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?

 

[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

 

[LankyWill]

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.

Sorry David i forgot what you said was gospel. Please except my sincerest of apologies as i forgot to remember to check your methods before thinking for myself...

 

[Dave OCD]

The only thing which bothers me is that over the years I've discovered many cases of seriously rusted armour under shrouds in outdoor situations and this with proper external type glands. In these cases the impedance has been high enough to render the armour useless as a protective conductor, this is usually in coastal areas where the salty air will corrode most things over time so in certain situations I feel better having that third core.

 

[davesparks]

Sorry David i forgot what you said was gospel. Please except my sincerest of apologies as i forgot to remember to check your methods before thinking for myself...

Not gospel at all, and I'm am fully in favour of thinking for yourself!

But throwing lines like 'my mentor always said' in to a technical discussion without any technical support for it and when a pretty definitive proof has been provided against that opinion (see darkwood's link above) isn't thinking for yourself!

My mentor always said that anything over 16mm had to have an additional cpc run with it, he also said that plastic CUs are for diyers. But that doesn't help the discussion at all.

 

[davesparks]

The only thing which bothers me is that over the years I've discovered many cases of seriously rusted armour under shrouds in outdoor situations and this with proper external type glands. In these cases the impedance has been high enough to render the armour useless as a protective conductor, this is usually in coastal areas where the salty air will corrode most things over time so in certain situations I feel better having that third core.

The third core won't prevent the armour from rusting, and once that has happened the cable is a write off and no longer safe for use.
Shrouds are a menace in any damp situation as they trap moisture. Obviously the ideal solution is to not have terminations outside wherever possible but a good quality Five part gland with adhesive line heatshrink is much better than a shroud these days.

 

[b55.v2]

"think i'll price it on using swa for cpc"

maybe take a Zs / R1R2 reading before doing that......

especially if the cable run cannot be visually inspected for its entire length......

 

[uksparks]

I like adhesive lined heat shrink

 

[uksparks]

maybe take a Zs / R1R2 reading before doing that......

especially if the cable run cannot be visually inspected for its entire length......

Yes indeed, I'll knock a rough price together for him based in certain things with conditions attached.

 

[shanky887614]

The third core won't prevent the armour from rusting, and once that has happened the cable is a write off and no longer safe for use.
Shrouds are a menace in any damp situation as they trap moisture. Obviously the ideal solution is to not have terminations outside wherever possible but a good quality Five part gland with adhesive line heatshrink is much better than a shroud these days.

should be using adhesive lined heatshrink shrouds outside.

shrouds are to keep the armour clean not dry

 

[davesparks]

should be using adhesive lined heatshrink shrouds outside.

shrouds are to keep the armour clean not dry

There's a lump of brass and a rubber seal between the armour and the shroud!

 

[shanky887614]

There's a lump of brass and a rubber seal between the armour and the shroud!

true but a lot dont fit bw glands outside they just use cw