BS7671:2018 DPC - Local Earth Electrode with TN systems

[ericmark]

I saw first hand the result when a contractor hit the cable to a housing estate braking only the neutral earth, the three phases were still connected, a radio amateur in the estate had a rather good earth and the guy wiring up his shed (shack) had bonded this wire to the 4 mm earth wire tie raped to the SWA feeding the shed, not a clue why the steel was not used. This 4 mm cable was reduced to balls of copper.

The main problem was he was only one in the estate to have a good earth, had everyone fitted multi earth rods with buried copper tape joining them the current would have been shared, and likely no problem, it was because only one person had the earth that the problem arose.

So if the DNO come and fit earth rods around the estate together within a day of each other then the TN-C-S will become PME and no problem, however if the consumer fits the earth rods they will be fitted over a long time, and until all the estate have fitted earth rods there is a clear danger.

 

[JohnW2]

.... a radio amateur in the estate had a rather good earth ... This 4 mm cable was reduced to balls of copper. ... The main problem was he was only one in the estate to have a good earth, had everyone fitted multi earth rods with buried copper tape joining them the current would have been shared, and likely no problem, it was because only one person had the earth that the problem arose.

I wouldn't say that the number of earths was the problem - the problem was surely that the earths (in this case just one) connected were of unusually low impedance. If the one earth had been a 'typical' domestic earth rod, not enough current would have flowed to melt any cable found in an electrical installation.

... however if the consumer fits the earth rods they will be fitted over a long time, and until all the estate have fitted earth rods there is a clear danger.

As above, if they only fit 'typical' domestic earth rods, there will be no significant danger, even if there is only one.

Kind Regards, John

 

[ban-all-sheds]

Are you sure enough about every soil condition, every minute of every day of every year after year after year in every place in the UK to be sanguine about that?

Are you sure that a very gradual, and haphazard, increase in the number of MEs will never make things worse for anybody? And if they might, are you sure that the system has been properly modelled by the BSI/IET, a proper risk assessment done, and that the outcome of that work shows beyond reasonable doubt that overall fewer people will suffer adverse consequences over the whole of the transition period than if the new requirement had not been introduced?

 

[JohnW2]

Are you sure enough about every soil condition, every minute of every day of every year after year after year in every place in the UK to be sanguine about that?

Of course not - that's why I restricted my comments to 'typical' domestic earth rods.

Are you sure that a very gradual, and haphazard, increase in the number of MEs will never make things worse for anybody?

Does "MEs" perhaps mean 'multiple earths'? If so ....

As I've often said, one of the first things I was taught at uni was that if a question included such words as 'never', 'always', 'none', 'all', 'no-one', 'everyone' etc. etc., the answer would invariable be 'no' since there are very few absolute certainties in the real world.

That aside, as I've said, I think it's probably still the case that the majority of UK electrical installations have (hopefully bonded) extraneous-c-ps and, so long as that remains true, it's quite difficult to think of ways in which it's likely that adding an additional 'earth electrode' to such an installation could/would "make things worse" for anybody.

Kind Regards, John

 

[bernardgreen]

A fairly extreme situation of difference in loads is therefore required to get a very high 'L-N' voltage in one of the house

One family away visiting relatives and one family trying to cook Christmas dinner on the other phase. Extreme difference in loads but a common situation.

 

[bernardgreen]

As above, if they only fit 'typical' domestic earth rods, there will be no significant danger, even if there is only one.

Whether there is significant danger depends, among other things, on the potential gradient in the soil around the earth rod.

 

[ban-all-sheds]

Of course not - that's why I restricted my comments to 'typical' domestic earth rods.

Nor I, which is why I am wondering why JPEL/64 are. Or seem to be.

That aside, as I've said, I think it's probably still the case that the majority of UK electrical installations have (hopefully bonded) extraneous-c-ps and, so long as that remains true, it's quite difficult to think of ways in which it's likely that adding an additional 'earth electrode' to such an installation could/would "make things worse" for anybody.

I don't know how many do still have them. Note that the use of them to provide an installation earth was disallowed a long time ago, the reason being that e-c-ps from incoming water pipes were being removed.

