Earth spike for out building
Earth spike for out building
I don't think there's a blanket "not advised".
PME Earthing does pose some challenges, which should be taken into consideration. Usual reasons for not using a PME earth might include:
otherwise you're normally quite free to use extend the main building's earthing out to an outbuilding.
- Andy.
If you had it you'd also use the outbuilding rebar, or steel piles or any other metal that was in the ground or in concrete in the ground that was part of the building. A lone spike as such is the least you can have.
Now you can and would extend the house earth to any old outbuilding if the supply was TT or TNS, the only problem is that a great many areas are wired with a supply earthing that is TNC-s (PME) and I presume you live in one of them.
So with PME the system earth is really the DNO neutral - and is very good as a CPC for blowing fuses if there is a live to earth fault, but it is not guaranteed to be at the same potential as the ground outside - the "terra-firma" earth voltage if you will. For garden tools, being pretty much always double insulated we do not care, but a supply to an earthed object that can be touched is a bit of a stickier wicket. The risk varies and so does the advice about how best to organise the earthing. Not all cases are equal.
The newest panic is some one in bare feet on wet grass touching a metal car door handle while it is plugged into an EV charger. Things like painted lamp posts are less of a hazard, and tend to be in part their own electrode. For years folk have wondered about mains supplies into caravans and boats.
As above a building with an earth floor, like a barn, or a damp concrete floor, like some garages, poses a similar hazard, but things that are free draining and hostile to bare feet (gravel perhaps) are less of an issue.
So what is in this outbuiding, how is it constructed and how is it to be used, and it may or may not be OK on a PME system, or best moved to TT.
Mike.
There is something of an urban myth that PME cannot be "exported", but as my learned friends have already stated, it needs to be done with a degree of caution.
Whilst I take the point about, let's say, damp floors, it is not unknown for the supply to a property to be in an outbuilding.
Looking at the point Andy made "not being able to create a reliable equipotential zone" ...
We no longer use this concept, and certainly from 17th Ed onwards BS 7671 recognises that there may well be no extraneous-conductive-parts. Hence, that particular "justification" is perhaps no longer relevant?
I thought the word "equipotential" still survives in BS 7671 (titles of regs 411.3.1 and 411.3.1.2 for starters). I'd certainly agree that there's no need for extraneous-conductive-parts to be present & bonded in order to achieve an equipotential zone - in many ways an all-insulating building with no metallic services is ideal - although there is still an equipotential zone between items of class I equipment via the c.p.c.s (hence the requirement for all exposed-conductive-parts within reach of each other to be connected to the same earthing system).
Where it gets messy is where there are parts that can introduce a potential but aren't practical to bond - e.g. damp concrete floors (you'd need a buried metallic grid under the entire floor area).
Granted the term "equipotential" is falling out of favour - not least because the situation isn't really one of equal potential - very often it's not entirely equal, and sometimes (e.g. during the clearance times for Earth faults) not anything like equal by perhaps hundreds of volts. Never the less the attempt is there to try to keep the voltage on accessible things as similar as we reasonably can in the circumstances. Bonding is still very much a requirement (where extraneous-conductive-parts are present) even if the method of shock protection is now just referred to as 'Automatic disconnection of supply'.
Your local DNO's rule on use of their PME terminal are usually a good read too - the likes of damp floors usually feature significantly.
- Andy.
Where it gets messy is where there are parts that can introduce a potential but aren't practical to bond - e.g. damp concrete floors (you'd need a buried metallic grid under the entire floor area).
BS 7671 only really covers that in one place now - Section 705 - although it's common practice in certain cases, e.g. for a shower block for a sports club or similar supplied from PME, as it's well-known that (probably very innocent and not hazardous) tingles might result.
I agree with the other points you make, although to be honest, if DNOs are worried about damp floors, how can we not worry about situations such as EV charging?
I always find it amusing when the use of the concrete floor rebar as bonding/ earthing is mentioned.
The chances of getting on site during the installation of the concrete floor in my experience is remote at best. Often the electricians won’t know that the job exists at this stage. Builders can’t even stop boarding over cables let alone involve us at the initial stages.
Then there is the questions of the type of rebar used, welded or tied with bits of ‘bendy’ metal, what is the resistance of the rebar is it a reliable earthed structure capable of accurate results?
Using the structural steelwork is a better option, often with very low resistance and often exposed when the electricians arrive on site.
This is something that is mentioned in EV charging, with the same questions, quality and availability of access to the rebar, I think may apply.
It may ease your worry to realise that the rebar does not need to be welded to get a decently lo electrode effect unless you connect to a very short length. - the connections between the bars via the bulk of the concrete is enough, and the fairly scratchy connections where the bars cross, - so the twist and tie rebar is still worth connecting to. It is worth a play with a meter if you get the chance.
Note however that a concrete encased electrode is not the best idea for lightning protection - a really high current, far higher than a few hundred volts of mains could ever push through, tends to crack the concrete.
It is a good idea, and required in some countries regs, but not ours, to make two connections to the buried metal, so that you can do a ring-round continuity test with an ohm meter between the two wires and have confidence that it has not rotted off or snapped below ground.
Mike
I always find it amusing when the use of the concrete floor rebar as bonding/ earthing is mentioned.
The chances of getting on site during the installation of the concrete floor in my experience is remote at best. Often the electricians won’t know that the job exists at this stage. Builders can’t even stop boarding over cables let alone involve us at the initial stages.
I worked on a particular job where they forgot to construct an earth mat for an HV substation during construction. It's unusual, but illustrates the point with a little humour. There was still a requirement for HV earthing, though, which had to be addressed ...
What I would say is that other countries do have a requirement for a foundation earth electrode without any problem. Once it becomes a requirement that's known and understood, there would be no real issue.
Yes the idea of multiple connections would seem to be a valuable one. But as I said putting this into practise on any site other than a prestigious one, or perhaps one governed by an architect seems unlikely in my experience.
builders tend to ‘crack on’ and let electricians worry about their job.
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