On 19/06/2019 09:17, Martin Brown wrote:
On 18/06/2019 21:42, ARW wrote:
On 18/06/2019 10:01, Andy Burns wrote:
Robin wrote:

Martin Brown wrote:

So how common is TN-C-S with a single PEN in new build?

I'm sure Adam can add more datapoints than everyone else here combined.

I am also sure that I am not sure. New builds in Epworth that I have
just worked on (one street). Half are TN-C-S with a 100A supply and
half are TN-S with a 80A supply.

Thanks. You would think it made more sense to do one or the other.

However I would say that 80% of newbuilds I have worked on in the last
20 years are supplied with a single core concentric cable.

So the incoming armoured cable has a live core and outer PEN sheath?

I expect it is against the wiring code but is there anything to prevent
the householder having a stake in the ground bonded to the household
protective earth in addition? It won't make a blind bit of difference
most of the time but if the neutral return ever fails then at least you
don't suddenly find every household appliance is live to the touch.

You can have an earth stake with a PME supply - it just becomes another
of the multiple earths.

(There was even a suggestion that the 18th edition might make it a
requirement - but that was shelved when they thought through the
impracticalities of that in a number of circumstances)

It is just after all another parallel multiple earth local to the
premises and although it cannot sink anything like the current that the
PEN can it does have the advantage of keeping the water pipes from
becoming live in the unlikely event of a fault.

For some values of live. Its one of the reasons TN-C-S installs are so
insistent on the need for maintaining an equipotential zone. The theory
being that live water pipes etc are only a risk if you are able to touch
them and something else that is at a true earth potential. If everything
you can touch is at much the same potential (even if that is 240V adrift
from true earth) there is no shock risk.

Round here single phase or neutral return fail faults are not at all
uncommon if a tree branch hits a line as it falls. Usually the people on
the phase at the top of four wires bear the brunt of such cuts. The
first wire snapping slows the branch down enough that the second or
third one down survives.

Interestingly the new fully insulated steel cored combined aluminium
cable can survive a whole tree falling on it sometimes although it bent
all the poles and ripped several mains supplies off houses along one
side of the road. You get some interesting mains faults in rural areas.

Yup, ABCs are more robust in general, and its also much harder to snap
just one of the conductors.


Last big one was when the milk tanker skidded on ice on the coldest day
of the year and took out a pole wrecking all mains and phones.



--
Cheers,

John.

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