swa

[switched on]

I've always used a 3 core armoured so it has its own internal earth. I always earth up the armoured to use the steel wire with a gland and banjo but i worry that if i don't use a 3 core and just a two core and the steel armoured as the earth is this ok?
Obviously safety is the key feature but when can you use the steel and just run a two core?? Just a funny subject i feel. Thanks alot

 

[pevvers]

Check guidance note 1, they have tables in there for all the different types of armoured cable which show which ones satisfy the adiabatic equation. Simples!

 

[switched on]

ideal i will check that out! just wandering if everyone else uses a seperate earth or just the armoured itself?? thanks

 

[pevvers]

I think a lot of people use it as they're not sure whether they can use the armour or not. It's easier to get a three core and use the extra core as your CPC than wonder whether the armour is adequate or not. You're normally stuffed if you're putting in 3 phase supplies though, and have to run the extra CPC, as I've only come across a 5 core once in the last 7 years. That was the only good part of the installation too, the rest was pants!!

 

[Bromd123]

is there a definative annswer on this as my lecturer at college has said that the armouring WOULD be used as the CPC through glanding and banjo's is this right?

 

[allsparkjonny]

there is a table somewhere on the internet that tells you....its to do with the csa and number of cores in the cable itself,you can go to a decent sized cable before the armourings will not actually have a combined csa large enough to be able to be used as an earth and a seperate earth would have to be put in.sorry cant find the table at the moment but hope this helps.

 

[allsparkjonny]

I think a lot of people use it as they're not sure whether they can use the armour or not. It's easier to get a three core and use the extra core as your CPC than wonder whether the armour is adequate or not. You're normally stuffed if you're putting in 3 phase supplies though, and have to run the extra CPC, as I've only come across a 5 core once in the last 7 years. That was the only good part of the installation too, the rest was pants!!

the smaller 3 phase supplies you can use the armourings no problem at all.its only when you get to the bigger stuff the combined csa of the armourings is insufficient.

 

[cornburn]

I was always told it's fine to use, but would need to check with the manufacturer of the cable (or their literature) for an equivalent CSA. For the price difference on new installs I can't imagine it's ever worth it, just use a 3 core and then you've got 2 earths, even better!

 

[Bromd123]

I was always told it's fine to use, but would need to check with the manufacturer of the cable (or their literature) for an equivalent CSA. For the price difference on new installs I can't imagine it's ever worth it, just use a 3 core and then you've got 2 earths, even better!

I suppose it would only start to be a problem when you get to the REALLY big ones (costwise)

 

[pevvers]

the smaller 3 phase supplies you can use the armourings no problem at all.its only when you get to the bigger stuff the combined csa of the armourings is insufficient.

Ah yes, I know this. But many people put in a 4 core, then think that they have to run a separate earth as they don't know that there are tables available to show you whether the armour is sufficient for the CPC.

is there a definative annswer on this as my lecturer at college has said that the armouring WOULD be used as the CPC through glanding and banjo's is this right?

Tables D.9, D.10a and D.10b of GN1, starting at page 147 are where you need to look.

If you're using PVC or XLPE cables up to a max operating temp of 70 degrees C, then you can go up to a 95mm using the armour as the CPC before you have to start running in an extra CPC.

BUT! If you're using XLPE SWA at max operating temperature of 90 degrees C, then things get a bit more interesting. You would not be able to use the armour as a CPC on the following cables: -

2C 10mm
2C, 3C and 4C 16mm
2C 25mm
2C 50mm
2C and 3C 70mm
2C, 3C and 4C 95mm
2C and 3C 120mm
And from 150mm all the way up to 400mm are not allowed.

I hope this helps!

 

[Bromd123]

Pevvers your a legend matey thanks!

 

[benji]

I have often wondered what would happen in a earth fault condition ?would it travel down the armour or would it travel down the additional cpc, but we know that it would go down the path of least resistance so if the SWA can,t carry the fault current the additional cpc must be able to carry the full fault current .Iv,e known very highly qualified elec.designers add a 6.0mm cpc as a suplimentary cpc because due to length of 50mm 4core the armour was just wasn,t big enough add the 6.0mm and it was ok .I could never work out how a fault current would decide which to go down .


yours benji

 

[pushrod]

is there a definative annswer on this as my lecturer at college has said that the armouring WOULD be used as the CPC through glanding and banjo's is this right?

even if you are using 3 core swa with one of the cores as the cpc you still need to earth the armouring at the supply end. ( i believe it is because it counts as an exposed conductor)

So yes the armouring would be earthed through the gland and "banjo" even if it was not being used as the means of earthng.

 

[pevvers]

I've known very highly qualified elec.designers add a 6.0mm cpc as a suplimentary cpc because due to length of 50mm 4core the armour just wasn't big enough

I would tend to disagree with adding the extra 6mm in there for these purposes. I have always been under the impression that if the armour cannot take the full fault current, then the additional CPC should be able to take it all.

I could never work out how a fault current would decide which to go down

The current would split in respect of the differing impedances. More current would flow down the path of least impedance. Just think of resistances in parallel, Ohms law and Kirchoffs theorem... Although it's been quite a while since I've done current in resistors in parallel myself, it's not hard!

 

[Sintra]

Check out the link for swa as cpc.

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