Sub Mains

[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 ??

 

[Spudnik]

I would say yes.

But that depends on the exact route the cable is taking.

 

[Harry7671]

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 .

 

[not clever]

RCD it or use swa.
Or make sure the t/e is below 50mm of all surfaces or clipped direct to surface or enclosed with in earthed trunk.

I'd go swa myself.

 

[Harry7671]

Thanks "not clever" i understand your input to abide for the 17th edition regs but i was led to believe sub main were excempt from this ?

Anyone else got any input this is a real grey area for me.

 

[not clever]

No grey area at all, 7671 is still relevant for submains.
Unless you can be sure that the t/e is surface mounted or in earthed metal enclosure or swa, then you'll be needing an rcd mate.
If the system is TT you'll need an rcd anyway.
I would also run a separate mec.

 

[old dog]

rcd is for additional protection as long as you have mechanical protection and can achieve disconnection times
you do not have to rcd however i would use swa bit more expensive but looks much more professional

 

[ian.settle1]

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.

 

[Spudnik]

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.

And this is where the problem arises.

Trouble is, ANY circuit, if not mechanically protected or buried less than 50mm etc etc, must be protected by a 30mA RCD.

So, if the submain is anything other than SWA and not more then 50mm, then its got to have one.

However, if the OP can fix the cable along a joist 50mm or more away from floorboards or ceiling, then i dont see the need.

 

[Harry7671]

Thanks everyone for your input

Looks like i will be using a 100ma rcd with a 63amp 60898 along side.

and yes i will be running a 10mm earthing condutor along side the sub

 

[amp]

Hi
why put an mcb with the rcd just use double pole rcd with feeds in and sub main out saves on mcb

 

[chocolate]

Hi.

15 metres of T&E is a long run from meter to main switch. Also T&E is not an ideal choice for a sub mains especially if its run under floors without any serious mechanical protection. You shouldn't be protecting this cable with a 100mA RCD. Not allowed according to 17th Edition. It's either a 30mA at the meter end and then forget using any RCDs on the consumer unit as this will lead to inconvenient tripping OR you should run a SWA from a switched fuse ( from the meter ) to the consumer unit. The SWA should be three-core 25mm2, and there's no need to run an additional earth conductor as you'll be using the armour and one of the cores to do just that.

Chocolate

 

[markspark]

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.

 

[Harry7671]

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.

Thanks mark very helpful indeed

 

[Harry7671]

unfortunalty im told no one makes 30ma time delayed rcd's , so the only round the problem to make sure cable is run 50mm away from ceiling and floors along the cable run ....

will be running swa in future as twe distrubution circuit wire in twe looks like a thing of the past!!!

no such thing as a 30ma time delayed rcd