PeterW Posted September 14, 2017 Share Posted September 14, 2017 Quick one for the sparks... Looking to run a second sub main CU in the integral garage on the house. Main CU is a double decker metal unit with all RCBOs. Garage will be fed from a 32A dedicated circuit. Question is .... as it's integral to the house, does the submain need to be metal or can it be a standard insulated plastic one...?? Link to comment Share on other sites More sharing options...
Nickfromwales Posted September 14, 2017 Share Posted September 14, 2017 Unsure, but is it worth the risk of it needing to be changed in the near future with regs shifts? Link to comment Share on other sites More sharing options...
Jeremy Harris Posted September 14, 2017 Share Posted September 14, 2017 Even if the regs allow a plastic CU in a garage (and I believe they do - but I don't have a copy of the latest amendment 17th Ed to check) I think it probably makes sense to fit a metal one. Garages tend to collect more flammable stuff than elsewhere, at least mine does, so anything that reduces the risk of fire, even only slightly, would seem to make sense. If I were fitting a new CU in my garage now, then I think I'd go for a metal one, just to be on the safe side, even though I think the risk from a fire starting in a properly installed CU, with tight and secure connections, is probably pretty tiny. 2 Link to comment Share on other sites More sharing options...
ProDave Posted September 14, 2017 Share Posted September 14, 2017 I would have a metal one in a garage anyway, much less likely to get damaged. The metal ones are only mandatory in "domestic" so some delving into that definition is needed to determine if it really needs to be metal or not. 1 Link to comment Share on other sites More sharing options...
Jeremy Harris Posted September 14, 2017 Share Posted September 14, 2017 36 minutes ago, ProDave said: I would have a metal one in a garage anyway, much less likely to get damaged. The metal ones are only mandatory in "domestic" so some delving into that definition is needed to determine if it really needs to be metal or not. Good point, I felt mine was vulnerable, so ended up building a cupboard around it, to provide a better degree of physical protection. I would have changed it for a metal one, if I was legally allowed to do so, but really couldn't justify paying for half a day of electrician's time just for something that was really me being a bit over-cautious. IIRC, BC weren't the slightest bit interested in the garage wiring at all, either. Even the very fussy interim chap I had, the one that moaned about just about everything else, never even looked at the garage. Any idea why metal CUs aren't mandatory for non-domestic use? If anything, it seems a bit counter-intuitive to not mandate them across the board when they made the change. Perhaps it was solely related to night time fire risk in houses, and the sort of logic within the IEE and other bodies that make up the regulatory group led to the conclusion that there was not so much need to protect non-domestic premises from the alleged CU fire risk. I still think they would have done a far better job to mandate better quality terminals, at that's where, in my view, the biggest problems are. I changed a pull switch in the old house a few months ago, and the terminal screws were so stiff in their threads that it was hard to tell when they were tight. One fooled me into thinking it was secure, but when I did a pull test on the wire to make sure it was OK I found that it was still loose. I bet crap like this is the real reason for the problems that were found with a few fires, along with badly designed gate-type terminals, that don't block off the opening when the gate is tightened, but with no cable in it. The relay bases I used in the heating control box were like that, and were a real pain, especially as I couldn't see the position of the gate when trying to put the wires in. Link to comment Share on other sites More sharing options...
PeterW Posted September 14, 2017 Author Share Posted September 14, 2017 Right so a small metal one with MCBs on the circuits is fine as there is a 32A rcbo in the main box ..? Come to think of it I've got a spare metal box that's our temporary supply ... just needs me to sort the MCBs. And I'm still considering @JSHarris idea of the contactor and emergency stop button ... Link to comment Share on other sites More sharing options...
Onoff Posted September 14, 2017 Share Posted September 14, 2017 Page 20/21 of the OSG is very confusing on the matter! 1 Link to comment Share on other sites More sharing options...
PeterW Posted September 14, 2017 Author Share Posted September 14, 2017 1 minute ago, Onoff said: Page 20/21 of the OSG is very confusing on the matter! And this is supposed to help me ..??? Link to comment Share on other sites More sharing options...
Onoff Posted September 14, 2017 Share Posted September 14, 2017 10 minutes ago, PeterW said: And this is supposed to help me ..??? You have a pm! Link to comment Share on other sites More sharing options...
PeterW Posted September 14, 2017 Author Share Posted September 14, 2017 Ta..! Link to comment Share on other sites More sharing options...
