Using armour as earth

[uksparks]

Hi,

ive been to look at a job today, used to be a pub a couple of years ago, now a house.

They have just gutted the kitchen and are going to refit.

There is s sub main (Henley blocks from head), 2 core 25mm SWA, run is approx 55 - 60 meters from head to board.

anyway, it needs a new board etc and a load of new circuits.

i just feel uneasy about using the steel wire on the SWA as the earth, I would rather have a 3 core or another separate earth.

ive not taken any readings as yet, only looked at it.

The only OCPD is the main fuse at present, I'd like a switched fuse put in.

what would you do regarding the earth? Would you use the steel?

 

[Leesparkykent]

I would install a switch fuse for the swa and nothing wrong in using the armour if the csa is adequate. Obviously if bonding is required to be connected to the EMT in the sub board then the CSA of the armour of the 25mm wouldn't be adequate.

 

[Pete999]

Providing the SWA is terminated properly I think you are safe using the Armour up to and including 95mm2 2 core SWA

 

[andyb]

As above, if it's suitable use the armoured.

 

[Leesparkykent]

It would make a difference if the swa was pvc or xlpe.

 

[Andy78]

It's part of its designed function, so yes no problem using it as a CPC.

 

[Pete999]

Providing the SWA is terminated properly I think you are safe using the Armour up to and including 95mm2 2 core SWA

I believe 25mm2 2 core PVC SWA PVC the armour is 60mm2

 

[davesparks]

Why do you think 2 core SWA is made if not for single phase circuits? Heck it's even got the right colour cores for the job, whereas three core is coloured for three phases.

The only reason to feel uneasy about using the cpc of the cable as the cpc under normal installation conditions is if you are incapable of calculating or terminating it correctly.

 

[Leesparkykent]

http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf

 

[Leesparkykent]

I think the only place 7671 states that the armour shouldn't be used is within a marina.

 

[Darkwood]

Here this says everything I want to on this matter

Use SWA as CPC, a guide to the acceptability of steel wire armour use as CPC.

 

[happysteve]

Here this says everything I want to on this matter

Use SWA as CPC, a guide to the acceptability of steel wire armour use as CPC.

Good article that, thanks.

I particularly like the tip about using two back nuts to sandwich the banjo when terminating in a plastic enclosure. Makes sense.

 

[uksparks]

There are water pipes, copper and heating pipes in the area where the board will be which are all copper. However, the water supply is bonded so would imagine it will be ok and not require further bonding.

its a bit of a pigs ear at the moment how it's fed as it's got Henley blocks off the head and the 25mm SWA is joined into a bit of 16mm twin and earth, so the actual CSA at present is at the weakest point which is 6mm on the twin and earth. So this needs doing properly through a switched fuse etc, no big issue.

i am more than capable of terminating the SWA properly, I don't mind using as such but prefer to use another core or separate earth.

My only query I suppose was what the equivalent CSA or the 25mm 2 core SWA equates to, Ive got a link somewhere, let me check.

 

[davesparks]

The tables in the above links should only not be treated as absolute or definitive. They only make use of the selection method, calculation by use of the adiabatic could still prove that the armour is suitable as a cpc in the larger cables

 

[LankyWill]

My mentor always said dont cheap out and get armoured with a cpc aswell. When it comes to future proof the extra moneys worth it. Can you run a 16mm earth along side the armoured?

 

[davesparks]

There are water pipes, copper and heating pipes in the area where the board will be which are all copper. However, the water supply is bonded so would imagine it will be ok and not require further bonding.

its a bit of a pigs ear at the moment how it's fed as it's got Henley blocks off the head and the 25mm SWA is joined into a bit of 16mm twin and earth, so the actual CSA at present is at the weakest point which is 6mm on the twin and earth. So this needs doing properly through a switched fuse etc, no big issue.

i am more than capable of terminating the SWA properly, I don't mind using as such but prefer to use another core or separate earth.

My only query I suppose was what the equivalent CSA or the 25mm 2 core SWA equates to, Ive got a link somewhere, let me check.

It's a pub installation, what do you expect it to be like???

It's going to be a slap dash mess that's been thrown in over the years and covered in stinking sticky yellow crap from the years of smoking in there.

Can you give a good reason why you prefer to not rely on the CPC designed in to the cable and instead add extra conductors. Especially considering that there is a strong theory that the majority of fault current will flow through the armour regardless of there being another conductor.

Do you also pull a g/y in to conduit or use 3 core MI for single phase?

 

[davesparks]

My mentor always said dont cheap out and get armoured with a cpc aswell. When it comes to future proof the extra moneys worth it. Can you run a 16mm earth along side the armoured?

It's not cheating out, it's using the cable the way it was designed and intended to be used.
Adding extra conductors outside of the cable is just an exercise in futility.

 

[uksparks]

As much as I would like to run an additional earth, it's a non starter, it's got to go a heck of a long way under floors, here there and everywhere, it really is a non starter.

 

[uksparks]

I'll price it up on using the armour I think.

 

[davesparks]

I'll price it up on using the armour I think.

You could always test it and calculate the CSA required and Zs requirements etc.
I know it's a crazy idea, but sometimes taking the technical approach actually works