Improving existing overhead supply to garage

[DHJ]

Hello, this is my first posting here though I've been browsing for a while. At my mum's house there's an overhead supply to a detached garage. The cable is deteriorating and I'm looking to replace it and improve the installation. I have read numerous posts about garage/shed installations and these have answered most of my questions but I'd appreciate advice on a few points.

Existing Installation
2.5mm^2 T+E wired directly from socket in kitchen, through wall, up & across to garage at 2.4m height, 3.2m span on catenary wire. Enters junction box in garage, one side to two 13A sockets, other side to striplight via switch. The sockets are used for charging rechargeable tools and very occasionally for a hoover or drill. The garage has block walls and a metal up & over door. There are no metal utility pipes.

I see the following problems with the existing installation:
No isolator or RCD for overhead cable (the window cleaner clonked it with a ladder once!)
No isolator, RCD or overload protection for garage.
No overload protection for lighting circuit.
No UV protection for outside cable.
No mechanical protection for overhead cable.
Overhead cable should be 3.5m or higher.

Supply earth type
Not sure whether it's TN-C-S or TN-S - see photo:

Plan A
The first-choice solution would presumably be a new RCD-protected circuit from the house CU with underground SWA and a garage CU, but the cost and disruption would be out of proportion to the limited use that the circuit gets, she'd rather disconnect and do without.

So I'm looking for a DIY alternative that's safe, adequate, legal and won't get flagged as a problem at future house surveys. I'm thinking along the following lines:
Plan B
RCD FCU from kitchen socket.
Either Hi-Tuf overhead from FCU to garage, or 3-core SWA overhead from FCU to garage via suitable weatherproof box.
In garage, to DP switch (metalclad or via metal box if using SWA), then to sockets 1 & 2 on radial circuit with switched 5A fused spur from socket 1 for light.

Questions
(1) Does Plan B seem sensible, given that Plan A is ruled out?

(2) Would the work be notifiable? I'm hoping it's covered by 1 (a,b) (replacing equipment & damaged cable) but would the RCD FCU count as new equipment?

(3) Hi-Tuf or SWA - TLC say both have UV protection and both are suitable for overhead supply supported with catenary wire. I haven't worked with either before. I get the impression from some posts here that SWA is tricky, but if it has clear advantages then I'll learn how to use it. I found this learn-by-your-mistakes topic: Use/purpose of Gland when installing garage consumer unit.

(4) Cable CSA - the TLC calculator suggests 1.5mm^2 would suffice for up to 20A on an 8m run, and this would be a 13A spur. I thought I'd be needing 2.5mm^2.

(5) Terminating cable at house end - where the cable exits the house wall, if I use SWA I'll need a weatherproof box. Would this 20 Amp Weatherproof Junction Box be suitable?
Can I fit the box directly over the hole and run T+E through the wall into a hole in the back of the box? Or do I need to run a weatherproof cable through the wall into the box via a gland - if so, what cable & gland would be suitable?
This is where I'm thinking Hi-Tuf would make life easier, run it from the socket through the wall and up & over to the garage. The 2.5mm^2 has a minimum bend radius of 66mm though - that's an ugly loop coming out of the wall. Also not sure how to protect it from being yanked out through the wall - does that RCD FCU have a cord grip?

(6) Terminating cable at garage end - for SWA could I terminate in metalclad DP switch or should I use a separate metal adaptable box & run the cores through to the switch?

(7) SWA armour earthing - earth at one end or both? (I'd be using 3-core SWA, one core as earth). Use M4 brass bolt through banjo, box & hoop/eyelet crimp, is that right?

(8) Testing - I have a multimeter and a neon screwdriver (I know...) Is there other worthwhile affordable test equipment, e.g. a plug-in socket tester?
I'll try to work through the relevant and practicable parts of the testing section of the online Whitfield book.
What's a safe way of testing RCD protection?

Many thanks for your time and patience.

 

[didthathurt]

That's a TN-C-S supply.

You could do as you propose. Hi-tuf is fine for an overhead catenary as yes, SWA does take some practise to work with and your post indicates you have no idea about how to do so (that external JB is so not useable as is for SWA). Overhead cables do not, in themselves, need RCD protection, it's the sockets in the garage that do. And you are correct, lights, when run from a circuit supplying sockets, should be run through an appropriately fused FCU (3A is normally reasonable).

