Gone West Posted July 19, 2019 Share Posted July 19, 2019 I have a 6mm2 three core SWA cable that I am running to my shed from the CU in the house. The supply to the house is TN-C-S. I was planning to use one of the cores as the earth and also the steel wire as the earth. I watched a video where it states that it is ok in most instances to use a two core and use the steel wire as the earth. In my case it is more complicated as my shed is metal. In the video it states for TN-C-S the earth should be 10mm2 for a metal shed but doesn’t explicitly say that it can be made up of 6mm2 copper core and the steel wire. I could use an earth rod for the shed but as he says on the video why bother when the DNO provides a perfectly good earth. What is the view of my plan? https://www.youtube.com/watch?v=NvENsVUhDUs Link to comment Share on other sites More sharing options...
dpmiller Posted July 19, 2019 Share Posted July 19, 2019 Wait for @ProDave but I would've thought TT was essential for this, it's a bit like a caravan? Link to comment Share on other sites More sharing options...
ProDave Posted July 19, 2019 Share Posted July 19, 2019 I would not export a PME earth to a metal shed. I would treat it as a "caravan" (for the same reasons) and connect it to a local TT earth (earth rod) with the metal of the shed bonded to the TT earth. In this case you connect the SWA to the PME earth at the origin with a normal SWA gland and Banko, but use a plastic gland at the shed end so it clamps onto the SWA sheath but makes no electrical conection to the SWA. 2 Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 19, 2019 Share Posted July 19, 2019 I'm with @ProDave on this, and would use 2 core SWA, with the armour connected at the supply end to the incoming PE and left unconnected at the shed end. Put in an earth rod and connect that to the shed and the earth block in the shed consumer unit. A simple garage consumer unit is ideal for this, as it will have an RCD plus a couple of MCBs, and you can then have properly protected lighting and power circuits at the shed end, with all the exposed metal bonded together and at local earth potential for safety. The snag with this arrangement is that you really need to have the installation tested, just to make sure that the wiring is OK and that the earth rod has a low enough resistance. However, unless you're on sand, or perhaps dry chalk, the chances are that the earth electrode resistance will be low enough. On the clay we have around here I can get around 50Ω to 70Ω or so pretty easily with a single rod. Anything under 200Ω is the general guidance, although for a 30mA RCD the earth electrode resistance can be as high as 1667Ω before the touch potential limit is reached. In my limited experience I've never seen an earth electrode with a resistance that high, though, so I suspect going without testing isn't likely to be a major risk. If push comes to shove then you're welcome to borrow my multifunction tester and measure the earth electrode resistance. It's not hard to do, and the same box of tricks will test everything else, like RCD trip times and current as well, so you can be assured the circuit's safe. 1 Link to comment Share on other sites More sharing options...
scottishjohn Posted July 19, 2019 Share Posted July 19, 2019 (edited) a question on earthing rods --what if you are on bedrock ? do you make a big hole and fill it with concrete with earthing rod in it? Edited July 19, 2019 by scottishjohn Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 19, 2019 Share Posted July 19, 2019 3 minutes ago, scottishjohn said: a question on earthing rods --what if you are on bedrock ? do you make a big hole and fill it with concrete with earthing rod in it? TBH, I've never yet encountered somewhere where there was nowhere to bang in a rod, but there are alternatives. The DNO and National Grid use buried earth mats or grids, and these can be made to work in difficult terrain. Every pylon is earthed like this, as are many power poles that have intermediate earths. Link to comment Share on other sites More sharing options...
Gone West Posted July 19, 2019 Author Share Posted July 19, 2019 Thanks for all the advice. Are there any comments on the video suggestions? Link to comment Share on other sites More sharing options...
Jeremy Harris Posted July 19, 2019 Share Posted July 19, 2019 15 minutes ago, PeterStarck said: Thanks for all the advice. Are there any comments on the video suggestions? John Ward is generally pretty sound (does drone on a bit though!). As you're looking at running power to a metal shed, and as there is a risk that there may well be a difference in earth potential between an exported TN-C-S/PME earth and the potentially exposed metal of the shed, I would definitely opt to wire the shed as a separate TT installation, and not connect the earth from the house to it. The metal of the shed should also be bonded electrically to the earth electrode and earth conductors within the shed end of the installation, with no connection to the house earth, other than to the metal armour of the cable (which needs to be insulated from the shed). The logic is the same as that applied to caravan electrical installations, or an electric vehicle charge point, in that there is a need to ensure that any metal that may be touched cannot rise above 50 VAC in the event of a fault. 1 Link to comment Share on other sites More sharing options...
Oz07 Posted July 20, 2019 Share Posted July 20, 2019 Slightly hijacking here but what is the pros/cons of having electric meter installed near garage then taking a feed from garage to house. I know Dave and Jeremy have similar arrangements and I have oppertunity to do this on next house. I would want power to garage anyway so need a cable regardless. I won some pretty thick cable off a job the other year. Offcuts on a warehouse. One advantage I can think is no need for consumer unit to be in a position 3m from meter in house. Link to comment Share on other sites More sharing options...
