Armoured Cable

[JCTucker]

I am in the middle of sourcing some armoured cable to run around 175m in a trench to a barn at the bottom of a field.

One simple question: Do I run twin core, and use the sheath as the earth or do I need twin and seperate earth? We have PME.

Thanks for any help you can provide.

 

[Qedelec]

Run 3 core and use one of the conductors as Circuit Protective Conductor. Mark it with Green/Yellow tape/sleeving and connect to the sheath armour via the Banjo.

 

[Adam_151]

2 core, because you'll be makeing the remote building TT, connect the armour to earth at the supply end, but keep it isolated at the load end, use an earth rod combined with appropiate RCDs

 

[JCTucker]

Things are never simple - two different answers! Are both methods okay under the regs?

 

[Qedelec]

Now look what you've done Adam, JC doesn't know if he's on his butt or his bonce.
You'll need to quote the regulation that stipulates this. Or at least state the advantages of making the remote property TT.

 

[ban-all-sheds]

So, let's see...

125m of 25mm² - 0.091 ohms.

An earth rod - three orders of magnitude greater.

 

[JCTucker]

So whilst we are awaiting the answer on 2 or 3 core, another question!

I am not planning on running anything significant off the supply....a couple of lights, occasional power tool, and sometimes a heat lamp drawing 500W, and I am trying to avoid going to 25mm. Would a 16 amp circuit give me enough, in which case I think I can use 16mm, and be under the 4% drop threshold.....does this make sense?

Edited to say I need 175 meters

 

[Adam_151]

So, let's see...

125m of 25mm² - 0.091 ohms.

An earth rod - three orders of magnitude greater.

Thats true ban, but I thought you were dead set against exporting a PME earth?, at 125m, there is likely to be a reasonable potenital between the ground and the cable earth...

 

[ban-all-sheds]

So whilst we are awaiting the answer on 2 or 3 core, another question!

I am not planning on running anything significant off the supply....a couple of lights, occasional power tool, and sometimes a heat lamp drawing 500W, and I am trying to avoid going to 25mm. Would a 16 amp circuit give me enough, in which case I think I can use 16mm, and be under the 4% drop threshold.....does this make sense?

Edited to say I need 175 meters

I don't think it makes any sense at all. 16mm² will put you on the edge for volt-drop, so if anything ever changed the requirements in the barn you'd have to re-dig a 175m trench, and replace 175m of cable. IMO, 25mm² should be the absolute minimum to consider, and that would only let you go to 30A. I've no idea how the cost ratchets up, but in your shoes I'd certainly look at how much difference 35mm² would make to the overall project.

. . . . .

PME? PME??

I dunno - next you'll be telling me that I should read original posts properly..

Anyway - don't think I was ever dead set against it - it always seemed to me to be a topic rife with differing opinions and no consistent answers.

There's a good article about this in this quarter's Wiring Matters† - last time I looked it wasn't online yet. The key factor is whether or not the detached building contains any extraneous-conductive-parts (see this for a definition).

If not, exporting the earth is fine. If it does, then you should either make it TT, or extend the equipotential zone to the outbuilding by applying main bonding to the e-c-ps. It does seem perverse not to make every effort to keep a low EFLI if you can.

Q: - if you did go that route, would the use of 4-core SWA, with one core as the main bonding conductor be OK?

†Well worth subscribing to, BTW - free to anybody who asks http://www.iee.org/Publish/WireRegs/questionnaire_new_update.cfm

 

[ban-all-sheds]

The IEE Wiring Matters article is now on-line:


http://www.iee.org/Publish/WireRegs...matters_electrical_installations_outdoors.pdf
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(Mod - I did not do this as an edit, as that would not have bumped the post, or triggered email notifications to people watching it)

 

[plugwash]

i don't think theres any problem with taking the main bonding for the new building from the submains earth provided its of adequate size.