DC run in SWA

[saint]

We need to run two strings from the roof via the loft space straight down the outside wall of the house and back inside under the stairs where the client wants his inverter. We want to use 4 core 4mm SWA for this outside run so ensure mechanical protection. I can't see anything that prevents this in BS7671. But i'm not sure about the actual steel armour. If we run AC we need to earth the armour but this can't be done for the DC.

Any thoughts ? Please!!

 

[fiardor]

Mechanical protection is different from earthing protection.

You dont need to earth the swa if you are running dc buddy.

 

[saint]

surely the sheath is earthed hen running AC in case the cable is damaged and the sheath becomes live from contact with the live cable within it? so whats the difference with DC?

 

[BruceB]

There is no problem earthing it - just connect earth to the armour at the inverter end. If you have a spare core in a cable you are meant to terminate it properly and take it to earth. I would treat the armour in the same way.

 

[saint]

it wouldn't disconnect the DC though. Am i missing something ??

 

[fiardor]

No saint you aint missing a thing.

Your original post asked about mechanical protection for dc which is fine.

If you earth the swa sheathing at the origin it will not protect against electric shock should the swa be cut or compromised in some way.

Ac earthing of swa is a different kettle of fish.

So the only protection you can provide for in dc swa is mechanical not electrical.

 

[saint]

ok my op was a bit mixed up. I just thought the armour could become live and not disconnect. I'm surprised this is allowed.

 

[moggy1968]

it will only become live if you cut both the +ve and -ve and then both come into contact with the armouring, but even then it will probably only affect that section of armouring as it easier for the current to then flow back through the cables rasther than through your shovel. Dc doesn't fault to earth like AC, it has to find it's negative.
Thats why when your stripping cable you can hold the positive or negative end without getting a shock, but not both at the same time as you will!!

 

[fiardor]

it will only become live if you cut both the +ve and -ve and then both come into contact with the armouring, but even then it will probably only affect that section of armouring as it easier for the current to then flow back through the cables rasther than through your shovel. Dc doesn't fault to earth like AC, it has to find it's negative.
Thats why when your stripping cable you can hold the positive or negative end without getting a shock, but not both at the same time as you will!!

LoL what he said....:45:

 

[anthanth]

25mm conduit down the side of the building, use 25mm copex to do any tricky corners.... Nice and easy!!!!!

 

[moggy1968]

yep, thats what we always do. looks far neater than SWA

 

[Gavin A]

yep, thats what we always do. looks far neater than SWA

we mostly do it with SWA as it's more hard wearing, and the installs are meant to be there for 30-40 years+, plus I like the safety of mechanical protection in areas like side passageways where 40 years of people carrying spades etc gives a fair risk that someone at some point could smack a spade or something similar into the conduit.