Right I have was looking at earlier posts from questions asked before about outbuildings and using an earth rod or the existing pme earth supply in a house. In a previous post someone put about if you lost the neutral in the road and someone was using an outside appliance with an earthed metal casing this could be dangerous so should be using an earth rod but can someone tell me what the difference between a socket In an outbuilding using the pme earth or a outside socket directly off of a normal house ring with a pme earth how is this any safer thanks !!!
Pme or earth rod
- Thread starter Rangers1882
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There is always an exceptional occurrence which will mean your installation could have been safer if done in a different way.
Is TT safer than TN?
What if your house was bigger and reached to where the outbuilding is?
Is TT safer than TN?
What if your house was bigger and reached to where the outbuilding is?
Let me guess, that comment was from bernard green?
There is NOTHING wrong with a PME supply.
There is NOTHING wrong with a PME supply.
Well exactly my point if putting an earth rod in is to stop this from happening how come we can have an outside socket off of the house ring surely this is exactly the same so why the big ho ha about earth rods to outbuildings ??
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The only time you need to install an earth rod at an outbuilding is when the outbuilding contains extraneous conductive parts and you don't want to extend the bonding from the house to outbuilding.
Both PME and earth rod systems have drawbacks, neither of them being anything to worry about.
Both PME and earth rod systems have drawbacks, neither of them being anything to worry about.
That was meant to be my point as well.
I'm sorry I failed to convey it well enough.
There is really no difference. Those who have concerns about PME earths being 'exported' outside of the building would, I assume, be as worried about a Class I load/appliance being used outdoors which was plugged into a socket connected to 'the house ring' as they would about any metalwork (or Class I appliances) in an outhouse which was connected to a PME earth.
Within the house, a supply fault resulting in the PME earth rising to a high potential (relative to 'true earth') should not be a problem, since bonding should ensure that no dangerous potential differences can exist between things connected to the PME earth and things which, in the absence of that bonding, would/could be at true earth potential. The moment one moves outside of that 'protected environment' ('equipotential zone'), whether into the garden or some outhouses, there is the possibility of a person being in contact with 'truth earth' (either literally, or by contact with pipes/structural metal coming out of the ground). If they were simultaneously in contact with an exported PME which happened to be at high potential because of a fault, they could theoretically experience a serious shock - and one that no RCD in the installation would protect them against.
It is a theoretically valid argument, but the risk involved is incredibly low. Supply-side faults resulting in seriously high potential PME earths are rare enough. The chances of someone touching an exported PME and true earth simultaneously at the very time when there was also such a (very rare) fault must be vanishingly small.
Properly bonding metal items (pipes, structure) which enter an outbuilding is an alternative to having a local earth rod, and that will reduce some, but not all, of the (incredibly small) risks associated with having an 'exported' PME earth.
I strongly suspect the risks associated with driving a car, or using ladders (let alone smoking, drinking, eating badly or having unprotected sex!) are dramatically higher than those associated with these theoretical problems of exported PME earths which some people get concerned about.
Kind Regards, John
If that occurred, then it would be dangerous. However a lost neutral isn't exactly safe for similar metal cased items indoors either.
With underground cables, it is exceptionally unlikely that the neutral would be cut with the line intact.
Virtually all items for use outdoors do not have earthed metal casings (class 1), and a substantial amount of indoor items don't either.
With an overhead supply on two separate conductors, it is somewhat more likely.
The type of earthing used depends on the circumstances - there is no answer which fits all. They all have advantages and disadvantages.
As I've just written, there shouldn't be a problem within the house provided it is a truly equipotential zone.
As you say, tools etc. used outdoors are rarely Class I. However, you'll find metalclad switches and sockets in many an outhouse - which are effectively 'Class I'.
Ironically, for those who worry about such things, the 'greatest risk' probably exists in relation to plumbing, rather than anything electrical - in the form of the 'outside tap', plumbed through the kitchen wall in copper, with a sodden area of soil (due to spillage from the tap) below it. (S)he who stands on that sodden ground with bare feet and touches the tap (inevitably connected to the PME earth if there is copper plumbing in the house) at the very moment when there is a 'lost neutral' is theoretically at some risk!
