Pme or earth rod

[bernardgreen]

Let me guess, that comment was from bernard green?

There is NOTHING wrong with a PME supply.

I don't recall making that exact comment. But I do stand by my opinion that there are places where PME may not be the best option.

 

[JohnW2]

To me a wooden shed in a garden backed by other gardens then PME however there are exceptions. ... Where beyond the garden is fields and there is a large distance between garden and house then TT is likely better ... With metal sheds we are really looking at the same as with a caravan and charging a car again thought needs to be given to location of earthed parts there needs to be a distance so items earthed to house supply like cast iron down spout can't be touched at the same time as the shed, caravan, or car ....

I certainly agree with you that there is no one-size-fits-all answer. However, as I tried to explain yesterday, I believe that the issue that worries some people is not really anything specifically to do with sheds, caravans or electric cars, nor, per se, about the possible provision of TT earths for such things - BUT, rather, much more generally about the theoretical risks associated with having anything 'touchable' outside of the house's equipotential zone which is in electrical continuity with the house's PME earth.

Your cast-iron downpipe would be an example, as would be many metal-plumbed outside taps or, say, outdoor Class I light fittings (or, of course, any Class I tools/appliances used outdoors, but that would be pretty unusual). In any of those situations, even in the absence of sheds, caravans, electric cars or TT electrodes, there are opportunities for people to come in simultaneous contact with things connected to the PME and things connected to 'true earth'.

Kind Regards, John

 

[ericmark]

Let me guess, that comment was from bernard green?

There is NOTHING wrong with a PME supply.

I don't recall making that exact comment. But I do stand by my opinion that there are places where PME may not be the best option.

The fact PME is not allowed for supply to caravans, boats, and petrol stations and it is not recommended where cars are charged outside a building it is very clear there are places where PME is not the best option.

My comment has always been with a house it is not always possible to know if the supply is TN-C-S or TN-S as if the supply neutral and earth are combined anywhere along the route between transformer and premises it becomes TN-C-S although the head may appear to be TN-S.

Where the estate has metal water mains and gas mains even if a neutral is lost the amount of earthed metal work limits the amount the earth/neutral will vary from true earth each house helping to reduce the amount it can vary.

Where the problem arises is where one house is the odd one out. This was the case with a radio ham where he had an earth mat in the garden for his aerials connected to a copper bus bar in his shed to which he connected his radio earth to. The electrician who wired up the shed ran a 4mm earth wire separate to main feed to the shed. And when some workers in the road hit the main feed to the estate his radio earth became the neutral/earth for whole estate and the 4mm earth wire melted. This however is a very rare thing and after the event the shed was left TT the earth wire was never replaced. However having a garden shed with an earth mat consisting of 4 x 3 meter earth rods and copper tape buried a foot down between them all in a domestic must be very rare.

 

[westie101]

and it is not recommended where cars are charged outside

That is not quite true.
I read a report last year that decided that there was no significant increased risk to charging cars so it was quite acceptable to do so.

This report also commented that the perceived risks for caravans etc. was never reached by any other form that by "it seemed the best thing to do" there was no study or investigation carried out to reach this decision.

In all of the above actual events were reported upon by DNOs to reach these conclusions

 

[JohnW2]

This report also commented that the perceived risks for caravans etc. was never reached by any other form that by "it seemed the best thing to do" there was no study or investigation carried out to reach this decision.

It is sometimes tempting to speculate that a lot of the rules, laws, regulations and guidelines related to matters of electrical safety may well have come about by a similar, not evidence-based, process!

Kind Regards, John

 

[ericmark]

Thank you westie101 I do seem to remember reading something about that but will admit I don't read all the stuff the IET sends me. Often my mag remains in the wrapper then binned without opening. Wiring matters I read but not the other one.

I do think the RCD has changed things a lot. Not sure the way we rely on it is good but all down to risk assessment and data from accidents and near misses.

I also cringe at the idea of putting diodes in the earth yet this is standard practice with boats.

I am lead to believe the today caravan bodies are no longer earthed to the mains supply and this would make a huge difference to the risks. The same may be true with the car body of electric cars. An isolation transformer in the charger could mean in real terms the car is class II.

The same with many boats rather than diodes in the earth using an isolation transformer although there is an earth wire in the supply cable it's not connected to the boat.

In my shed all switches are plastic so are all sockets and on a wooden shed really does not matter what earth is used. However most sheds are likely DIY wired and without a loop tester or earth rod tester it would be hard to test any earth rod so in many cases really no option.

 

[JohnW2]

I do think the RCD has changed things a lot. Not sure the way we rely on it is good but all down to risk assessment and data from accidents and near misses.

That's true in general, but I don't think it's in any way relevant to what we're discussing. The RCDs found in domestic installations would surely not afford any protection against the sort of (tiny) risk we're discussing, would they?

Kind Regards, John

 

[JohnW2]

In my shed all switches are plastic so are all sockets and on a wooden shed really does not matter what earth is used.

Is it necessarily that simple? Can you guarantee that no-one will ever plug a Class I load into any of those sockets?

Kind Regards, John

 

[stillp]

and it is not recommended where cars are charged outside

That is not quite true.
I read a report last year that decided that there was no significant increased risk to charging cars so it was quite acceptable to do so.

I'm told that at a particular motorway service area, a significant voltage has been measured between the body of an electric vehicle connected to the public charging point (which has its own earth rod) and the metal structure of the adjacent building.

 

[bernardgreen]

One place where PME can be be troublesome is in old stone cottages where dampness in walls without a damp proof course provides a path for extraneous potentials ( true ground ) to be introduced into the inside of the building.

I am not monitoring this in my cottage but have avoided sinking metal boxes into the stone walls having seen the deterioration of metal boxes in damp walls in buildings with PME supplies.

 

[JohnW2]

I'm told that at a particular motorway service area, a significant voltage has been measured between the body of an electric vehicle connected to the public charging point (which has its own earth rod) and the metal structure of the adjacent building.

Do you have any information regarding the magnitude of this 'significant' potential difference?

Kind Regards, John

 

[stillp]

Yes.

I'm not going to post it because the cause is under investigation.

 

[JohnW2]

Yes. I'm not going to post it because the cause is under investigation.

Fair enough. Given that "the cause is under investigation", do I take it that there is at least a suggestion/possibility that it may be due to 'something being wrong', rather than that the potential difference is a natural consequence of the arrangement which exists?

Kind Regards, John

 

[stillp]

do I take it that there is at least a suggestion/possibility that it may be due to 'something being wrong', rather than that the potential difference is a natural consequence of the arrangement which exists?

Perhaps you do, but (IMO) you're probably wrong!

 

[JohnW2]

do I take it that there is at least a suggestion/possibility that it may be due to 'something being wrong', rather than that the potential difference is a natural consequence of the arrangement which exists?

Perhaps you do, but (IMO) you're probably wrong!

I take that as a 'no' then - I was merely asking a question!

However, it seems a bit odd that the answer would be 'no' to "... at least a suggestion/possibility that it may be due to ....", given that an "investigation of the cause" being undertaken - unless, of course, it is one of Sir Humphrey's purely 'cosmetic' or 'sweeping under the carpet' investigations

Kind Regards, John