In article ,
Andy Wade wrote:
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.

Which means plastic is safer.

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

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.

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.


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.

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.

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.

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.

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.

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.

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.

Please define 'live'?

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/.

What most set out to do is confuse.

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

More semantics.

--
*I love cats...they taste just like chicken.

Dave Plowman London SW
To e-mail, change noise into sound.