Again, however, truly minute risks!
Kind Regards, John
Thanks for your replies you have summed up my thinking on this subject it was just bugging me why every time I read about an outbuilding I would see posts about using an earth rod instead of the house pme supply which I personally saw not a lot of gain as no one has come up with a specific regulation that can back these thoughts up with !!!
Can I also just ask as you said an alternative to using an earth rod you could bond extraneous conductive parts could you bond them from the exported pme into the outbuilding or would this have to come from the met in the house thanks
You must.
They must be bonded to the MET in the house but you may use the armour (copper equivalent size) or/and a core of the cable.
This must be done with an appropriately sized conductor (probably 10mm²) which, for PME, is dependent on the Neutral supply conductor size.
It is this which may mean using a TT earth is preferred at the outbuilding because of cost or inconvenience but not necessarily desirable in itself (although some seem to like it).
EFLI has answered your specific question, but as the person who mentioned the bonding of extraneous-c-ps 'as an alternative' to using a local earth rod, I should remind you that I also said that although that reduces some of the risks that some people worry about, it does not reduce them all.
That is because, whilst one can bond metal extraneous-c-ps, one often cannot (without going to extreme lengths) effectively bond a floor which may be damp and in contact with soil, and one certainly cannot (without going to ludicrous lengths) effectively bond the soil of the surrounding garden.
I have a shed in my garden (the one small window of which is often covered by junk!) which has a metalclad light switch just inside the door - so many people entering operate (hence touch) that switch before entering the shed, whilst they are standing on the soil outside. If the metal case of that switch were connected to a PME earth (which it isn't, since I don't have a PME supply), the soil was wet and their feet were bare or semi-bare, then they would be at risk of the vanishingly improbable event I discussed - even if any metal extraneous-c-ps within the building were adequately bonded.
Kind Regards, John
The regulations require that any extraneous-conductive parts in an outhouse are adequately bonded to the 'origin of' (aka the Main Earthing Terminal, MET) the 'earthing system' of the electrical installation within that outhouse, but don't mind whether that 'earthing system' is a local 'TT' one (i.e. local earth rod) or the DNO-supplied (TN-C-S/PME or TN-S) earth used in the main house.
The regulations do not seem (yet!) to have considered the other potential issues I have mentioned.
As you have seen, opinions differ. Since the only views you were getting were 'PME is fine', although it's not an issue which particularly worries me, I have attempted to play Devil's Advocate by explaining the thinking behind some of the concerns about exported PME earths that some people have.
Kind Regards, John
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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 because of the voltage gradient that with likely exist under fault conditions.
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 but with a caravan for example fire regulations will on a caravan park ensure a reasonable distance between caravan and any building so using a TT earth is required. This will also reduce erosion of steady legs. I would still use PME if less than 3 meters from house but over that likely TT in the main because of corrosion more than safety also care is required in selecting material the rod is made from because of electrolysis. Had this problem with narrow boats copper coated is no good needs to be steel and need testing on a regular basis. Ali boats are a real problem often diodes used in the earth cable or isolation transformers.
So what I am saying there is no one size fits all and consideration must be given to shed type and environment. My caravan when hucked up to charge batteries is connected to PME supply as the gas and electric meters can be touched at the same time as the caravan so TT would be dangerous under fault conditions.
Where beyond the garden is fields and there is a large distance between garden and house then TT is likely better because of the voltage gradient that with likely exist under fault conditions.
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 but with a caravan for example fire regulations will on a caravan park ensure a reasonable distance between caravan and any building so using a TT earth is required. This will also reduce erosion of steady legs. I would still use PME if less than 3 meters from house but over that likely TT in the main because of corrosion more than safety also care is required in selecting material the rod is made from because of electrolysis. Had this problem with narrow boats copper coated is no good needs to be steel and need testing on a regular basis. Ali boats are a real problem often diodes used in the earth cable or isolation transformers.
So what I am saying there is no one size fits all and consideration must be given to shed type and environment. My caravan when hucked up to charge batteries is connected to PME supply as the gas and electric meters can be touched at the same time as the caravan so TT would be dangerous under fault conditions.
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