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

JohnW2 · 12 Oct 2012

westie101 said:
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

Eh? As I said, in the very worst possible situation (unless you folk somehow let HV onto the LV network

), of 230V between the connecting cable and earth, one would be very unlikely to get more than about 12A flowing along that cable and down the electrode (at an electrode impedance around 20Ω - pretty unusual for a domestic rod). To get the ~60A needed to require a 10mm² cable would be incredibly unlikely with a domestic rod, wouldn't it?

What sort of current do you envisage you could get to flow down the rod - and under what circumstances?

Kind Regards, John

bernardgreen · 12 Oct 2012

Hi Westie

I am in a slightly sarcastic mode tonight.

It the external faults that concern me, especially as some of the "electricians" I have met seem to think an external fault presents no hazard to the home owners.

I agree the average ground rod is not going to sink heavy currents but a one ohm rod (*) could be burdened with 230 amps worst case.

(*) A metallic water pipe becomes an earth rod when bonded to the MET

JohnW2 · 12 Oct 2012

bernardgreen said:
JohnW2 said:

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 -

Click to expand...

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.

As I've said, I imagine that it's probably the smallest that most folk would contemplate using (certainly if they view it as bonding to an extraneous-c-p), but (as I've been saying to westie) it's far fatter than can really be justified solely in terms of the maximum current likely to flow into a domestic earth rod under the worst possible faulst conditions. I would imagine that reduced susceptibility to (electrically significant) mechanical damage is (together with 'comfort') one of the main reasons for using cable that 'fat' in the situation we are talking about.

Kind Regards, John

westie101 · 12 Oct 2012

I would really not try to predict the impedance of any rod so would always work on it being somewhat lower than you suggest.

Data I have (which I'm sure I've put on here in another thread) suggests in some soils a 1.5 m rod would have an impedance of 10 ohm.

JohnW2 · 12 Oct 2012

bernardgreen said:
I agree the average ground rod is not going to sink heavy currents but a one ohm rod (*) could be burdened with 230 amps worst case.
(*) A metallic water pipe becomes an earth rod when bonded to the MET

Indeed, but I am talking in context - that context being an earth rod installed in relation to an outbuilding to which a TN-C-S earth has been exported. I'd be amazed if that was ever anything like as low as 1Ω.

The fact that extraneous-c-ps like water pipes can have much lower resistances than the usual domestic earth rod is, of course, the reason why regs require a minimum of 10mm² for main bonding conductors - but that's not what we are talking about.

Kind Regards, John.

JohnW2 · 12 Oct 2012

westie101 said:
I would really not try to predict the impedance of any rod so would always work on it being somewhat lower than you suggest. Data I have (which I'm sure I've put on here in another thread) suggests in some soils a 1.5 m rod would have an impedance of 10 ohm.

OK - 2.5mm² cable then

Kind Regards, John

bernardgreen · 12 Oct 2012

getting back the question.

It seems acceptable to export it ( in-advertantly ) to gas meters and external metalic service pipes but not to garden tools.

No doubt some one will say that bonding services does not export the PME earth and that it only prevents extraneous potentials from entering the equipotential zone.

JohnW2 · 12 Oct 2012

bernardgreen said:
getting back the question. ... It seems acceptable to export it ( in-advertantly ) to gas meters and external metalic service pipes but not to garden tools.

I don't know about 'acceptable', but it's certainly a universal phenomenon. As you often point out yourself, one of the most common visible manifestations of an effectively exported earth is the outside water tap (if plumbed with metal pipework). For that situation, and certainly with TN-C-S, there is a theoretical argument that such taps (i.e. the internal equipotential zone) should be bonded to an earth rod in the immediate vicinity of the tap - so as to minimise foot-hand PDs for those using the tap (maybe with bare feet as well as hands).

Kind Regards, John

bernardgreen · 13 Oct 2012

With the unpredictable and then constantly varying ground conditions it is a hopeless task to reliably "bond" the lawn into the equipotential zone using a single earth rod.

It would be interesting to look at voltage gradients between two adjacent lawns "bonded" to PME "earths" that were derived from neutrals in different street cables.

