Armoured Cable Query

[Dougie212]

Hi there.
We are having some work done in our garage and as a result the garage consumer box is going to be moved.

The box is supplied from our main consumer unit by a 3 core armoured cable. The steel strands of the cable have been cut short around where the cable enters the box. My friend seemed to think that the armour should be connected to earth in the garage box.
Before I start hunting for the number of the chap that fitted it and having a rant, should the armour steel be connected the earth in the garage unit or not? Thanks in advance.

 

[EFLImpudence]

Not necessarily.

Pictures would help.

 

[DetlefSchmitz]

It should be connected to earth at the source end (the main CU). At the other end the third core is providing the earth (I assume!).

Cutting the armour back is a bit rough though. It should be properly terminated in a gland.

 

[ban-all-sheds]

Before I start hunting for the number of the chap that fitted it and having a rant

Was he actually an electrician?

Cutting the armour back is a bit rough though. It should be properly terminated in a gland.

 

[Dougie212]

Thanks guys.... I'd read on google about it being terminated at the source end but it had sent me into a bit of a panic...
I may still give him a ring though as like you say, the armoured steel is just cut where he's trimmed the outer cover back. Looks a bit rough, if nothing else, as a non spark, I'd have maybe taped that cut so it at least looked a little better.

Knew the forum would come to the rescue..!!

 

[bernardgreen]

If the "Earth" for the garage electrics is exported from the Main Earth Terminal in the house then the armour can be used as the Earth in parallel with one of the cores and if so it can be connected at the garage end.

If the "Earth" for the garage electrics is derived from a ground rod and not exported from the house then the armour MUST be earthed at the house end and must NOT be connected to anything at the garage end.

 

[ban-all-sheds]

Did you see this week's Grand Designs?

 

[Dougie212]

... No.. Not yet..

 

[Lectrician]

If it was done neatly, not butchered, you would assume it had been done to divorce the earth. If so, pointless using 3 core.

This thread needs photos

 

[ban-all-sheds]

... No.. Not yet..

That was in reply to Bernard.

I know what he's getting at - potential difference between two points on the ground.

But this week's GD featured a house 60m wide, so I wonder how he would have people deal with a (presumably) single equipotential zone covering such a distance. The house had exposed structural steel, BTW.

 

[bernardgreen]

By ensuring all extraneous conductive materials in contact with the ground are bonded to the MET a 60 metre wide equipotential zone can be created. The size of the ground area involved would give reason to consider the sizing and current carrying capacity of the bonding cables. In a typical residential area normal bonding requirments would almost certainly be adequate. Close proximity to an electrified railway line, power station or a factory with heavy electrical loads may require additional work to ensure the equipotential zone inside the building is protected from steep potential gradients in the ground around and under the building.

It would have to be one equipotential zone covering the whole building. That said a very large building can be divided into two ( or more ) equipotential zones with maybe different potential to ground but there then has to be transition zones if people have to move from one equipotential zone to another. Transition zones are devoid of all conductive surfaces and wide enough that it is impossible to touch any conductive surface in any equipotential zone while passing through the middle of the transition zone.

A house that is not on a TT electrical supply is an equipotential zone ( referenced to the MET ) surrounded by another equipotential zone, namely the ground and all conductive items on the ground referenced to the ground. Hence the "earth" from the MET shpould not be taken into the equipotential zone that is the garden.

Provided the potential on the MET is maintained to be within a few volts of the true ground potential there is no real need fro a transition zone at the kitchen door into the garden. But if for any reason the potential of the MET rises to a dangerous high voltage then ? ? ? ( think about the out door water tap etc etc )

 

[ban-all-sheds]

So when you wrote this:

If the "Earth" for the garage electrics is exported from the Main Earth Terminal in the house then the armour can be used as the Earth in parallel with one of the cores and if so it can be connected at the garage end.

If the "Earth" for the garage electrics is derived from a ground rod and not exported from the house then the armour MUST be earthed at the house end and must NOT be connected to anything at the garage end.

did you know that the garage was more than 60m away?

 

[ericmark]

I remember working on a power station where armour was only ever connected at source that was what the design engineers had stipulated. Seemed rather rough to me but clearly it does not have to be connected. In fact in radio work often it is only connected one end.

Personally I would always use a gland and use SWA as well as copper for the earth if taking the earth from the source. However it could be garage is TT and does not use house earth.

So step one is read the minor works or installation certificate and see what that says before saying if correct or not.

 

[bernardgreen]

did you know that the garage was more than 60m away?

No I didn't....did you ? Or are you a bit confused by someone having mentioned a 60 metre wide house ?

 

[ban-all-sheds]

No - I'm confused by you saying on the one hand that if the earth for the garage electrics is derived from a ground rod and not exported from the house then the armour must be earthed at the house end and must not be connected to anything at the garage end, and on the other that a single (60 metre wide, although the specific size is not relevant here) equipotential zone can be created.

If the garage has a TT installation then the armour is an ecp, and maybe it should most definitely be connected to the garage earth terminal.