Neither you, nor I, know how many do, but I would have thought that knowledge essential to any engineering organisation seeking to mandate a haphazard, gradual increase in the presence of earth rods like this.

Put it this way - if they have not properly modelled it, and not done a proper risk analysis, and not done a proper cost-benefit analysis, then they have shamefully abrogated their responsibilities.

 

[JohnW2]

I don't know how many do still have them. Note that the use of them to provide an installation earth was disallowed a long time ago, the reason being that e-c-ps from incoming water pipes were being removed.

Indeed, and I'm sure that the proportion of properties supplied via metal water pipes continues to decrease. However, I suspect that the proportion still supplied via metal gas pipes continues to be quite high.

However, I'm not sure that this is necessarily a major part of the decision making process. I was merely pointing out that when there are bonded extraneous-c-ps, adding a 'typical' earth rod to the installation is not going to do any harm. Probably more important, even if there are not any extraneous-c-ps, anything remotely describable as a 'typical domestic earth rod' is not going to present any risk. One would probably have to have a Ze below 10Ω before had to even start worrying.

Neither you, nor I, know how many do, but I would have thought that knowledge essential to any engineering organisation seeking to mandate a haphazard, gradual increase in the presence of earth rods like this. ... Put it this way - if they have not properly modelled it, and not done a proper risk analysis, and not done a proper cost-benefit analysis, then they have shamefully abrogated their responsibilities.

Such decisions obviously should be made on the basis of as much information/ data/ 'evidence' as possible, but it is worth noting that the "engineering organisations" in a number of other ('sophisticated') countries made the same decision a long time ago.

If they have satisfied themselves that there are enough risks associated with loss of TN earths to justify consideration of requiring earth rods, if one believes (which you probably don't) what I wrote above of the lack of downsides of 'typical domestic rods', then it's likely that the risk analysis would come out in favour of the rods. If they really had concerns, I suppose they could mandate a minimum Ze for these rods!

Kind Regards, John

 

[JohnW2]

One family away visiting relatives and one family trying to cook Christmas dinner on the other phase. Extreme difference in loads but a common situation.

True, but since most things in the unoccupied house would be switched off, so not much equipment (and obviously no human beings, since they would be absent) would be subjected to the ~400V, and the other family's miserable failure in their attempts to cook their Christmas dinner would presumably result in their rapidly getting the fault attended to.

It's also worth noting that the comments I made in response to secure's comment about "400V" relate to the loss of neutral (not necessarily with simultaneous loss of earth connection), and the effect of that on L-N pds - so is equally applicable in TN-S, TN-C-S and TT installations. It's therefore totally irrelevant to this discussion about earthing arrangements.

Kind Regards, John

 

[securespark]

The way I understand it, there will always be a gradient in the ground.

It's for this reason you can't connect utility pipes to earth electrodes.

 

[JohnW2]

The way I understand it, there will always be a gradient in the ground.

Indeed so.

It's for this reason you can't connect utility pipes to earth electrodes.

I don't really understand that. Indeed, in my TT installation, the earth electrode is inevitably connected, via boding, to utility pipes.

Kind Regards, John

 

[stillp]

Nor I, which is why I am wondering why JPEL/64 are. Or seem to be.

Have you commented on the DPC yet?

 

[JohnW2]

Have you commented on the DPC yet?

Whether he yet has or not, I suspect that it would not achieve anything in relation to the matter BAS was referring to.

I find it hard to believe that they would abandon a fairly major proposed change in the regulations (which must already have been considered and discussed at length by the committee) because someone had suggested during the consultation process that there might be an inadequate evidence base to justify the change, don't you?

I'm not suggesting that that is a reason for not commenting, but it's just a statement of what I see as the probable realities of the situation.

Indeed, I would be interested to know, from your experience, how common is is for significant changes of any sort to be made to a draft Standard as a result of comments received during the public consultation process.

Kind Regards, John

 

[ban-all-sheds]

Probably about as often as a government changes its mind following a consultation process.

 

[stillp]

how common is is for significant changes of any sort to be made to a draft Standard as a result of comments received during the public consultation process

IME, quite common.