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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> 4-core armoured cabling https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1247503012 Message started by Trotty on Jul 13^th, 2009, 5:36pm

Title: 4-core armoured cabling
Post by Trotty on Jul 13^th, 2009, 5:36pm

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!

Title: Re: 4-core armoured cabling
Post by Zambezi on Jul 13^th, 2009, 5:56pm

Firstly welcome to ATT
You are not allowed to tackle wiring outside without notifying building control or using a Part P registered competent person. (formalities out of the way).
The armoured cable will not be 20mm 4core, I guess you have measured the outside diameter of the whole cable? Normally cable is rated/sized by measuring the cross sectional area of a cable. it is probably 2.5mm sq. 4 core if it is only 20mm in diameter. Sometimes the size of the cable is printed on the cable, if you have a look it may say something like 2.5MM SQ x 4C PVC and the manufacturers name.

You mention getting voltages of 240V and 120V, how are you measuring these? Are you testing between red and black cables to get 240V or red to earth, black to earth etc?

How far is the shed from the house?

What are you planning on supplying in the shed?

Do you know what your earthing arrangement is at the main intake (fuse)? I think there is a guide somewhere on here to identifying if it is TT (earth rod), TN-C-S (PME), TN-C, TN-S.

More answers when you get back to us with more info.

Title: Re: 4-core armoured cabling
Post by Goodsparks on Jul 13^th, 2009, 6:07pm

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

Title: Re: 4-core armoured cabling
Post by Goodsparks on Jul 13^th, 2009, 6:07pm

Sorry Zamb. ;D

Title: Re: 4-core armoured cabling
Post by Zambezi on Jul 13^th, 2009, 6:36pm

No worries Goodsparks 8-)

Title: Re: 4-core armoured cabling
Post by Trotty on Jul 13^th, 2009, 10:49pm

Hello fellers

Many thanks for getting back so soon. You're right - I was approximating the cable diameter thinking that might throw a clue! The cable says 4 x 1.5sq stranded and there is a 600/1000v rating too.

The voltages I recorded were taken with a Weidmuller Master check 3.2. I'm not sure how to say it differently to the first post but I applied one 'probe' on the black wire the other on the red and it showed 240v. I then applied one 'probe' on the blue wire the other on the red and it showed 120v.

In terms of what is to be supplied in the shed – a couple of lights (60w) and a computer, CD player possibly a small fridge.

The shed is about 80 feet from the house.

I afraid I can't say for sure what the earthing arrangement at the main intake is but the house appears to have a pretty straightforward (apart from this cable!) up-to-date system if that is any indication of the earthing method.

Thanks very much again for your warm welcome and input.

Howard

Title: Re: 4-core armoured cabling
Post by Lectrician on Jul 14^th, 2009, 7:13am

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

Title: Re: 4-core armoured cabling
Post by Trotty on Jul 14^th, 2009, 12:23pm

I can access the RCD unit and the socket next to it but the armoured cable comes in behind a fitted unit which I'd have to break to remove. It's hard (for me) to tell exactly what the arrangement is but I have attached a couple of pics in case that helps. It's probably more obvious to you what's going on.

Just FYI - when the cable at the outbuilding was attached before, the socket and light there worked fine...

[gallery]Howard_Ely/1247571889.jpg[/gallery][gallery]Howard_Ely/1247571770.jpg[/gallery]

Title: Re: 4-core armoured cabling
Post by LS on Jul 15^th, 2009, 1:35am

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?

Title: Re: 4-core armoured cabling
Post by Lectrician on Jul 15^th, 2009, 7:20am

It is still difficult to see what they have used the yellow and blue for, if anything.

Can you turn OFF your consumer unit, and then test between the yellow and blue of the SWA to the armour of the SWA on the OHM setting on your meter? If you have very low ohms between the yellow and the armour, we can pretty much say that the yellow is a supplementary earth, and the blue is likely un-used.

If there is a high or no reading, then the yellow and blue are either unused, or connected to something else where the SWA enters the building. Ideally we need to see where this SWA is terminated in the building.

Title: Re: 4-core armoured cabling
Post by Trotty on Jul 15^th, 2009, 10:19am

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

Title: Re: 4-core armoured cabling
Post by Trotty on Jul 18^th, 2009, 9:26pm

Quote:

Can you turn OFF your consumer unit, and then test between the yellow and blue of the SWA to the armour of the SWA on the OHM setting on your meter? If you have very low ohms between the yellow and the armour, we can pretty much say that the yellow is a supplementary earth, and the blue is likely un-used.

If there is a high or no reading, then the yellow and blue are either unused, or connected to something else where the SWA enters the building. Ideally we need to see where this SWA is terminated in the building.

Yes - I've done this now. There's a very low OHM reading between the yellow and the armoured shield (0.004) and a reading of 1.0 between the blue and the armour. I'd conclude that the earth is probably valid and the the blue is probably redundant. Would you agree?

When wiring the cable in the outbuilding with a gland, is it done to connect the armour tab connection to the earth?

Many thanks

Howard

Title: Re: 4-core armoured cabling
Post by Zambezi on Jul 19^th, 2009, 12:37pm

wrote on Jul 18^th, 2009, 9:26pm:

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).

Title: Re: 4-core armoured cabling
Post by Trotty on Jul 19^th, 2009, 1:10pm

Hello

I've checked the earthing system alongside your online guide and I'd be confident that my house follows the TN-S model. How does this affect matters now?

Many thanks

Howard

Title: Re: 4-core armoured cabling
Post by Lectrician on Jul 19^th, 2009, 3:47pm

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).

[edit]A quick note also in regards the forum software:

To quote a user, rather than click reply, you can click 'quote'. This places the users message in a box.

If you want to quote random bits of text, you can highlight the text and click the quote button on the post page (12th button along the top below the subject box) - right click the button for info on use.

Alternatively, just place the tags in your post {quote}your quoted text{/quote}. Replace the {} with [].[/edit]

Title: Re: 4-core armoured cabling
Post by Zambezi on Jul 19^th, 2009, 7:23pm

Lectrician wrote on Jul 19^th, 2009, 3:47pm:

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 'pherana 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).

[edit]A quick note also in regards the forum software:

To quote a user, rather than click reply, you can click 'quote'. This places the users message in a box.

If you want to quote random bits of text, you can highlight the text and click the quote button on the post page (12th button along the top below the subject box) - right click the button for info on use.

Alternatively, just place the tags in your post {quote}your quoted text{/quote}. Replace the {} with [].[/edit]

Thanks Lec ;D ;D