Yet more on PME earth export

[RF Lighting]

What's wrong with just connecting everything together?

If the pipework remains good, you effectively got a PME supply. If the pipe is removed, you've still got your rods, and providing your installation is configured as a TT installation, then that's fine too.

If you're concerned about exporting a PME earth, which I understand if you have a metal framed greenhouse, then export the house earth to earth the SWA as far as the greenhouse but insulate it at the greenhouse, and create a local TT installation.

 

[JohnW2]

What's wrong with just connecting everything together? If the pipework remains good, you effectively got a PME supply. If the pipe is removed, you've still got your rods, and providing your installation is configured as a TT installation, then that's fine too.

It's all about the situation in which the pipes remain good (hence, as you say, the installation effectively PME) coupled with this extremely rare scenario, which worries some people, of the PME earth rising to a high potential above true earth, exported into a garden which, with the best will in the world, cannot be constituted as an equipotential zone.

If you're concerned about exporting a PME earth, which I understand if you have a metal framed greenhouse, then export the house earth to earth the SWA as far as the greenhouse but insulate it at the greenhouse, and create a local TT installation.

Whether I am particular concerned or not (and I do have a metal framed greenhouse) it is, indeed, that very small risk that I have sought to eliminate.

What I have done is effectively the same as the conventional approach you suggest, except that I have shifted that 'local TT installation' into my cellar - and I've explained the reason. As I said, there are several final circuits, supplying assorted outdoor lighting and sockets (all of which carry the same, very improbable, risk as a greenhouse, given that one can't make a garden into an equipotential zone), as well as the greenhouse. The only alternative I could think of would be to provide each final circuit with its own RCD (or RCBO) and earth rod, scattered about all over the garden - an approach which I would not regard as attractive, or desirable. As I asked EFLI, what would you do? (or are you perhaps just not concerned about exporting a {probably} PME earth into a garden?)

Kind Regards, John

 

[EFLImpudence]

Firstly, the garden MET is definitely extraneous and not exposed (in the electrical sense).

Secondly, isn't the reason for TTing an outbuilding usually just a necessary second choice because of the difficulty/impracticability/cost of adequately bonding to the house, whereas here it would be particularly simple.

However, because of your situation with the water pipe connecting your installation to the neighbours PME, I suppose it does offer a solution.

My doubts may just be because I am not used to the immenseness of W2 Towers and were this part of the building 'next door' no one would think anything of it - although why would it be done or why is it not done?
Shall I TT my living room 'just in case'?

Anyway, you have obviously thought about it and knew all that before you asked.

 

[RF Lighting]

I don't really have an issue with exporting PME supplies*.

I get there's a small risk that the neutral derived earth could rise slightly above true ground potential, and there's the risk of the CNE going open circuit, but you could just as likely have an earth fault and a failed RCD on a TT supply.

Both systems have their risks and short comings. I don't get why certain people give PME such a hard time but forget that TT (and even TN-S!) supplies are not fool proof.





*There are certain rare circumstances where it wouldn't be desirable.

 

[JohnW2]

Firstly, the garden MET is definitely extraneous and not exposed (in the electrical sense).

As you will have seen, that was certainly my view. However, I also accept the fact that the earth block might also be considered as 'exposed', given that it is connected to the CPCs of this part of the installation.

Secondly, isn't the reason for TTing an outbuilding usually just a necessary second choice because of the difficulty/impracticability/cost of adequately bonding to the house, whereas here it would be particularly simple.

I agree that's often the reason, even though I confess that I do not fully understand the reason for requiring a high CSA bonding to the outhouse, or whatever, when the only local path to earth (at the outhouse) is going to be of relatively high impedance (in comparison with a TN earth). However, another reason is in order to 'avoid exporting a PME earth' for the reasons of safety which we know concern some people.

However, because of your situation with the water pipe connecting your installation to the neighbours PME, I suppose it does offer a solution.

I don't think that my apparently 'unintended' connection to the neighbour's PME earth is relevant. I would be saying exactly the same if my house had a 'deliberate' PME installation. Do you see a difference?