Onoff Posted July 20, 2019 Share Posted July 20, 2019 I'd have posted last night but discovered that 1 pint of Peroni and 3 of Punk IPA lead to a very, very unwell @Onoff. Worth looking at John Ware's 2005 IET publication that WAS the standard for this sort of thing and bits still relevant. outdoor-electrical-installations.pdf I can't remember but do you have an overhead supply? On poles? If so, like us here it may have been upgraded from a TT supply to have intermediate down straps periodically tied down to earth at the pole base (ideally at every connection point) making it PME TN-C-S. The "M", multiple, deriving from the many earth straps along the line. PME supplies are generally always TN-C-S but TN-C-S is not always PME. It might be the sub station feeding the property is so close there are no multiple points tied down to earth. Since they upgraded the overhead supply here all new builds get a TN-C-S supply. I was quite happy with my old, two wire TT supply and rod set up. A few years back we went on holiday and a tree came down on the overhead between the pole and the house. The BiL looking after the house called the DNO and they repaired (bodged, but that's another story) the cable. I came back to find a shiny new PME sticker in the fuse board. So I'm on TN-C-S but still with my TT rod. I'm keeping my rod for the simple fact some time back, a gang came along the road one day and CUT all the down straps off the poles with a petrol grinder. That would likely have left a number of people with pure TN-C-S installs with very high Ze. Of course nothing was done and new straps got fitted (until they're nicked again). It was around the time from memory that half of Dartford got taken out for a week by similar thieving scum! You're quite welcome btw anytime to my MFT and I'd TT the metal shed too and fit a garage unit with local RCD protection. The shed is potentially an extraneous conductive part don't forget however you do it. What are you feeding it from in your cu? Isn't your SiL a sparks? Link to comment Share on other sites More sharing options...
ProDave Posted July 20, 2019 Share Posted July 20, 2019 I kept my meter at the boundary. I have a CU in a second meter box to feed the shed static caravan and outside socket, and then a 3 core 16mm SWA fed from an 80A switch fuse to the house. I suplimented my TNCS earth with a local rod at the house end as well. And the caravan is on it's own TT earth. Link to comment Share on other sites More sharing options...
Gone West Posted July 20, 2019 Author Share Posted July 20, 2019 13 minutes ago, Onoff said: I'd have posted last night but discovered that 1 pint of Peroni and 3 of Punk IPA lead to a very, very unwell @Onoff. Hope you're feeling better now. Thanks for the link. My son-in-law's friend is the sparky who signed off my house installation and will sign off this work. The supply is indeed PME (TN-C-S) according to the sticker. The three core 6mm2 SWA is connected to the house CU through a 45A RCBO and is connected to a temporary box on a post in the garden at the moment feeding the sewage treatment plant. I will be moving it from the post to the shed. Are you saying I could use the SWA earth and a TT earth together, without problems as the TT is just another PME earth? Link to comment Share on other sites More sharing options...
Onoff Posted July 20, 2019 Share Posted July 20, 2019 1 hour ago, PeterStarck said: Are you saying I could use the SWA earth and a TT earth together, without problems as the TT is just another PME earth? Just had a giant fry up and Lucozade Sport! Feeling much better ta. No, not saying that at all. I'm on a TN-C-S supply (now) at the house, just happen to have the original TT rod too which is only now become pretty much another "M" on the multiple earth line but its mine. Leaving my rod in place could have a protective effect should the supply earth fail in some way or become degraded as aforementioned. The DNO supply you with a TN-C-S system to your one building, the house. If you were to use their earthing system it's colloquially known as "exporting the TN-C-S". If you asked them officially they might not even let you. So don't ask them! Honestly, just rod the shed end. Only connect the CPC and armoured at the house end. That'll protect the swa itself. This may help. https://professional-electrician.com/technical/stroma-certification-supply-chain/ 1 Link to comment Share on other sites More sharing options...
Onoff Posted July 20, 2019 Share Posted July 20, 2019 More reading for you! Their method of terminating in a Class 2 enclosure pre the main board in the shed is proper belt and braces. Most just cut back the armoured strands, whack a bit of heat shrink over it and put in a plastic stuffing gland. https://professional-electrician.com/technical/armoured-cable-supplies-outbuildings/ Link to comment Share on other sites More sharing options...
Temp Posted July 21, 2019 Share Posted July 21, 2019 The IET have a guide "ELECTRICAL INSTALLATIONS OUTDOORS: A SUPPLY TO A DETACHED OUTBUILDING". Not sure if link will work... https://www.google.com/url?sa=t&source=web&rct=j&url=https://electrical.theiet.org/media/1695/electrical-installations-outdoors-a-supply-to-a-detached-outbuilding.pdf&ved=2ahUKEwjp3-2y28TjAhUZi1wKHe0DCrMQFjAAegQIAxAB&usg=AOvVaw383FWw05Q6zVowQqp2B-0o&cshid=1563668016887 Link to comment Share on other sites More sharing options...
Temp Posted July 21, 2019 Share Posted July 21, 2019 13 hours ago, Onoff said: Honestly, just rod the shed end. Only connect the CPC and armoured at the house end. That'll protect the swa itself. This is what I got.. SWA cable from MCB in house CU. Braid connected to earth in the CU at the house end only. Earth rod at shed end. Garage CU in the shed with RCD for power sockets. Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now