Electrician advised against just updating the fuse box

JohnW2 · 17 Feb 2018

bernardgreen said:
Yes, deriving an "Earth" from a Neutral does concern me. Reason for that is experience of Neutral bounces and high currents flowing to the ground rod from the PME "earth".

I'm a bit confused, since you are talking about PME and a 'ground rod' in the same breath.

It's really the possibility of very low impedance routes to earth via extraneous-c-ps which is really the worry in that respect, rather than any 'ground rod' ...

... with a typical domestic TT electrode ('rod') like mine, even in the middle of a very wet spell, you'd be hard pressed to get more than about 5A flowing into it, even if one connected it directly to 230V.

If (as is becoming increasingly common) one's water supply and (if one has one) gas supply arrive in plastic (at least, until just before they enter the building), then I don't think that there is really any need for concern about 'high currents' flowing (anywhere) as a result of a PME CNE fault.

Kind Regards, John

bernardgreen · 17 Feb 2018

JohnW2 said:
'm a bit confused, since you are talking about PME and a 'ground rod' in the same breath.

Because I had a ground rod, 6 foot of stainless steel tube, one inch diameter ( or larger ~~ memory fails ) driven into the bank of the stream amd PME Earth. The rod was as back up with the full agreement of the DNO ( or was it they who suggested it. ).

Lost neutrals and resulting ""earthed" CPC at 230 voltsi have caused fires.

JohnW2 · 17 Feb 2018

bernardgreen said:
Because I had a ground rod, 6 foot of stainless steel tube, one inch diameter ( or larger ~~ memory fails ) driven into the bank of the stream amd PME Earth. The rod was as back up with the full agreement of the DNO ( or was it they who suggested it. ).

Fair enough (and, of course, the expectation is that in only just over 4 months' time, it will actually be a requirement of BS7671, so presumably no need to get any agreement/permission from the DNO!).

As for having it as a 'backup', maybe yours would, but most people do not have convenient nearby streams, and even a good 'normal' domestic earth rod would probably not make any appreciable difference to the potential of CPCs etc. in the event of a 'lost PME neutral'.

bernardgreen said:
Lost neutrals and resulting ""earthed" CPC at 230 voltsi have caused fires.

As I recently wrote, any risk of fire in such circumstances must result from extraneous-c-ps with very low impedance paths to true earth, not from earth rods - since you'll never get enough currents flowing into a domestic earth rod to even noticeably warm up any of the protective conductors (CPCs and bonding conductors), let alone start a fire.

Maybe the main potential extraneous-c-ps (water and gas supply pipes) should be 'banned' (by requiring 'insulating sections to be inserted)?

Kind Regards, John

Risteard · 19 Feb 2018

ban-all-sheds said:
How can you condemn something which has not failed anything?

I can condemn the aspects of it which are bad through my observations. It still doesn't mean that the Report is a pass or fail assessment of the installation because that isn't its purpose. It simply makes recommendations about what should be improved/investigated and what the urgency is for this to be addressed.

Have you ever filled out one of these reports?

ban-all-sheds · 19 Feb 2018

You weren't talking about "condemning aspects", you were talking about "condemning" entire installations:

Risteard said:
With respect plenty offer an honest appraisal of the installation. The only reason I condemn most installations following periodic inspection and testing is because most are complete and utter ****e.

On account of them being complete and utter ****e.

By all means continue to protest that an installation which you have condemned for being complete and utter ****e has not "failed" the inspection because that word is not in the EICR.

Meanwhile, those of us in the real world, can get on quite happily with the notion that an EICR stuffed full of C1s is a "fail".

Risteard · 19 Feb 2018

ban-all-sheds said:
You weren't talking about "condemning aspects", you were talking about "condemning" entire installations:

On account of them being complete and utter ****e.

By all means continue to protest that an installation which you have condemned for being complete and utter ****e has not "failed" the inspection because that word is not in the EICR.

Meanwhile, those of us in the real world, can get on quite happily with the notion that an EICR stuffed full of C1s is a "fail".

