RCD location

[geoffhodges]

I am shortly going to fit a power supply to a new garage.

I think I understand the basic logic of fitting an RCD to protect a circuit where some, if not all, of the cable is buried in a wall. Can I fit the RCD directly downstream of a plug-in MCB located in an old Wilex CU fuseway?

If this RCD is in the house rather than the garage, downstream can I also fit a Garage CU (RCD plus 2 MCB) within the garage for splitting/isolating the lighting and power circuits? My understanding is you shouldn't fit two RCD's to protect the same circuit.

 

[Spark123]

What sort of cable is it which is to be burried in the wall? Some cable types do not need to be 30mA RCD protected.
You only generally need one 30mA RCD per relevent circuit i.e. one supplying socket outlets.
You can fit an RCD in an external enclosure if you wish, however it may not be required.
It would be better imo to have the 30mA RCD in the garage CU and the submain cable as such that it doesn't need 30mA RCD protected.
This work will be notifiable to your LABC under part P of the building regs.

 

[geoffhodges]

What sort of cable is it which is to be burried in the wall? Some cable types do not need to be 30mA RCD protected.
You only generally need one 30mA RCD per relevent circuit i.e. one supplying socket outlets.
You can fit an RCD in an external enclosure if you wish, however it may not be required.
It would be better imo to have the 30mA RCD in the garage CU and the submain cable as such that it doesn't need 30mA RCD protected.
This work will be notifiable to your LABC under part P of the building regs.

Thanks for the info!

The cable will probably be ordinary 4mm2 twin&earth. From the CU/RCD, the cable will enter the house loft via about 2m of plastic conduit, buried less than 50mm in a wall. Apart from the underground link (via plastic conduit) to the detached garage, the rest of the cable can be surface mounted.

The main point of my first question is can the RCD be fitted downstream of the CU MCB? I have seen one or two articles where the RCD is UPSTREAM of the CU.

 

[electronicsuk]

Plastic conduit will not provide adequate protection for twin and earth cable buried underground, concrete or otherwise. You will need to use something like Steel Wire Armoured, and this will helpfully alleviate the need for RCD protection upstream of the garage, as you can bury it less than 50mm in the wall without the need for one. This will then mean you can have a small CU in the garage and provide the RCD protection there.

 

[geoffhodges]

Plastic conduit will not provide adequate protection for twin and earth cable buried underground, concrete or otherwise. You will need to use something like Steel Wire Armoured, and this will helpfully alleviate the need for RCD protection upstream of the garage, as you can bury it less than 50mm in the wall without the need for one. This will then mean you can have a small CU in the garage and provide the RCD protection there.

Many thanks for the info.

I was intending to use bricks & coping stones to protect the underground plastic conduit.

 

[electronicsuk]

Sorry Geoff, this probably would be enough to stop most eople sticking a spade through the cable, but as far as the regs are concerned it needs to be an earthed metallic covering.

 

[Spark123]

Agree with Matthew, would be better done in SWA.
This means the cable doesn't need to be RCD protected (unless you're on TT etc) and the RCD can therefore go in the consumer unit at the garage end.

 

[geoffhodges]

Sorry Geoff, this probably would be enough to stop most eople sticking a spade through the cable, but as far as the regs are concerned it needs to be an earthed metallic covering.

The old garage installation actually has the supply cable within a old, thick walled, steel gas pipe buried just below the surface, tightly sandwiched between the drive and the patio.

I have seen no mention of using steel conduit in my research for the new installation. Some of the current books (pre 17th edition IEE?) on the subject, clearly show plastic pipe protected by bricks/coping stones, although they do mention using SWA cable as alternative!

 

[geoffhodges]

Agree with Matthew, would be better done in SWA.
This means the cable doesn't need to be RCD protected (unless you're on TT etc) and the RCD can therefore go in the consumer unit at the garage end.

Whatever method I use for the underground section, is it permissable for the cable/conduit to enter the garage via a short, right-angled, duct buried in the foundation? I read somewhere that the cable/conduit should rise up on the outside of the garage wall to a height of 150mm above the DPC, before going through the wall to the interior.


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Moderator's note:

Geoff please use the "reply" button not the "quote" button
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[davelx]

Dear Moderator 11 - your posts are wasting a lot more space than his quotes.