Garage supply

[Mabro22]

Does a new cable that’s supply a garage need to rcd protected at the consumer unit inside or can it be supplied off the non rcd side of the
consumer unit?

The garage will have a rcd consumer unit inside.

Any help would be appreciated. Thanks

 

[pc1966]

Depends on the cable type and how it is run.

If it is SWA then the earthed armour means no need for RCD protection for that cable.

Flexishield is a similar idea, but the aluminium foil layer has a modest limit for any MCB so it can safely disconnect.

If it is any other cable type it has to be such that it can't realistically be penetrated by accident, so not buried less than 50mm from any surface, and where outside either protected by duct/conduit or run in a manner where it is safe from any realistic risk of damage from gardening activities, etc.

 

[Mabro22]

Depends on the cable type and how it is run.

If it is SWA then the earthed armour means no need for RCD protection for that cable.

Flexishield is a similar idea, but the aluminium foil layer has a modest limit for any MCB so it can safely disconnect.

If it is any other cable type it has to be such that it can't realistically be penetrated by accident, so not buried less than 50mm from any surface, and where outside either protected by duct/conduit or run in a manner where it is safe from any realistic risk of damage from gardening activities, etc.

Thank you, I thought the same but just making sure ??

 

[telectrix]

unless it's TT earthing. then the cable will need RCD for fault protection.

 

[pc1966]

As @telectrix points out, if the supply is TT then you need some sort of RCD as a cable fault is unlikely clear otherwise. But in that case you ought not to have a non-RCD side in the first place!

 

[pc1966]

Just to add, you might have a non-RCD side (no 30mA instant RCD) but with a 100mA delay RCD incomer in a TT system.

So yes, it could be a little more complicated...

 

[nicebutdim]

Just to add, you might have a non-RCD side (no 30mA instant RCD) but with a 100mA delay RCD incomer in a TT system.

So yes, it could be a little more complicated...

Was just about to ask if upfront 100mA time delayed RCD would be considered suitable protection on a TT installation?

 

[pc1966]

Was just about to ask if upfront 100mA time delayed RCD would be considered suitable protection on a TT installation?

It would be OK for SWA cables, etc, that don't normally need RCD, as it would allow a cable fault to disconnect the supply even though the TT earth is not low enough impedance to trip the sub-main OCPD.

But it would not allow non-shielded cable types in vulnerable locations as if you nailed it the resulting metalwork would be capable of giving a fatal shock before the 100mA delay RCD disconnects.

 

[Mike Johnson]

Wouldn't the additional protection requirement then apply after the 100mA delay RCD, therefore additional 30mA RCD's will be required.

 

[nicebutdim]

Wouldn't the additional protection requirement then apply after the 100mA delay RCD, therefore additional 30mA RCD's will be required.

To be specific, I'd wondered if 100mA was suitable as upfront protection, with circuits from garage board being protected by 30mA RCCB.



Let's assume a TT installation where all circuits are afforded 30mA protected by RCBOs, with upfront time delayed 100mA RCCB providing additional protection. Would it be better to:

A) Protect armoured run to via MCB, with RCBO board at outbuilding end.

or

B) Protect armoured run with RCBO and use MCB protection at outbuilding?

My thinking leans towards the former, but I have insufficient knowledge to suggest that either method would be a better set up.

 

[pc1966]

Let's assume a TT installation where all circuits are afforded 30mA protected by RCBOs, with upfront time delayed 100mA RCCB providing additional protection. Would it be better to:

A) Protect armoured run to via MCB, with RCBO board at outbuilding end.

or

B) Protect armoured run with RCBO and use MCB protection at outbuilding?

The OP said they have a garage CU with RCD there, so I would be going for a MCB (or switch-fuse) to feed SWA so only the one 30mA "additional protection" RCD to trip/reset, and near where it is used.

Also slightly less leakage current as no long run of SWA on it, though I suspect that is trivial, probably only a few hundred pF per meter of SWA.

If the garage CU was a RCBO board then a leak tripping the sockets won't plunge it in to darkness by taking out the lights, though a hard fault would still probably take out the MCB feeding it (due to the poor selectivty of cascaded MCBs).

 

[pc1966]

What I would not want is the garage feed coming off the RCD-side of my home CU, as any fault out there (which is more likely due to water, etc) is going to take out my other home circuits.

Hypothetically, as I have an all-RCBO board. Also I'm in a 3rd floor flat so a garage supply is a bit unlikely!