Jeremy Harris Posted September 15, 2017 Share Posted September 15, 2017 @PeterW, if you opt for a contactor and E stop system, then remember to add another 6A MCB in the CU for the control circuit. That way you can run the loop for the E stop control circuit in 1mm² with the cable adequately protected. You can run the E stop as a single run around the garage to each button, as they are normally closed, so opening any one in the loop will turn off the contactor. I added an illuminated DP switch to the E stop loop feed, just to be able to isolate all the sockets, except one that runs the garage door, as a quick way to just make sure all the power is off when I'm not using the workshop. Link to comment Share on other sites More sharing options...
PeterW Posted September 15, 2017 Author Share Posted September 15, 2017 Thanks @JSHarris I was pondering how the contactor loop was configured ..! Link to comment Share on other sites More sharing options...
Jeremy Harris Posted September 15, 2017 Share Posted September 15, 2017 @PeterW, it nearly caught me out! I have two radial socket runs either side of the workshop, plus another radial run to an outdoor double gang socket for garden stuff. These are each on 20 A MCBs. I ran runs of 2.5mm² around for the sockets, in 20mm conduit, together with a run of 1mm² for the E stop loop. We then realised (the electrician and myself) that we'd need another 6A MCB to protect the E stop loop cable. We did debate just running it from the 6A lighting MCB, which would have been OK, as the contactor load is tiny (just a few mA), but I decided that I'd rather not have the power to the sockets go off if there was a lighting fault. By luck, there was a spare single module way in the box, so the E stop now has a dedicated MCB. When it came to wiring things up, the 20 A MCBs that protect each radial feed the DP contactor line connections, with the contactor neutral connection coming from the box neutral bus. The individual radial feeds are therefore each DP switched by the contactors and the MCBs protect both the cable and the contactor. This means that when the E stop is operated, all the power circuits are properly DP isolated. The exception is the single socket for the door power that runs from an MCB directly, not via the contactors. I used 25 A DP contactors, that are single module width. The box has a 40A DP B curve RCBO at the front (it's fed from a 6mm² length of SWA from the outdoor distribution box that has a 40A C curve DP RCBO at the feed end). The reason for choosing a C curve on the remote feed end was really just to provide adequate protection for the 6mm² SWA without nuisance tripping. I wanted to be reasonably sure that the workshop end RCBO would trip first if there was a current overload fault condition. Link to comment Share on other sites More sharing options...
dpmiller Posted September 15, 2017 Share Posted September 15, 2017 You could also go with a cheap DOL starter which then gives the opportunity to have all and any kill switches at SELV. Link to comment Share on other sites More sharing options...
Nickfromwales Posted September 15, 2017 Share Posted September 15, 2017 10 hours ago, PeterW said: Right so a small metal one with MCBs on the circuits is fine as there is a 32A rcbo in the main box ..? Come to think of it I've got a spare metal box that's our temporary supply ... just needs me to sort the MCBs. And I'm still considering @JSHarris idea of the contactor and emergency stop button ... Why don't you put RCBOs in the garage CU and feed that CU off an MCB in the house? That way you only lose one circuit at a time in the garage rather than the lot in one go ? Link to comment Share on other sites More sharing options...
PeterW Posted September 15, 2017 Author Share Posted September 15, 2017 4 minutes ago, Nickfromwales said: Why don't you put RCBOs in the garage CU and feed that CU off an MCB in the house? That way you only lose one circuit at a time in the garage rather than the lot in one go ? Cos I have OCD and I've got a perfect layout for my house CU that is all RCBO...!! Link to comment Share on other sites More sharing options...
Nickfromwales Posted September 15, 2017 Share Posted September 15, 2017 Get a matching MCB you loon, all that'll be missing is the test button ? Saves you going back to the house to reset when you plug something in and a spike throws the RCBO. Link to comment Share on other sites More sharing options...
Nickfromwales Posted September 15, 2017 Share Posted September 15, 2017 And youll then also have one RCBO in the bank Link to comment Share on other sites More sharing options...
Jeremy Harris Posted September 15, 2017 Share Posted September 15, 2017 (edited) FWIW, my logic was that if an earth fault tripped out the whole workshop, then it wasn't going to be a significant problem, unlike in the house, where having half the house go out because of a single appliance earth leakage fault would be annoying. I could have gone for RCBOs instead of MCBs, but at the time I already had a box of MCBs available, so just used them. By chance they all match, too, and they match the 40 A DP RCBO. It's just the contactors that don't match, so might offend those who like to see a perfectly matching row of DIN rail devices in a box....................... Edited September 15, 2017 by JSHarris 1 Link to comment Share on other sites More sharing options...
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