This is, however, most certainly notifiable. Note f of table 1 of Approved Document P states, in relation to the sort of work you are proposing;

"Only if the existing circuit protective device is suitable and provides protection for the modified circuit, and other relevant safety provisions are satisfactory".

You are proposing to change the circuit cabling on a circuit which has inadequate protection and contravenes other safety provisions. If your kitchen/downstairs sockets are not RCD protected at the CU then, as part of this work, you really should be upgrading that level of protection to bring the installation up towards current standards. I don't think you can try to sidestep that one as you could not justify just replacing what you have as it is such a deficient arrangement. What you are actually proposing is to rip out the old supply and provide a new, upgraded supply.

I know it always sounds a bit harsh when it's said, but you really need to consult a pro locally to examine what you have overall to advise on what work should be carried out and either a self-certifier or LABC should be involved.

 

[holmslaw]

..

 

[DHJ]

holmslaw - thanks for the link, I already have a copy of that article & the garage end of my Plan B is based on it.

That's a TN-C-S supply.

Thanks. Does that impose extra requirements on this job?

SWA does take some practise to work with and your post indicates you have no idea about how to do so (that external JB is so not useable as is for SWA).

I'll read that first bit as "welcome to the forum." Are TLC misdescribing that box when they say "Ideal for external jointing of flex , Hituf and 3 / 4 core SWA cables"? What would be a better alternative?

Overhead cables do not, in themselves, need RCD protection

Yes I'd gathered that this isn't mandatory, I was thinking though that it could provide valuable protection e.g. if next time the window cleaner catches the cable with a sharp edge of an aluminium ladder.

This is, however, most certainly notifiable. Note f of table 1 of Approved Document P states, in relation to the sort of work you are proposing;

"Only if the existing circuit protective device is suitable and provides protection for the modified circuit, and other relevant safety provisions are satisfactory".

Thanks for your comments on this. The points that note (f) relates to are:

Adding lighting points (light fittings and switches) to an existing circuit
Adding socket-outlets and fused spurs to an existing ring or radial circuit

I don't think my Plan B involves doing either of those, though you could argue over the semantics: does putting a fuse into an existing unfused spur count as adding a fused spur? The intention of those points seems to be to cover extending existing circuits, which I wouldn't be doing. But I accept that it's debatable whether my plan is covered under 2B 1(a,b) (replacing equipment & damaged cable) so I may have to accept that it's notifiable.

You are proposing to change the circuit cabling on a circuit which has inadequate protection and contravenes other safety provisions. If your kitchen/downstairs sockets are not RCD protected at the CU then, as part of this work, you really should be upgrading that level of protection to bring the installation up towards current standards. I don't think you can try to sidestep that one as you could not justify just replacing what you have as it is such a deficient arrangement. What you are actually proposing is to rip out the old supply and provide a new, upgraded supply.

What I'm proposing is replacing a damaged cable and inserting two fuses, an RCD and a DP switch into an existing spur. I'm intending thereby to put right the inadequate protection. Are there other safety concerns that you believe need to be addressed, that have not already been mentioned? I'm here to learn.

Are you saying that all downstairs sockets must be RCD protected at the CU?

If my Plan B were implemented competently and any required notification & inspection undertaken, would you still consider it a deficient arrangement and if so, in what ways?

Thanks again for taking the trouble to reply.

Dave

 

[didthathurt]

Definitely notifiable. There is legal precedent that states that any regulation which is intended to be restrictive must be read in a restrictive manner. Part P is intended to restrict the amount of work done outside of it's notification requirements so, if there is a semantic debate it must fall on the side of notification.

You are adding a fused spur in the garage for the light and you are effectively removing the unfused spur and replacing it with a fused spur.

The JB would be OK for the cores of SWA but SWA must be terminated via a gland attached to an enclosure within which that JB would be mounted.

The regs say all sockets likely to supply outdoor equipment must be RCD protected - in practise this is interpreted as all downstairs sockets. You could replace the ones near the doors with RCD sockets and label the rest as not for use with outdoor equipment. When I referred to a deficient arrangement I was referring to the garage supply and this aspect, not necessarily the whole installation.

Your plan B would correct the situation re the garage supply, a pro spark would need to look at the whole circuit not just that spur though.