OT but coincdentally, today I had a letter from the gas co. telling me
that they want to come and change my gas meter for a newer one in a
couple of weeks. Having read this thread I went and checked if the
existing meter has an earth bond anywhere - and yipeee (sarcastic) it
hasn't. Being pessimistic, does this mean the gas fitter could come
along and refuse to install the new meter (after disconnecting the old
one of course) because of this lack of an earth bond? The distance
from gas meter to CU and earthbond is about 3 feet!
Wouldn't this be one of those cases where there's no compulsion to
upgrade an existing installation to meet modern regs? Surely
cross-bonding of the incoming services to the main earth hasn't always
been compulsory (at least, not judging by the number of times I've had
to install it myself!)
Watch the guy like a hawk, and make sure they don't leave leaks. I don't
know if the twit who changed my meter was a CORGI (looked more like a
quickly trained temp), but we had to call out Transco when we started
smelling something gassy soon after.
Secondly, they have to reignite the boiler after completing the work.
This is the point where he may shake his head (if qualified to do so),
mutter something about poor servicing and ventilation - and plant an
expensive for this time of year 'do not use' notice capping the pipe as
Happened to me :-(
Can't you put them off until the spring?
The requirement is that the house pipework is bonded on the consumers
side of the meter as soon as is practical after it enters the house.
Often this is actually implemented by effecting the bond in the external
meter box if you have one, however this is not actually the letter of
the requirement since it is not after entry as such.
Meter is in a semi-concealed box at side of house. A 28mm copper pipe runs
up the wall and across the bedroom floor then into a landing cupboard where
the boiler is housed.
I can join the earth cable to this copper pipe in the bedroom without too
much trouble. This will be my 'as soon as practical' solution. It was the
thought of having to route it across the bedroom and down the wall which
prompted my original question.
I'm doing this to conform to regs. I don't really understand why.
Thanks for all the responses.
It really needs to go where the pipe comes into the house. Unless you can
prove it is a continuous length from there to where you propose fitting
Well, consider the fact that the pipe from the street is plastic. A metal
one running through earth would provide an earth of sorts - but plastic
won't. The copper within the house is connected to what? A boiler, gas
fire, cooker etc all of may also be connected to electricity. If one of
those developed a fault it's conceivable the electricity could get to the
pipe. And make all the copper pipe 'live'. I know it's unlikely but
protective measures have to allow for every possibility.
*The average person falls asleep in seven minutes *
Dave Plowman email@example.com London SW
Which means plastic is safer. If the gas pipe were metal, in contact
with earth, then it would constitute an "extraneous-conductive-part" in
the language of BS 7671 - i.e. it's capable of importing a potential
(voltage) which might be different to that of the house electrical
earthing. The extreme example of this is a house on a PME supply where
the supplier's neutral connection has broken and all the electrical
earths are live at 230 V relative to an unbonded service pipe which
remains at the real local ground potential. Hence the requirement for
*bonding* (not "earthing" or "earth bonding") all incoming services to
equalise their potentials. *Bonding* reduces the risk of dangerous
touch voltages appearing between different accessible bits of metal.
Now, plastic service pipes don't constitute extraneous-conductive-parts,
and nor does the metal installation pipework downstream, provided it
doesn't come into contact with the ground. Hence, if you read BS 7671
literally, there's no requirement to bond metal gas or water pipework
fed from plastic service pipes. However the IEE, in its practical
advice (e.g. the OSG), continues to recommend that metal installation
pipework is bonded, probably because of the risk of some ground contact
occurring - contact with damp walls or grounded structural metalwork and
that sort of thing. The upshot of this is that it will be very
difficult to convince anyone doing formal inspection and testing that no
bonding is necessary.
But the protective measure there is the *earthing* of the electrical
equipment concerned  which ensures that the supply is quickly cut off
(by fuse, MCB or RCD) when such a fault occurs. Note that earthing does
*not* prevent the metalwork becoming live - during the fault a
significant fraction of the mains voltage will be dropped across the
relevant circuit protective conductor (CPC) - it just ensures that it
won't stay live for very long.
It really is quite important to understand that earthing and bonding are
quite different things, even though the same piece of wire can sometimes
carry out both jobs. Talking about "earth bonding" is strongly
deprecated, even though the sort of bonding we're talking about here is
 Or the use of Class 2 equipment (sometimes incorrectly called
Yes, have everything in the house made of plastic and the risk of
Round here many are still iron barrel from house to street main - they
simply pushed plastic through it when they changed. How it is sealed to
the iron barrel I have no idea - but there is electrical continuity across
the gas meter to the pipe which disappears to the street.
Think you're splitting hairs. Incoming services are bonded to the same
'earth'. Which may or may not be a true earth - whatever that is.
So you're happy to hypothesise about the neutral failing in a PME
installation but not an appliance ground being faulty? I know which one I
reckon is more common.
Please define 'live'?
What most set out to do is confuse.
*I love cats...they taste just like chicken.
Dave Plowman firstname.lastname@example.org London SW
Although Transco often use insulating inserts on the supply side of the
meter, with the aim or preventing earth leakage and diverted neutral
currents flowing in their metal pipes. (See 4.2(iii) and 4.3 in the OSG.)
No, I think that understanding the conceptual difference between
earthing and bonding is key to understanding the subject. It's
unhelpful when people muddy the water by referring to bonding as
earthing or using woolly terms like "earth bonding." Think of earthing
as being active protection, while bonding is passive.
True, and ...?
... you've just demonstrated the need to define an equipotential zone by
bonding, so that the local ground potential is made largely irrelevant.
That's unfair. The PME neutral failing brings immediate danger
(mitigated by the bonding) but the o/c CPC situation requires a second
fault (insulation failure) to develop before a dangerous situation
arises. And that's why ... EaWR ... preventative maintenance ... etc.
In this context just "at a hazardous voltage" (wrt surroundings) will
do; above the ELV limit; >50 V AC.
In a circuit where the phase and CPC are the same size (and simplifying
by assuming that Ze << Zs) then during an earth fault an
exposed-conductive-part may rise to half mains voltage until the
protective device operates. If the circuit's wired in T&E with a
reduced size CPC then the voltage is higher, being Uo * m/(1+m), where m
is the ratio of L&N size to CPC size. 4 mm^2 is the worst case with an
m of 2.67, giving a fault voltage of around 170 V.
Isn't accurate use of language important in all technical disciplines?
In the present field there's a well defined vocabulary and no excuse...
My understanding was that the point of connection to piping should be
accessible so that the presence and security of a pipe clamp and cable could
be checked in future.
If this point would be under flooring at the point of entry into the house,
shouldn't the connection deliberately be made in a cupboard or other
downstream more accessible location anyway??
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