Hello

I have a 20mm, 4-core armoured cable running off a spur from the house ring main. Before it exits the house it passes through a fused (13amp), RCD (30 mA) spur unit. The core colours are: red, black, blue and yellow.

At the other end, my multi-tester indicates that the red and black wires show 240v and the red and blue 120v.

The previous owner of the house used this cable for a couple of sockets and lights for an out building. I would like to do the same. (I do understand that I will have a max 13 amp load.)

Could you please advise me how the wiring should be configured in the outbuilding with the scenario as described above? There's no plumbing or metal areas in the building. I have carefully read the new DIY wiring legislation and it seems I'm permitted to tackle this kind of job. I'm not daft but not a technical wizard either – so be gentle!

Many thanks!

Hey Trotty,

Need to re check that armoured. It won't be 20mm (size indicates the area of the conductor not the overall cable) it'll most likely say 4 x 1.5, 4 x 2.5 or 4 x 4 mm embossed on the side of the sheath. The later number being the cable size. The size, combined with the overall length from its origin (First consumer unit in the house) will limit what you can safely draw from it.

The voltages you are measuring could mean a few things... Most likely are that either the blue conductor is connected to a load at one end (lights ?) and the 120 volts you are measuring between red and blue are through the filament of a lamp or something, alternatively theres a neon indicator or such like in the circuit - built into an isolation switch or connection unit.

Really need to figure these out before we can move on to new connections, system earthing and the 'nuts and bolts' of it.

Post up those details and we'll see what can be usefully added.

Boring bit.
Strictly speaking the new installation does fall within the scope of part P, that combined with the way you've described the existing wiring system suggests in may be outside of your competence. Whoever does the work (you or competent contractor) needs to ensure that is suitably designed and installed and then correctly tested and certified using the appropriate kit.

Paul

No worries Goodsparks  

What is currently connected at the far end of the cable?  Describe the connections.

Can you not see at the origin of this cable which cores are connected to what?

Earthing Type

Seeing as you already have an RCD protected and fused-down electrical supply to the garage in an armoured style cable, you already have basicaly what you need. Do I assume that the sockets and lights in the outbuilding are no longer there, hence your query as to how to wire internally the shed?

Quote:


That's right. Although the previous installation worked fine, I obviously I want to make ABSOLUTELY sure that:
a) The PD of 120v I'm getting across the red and blue wires is not significant and the blue is redundant (and can be taped off)
b) That the earth (yellow) wire is valid/connected

I could just trust that the previous installation was all logically done (as it seems to be) i.e. red-live, black-neutral, yellow-earth and blue-unused. But I want to be confident before I do the job and get it inspected. I'll try Lectrician's suggestion to take a reading with the CU switched off if this will lend weight to the idea that the yellow is a valid earth etc.

Many thanks again!

Howard

Quote:

It depends on what type of earthing arrangement you have got, which is why we asked you to check what you have. Some earths can be exported, by earthing the armouring in the property and shed (using the ring/tab (banjo) and the earth wire (yellow wire in your case)). Some can't be exported, so you would connect the yellow in the house and the armour using the ring/tab (banjo) to earth but not connect them to anything in the shed, then you whack in an earth rod at the shed and connect all your local earths to the rod (keeping the house earth and shed earth separate).

TN-S can be exported without issue.

It is important to ensure the armour is earthed at the supply end, which by doing the test you did, we can fairly safely assume it is.

It is good practice to also take a fly lead from your armours 'banjo' clamp or 'Piranha nut' to the earthing terminal of what ever the SWA is supplying, as well as your yellow core (which you will be sleeveing with green/yellow sleeve).






Edited: