BS7671:2018 DPC - Local Earth Electrode with TN systems

[JohnW2]

Nor me. But I am open to the suggestion that better maintenance would reduce risks to a level low enough to not warrant other measures being put in place to reduce them still further.

Indeed - but, like me, you are not Bernard.

I'm actually not clear as to what sort of circumstances lead to the 'lost neutrals' we are always hearing about. On the face of it, it's quite hard to imagine how one would (completely) loose just the neutral. I therefore don't really know to what extent 'maintenance' might be able to reduce the risk. As for TN-S, they could, I suppose, replace all the cables in the country, but I suspect that would have more than a slight effect on electricity bills!

Kind Regards, John

 

[JohnW2]

And, with the disclaimer that I still haven't looked at the DPC yet, might there now be a practical difference with the requirement to disconnect earthing of alternative supplies?

I haven't looked properly at that yet, either. However, the comment in the introduction suggests that it might just be removal of the prohibition of 'switching' earths/PENs in that situation, rather than a requirement to do it. However, that's pure speculation based on reading just one sentence. I'll have a look when I have a moment.

Kind Regards, John

 

[ban-all-sheds]

I must get on the case before the comment window closes - I really want to ask them where the evidence, and CBAs are, that have led them to remove labelled-socket exemption for no RCD protection, to require arc-fault breakers, and probably others.

 

[JohnW2]

I really want to ask them where the evidence, and CBAs are, that have led them ..... to require arc-fault breakers, and probably others.

That one does seem rather odd. Is this perhaps the LFB at work again?

Kind Regards, John

 

[securespark]

Yes, I wondered about that, too.

Me too!

 

[securespark]

Me too!

John
For the record, I have seen a lost neutral on a DNO supply twice in my working career. Both times a voltage exceeding 400 appeared on the customers' supplies.

 

[securespark]

What went wrong there?

 

[JohnW2]

John ... For the record, I have seen a lost neutral on a DNO supply twice in my working career. Both times a voltage exceeding 400 appeared on the customers' supplies.

It obviously happens, but I imagine it's very rare. As I asked, have you any idea what sort of circumstances result in the loss of just the neutral - with underground supplies (the last part of my overhead supply is in singles, so loss of just N would be quite easy!).

>400V on what, and relative to what?

Kind Regards, John

 

[JohnW2]

What went wrong there?

It looks like you replied, with a quote, to your own post

Kind Regards, John

 

[securespark]

Thanks. Proof that I am finally losing my marbles...

 

[securespark]

>400V on what, and relative to what?

Have searched for pictures and the story here but alas, I cannot find it.

But the readings came from a socket outlet via an MFT.

 

[bernardgreen]

For the record, I have seen a lost neutral on a DNO supply twice in my working career.

Put me down for three that I can recall.

One at a hill top site ( radio comms ) with a near zero ohm ground electrode. Four wire overhead, Neutral and one Phase snapped

One on the supply to my previous house. under ground cable failed, ( a few months later the same cable lost a phase which reusulted in a lot of voltage bouncing on the Neutral

One at my uncle's house which led him into converting to a TT supply with a VOLB

 

[bernardgreen]

ave searched for pictures and the story here but alas, I cannot find it.

Are these what you are looking for

http://www.dailymail.co.uk/news/art...ters-called-71-homes-massive-power-surge.html

https://www.fire-magazine.com/castleford-explosion-injures-firefighters-footage-released

 

[securespark]

Actually, I was searching my own picture collection and this forum for the thread I thought I wrote about it.

 

[JohnW2]

Have searched for pictures and the story here but alas, I cannot find it. ... But the readings came from a socket outlet via an MFT.

One can postulate a situation ....

... if two houses are on different phases but fed by the 'same' neutral, if that neutral breaks upstream of the houses (i.e. leaving the two house's Ns connected, but not connected to the 'supply neutral'), the the switched-on loads in one house will be in series with those in the other house, with the full phase-to-phase voltage (440-440V) across those loads-in-series. If the switched-on-loads in one house are very high in relation to those in the other house, most of that phase-to-phase voltage will appear between L and N (e.g. as measured at a socket) in that second house (the one presenting the greater 'load impedance') - with only a very small "L-N' voltage in the other house (the one with the 'large loads').

A fairly extreme situation of difference in loads is therefore required to get a very high 'L-N' voltage in one of the house. In the more likely scenario of similar switched-on loads in the two houses, the phase-to-phase voltage would be roughly equally shared between the houses, hence with some 200-240V 'L-N' in both houses.

Kind Regards, John