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Reconnecting Outbuilding Electrics Following Storm Arwen Damage.

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Raspberry197 | 10:03 Fri 12th Aug 2022 | Home & Garden
18 Answers
I'm new on here, I live in quite a remote area (highlands).
In 2016 I had a timber outbuilding (workshop) constructed to the rear of my house, approx 12m away.
Concrete base, walls bolted to a line of bricks, similar to a stable building.
I had a 5ft led strip light and an outside pir led spotlight fitted professionally at the time.
All were tested and written off by a qualified local(ish) electrician at that time.
I asked for a twin pole light switch to be fitted in my hall, so I could switch either of the shed lights on or off from within my house. No switches in the shed.
This involved a trench from my house to the shed, two black armoured (SWA) cables and an earth cable were led from my house to the shed.
Terminated in the shed at a Hagger consumer unit. The lights worked of a 6v circuit breaker, I also had a twin socket led from a 16v circuit breaker. The lights were wired with 1.5mm cable, the twin socket 2.5mm.
Because I wanted the only light switches to be in the house, the thinner of the SWA cables used was 4 core for switching.
The thicker of the SWA cables was the main supply and 3 core.
Anyhow, Storm Arwen flattened the shed with gusts well over 100mph last winter and I islolated the shed circuit from the house last November.
A few hundred hours later I have the shed, or most of it at least, reinstated, I had to replace about 25% of the timber walls, joists and onduline roofing panels.
Obviously nothing has been damaged or electrics disconnected within the house end of the electric circuit, and the SWA cables were strong enough to survive.
My question is this?
I don't want to bodge anything, but my electrician lives miles away and he is away from home just now working on a new build estate until November. I'm not sure where a couple of the switching wires from the 4 core SWA cable should go at the shed end.
I dont want to just 'have a go' and cause any damage so I'm looking for advice from any qualified electrician who might be able to help.
The 4 core swa colours are Blue (neutral), Grey L3), Black(L2) and Brown(L1) as it's a switched live circuit.
I've reconnected the 3 core SWA cable and the twin switched socket is working.
The 3 core enters the shed consumer unit from the bottom right, the 4 core from the bottom left and I also have a single earth wire, all led underground from the house.
I assumed the black on the 4 core swa was earth, as black was earth on the 3 core, but realised that it isn't.
So I have the brown (L1) leading to the top of the lighting 6V circuit breaker and three left over.
If Blue is neutral it may connect to the neutral bar? Leaving me with the Black and Grey.
I have renewed the 1.5mm cable runs to the led strip light and have swapped the spot light pir for an led bulkhead light outside.
I've sleeved both the earth wires from the twin and earth cables and put them onto the earth bar, the blues to the neutral, along with the 2.5mm lead to the twin socket.
So I now have the two live (browns) from the lights and the Black and grey from the 4 core swa cable to marry up somehow?
Having just joined to day I'm unsure if photos can be uploaded but have the existing twin pole switch wiring and photos of outside which may help.

I do realise that I should get a qualified professional to connect it, but due to my remote location, the fact the guy who installed it lives quite a distance away, and he is also working away from home until November, combined with the fact that it's not a new installation, I'm purely trying to reinstate what was correctly done and has been working without issues for the past five years. I'm a short distance away from reconnection at the moment. This is my final hurdle.

Apologies for the long winded rant, but it's all related to my current dilemma.

I'll look into how I attach some photos.. If I can, having just joined this morning.



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As I suggested above, the "fourth" wire in the 4-core SWA would be used as an earth. The switch does need an earth... not to make the switch work, but for safety in case a metal switchplate is used. Plus... an earth at the switch is better for testing the EFLI (earth fault loop impedance) The steel "armour" should be earthed too. A fault could develop where the...
17:27 Fri 12th Aug 2022
Question Author
Inside the shed..

Twin pole switch connections in house..

You can't attach photos but if you upload them to a photo hosting site then you can link to them.

That was a lot of info. Normally I'd advise as I've a little experience but no official qualification. But that was a lot and one probably needs the job if front of them to check things. I wish you luck getting a speedy reply from someone qualified.
Question Author
Twin pole switch connections in the house..

I wish I'd taken a photo of the shed consumer unit connections prior to the storm damage, then wouldn't have this problem ;)
Ah I see you replied before I finished typing :-)
Wait for The Builder to see your post
Question Author
Thank you Old_Geezer. It's basically two lights on a 6v CB with the switche in the house, instead of beside the light units.
It was expensive at the time, with all the extra armoured cable cost, but it had been working fine.
Question Author
Welcome, Raspberry.
If I were there, I'm sure I could trace everything easily.
I know these things are very difficult to describe. Pictures normally help, but in this case I'm afraid they make it worse.
The pics of the inside of the consumer unit are, to put it mildly, a bit of a cause for concern. None of it makes any sense.
I'm not saying it's unsafe...... just completely "unconventional."

I can understand the pic of the 2-gang switch in the house.
Brown... live feed.
Black and grey... switched LIVE feeds to the PIR light and the striplight (in the shed.)
(No neutral is involved here.)

What I would like to know is exactly how the shed supply (LIVE/Neutral/Earth) is taken down to the shed.
I guess the 3-core SWA is doing this, but where does that come from?

If so... the shed consumer unit should contain ...........
LIVE/neutral/earth from the 3-core SWA.
A 16A MCB (miniature circuit breaker) for the socket... with L/N/E going to the socket.
A 6A MCB for the lighting. L/N/E going to some kind of distribution to the two lights. (Junction box?)

