I have seen a number of posts where it is suggested that sheds should have TT supplies. Having read the very good explanation from the IET by John Ware in wiring matters. At the end of the article it does say “Where the garage contains an extraneousconductive-part such as a metal water pipe Among the options open to the installation designer are to make the installation in the garage part of a TT system or to provide main equipotential bonding to the extraneous-conductive-part in the garage.” I agree with what he says this was under the old 16th Edition with 17th Edition 4220.127.116.11 is the regulation I have been looking at where it states “Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively.” In a small garden when for example the lawn mover could as easy be plugged to the house as shed then the earthing systems for both should be the same.
Although I can see there may under extreme cases be a need to turn the supply into a TT supply under most cases I think this would be at best unnecessary and at worse darn right dangerous and I question if it is right to suggest on a DIY site feeding a shed as TT.
If I was doing the job I would make a risk assessment with consideration of where the shed was and how the gardens were arranged and would be able to measure the resistance of any earth rod used and work out current flow with worse case scenario and may combine both earth rod and board earth and would consider voltages gradients and how deep the earth rods would need to be for safety. Also the ability to test the rod once fitted. But I don’t think a DIY person would be able to work this out and even if he had registered the work under Part P. I would not be convinced that the staff I have met from the LABC could work this out even if they are in theory in charge of the site health and safety.
So on a DIY site I don’t think the idea of TT supply to outbuilding from a TN-S house supply should be suggested.
I will be interested in any comments. It is of course not black and white and is a private opinion. I would hope any comments would include sources of why the suggestions are being made. i.e. any 17th Editions reference or other IET publication.
On the subject of outbuildings containing swimming pools the eca say If the swimming pool forms part of an installation and the supply to that installation is PME, then all metalwork and pipes should be separated from the rest of the installation and connected to an earth electrode. If this is not practical, then a metal grid will need to be installed in the floor around the pool and shower areas and supplementary bonded. In either case, the installation should be protected with an RCD.
There can be no direct source referring to sheds and garages having to have an earth rod as this would be plain wrong. Something has been misinterpreted somewhere. I'm suggesting that misinterpretation to be between outbuildings and outbuildings containing a special location.
Thank you Kuzz for your help. It seems from another post the ECA are at fault and they had informed their members that under the 16th Edition you must use an earth rod and have said under the 17th Edition either exported TN-S or TT can be used. Not being a member of ECA I was unaware of this and frankly a little surprised as my organisation the IET have been producing a magazine called wiring matters and in that an article by John Ware advocated that only where there were exterior conductive parts would a TT system be used and even then it is not the only option and using 10mm² earth cable a TN-S system can still be used.
The article does quote the regulation numbers 130-07-01(132.7), 412-05-01(417.3), 413-02-02(418.104.22.168), 413-02-03(422.214.171.124), 522-06-03(522.8.10), 522-10-01(522.10.1), 542-02-04(542.2.4), 721-01-02(610.4), 741-01-01(610.6) and 743-01-01(631.1) of course these have all now changed so it takes some wadding through but I can’t see any significant changes in the regulations quoted with the exception of what can be used as an earth rod 542.2.4 so it seems someone in the ECA had made an error reading the regulations.
Sorry to say this is common and even the IET have made mistakes minor I know but I have an argument about a socket being mounted too low as the guide to 16th Edition quoted 450mm instead of the 400mm quoted in Part M. And I have been guilty of not reading the regulations myself but accepting the word of a colleague as to what is allowed I thought I could use 100 meters of cable in a ring main for years may be with the older imperial cable you could have used 100 yards but as little as 10 years ago earth loop impedance meters were hardly used. I still see electricians admit they have not got a copy of 17th Edition so hardly surprising there are mistakes. And one misses things like caravans being TT unless you work with caravans you are unlikely to look it up. My 2391 exam referred to stables and I was really uneasy as I could remember there were different rules for agricultural and horticultural premises but not exactly what they were passed anyway but I think that’s sneaky.
Anyway now cleared up there is no need to recommend TT supply to sheds and garages.
All best Eric
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