Outdoor earthing/bonding

[JohnW2]

Although related to the current ‘cross bonding’ thread, I’ve spun this off into a separate thread, since it’s essentially a separate and circumscribed issue.

The issue of earthing/bonding seems to take on new dimensions when one gets outdoors – e.g. sockets in gardens, greenhouses and sheds etc. Short of ensuring that there are no exposed-conductive-parts and trying to ensure that only Class II equipment will be powered from such sockets, I’m not really clear as to what one can or should do about protection (apart from ensuring that the circuits are RCD-protected). Effective ‘bonding’ of the wet walls of a timber shed or all of the soil in a garden is clearly not a viable option!

A little while ago, I saw a socket which had recently been installed (by an electrician) in a friend’s greenhouse (metal framed, with the metal frame sitting on/attached to a low brick wall) and noticed a G/Y cable connecting the socket to the frame of the greenhouse. In common sense terms, that struck me as being not-too-clever an idea, primarily because it means that someone could be touching the metal frame whilst standing on the ground (potentially with bare feet!). In the absence of any ‘bonding’ between frame and soil, that could obviously represent a hazard under some fault conditions. In effect, the electrician had turned the greenhouse frame into a giant exposed-conductive-part, with an ('unbonded') path to true earth nearby.

There is also the issue that if the greenhouse’s frame had some meaningful degree of connection to the soil (e.g. wet bricks), the SWA’s armour (and CPC if it had one) would be introducing another variant of ‘earth potential’ into the house’s electrical installation. However, I suppose that's no worse than having bonded water supply pipes. ‘Earthing’ the greenhouse frame to a local electrode would enhance that problem, if the G/Y cable remained in place.

I suppose that one solution would be to not connect the SWA’s armour/CPC at the greenhouse end but, instead, to connect the socket to a local earth electrode and ‘bond’ this to the greenhouse frame - i.e. create a local TT installation.

When it comes to ‘garden sockets’ (i.e. not in a greenhouse/shed etc.) the problems would seem to be essentially insuperable, so I suspect that reliance on use of Class II equipment is about the best one can do (apart from the RCD protection).

Any comments, or help in understanding how these situations are usually dealt with?

Kind Regards, John.

 

[bernardgreen]

I am buying a cottage next to a shop. Until recently they were all one property where the shop keeper lived n the cottage. There was a single meter in the shop and the cottage took its power from that meter. That is a TT supply as the supply was overheard until recently.

I am getting a new supply installed for the cottage which will be PME.

Problem found when checking with the DNO about arrangments for the new meter. There are water pipes linking between the cottage and the shop which will be having sub metered water from my supply for at least a year.

Because of that either
[1] the shop has to convert to PME.
[2] my new supply has to be TT or
[3] the metallic pipes linking the buildings have to be replaced with plastic for at least a metre.

and great care will be needed to ensure that no earthed or bonded parts of the cottage can make contact with any earthed or bonded parts of the shop.

I think having a TT for the new supply is the "safer" option.

 

[JohnW2]

Problem found when checking with the DNO about arrangments for the new meter. There are water pipes linking between the cottage and the shop which will be having sub metered water from my supply for at least a year.
Because of that either
[1] the shop has to convert to PME.
[2] my new supply has to be TT or
[3] the metallic pipes linking the buildings have to be replaced with plastic for at least a metre.
....and great care will be needed to ensure that no earthed or bonded parts of the cottage can make contact with any earthed or bonded parts of the shop.

Why? Am I missing something? What you are describing (when viewed from the perspective of either shop or cottage) doesn't sound materially different from the situation I have here (which I described again yesterday) - namely that it appears that my TT installation is joined to (and 'benefits from') the DNO's TN earth in an adjacent property - by virtue of underground water supply pipes and associated 'main bonding' in both properties. Provided that the infamous bonding is in place, I don't really see what the problem is.

Kind Regards, John.

 

[bernardgreen]

The problem is that the DNO's new supply estimate and planning people have raised the concern that a pipe bonded to a PME "earth" at one end and earthed to an earth rod at the other end could create a problem if there was a fault that caused high fault currents to flow between the two supplies

No I do not see any problem as safety devices will operate and cut power irrespective of where and how the current reaches earth or ground or the combined neutral/earth on it's way back to the star point.

 

[JohnW2]

The problem is that the DNO's new supply estimate and planning people have raised the concern that a pipe bonded to a PME "earth" at one end and earthed to an earth rod at the other end could create a problem if there was a fault that caused high fault currents to flow between the two supplies
No I do not see any problem as safety devices will operate and cut power irrespective of where and how the current reaches earth or ground or the combined neutral/earth on it's way back to the star point.

As I said, I don't see a problem either, and that the situation you describe is almost exactly the same as I have in my house. Furthermore, as we discussed here recently, some countries require a local earth electrode with a PMS system - and. let's face it, 'PME' means 'PME', so all an additional 'E' does is to add to the 'M'!

If I were in your position, and actually wanted a PME supply, I think I would be arguing with the DNO - but, frankly, I'd probably prefer to have TT anyway!

Kind Regards, John.

 

[Electrifying]

If there was a problem with the supply neutral (high resistance joint/loss of...), then all the operational current from the cottage will flow via the 'Main Protective Bonding Conductors' to the nice earth rod at the shop.

This will raise the potential on all metalwork in both premises and the Protective Device (RCD) won't trip!!

So, happy days.

The three solutions that you have been offered make sense.

 

[JohnW2]

If there was a problem with the supply neutral (high resistance joint/loss of...), then all the operational current from the cottage will flow via the 'Main Protective Bonding Conductors' to the nice earth rod at the shop.
This will raise the potential on all metalwork in both premises and the Protective Device (RCD) won't trip!!

That's true. I was only thinking of things from the point of view of Bernard's cottage, with its proposed PME supply - in which context a lost neutral would presumably have the same effects whether the shop (and pipework connections thereto) existed or not (in fact, the shop and it's earth electrode would preaumably marginally 'improve' the cottage's situation). The problem with the real situation is that of the shop, which would also 'enjoy' the raised potential of the cottage's metalwork.

Mind you, I suppose the scenario you describe shouldn't actually pose a hazard if all required bonding was in place, and adequate - and the occupants of the cottage would presumably be immediately aware of a problem, since not much of their equipment would work, or work satisfactorily, if the only return path for their supply were via an earth rod (and bonded water pipes!).

The 'similar' situation I have is obviously a bit different, since the assumed connection between the two properties is via a long length of underground metal pipe - but I am on the 'receiving end' in this case. If my neighbour's supply is PME, and if they lost their neutral, their installation would have all of that buried pipe available as a return path before it got to my property - but, particularly since the impedance of the pipe would be a lot less that that of its connection to earth, my MPB, Earthing Conductor and Electrode would inevitably end up carrying at least some of that return current (hence the potential of my CPCs and metalwork would rise, at least to some extent) - so do you think the situation I have should be regarded as being a significant problem?

Kind Regards, John.

 

[JohnW2]

This thread has got a bit sidetracked by Bernard's cottage and some subsequent drift back into the 'earthing vs. bonding' arguments, which I had hoped to avoid by starting a new thread.

I wonder if anyone has any comments/thoughts in relation to my initial post, particularly the bonding of metal greenhouse frames. I'm still uneasy about what I saw in my friend's house and need to decide whether my concerns are justified, and therefore whether I should mention it to him.

Kind Regards, John.