Lack of guidence about when ok / not ok to export PME

[Adam_151]

Is it just me?

There seems to be a lot of hear-say, trade counter talk, indignant opinion and a lack of official guidence and logical reasoning regarding this point?...

Not looking for guidence on a particualar situation, just has serveral times where I've had to make the PME/ TT decision and realised that there isn't a lot of guidence on the issue in general.

 

[studentspark]

I have tied myself in knots over this before.
You have to forget all the hearsay and follow the guidance, it is there.

 

[JohnW2]

Is it just me? There seems to be a lot of hear-say, trade counter talk, indignant opinion and a lack of official guidence and logical reasoning regarding this point?... Not looking for guidence on a particualar situation, just has serveral times where I've had to make the PME/ TT decision and realised that there isn't a lot of guidence on the issue in general.

No, it's not just you - this is, IMO, a very confused/confusing area. I think the main problems are (a) that there is no 'one size fits all' answer and (b) in many/most situations, the answer will not be clear-cut, but a balance between pros and cons.

I don't think there is any doubt that exporting a TN-C-S earth creates definite potential hazards if the situation is such that a person can simultaneously be in contact with that earth and some other version of 'true earth'. Whilst one can attempt to achieve an equipotential zone (at exported earth potential) within an outbuilding, problems will arise if a person standing on earth outside the outbuilding can come in contact with things bonded (or otherwise connected) to the exported earth. I'm not convinced that TN-S is necessary immune from such potential issues, either.

I may be wrong but, as far as I can see, there is probably nothing stopping one doing both - i.e. having an exported earth which is also connected to TT electrodes in the vicinity of the outbuilding. With that, the ground surrounding the building would become a bit more equipotential with the exported earth (thereby reducing the problem mentioned above) - and, as far as the 'PME' is concerned, this would really just be anither 'M'. However, I suspect that suggestion will prove controversial, if not just plain wrong!

Kind Regards, John

 

[westie101]

and, as far as the 'PME' is concerned, this would really just be anither 'M'. However, I suspect that suggestion will prove controversial, if not just plain wrong!

From the DNO point of view there is nothing specifically preventing this, subject to using cables of a sufficient size.
I would suggest, without looking, that BS7671 or documentation associated with it will probably say no.
But at the end of the day it is up to the designers to show compliance with regulations so it falls to the technical knowledge of the individual to decide!

 

[JohnW2]

and, as far as the 'PME' is concerned, this would really just be anither 'M'. However, I suspect that suggestion will prove controversial, if not just plain wrong!

From the DNO point of view there is nothing specifically preventing this, subject to using cables of a sufficient size.

I'm not sure what 'sufficient size' would mean in that context, given that (even with 230V applied) one would be hard-pressed to get more than a handful of amps down any TT electrode likely to be encountered in a domestic property.

I would suggest, without looking, that BS7671 or documentation associated with it will probably say no. But at the end of the day it is up to the designers to show compliance with regulations so it falls to the technical knowledge of the individual to decide!

I think you're right that BS7671 doesn't like connections between different earthing systems. However, to my mind and in this context, that becomes a rather odd position in the presence of 'PME'!

Kind Regards, John

 

[westie101]

I'm not sure what 'sufficient size' would mean in that context

The minimum size required by the regulations that we enforce, i.e. 10mm2 for a service neutral copper equivalent size of 35mm2

 

[westie101]

Interestingly (or not) we bought a touring caravan earlier in the year and when we use it on site with an Electric Hook Up I always attach a nominal earth rod to the chassis (as I did when we used a trailer tent in similar circumstances).

It just gives me piece of mind for the loss of the site earth.

 

[JohnW2]

I'm not sure what 'sufficient size' would mean in that context.

The minimum size required by the regulations that we enforce, i.e. 10mm2 for a service neutral copper equivalent size of 35mm2

Yes - but, given what I said about the sort of current one can get down a domestic TT electrode, doesn't that represent an example of the 'blind following of regulations, without consideration of the electrical/engineering principles involved' of which you are often critical?