NOTE for general readers...... The risk of danger from the external tap is for the majority of people very small and will only become significant when there is a fault in the house or the supply network. That said I know one gas meter reader who wears gloves if he has to clean the glass on an external meter before he can read it.

Electrifying · 13 Oct 2012

We don't half get some babbling when you pair start on about something.

Bottom line is, the 'Electrician' who is contemplating running power to an outbuilding will weigh up the conditions as to whether he should use the 'system' earth or supply the outbuilding with it's own 'TT' earth.
He wouldn't use both.

There are a number of things to take into account and a good electrician will assess the 'supply' and 'existing installation', the construction of the outbuilding, including the floor, and the presence of any 'extraneous' parts.

Interestingly, if you run a 4.0mm power supply into an 'attached' garage, no-one bats an eyelid................move the same garage away from the house by one metre and everyone starts flapping about 'exporting' earths.

JohnW2 · 13 Oct 2012

Electrifying said:
Bottom line is, the 'Electrician' who is contemplating running power to an outbuilding will weigh up the conditions as to whether he should use the 'system' earth or supply the outbuilding with it's own 'TT' earth.

Exactly - that's why I wrote, in the second message in this thread:

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.

Electrifying said:
He wouldn't use both.

Maybe not - but that doesn't necessarily mean that it would never be the theoretically best option for the particular situation. On the face of it (given that RCD protection is obviously required), "if in doubt, use TT" would seem to be a reasonable rule of thumb - but there are undoubtedly situations in which the considered decision will be to use the 'exported' earth.

Electrifying said:
There are a number of things to take into account and a good electrician will assess the 'supply' and 'existing installation', the construction of the outbuilding, including the floor, and the presence of any 'extraneous' parts.

Indeed - that's why there is no 'one size fits all' answer. Also to be considered, as I said, is whether any conductor in the local equipotential zone (if one can be created) can be touched by anyone outside of it.

Electrifying said:
Interestingly, if you run a 4.0mm power supply into an 'attached' garage, no-one bats an eyelid................move the same garage away from the house by one metre and everyone starts flapping about 'exporting' earths.

Don't you believe it - I'm sure I've seen people 'flapping' about 'damp floors' etc. in attached garages! For what it's worth, it's never me who raises the issue of 'exported earths'.

Kind Regards, John

JohnW2 · 13 Oct 2012

bernardgreen said:
With the unpredictable and then constantly varying ground conditions it is a hopeless task to reliably "bond" the lawn into the equipotential zone using a single earth rod.

Of course. Indeed, it would be hopeless with any arrangement likely to be contemplated in a domestic garden. That's why one cannot, in practice, do anything about the theoretical (tiny) hazard of an earth exported to a Class I portable appliance used outdoors.

However, that's different from seeking to make a small area of ground within touching distance of an outside tap roughly equipotential with the tap, which is far more attainable. Indeed, if one really wanted to, one could use a conductive 'mat'.

However, as you say, the risk presented by any sort of 'exported earth' is so minute that I think it gets far too much 'air time' - I've yet to be convinced that anyone has ever been seriously injured or killed as a direct result of a high potential PME 'earth'. I'm sure that serious injury and death due to mechanical injury in gardens and outhouses is a topic far more deserving of discussion (and attempts to minimise) - you've only got to look in any A&E department on the first decent weather weekend in a year to see that!

Kind Regards, John

Spark123 · 13 Oct 2012

westie101 said:
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.

Knowing my luck if I tried that I'd go straight through the SWA feeding the site

JohnW2 · 13 Oct 2012

Spark123 said:
Knowing my luck if I tried that I'd go straight through the SWA feeding the site

... and me! Anyway, isn't that a standard method of locating buried live cables?

Kind Regards, John

bernardgreen · 14 Oct 2012

Electrifying said:
We don't half get some babbling when you pair start on about something.

Healthy debate, can sometimes lead to changes for the better.

Electrifying said:
he should use the 'system' earth or supply the outbuilding with it's own 'TT' earth.
He wouldn't use both.

If there are any services that need to be bonded then TT is the only cost effective way. Using the "system" earth will require ( by BS7671 ) a 10mm earth wire without joints. ( as in the house MET bond to to a service pipe ) That would make the cost of SWA un-necessarily expensive if the current loading could be safely carried on 4 mm

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

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