andrewpreece wrote:
Hello Folks,
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?
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
L-to-earth-conductor faults).
(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?
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.
(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??????
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
undesirable, even).
(4) I understand that suplementary bonding is not required in anyother
situation, i.e. kitchens and toilets????
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!)
(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.
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.
thanks for any answers or observations,
Y're welcome - Stefek
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