My doubts may just be because I am not used to the immenseness of W2 Towers and were this part of the building 'next door' no one would think anything of it - although why would it be done or why is it not done? Shall I TT my living room 'just in case'?

I'm not sure I follow any of that - perhaps you could clarify? I think I'd probably be saying the same if I lived in a tiny house with a few electrical bits and pieces in the garden.

Please also see my response to RF's latest post (which I'm about to write - so you may have to wait for a few minutes )

Kind Regards, John

 

[JohnW2]

I don't really have an issue with exporting PME supplies*.

Frankly, nor do I - I think that crossing roads and travelling in cars represent significantly greater risks to me and mine. However, as with so many questions I ask, what I'm attempting to do is 'Jobsworth-proof' my electrical installation as much as I can against those who may feel differently - not the least, just in case I'm not around to defend the situation if/when it is ever challenged.

I get there's a small risk that the neutral derived earth could rise slightly above true ground potential, and there's the risk of the CNE going open circuit,...

Those who have these concerns are seemingly worried about something much worse than a rise of a PME earth to 'slightly above' true earth potential, coupled with the fact that a current flowing through a person from such an elevated potential 'earth' to true earth would usually not benefit from any RCD protection.

but you could just as likely have an earth fault and a failed RCD on a TT supply.

You could, but that's rather different.

Both systems have their risks and short comings. I don't get why certain people give PME such a hard time but forget that TT (and even TN-S!) supplies are not fool proof.

As I've said, I'm not taking sides - merely trying to arrange things so as to minimise the chances of anyone being dissatisfied with it - albeit I realise that one cannot please all the people all of the time!

Kind Regards, John

 

[EFLImpudence]

I don't think that my apparently 'unintended' connection to the neighbour's PME earth is relevant. I would be saying exactly the same if my house had a 'deliberate' PME installation.

Oh. I thought that was your main concern.
I may have been labouring under a misapprehension all along.

Do you see a difference?

I thought the loss of neighbours supply neutral was the worry which obviously would not matter without the bonding.

I think I'd probably be saying the same if I lived in a tiny house with a few electrical bits and pieces in the garden.

Probably you would.

 

[JohnW2]

I don't think that my apparently 'unintended' connection to the neighbour's PME earth is relevant. I would be saying exactly the same if my house had a 'deliberate' PME installation.

Oh. I thought that was your main concern. I may have been labouring under a misapprehension all along.

Not at all - again, apologies if I wasn't clear. The concern (as I've said, primarily of some others, rather than myself) relates to the fact that I effectively have a PME earth. Whether that PME earth derives from my cutout or the neighbour's cutout (via some pipework) is irrelevant. The only reason I mentioned the 'unintentional PME' is, I suppose, becuuse if I were certain that I had a 'pure TT earth' (with no suggestion that it was connected to anyone's PME earth), then there would be no issue (of 'exporting a PME earth) to address.

Do you see a difference?

I thought the loss of neighbours supply neutral was the worry which obviously would not matter without the bonding.

The worry (as I've said, for some people, not really me) would seemingly be the loss of the supply neutral from which my installation's earth is derived - again, regardless of whether the 'earth' connection arises in my house or my neighbour's.

Kind Regards, John

 

[ericmark]

How odd just written about this on another post.

To my mind either the out buildings need to share same earth or have a local earth and rule of thumb if out building less than 6 meters from house then share the earth and if over 6 meters then use it's own rod.

As to houses I would say under 10 meters need to be same earth over 10 meter does not matter.

However the situation you have with gas and water linking the house does mean you really share an earth but clearly should not be shared in this way.

I was always told it was up to the DNO to decide what earth system a premises should have and from what you say I can see good reasons why this should be the case.

The caravan and electric car however present another problem as they are required to have TN-S or TT supply and clearly if parked within 6 meters of the house that would mean whole house would also have the same earth system. In turn this could also mean when some one buys an electric car the whole estate would need to change to a TT system.

