Extend RFC to a detached building/garage/shed?

[Spoon]

You brainy guys may be able to help with this thought I was having.
There has been a thread recently about 4mm radial from ring main.

This got me thinking..... (No funny comments)
So my question is: If you had a building/garage/shed, say about 5 meters away from a house, is it acceptable to extend one of the RFC's to this building?

I know its best to have it's own circuit, but just for this conundrum lets say there are no spare ways in the CU.
Yes you could use a 13A fused spur, but what if you wanted to have a few appliances in there that would draw more than 13A.

You would also be within the 100m area as well.

 

[Pretty Mouth]

The cpc wont be a ring though will it?

Would that matter? Assuming cpc meets adiabatic requirements throughout the circuit, and is taken to every point in the circuit?

 

[Spoon]

Would that matter? Assuming cpc meets adiabatic requirements throughout the circuit, and is taken to every point in the circuit?

This is where I wasn't sure so suggested a 5 core, hoping someone would correct me if I was wrong.

 

[pc1966]

Regarding the isolation of the supply to the garage/shed, you could use a 4 pole changeover isolator. So in the 'off' position the garage/shed supply is isolated..

That is a good point as you don't want the ring opened to isolate! Problem is many are centre-off which is not what you really want.

This might be usable (4P with NO/NC contacts), but you would need a separate box and knob to complete it. at 25A it ought to match the 2.5mm cable's rating:

CA20A223/GBA001*FT | Kraus & Naimer, 4P 2 Position 60° Double Throw Cam Switch, 25A | RS

uk.rs-online.com

 

[Pretty Mouth]

This is where I wasn't sure so suggested a 5 core, hoping someone would correct me if I was wrong.

I'm not sure myself tbh, which is why i worded it as a question. Have to check the regs. I was led to believe that a ring could be run in conduit or trunking, using containment as CPC, where there's no need for it to make a ring. Would the same not apply in this case?

 

[Spoon]

That is a good point as you don't want the ring opened to isolate! Problem is many are centre-off which is not what you really want.

This might be usable (4P with NO/NC contacts), but you would need a separate box and knob to complete it. at 25A it ought to match the 2.5mm cable's rating:

CA20A223/GBA001*FT | Kraus & Naimer, 4P 2 Position 60° Double Throw Cam Switch, 25A | RS

uk.rs-online.com

https://cpc.farnell.com/europa-components/lbc324p/isolator-change-over-32a-4p/dp/MC02638

This says its a 4 pole changeover isolator but the data sheet doesn't???? Or I'm tired and missing something.

 

[pc1966]

This is where I wasn't sure so suggested a 5 core, hoping someone would correct me if I was wrong.

The problem with not having the CPC in a ring as well is for testing: you won't easily detect an open on the CPC and so a reduction in its fault capacity & reliability.

But you could run an external CPC with 4-core SWA and have the CPC loop through the armour and back via the separate CPC (or 5th core).
[automerge]1590442220[/automerge]

This says its a 4 pole changeover isolator but the data sheet doesn't???? Or I'm tired and missing something.

Centre-off! You don't want the ring left open.

 

[Pretty Mouth]

Aha. Here's the relevant reg:

543.2.9 Except where the circuit protective conductor is formed by a metal covering or enclosure containing all of the conductors of the ring, the circuit protective conductor of every ring final circuit shall also be run in the form of a ring having both ends connected to the earthing terminal at the origin of the circuit.

 

[Spoon]

Centre-off! You don't want the ring left open.

Ta mate.. Thought it was 2 position.

 

[telectrix]

Aha. Here's the relevant reg:

543.2.9 Except where the circuit protective conductor is formed by a metal covering or enclosure containing all of the conductors of the ring, the circuit protective conductor of every ring final circuit shall also be run in the form of a ring having both ends connected to the earthing terminal at the origin of the circuit.

so the armour of SWA is "a metal covering containing all of the conductors of a ring"?????

 

[pc1966]

so the armour of SWA is "a metal covering containing all of the conductors of a ring"?????

I would say so. And as I guess you need accessible junction box then you would have a means of checking the 2nd ring's CPC continuity.

But more importantly, what do we call this arrangement?

An hourglass circuit? It is a curious way of passing the time discussing it...

 

[Pretty Mouth]

so the armour of SWA is "a metal covering containing all of the conductors of a ring"?????

I'm not sure it would be. The reg does seem to back up Pete's post.

Would it be a reasonable departure from the regs to extend the ring to the outbuilding using 4C SWA, using armour as CPC? Again, assuming adiabatic, max zs, etc met. It would add an extra layer or 2 to the testing, but I don't think it would be less safe.

 

[FatAlan]

That is a good point as you don't want the ring opened to isolate! Problem is many are centre-off which is not what you really want.

This might be usable (4P with NO/NC contacts), but you would need a separate box and knob to complete it. at 25A it ought to match the 2.5mm cable's rating:

CA20A223/GBA001*FT | Kraus & Naimer, 4P 2 Position 60° Double Throw Cam Switch, 25A | RS

uk.rs-online.com

Very good point. You don’t want an isolator that in effect breaks the first part of the ring into two legs and cuts off the latter part.

 

[Lucien Nunes]

It would add an extra layer or 2 to the testing, but I don't think it would be less safe.

I think those aspects are connected. Once you have installed a non-standard configuration, other people might have to test, modify and repair it. If they don't understand it or make incorrect assumptions about it, they might not test it correctly (or at all, if they keep getting nonsense readings) hence it might become less safe.

Personally I would be disappointed to find a ring extended to an outbuilding simply for economy, however if the lengths and loading are sensible, I would accept it. There is a risk that if the garage run adds much length at one end of the ring, and is perhaps only occasionally used, most of the house loading would end up on one leg. But I also accept that it is very rare for a correctly-installed ring cable to be overloaded under normal operating conditions.

The 5-core idea is a bodge. There's no reason to do anything that weird with the CPCs when you could just use 7-core SWA and give both legs three cores. I would be tempted to connect the armour to the spare core at both ends but both to one CPC at one end only, so that the readings come out correct for equal-size conductors.

 

[littlespark]

To be able to isolate the shed, instead of spurring from a 13A fuse, could you spur from a garage board or enclosure, in the house, with maybe a 32A breaker? And going out in 4mm? The supply side coming from the rfc

Wouldnt look terribly great in the house though.

 

[Pretty Mouth]

I think those aspects are connected. Once you have installed a non-standard configuration, other people might have to test, modify and repair it. If they don't understand it or make incorrect assumptions about it, they might not test it correctly (or at all, if they keep getting nonsense readings) hence it might become less safe.

I hear what you're saying Lucien, though I believe the skills needed to recognise a circuit installed in this manner are quite basic. They would be the same skills needed to recognise and locate parallel paths across the cpc of a ring final. With it being documented as a departure, I would question the competence of anyone unable to understand it.