) current flowing in their pipes. It won't help if
this is shorted out :-(
My incoming gas main is iron barrel which is imbedded directly in the
earth. A few years ago, they inserted a plastic pipe in it - very clever -
no disruption to the garden path.
I'm totally confused about all the various options, so simply bonded
absolutely everything together - including a vast earth rod. But not the
incoming leccy SWA main sheath as it looked not that sanitary.
It measures ok. :-)
*If only you'd use your powers for good instead of evil.
Dave Plowman email@example.com London SW
6mmsq is undersized by current Regs thinking: the main earthing
conductor (from service-head-thingy to earthing block, and on to CU
earthing terminal) is s'posed to be 16mmsq, and the main bonding
conductors (to water, gas, CH and so on) to be 10mmsq. I'm not sure of
the contractual/legal position on fiddling with the earth connection
provided by your supply company, but ideally you'd want them to do the
6mmsq -> 16mmsq upgrade rather than d-i-y, even for The Best Of Reasons
and with the Best Of Intentions. They may take the opportunity to turn
you into a TN-C-S (PME) installation while they're about it, as this is
the Modern Fashion (and does reduce the earth loop impedance, i.e. make
it yet more likely that fuses will blow nice and quickly in the event of
On the what-I-take-to-be-totally-standard-pattern clamps I have... the
'clamp section' indeed doesn't move. You wrap the strap round the pipe
and under the end of the screw. As you tighten the screw down (yes, with
the locknut backed off ;-) the two sections of the strap - the captive
start and the part you just threaded through - both deform under
pressure from the screw, pulling the strap tight against the pipe. Since
there's not a great deal of movement, I find it useful to keep the loose
end of the strap pulled tight with pliers, by rolling the outside of
their jaws against the side of the clamp, until the screw's far enough
in for the strap to grip tight against the pipe. And you can't really
reuse the clamps - once the strap's bent in, especially the captive end,
it ain't going to spring back when you slacken off the screw. Other than
that - I can't offer any pearls, or even bits of frosted glass, of wisdom.
That's about it. The bathroom stuff is all about *supplementary*
bonding, so that all the bits of metalwork which are capable of
introducing a potential are at the *same* potential. That it ends up
being earth potential is incidental (and in some circumstances
You understand rightly. It's the wetness and nakedness (most
importantly, bare-footedness, though an electrocution path might pass
through any other parts of the skin which rest firmly against the
metallic items at differing potentials) of the
(immediately-post-)bathing body which are the main causes of the lowered
resistance which increases the risk of shock: so it's baths and showers
we care about. If you take your imitation of 'The Naked Chef'
overliterally, you may want to indulge in supplementary bondage around
your kitchen sink, but that's a matter of personal taste, rather than
strict regulation. (Quiet at the back there, Simpkins!)
You done good: a separate earthing block is preferred practice, since it
makes testing of earth impedance easier. (Our reading today is from
section 4.10 of the On-Site Guide: the lord Cook spake thusly unto us,
"[whilst] the earthing bar is sometimes used as the main earthing
terminal; however, means must be provided in an accessible position for
disconnecting the earthing conductor to facilitate testing of the
earthing". For so requireth fivehundredandfortytwo-dash-ohfour-dash-ohtwo.
Hmmm. I'm thinking about leaving it all now, it's too much trouble for no
Read my supplementary message where I correct myself, I think I have 10mm2,
not 6mm2 as I said. It's close enough for country work, ain't it!
Everyone says the same thing - that the straps deform under the screw and
tightens on the pipe, but my straps don't deform at all, you can undo them
just see a faint circular scrape mark where the screw contacted the strap,
even if you apply an unfeasible amount of torque to the screw. Nor does the
tighten even slighly, as there is no force acting on the strap which could
I am coming to believe that these clamps must be disassembled then
in a different arrangement to that they were purchased in before they will
have pulled one apart and rearraged it and will test my theory tomorrow.
While we're on earth clamps, whats the preferred method of dealing with
the embossed aluminium safety labels with a slot at both ends?
a) Leave both ends captive on the strap round the pipe (but label is too
long for 15mm pipe)
b) Leave one end captive & the other flapping in the breeze.
c) Trap it onto the free end of the strap (by folding the spare strap)?
d) Toss it in the bin (Noooo!)
Not having a clue (about this) myself, I found it could be _almost_
neatly attached to the strap and through the loose end of the strap
whilst passing over the top of the clamp/screw. i.e. capping the screw
meaning you could not undo the screw without first removing the
Just seemed like the logical way of doing it in today's "safety
I am informed that the label should come off the strap and be attached to
screw before the clamp is assembled. This is another thing they didn't tell
me when I bought the clamps - that's now two bits that have to be
and reassembled in a different position. The rationale is that the warning
should be easily readable. Of course, I now have 5 clamps all
clamped badly onto pipes because I didn't reposition the sliding clamp bit
with the labels still attached to the strap and the labels obscured 'cause
where they were when I bought them!
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