It isn't. It will involve an assessment that the electrical installation is in an unsatisfactory condition to remain in service in its present state, however.

ban-all-sheds · 19 Feb 2018

Risteard said:
It isn't. It will involve an assessment that the electrical installation is in an unsatisfactory condition to remain in service in its present state, however.

Meanwhile, in the real world, most people will get on quite happily regarding that as a fail.

Risteard · 19 Feb 2018

ban-all-sheds said:
Meanwhile, in the real world, most people will get on quite happily regarding that as a fail.

City & Guilds wouldn't agree with you.

ban-all-sheds · 20 Feb 2018

Meanwhile, back in the real world of bulbs, live wires, earth wires etc....

bernardgreen · 20 Feb 2018

JohnW2 said:
Maybe the main potential extraneous-c-ps (water and gas supply pipes) should be 'banned' (by requiring 'insulating sections to be inserted)?

Or use them as the Earth "rod" on a TT installation. Full circle back to the days before PME and cost saving combined Neutral and Earth in the local network

JohnW2 said:
any risk of fire in such circumstances must result from , not from earth rods

The extraneous-c-ps with very low impedance paths to true earth ( metallic water mains etc ) is seldom obvious until the bond wires melt. Do the regulations / guide lines require the impedance to ground of a water pipe be measured before the decision to bond is made ?

securespark · 20 Feb 2018

ban-all-sheds said:
Meanwhile, back in the real world of bulbs

Why would we be interested in just that part of the lamp on its own?

ban-all-sheds · 20 Feb 2018

Meanwhile, back in the real world...

JohnW2 · 20 Feb 2018

bernardgreen said:
Or use them as the Earth "rod" on a TT installation. Full circle back to the days before PME and cost saving combined Neutral and Earth in the local network

Dangerously non-future-proof. It hasn't happened in my village yet, but there's hardly a time when one of the nearby villages isn't having its water supply pipes replaced with plastic ones.

bernardgreen said:
The extraneous-c-ps with very low impedance paths to true earth ( metallic water mains etc ) is seldom obvious until the bond wires melt. Do the regulations / guide lines require the impedance to ground of a water pipe be measured before the decision to bond is made ?

No - and they couldn't, since there is never a question of a 'decision to bond'. If an extraneous-c-p enters a building, it must be bonded, no matter what impedance to earth it represents.

If there were any such regulation, it would have to be (as I suggested) to 'ban' very low impedance extraneous-c-ps (by requiring an 'insulating section) - I suppose theoretically possible when the extraneous-c-p is a water pipe, but a rather different matter if it is 'structural steel'!!

I suspect that, even though you can quote examples, bonding conductors melting because of PME neutral faults is so rare that it has not been considered necessary/appropriate to take any action.

Kind Regards, John

bernardgreen · 20 Feb 2018

JohnW2 said:
No - and they couldn't, since there is never a question of a 'decision to bond'

Maybe there should be a decision to bond or not to bond depending on the impedance of the extraneous pipework.

JohnW2 said:
is so rare that it has not been considered necessary/appropriate to take any action

is it that rare. ? How many of the "power surge" incidents are in reality a broken Neutral on the local network ( often due to metal theft ) causing Neutral to Ground currents beyond the current carrying capacity of the bonding connections..

JohnW2 · 20 Feb 2018

bernardgreen said:
Maybe there should be a decision to bond or not to bond depending on the impedance of the extraneous pipework.

I'm surprised to see you suggesting that, because if one was allowed to, and did, 'decide not to bond', that would result in potential hazards (due to the lack of an equipotential zone) - which I strongly suspect would represent a greater risk that posed by the possibility a bonding conductor melting.

bernardgreen said:
is it that rare. ? How many of the "power surge" incidents are in reality a broken Neutral on the local network ( often due to metal theft ) causing Neutral to Ground currents beyond the current carrying capacity of the bonding connections..

I don't know any 'chapter and verse'. However, 'broken neutrals' are pretty rare to start with, and, even when it does happen, it is probably not often going to result in bonding conductors melting.

Maybe someone who works for a DNO could tell us how often they have seen or heard of bonding conductors melting for this reason? It's one of those occasions when we miss westie.

Kind Regards, John

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Electrician advised against just updating the fuse box

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