UK Help / Advice required

[pdab09]

As an old time City & Guilds electrician who last worked as such back in the1960s I realise I’m a bit stone-age to say the very least. That said my daughter just installed a decent shed in the garden of her new home and rather conveniently there is an un-made SWA cable laid from the house distribution board to the shed and I’ve been asked to get it all up and running for her. From her pictures of the loose cable I think it’s 2 core 4mm SWA but I’ve not seen it as she lives a considerable distance away (this will be a long weekend trip / job). Now I intend to install a small, metal consumer unit with an RCD, two lights and a 2-gang 13A socket outlet on MCBs. I’ll most likely use plastic conduit and metal clad fittings. Where I’m a little uncertain is where I pick up the mains feed from the house. My daughter can’t see the SWA cable and I’m guessing it tucked away in the house distribution unit. Never used a RCBO but I was thinking of an RCBO and a DIN rail, double pole isolator switch. Also, believe the household distribution unit comes under Part P – is that correct. Any pointers / advice appreciated.

 

[westward10]

I would be dubious about connecting this cable without knowing it's installation history, I assume it is underground. If you do decide to use it then it needs testing for insulation resistance and continuity.

 

[Julie.]

As an old time City & Guilds electrician who last worked as such back in the1960s I realise I’m a bit stone-age to say the very least. That said my daughter just installed a decent shed in the garden of her new home and rather conveniently there is an un-made SWA cable laid from the house distribution board to the shed and I’ve been asked to get it all up and running for her. From her pictures of the loose cable I think it’s 2 core 4mm SWA but I’ve not seen it as she lives a considerable distance away (this will be a long weekend trip / job). Now I intend to install a small, metal consumer unit with an RCD, two lights and a 2-gang 13A socket outlet on MCBs. I’ll most likely use plastic conduit and metal clad fittings. Where I’m a little uncertain is where I pick up the mains feed from the house. My daughter can’t see the SWA cable and I’m guessing it tucked away in the house distribution unit. Never used a RCBO but I was thinking of an RCBO and a DIN rail, double pole isolator switch. Also, believe the household distribution unit comes under Part P – is that correct. Any pointers / advice appreciated.

You will need to fully test the cable prior to anything, if it's OK, don't use a RCBO use a MCB in the cu if you can.

If not, you need to split the tails and take it into a kmf and feed the shed from that.

Personally I would use RCBOs rather than a RCD and MCBs in the shed.

The only reason to use a RCBO at the consumer unit is if the Zs is too high for the MCB and you need the RCD aspect for fault protection. (In that case you would need to have a kmf first, then an RCD - if you can't get the RCBO in the cu)

Given the load, and cable size (4mm) you should be able to select a MCB rating to suit the load and Zs.

If it's in England/Wales yes it comes under part P.

 

[westward10]

An RCD at source can be omitted if ADS is satisfied without it.

 

[Simon47]

If it's 2core then that means using the wire armour for earth - or you TT the shed, or you need to run a separate earth.
Assuming disconnect times can be met, it doesn't need RCD protection - meaning MCB at source end and RCD can be fitted at load (sged) end where it's more convenient to reset. For discrimination, consider a fuse. Or, you might just omit fuse/MCB for socket (use RCD socket) in shed and just fuse down the lights (could use a switched FCU.)

"Part P" is irrelevant ! What matters is wgether the work is notifiable. If you are lucky then it might be possible to not create a new circuit - e.g. there might be a circuit you could "modify". If that's not the case, e.g. you need to add an MCB, then you either need to notify the local authority building control (LABC) dept. before starting work, or you need to defer the work to someone who can register via their scam.

 

[westward10]

If it's 2core then that means using the wire armour for earth - or you TT the shed, or you need to run a separate earth.
Assuming disconnect times can be met, it doesn't need RCD protection - meaning MCB at source end and RCD can be fitted at load (sged) end where it's more convenient to reset. For discrimination, consider a fuse. Or, you might just omit fuse/MCB for socket (use RCD socket) in shed and just fuse down the lights (could use a switched FCU.)

"Part P" is irrelevant ! What matters is wgether the work is notifiable. If you are lucky then it might be possible to not create a new circuit - e.g. there might be a circuit you could "modify". If that's not the case, e.g. you need to add an MCB, then you either need to notify the local authority building control (LABC) dept. before starting work, or you need to defer the work to someone who can register via their scam.

You can utilise the armour as the cpc.

 

[Simon47]

You can utilise the armour as the cpc.

As I wrote : "that means using the wire armour for earth". But you have to determine the effective CSA and design the circuit accordingly.

 

[westward10]

As I wrote : "that means using the wire armour for earth". But you have to determine the effective CSA and design the circuit accordingly.

The chances of the armour not being adequate for use as a cpc are pretty remote.

 

[Simon47]

Oh yes, for good measure are there any extraneous conductive parts in the shed ? If there are then things get "a bit more interesting" design wise.

 

[pdab09]

Oh yes, for good measure are there any extraneous conductive parts in the shed ? If there are then things get "a bit more interesting" design wise.

Yep - just found out it's an aluminium panel shed! Guess that'll have to be bonded?

 

[Julie.]

Yep - just found out it's an aluminium panel shed! Guess that'll have to be bonded?

Given that, it probably makes sense to TT the shed, unless you can be sure it doesn't act as an extraneous-conductive-part. (The SWA won't have sufficient cross section for ECP bonding.)

Just being metal doesn't necessarily mean that it could introduce a potential from outside, it could merely be a piece of metal (we don't bond cutlery for example), although likely to be a Exposed conductive part, so may require bonding, but not needing such a large conductor cross section.