Upstream RCD's

[pig]

Hi all,
I am currently planning on designing an installation for a conservatory to be fitted with lights, floor sockets, and underfloor heating. My plan was to take a 6mm T+E from the Hager consumer unit under the stairs, protected at 32A, which would then run above the ceiling of the room adjoining the conservatory. My plan was to bring the cable down the wall of this room and drill directly through into the conservatory and have a small consumer unit in there with RCBO's for lights, sockets, and underfloor heating. My dilemma is that as the distribution cable will be chased into the wall on its way to the conservatory, RCD protection at the source (under the stairs) would be required. This would then mean there would be two 30mA RCD's in series. I understand that RCD protection for the sub-main may be overcome by using steel conduit or SWA in the wall but I would have liked to avoid this if possible and wondered if it can somehow be done with a higher rating RCBO at the source like 100mA or a time-delayed RCD? The wall is only single brick thick so burying at a depth greater than 50mm also isn't an option.

Many thanks

 

[bill01803]

Cables in walls require 30mA protection if less than 50mm so if you want to have rcbos it looks like steel conduit to me

 

[Midwest]

Why can’t you use swa? What does your CU under the stairs compose of?

 

[Julie.]

For a rcd to sit above another it must have both a higher trip value, and a longer operating time, as when there is a fault there is likely to be a substantial current - 100, 200 or more amps, whatever the value it is likely to be well in excess of the trip setting 30mA or 100mA - so both devices will be tripping instantly ~10ms

Use armoured cable, or conduit, as with twin&e you must use 30mA no time delay.

Or use mcbs rather than rcbo and just accept an earth fault would trip the rcd in the main cu - just like any other rcd split board.

 

[Baddegg]

Agreed, go with mcbs it’s still part of the house so just rcd at the board......it’s not like it’s at the end of the garden

 

[telectrix]

or you could feed the sub from a RCBO in the main CU, so a fault in the conservatory won't take out rest or part of house. all depends on the config. of the CU. if it's got a non -RCD way

 

[davesparks]

Why not just run new circuits back to the CU? If you're creating that cable route anyway then it will be just as easy to run a lighting and power circuit.

Or even just extend the local lighting and power circuits?

 

[lozarus]

You may be over engineering it. Depending on the house and layout of course.

As a similar scenario, my outbuilding has an upfront RCD at 30mA at the house CU, cabled and protected accordingly. In the outbuilding (or annex as its best described) is a CU with a main switch + MCB configuration, used purely for the distribution - it supplies annex sockets, UFH, conservatory sockets, lighting etc.

It's a layout that bothers some, as one RCD covers multiple circuits. However, in my layout it really is irrelevant. If the upfront RCD tripped, taking out my sockets, UFH and lighting, the layout of my house means thay the lighting from the main house easily lights enough of the annex up, for me to return to the main CU in safety. For extra cover, I have installed a discreet recessed emergency light near the annex CU.

Can your layout work in a way that allows the RCD to be upfront perhaps?