Earth wire from consumer unit.

On Mon, 5 Nov 2007 19:56:49 -0000, "Mark"

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!
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dave wrote:

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!)
David
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Lobster wrote:

There is no compulsion if you are not making changes. However, once you make a change anywhere then you ought to do it.
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John.

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dave wrote:

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 well.
Happened to me :-(
Can't you put them off until the spring?
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Adrian C

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Mark wrote:

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.
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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.
mark
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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 it.

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.

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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Dave Plowman (News) wrote:

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 [1] 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 earth/ed/.
[1] Or the use of Class 2 equipment (sometimes incorrectly called double-insulated).
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Andy

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Yes, have everything in the house made of plastic and the risk of electrocution diminishes.

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.

More semantics.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Dave Plowman (News) wrote:

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...
--
Andy

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"Mark" wrote

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??
Phil
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