Dave Plowman (News) wrote:
In article ,
Andy Wade wrote:

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.

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.)

Think you're splitting hairs.

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.

Incoming services are bonded to the same 'earth'.

True, and ...?

Which may or may not be a true earth - whatever that is.

.... you've just demonstrated the need to define an equipotential zone by
bonding, so that the local ground potential is made largely irrelevant.

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.

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.

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'?

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.

More semantics.

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