On 18/06/2019 22:27, Roger Hayter wrote:
John Rumm 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.

AFAIK PME has been the /default/ for new, ordinary connections for
quite some time.

me too, with "quite some time" being from the late 60's onwards, except
overhead supplies?

Overhead supplies have often had PME added later. This place (overhead
supply) was fully rewired in the mid 80's I believe, and was TT then.
However at some time in the 10 to 15 years after that, the network must
have been upgraded to PME since the cutout now sports a yellow PME label
(well spotted Adam!)

What would be the reason for not using PME with overhead supplies in the
first place? Were some ot the neutral wires too long to have a low
enough resistance? Or was it just that most overhead supplies were put
in before PME was fashionable? Our overhead supply is PME but it was
redone in the 80s.

I would guess that the neutral was designed to be just that, and not a
PEN conductor, and it was only earthed only the once at the sub station
in the same way that a TN-S supply would be. Hence there would be a much
greater risk if the neutral was cut anywhere along the route.

As to why, perhaps because it is more expensive to PME an overhead
supply, or simply they did see the need at the time, and considered that
leaving it TT was ok.

--
Cheers,

John.

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