In any event, I don't imagine that anyone connecting an exported earth to a TT electrode would dream of using anything smaller than 10mm² - so I guess that your regs would almost inevitably be satisfied! Indeed, one way in which one might be able to 'square' this with BS7671 would be to regard the TT electrode at the outbuilding as an 'extraneous-conductive-part' (rather than as a 'earth electrode'), and therefore connect it to the exported earth via a 'bonding conductor' (for which 10mm² would again be the minimum), rather thasn an 'earthing conductor'!

Kind Regards, John

 

[JohnW2]

Interestingly (or not) we bought a touring caravan earlier in the year and when we use it on site with an Electric Hook Up I always attach a nominal earth rod to the chassis (as I did when we used a trailer tent in similar circumstances). ... It just gives me piece of mind for the loss of the site earth.

I guess that 'says it all' ... and I presume you connect it with a conductor of at least 10mm²?

Kind Regards, John

 

[westie101]

but, given what I said about the sort of current one can get down a domestic TT electrode, doesn't that represent an example of the 'blind following of regulations, without consideration of the electrical/engineering principles involved' of which you are often critical?

Yes but no but!
By starting from the result it is not difficult to realise it is about minimum current carrying requirements in fault conditions. Both internally and externally.

Rather than "blind" following it is in this case about understanding and agreeing.

 

[bernardgreen]

This is a question with no easy fit all answer.

It does seem sensible to connect the CPC to true ground and at first glance this cannot present any hazard. On second glance it seems there is a perceived hazard when true ground gets inside the equipotential zone and this is prevented by main bonding all incoming services to the CPC via the MET.

So on the converse the potential of the equipotential zone could be perceived to present a hazard if it got into the zone of true ground.

My cottage and the adjacent shop were once a single property with a TT supply. When I bought the cottage I had to separate the two and install a new supply to the cottage. This is ( for the time being ) a PME TN-C-S. It was made very clear that there could be no connection, intentional or accidental, between my CPC and the CPC of the shop. To the extent that the common water supply ( all metal ) had to be split and isolated.

 

[westie101]

It was made very clear that there could be no connection, intentional or accidental, between my CPC and the CPC of the shop. To the extent that the common water supply ( all metal ) had to be split and isolated.

I would suggest, in part, that it is because they are two separate properties under different ownership.

 

[JohnW2]

but, given what I said about the sort of current one can get down a domestic TT electrode, doesn't that represent an example of the 'blind following of regulations, without consideration of the electrical/engineering principles involved' of which you are often critical?

Yes but no but! By starting from the result it is not difficult to realise it is about minimum current carrying requirements in fault conditions. Both internally and externally. Rather than "blind" following it is in this case about understanding and agreeing.

Hmmm. A return "yes, but", I suppose!! .... As I said, under the worst-case fault conditions (230V applied), the maximum current flowing down a domestic earth electrode would be very low - I would have guessed around 10A-12A tops and much more likely in the range 2A-5A. If you agree with that, how does your understanding result in your agreeing with a 10mm² minimum cable size - would not, in engineering terms, 1mm², or even less, be closer to the mark?

Whatever, I've agreed that (I hope) no-one would really consdier using smaller than 10mm² - so there's a fair bit of safety margin in there, since the maximum possible fault current (with 230V applied) would then be less than the cable's CCC down to a Ze of around 4Ω!

Kind Regards, John

 

[bernardgreen]

In any event, I don't imagine that anyone connecting an exported earth to a TT electrode would dream of using anything smaller than 10mm² -

Why 10mm² when the earth leakage protection will operate on a 30 mA earth fault current. And if that doesn't then an MCB will operate.

That sounds right, until one looks at the circuit of a PME "earth" connected to a ground rod. There are no protective devices in that circuit. Not matter what current flows through it no safety device with operate. So 10mm² is a good size to use.

 

[westie101]

I specified internal & external faults
e,g,
loss of neutral on the service cable
loss of neutral on the main
both of which could lead to load current being carried by that electrode and connecting cable