On 19/06/2019 10:45, John Rumm wrote:
On 19/06/2019 09:17, Martin Brown wrote:
On 18/06/2019 21:42, ARW wrote:
[snip]
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.

Probably not a lethal shock but enough to be unpleasant.

I wouldn't trust my flagstone floor to be at anything other than local
ground potential. And I have had a small shock from a telescope that had
mains live leakage despite being up a wooden ladder at the time.

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.

Careful examination of the poles shows that they are now PME even if all
the houses round here all have a copper stake driven into the ground
from the original TT install where there were four independent wires any
one of which could be snapped independently (although usually it was the
top wire that got snapped moving neutral toward the remaining phases).

It was particularly exciting in wet weather with the remains of the
perished rubberised fabric hanging down and flapping in the breeze
arcing and sparking as it touched other conductors.

--
Regards,
Martin Brown