H
Harry7671
Does a 16mm TWE sub main feeding a consumer unit with 2 x RCD's and a main sw, wired under floor boards need rcd protection ??
Discuss Sub Mains in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net
Thank you for your reply
The cable would run from the mains position (ground floor) up in boxing approx 100mm deep 200mm width then underfloor boards approx 15 meters (joists run the right way so does not need to run through joists but along side) to the other end of the house consumer unit.
i was led to believe Sub mains do not have to be rcd protected as the board its feeding has 1 main sw and 2 x rcds.
does anyone know the regs on this , ive looked but cant find anything .
If you did fit an RCD it would need to be a100mA as if you have 30mA ones upstream of it then the would be no discrimination between them if you protected the 16mm by a 30mA RCD.
we;ve encoutered this exact problem at work,( local authority housing), and the answer we got from niccy bod was, unless the cables are buriied more than 50mm ( not likely) or run in trunking ( looks naff) than 30ma rcd is required, as for discriminastion, you can use a time delayed rcd and still achieve disconnection times! hope this helps.
The requirement for protecting cables with an RCD when they are less than 50mm from the surface is for walls only though?
I know that I would personally favour an SWA anyday on sub-mains and have a dislike of the use of T&E for this purpose. Also the electricity boards will not accept SWA as a sub-main prior to metering if that is relevant here. But strictly speaking the RCD protection is only necessary for sub-mains within walls. The use of an RCD on the sub-main will also contravene 314:1 All circuits of an installation should not be connected to a single upstream RCD in order to avoid unwanted tripping, avoid hazards and minimize inconvenience in the event of a fault (reg 314.1)
I hope this isn't a spanner in the works ;-)
Plus, even if there was a time-delayed 30mA RCD it wouldn't meet the requirements anyway as 30mA RCD's are for fault protection and to limit the voltage/current flowing into someone during a fault or direct contact, but the time delay would not allow this to happen would it?
what a good post, i have read the thread from start to finish, here is your answer in a nut shell.
quality
regards electro
hi all, just been reading this thread and im not trying to hijack but i have found this really usefull! cheers! i have to do a sub main tomorrow and was wondering the same. i intended on using swa but my questions are..
1)will i need an isolator at the mains cut out possition? my sub is to another room but only 5m in length.obviously length is not important/part of discussion
2) what size swa? i am led to believe 7671 states 25mm tails but will i need 25mm? can i use 16mm as it will more than suffice considering single phase supply only provides 80A?
3) what size isolator at mains to cover appropriate swa size?
4) if swa is 3 core, i take it no seperate earth is needed as mentioned above? plus, surely a main earth as part of a sub would need mechanical protection?
sorry to but in and hope i dont cause us to go off topic! thanks, martin
oh, and another..if cu is plastic, should i just earth sheath at cut out end and tape neatly at cu or should i put clamp on for the sake of it?
Reply to Sub Mains in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net
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