The 4-core SWA would connect into this junction box to provide the switching arrangements in the house.
There are many ways of wiring this....... I've suggested just one of them.
Just a point of information, the Circuit Breakers are rated in amps, not volts. They trip when the specified current (in amps) is exceeded.
Having said all that........... I still can't figure why it's a 4-core SWA.
Switching should involve one LIVE plus two SWITCHED LIVES.
I am indebted to M'Lud for his clarification ;o)))
The Builder, perhaps the electrician who first installed the system only had 4 core cable but utilised it as 3 core?
Thanks Vulcan.
I think I've answered my own question now.
The BLUE must be acting as an EARTH.
The GREY, BLACK and BROWN are the switch wires.

I can only assume that since the pic of the switch shows normal 3-core, but there is no actual detail of the connection to the SWA.
Question Author
Thank you all for your help, it's much appreciated indeed.

It was my error referring to the CB's as 6V and 16V to start with.

They are indeed MTN 106 B6 (6 amps) for lighting and MTN 116 B16 (16 amps) for the twin socket.

The three core SWA cable, on the right supplies the power. I reconnected this as I could easily see that the black had been earth, brown live and grey neutral. As soon as this was reconnected along with the 16amp CB to the twin socket, it worked.
I'm sure the purpose of the 4 core swa cable is to take the power to the house switches then back again to the lights.

My problem lies with the switching to the 2 pole switch within the house, and reconnecting that from the shed end.

There was a twin joining terminal block inside the shed mini-consumer unit but the wires were torn from that when damaged.

Inside the house, both SWA cables enter below a 17th edition consumer unit and have their own CB there. They are terminated and led directly up to the twin light switch in the hall from there by the grey cable seen in the photo of the light switch.

I also think that the steel braiding around the armoured cable acts as an earth, when clamped to the steel mini-consumer unit in the shed. but there is also a separate yellow/green earth wire available, which was also snapped when damaged.
Belts and braces are fine by me though, if this is the case.

The suggestion of using 4 core swa cable, as no 3 core available is also a good call, meaning one would be dormant (unused).

There was no junction box, apart from one outside, where the bulkhead light was attached to its supply cable.

As I suggested above, the "fourth" wire in the 4-core SWA would be used as an earth.
The switch does need an earth... not to make the switch work, but for safety in case a metal switchplate is used.
Plus... an earth at the switch is better for testing the EFLI (earth fault loop impedance)

The steel "armour" should be earthed too.
A fault could develop where the armour might become LIVE.
If the armour is not earthed, then safe disconnection won't happen (the Trip/MCB will not operate...... and the shovel that hits it will become LIVE.)

In the shed consumer unit... a 1.00mm twin core (plus earth) should run from the unit to both lights. (From the same MCB if you like.)
This will supply the lights with L/N/E.
Use the lights as a junction box (inside each light obviously.)

Terminate the 4-core SWA with a junction box.
Run the BROWN together with the GREY up to one of the lights.
Run another BROWN (same connection) together with the BLACK up to the other light.

Inside Light 1 .......... connect BROWN to the BROWN from the consumer unit. This will take a LIVE feed back to the house switch.
Connect the GREY to the lamp. This will pick up a SWITCHED LIVE back at the house to fire the light up.

Inside Light 2 ... do the same... but this one will be BLACK.

Both lights will now have a supply of N and E ... from the shed consumer unit ... plus a SWITCHED LIVE to make it all go.

As I said above... there are a hundred ways of doing this. This is just one of them.

From what you say... connectors inside the consumer unit could possibly have been used as junction boxes.
You could do that again, or use the method I described.

Question Author

The Builder, thank you so much, that's way more detail than I expected to find, and explained precisely with it :)

I don't see the need to strip out or re-wire any part of the circuit or switches located inside the house, as they weren't damaged, and it actually appears to be making sense to me now.

The 3 core SWA cable is supplying the mains power, and that's been reconnected and is working the twin socket.

The earth cable that runs from the house along with the two armoured cables threw me, as I thought that would have been used as an earth for the 4 core cable. But I'll connect that to the earth bar at the shed end as it can't be anything else, as you have covered everything.
I'd thought the blue (4 core) would have been neutral, it being an earth hadn't occurred to me. I see now it's sleeved green and yellow into the top right of the twin pole switch though.

So, just to confirm, for the switched lighting... the 4 core armoured cable at the shed end should be connected as follows..
Brown (L1) to the top of the 6A MCB (where I'm sure it was before).
Blue (sleeved yellow and green) to earth.
Black (L2) to one of the light's brown live 1.0mm wire.
Grey (L3) to the other light's brown live 1.0mm wire.
Both light's blue wires to neutral bar and both sleeved earths to earth bar.

I think the twin block connector in the mini-consumer unit (shed) will have been to join Black to one light's brown wire and Grey to the brown on the other light. Which one will be determined by the side of the two gang indoor switch that has the grey or black on the bottom (switched side) of the switch.

I have run new 1.0mm twin and earth grey cable to both lights (it was 1.5mm before) but I thought 1.0mm could handle a single led light unit.
The cable to the twin socket (from the 16A MCB) was 2.5mm, this hasn't been replaced as it wasn't damaged.

Thank you once again for your time and the expertise which you've shared.

You're very welcome, Raspberry. Glad we could help.
Welcome to AB :o)
Raspberry....I have concern regards the swa termination at the shed DB...and the lack of circuit id within same. These terminations should be sleeved, or at least taped in the appropriate colours for L..N..E. As there is no 30ma RCD protection installed within the shed we can only hope there is protection at the source. My advice ..get it checked out by a registered sparks.This shed is clearly outside the equipotential zone and could could give you a nasty surprise under fault conditions.

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