However if one does a risk assessment then in a normal house the TN-C-S is safer than TT assuming the DNO have made it PME so to require the whole estate to move to TT because one guy buys an electric car would be crazy.

I did bring up the location of caravans to a house and I argued that when parked close to house then TN-C-S would be better option. However two things were pointed out. One was the TT is only for caravan sites and second was caravans should not be parked close to a building because of fire risk. However with electric cars one would clearly be expected to park it in the garage where that is part of the house.

As to fire risk I look at the planes and laptops both which have had problems with NiMH batteries bursting into flames and I would not be happy with an electric car on charge in my integral garage?

 

[westie101]

I always follow these PME threads with interest as i, as you know, sit on the other side of a fence

However if one does a risk assessment then in a normal house the TN-C-S is safer than TT assuming the DNO have made it PME

I found interesting as some of the concerns about PME are outside the remit of, IMHO, a proper risk assessment
Whilst the effect is taken into account, the other important part of the assessment is likelyhood. Which as I'm sure I've commented on before is very, very low so as to make the overall risk low!

If there is so much concern over DNO provided earths why do we see so many electricians reporting low ELI and requesting an earth from us (three today in my patch alone)
Why do we see comments on here suggesting folk with poor earthing ask their DNO for a PME earth (though, again, as I've said just ask for an earth rather than specify what type)

Oh and lastly to John W2, it's your house, what jobsworth do you think could inspect your installation without your permission, even a DNO employee with a right of access can't

 

[JohnW2]

To my mind either the out buildings need to share same earth or have a local earth and rule of thumb if out building less than 6 meters from house then share the earth and if over 6 meters then use it's own rod.

The discussion isn't really about whether buildings should share an earth but, rather, about the fact that some people are concerned about exporting a TN-C-S earth to a place which is, or may be, outside of the installation's equipotential zone. There is obviously no practical way that one can 'bond a domestic garden' (to bring in into the installation's equipotential zone) - so, if there is any possibility of someone standing on the ground (maybe with bare wet feet) touching any exposed-conductive-parts (or exposed bonded parts), some people feel that it is not 'safe'/advisable for those parts to be connected to a TN-C-S earth.

I understand that argument, which seems theoretically correct as regards the (exceedingly improbable) risks - although, as I've said, I personally feel that the level of risk is so low that it doesn't really concern me personally.

Kind Regards, John

 

[westie101]

the installation's equipotential zone

Great bit of theory is that!!
Often one of the first indications of a lost neutral fault is a report of mild shocks from within the houses affected!

 

[JohnW2]

Whilst the effect is taken into account, the other important part of the assessment is likelyhood. Which as I'm sure I've commented on before is very, very low so as to make the overall risk low!

Exactly. That's why I've said that I'm personally not concerned. I believe that I face considerably greater risks in everyday life every day.

Oh and lastly to John W2, it's your house, what jobsworth do you think could inspect your installation without your permission, even a DNO employee with a right of access can't

Any electrical installation is likely to be inspected sooner or later (with the householder's permission). As I've said, probably my greatest concern is that this might happen when I'm not around to put my case and defend the situation!

Kind Regards, John

 

[JohnW2]

the installation's equipotential zone

Great bit of theory is that!! Often one of the first indications of a lost neutral fault is a report of mild shocks from within ouses affected!

Fair enough, but you know what I'm saying Anyway, aren't you the one who often talks about Faraday Cages

Maybe I should ask this of an electrician, rather than a DNO man, but how would you feel if somewhere in a house (with a TN-C-S supply) where people were likely to stand with bare wet feet (a shower tray?) was lined with (unbonded) metal connected to an earth rod outside? That might sound silly, but it really isn't that far off what could happen when a TN-C-S earth is exported into a garden, is it?

Kind Regards, John

 

[westie101]

Any electrical installation is likely to be inspected sooner or later

Who by? The LA's don't have the resources or remit, the schemes don't have the power as I've said it is nothing to do with the DNOs unless it is seriously dangerous or affects other customers. The only time it could occur is following a death caused by electricity.