Any Electricians ?

[Mark Hennessy]

The risk from PME supplies concerns an open-circuit on the Neutral, ahead of the RCD on your incoming mains. What happens is that everything in your house, including earth, goes high. The RCD doesn't trip out because it's lost power. So, you have a house that seems dead, yet by simply touching the metal case of any electrical device, you could get a nasty shock.

Interesting what you say there: it may explain a scenario I witnessed many years ago on a new-ish build house - presumably, therefore, with PME.

There you go Lawrence, it does happen

The upshot was that they had to dig a large trench in the road - about 20 metres long, back to a 'T' junction in the road - before they found the fault; it went on for about 3 days and nights. Being curious, one evening I spoke to the person supervising the work: "Real headache, this one. We've just got to keep digging back from the house until we find the trouble." Which they eventually did: an O/C neutral in a bitumen-filled junction box, so I was told.

Good on them for finding it!

My understanding is that when this happens, the supply companies often take the easy way out - and convert the property to TT, using a local earth spike. I wonder if this was before RCDs became commonplace?

Alternatively, perhaps it's safe to bet that these days, everything comes down to money

Peculiar thing was that several adjacent houses should have been also affected, but only this one house appeared to experience the fault. Perhaps the occupants of the other houses hadn't noticed, but I find that difficult to believe.

That is interesting, but I guess it depends on the exact detail of the wiring underground. Of course, it's common for houses to be on different phases, so perhaps only every 3rd house might have been affected. But you just never know - depending on when it was, perhaps there was much less electrical stuff in the houses, perhaps much more Class-II stuff? There is clearly potential division going on, so maybe other houses didn't have quite as many volts on their earths?

Luckily, we have TN-S, the Rolls Royce of electricity supplies. I suspect Alan does too... When they changed the termination on my incoming supply, the new one contained a 100A fuse - 3 times the previous one. When I expressed surprise the guy said that the incoming cable is good for several hundred amps - it apparently is many times bigger than modern cables. "They don't make 'em like they used to, mate!"...

Cheers,

Mark