Can we use the armour of an SWA as the CPC???

[somersetsparks]

Bryanh - I don't really get your post, 3 core and 5 core are available just not necessary most of the time. Using the armour as the earth is cheaper and more practical in most cases

 

[bryanh]

The point was some were saying they dont like using the SWA as a CPC, What I was saying was they could include a CPC within the SWA cable just as they do in T&E etc 2.5+1.5 not a full size CPC then you are covered both ways

 

[somersetsparks]

Ahh I understand, however it would be pointless - The armour provides this function, the only time it would be useful is on cables which dont meet the minimum CSA of armourings. I don't understand the covered both ways bit though - You are covered if you install a proper and continuous compliant earth, in most cases the armour provides this, in fact I don't see how people are happy to trust a bit 1.0 cpc in a screw terminal which has probably chewed it down to 0.5mm on a lighting circuit, but frown upon using the mass of armourings, terminated in proper glands? beats me

 

[johnboy6083]

the only time i wouldnt use Armour as CPC was a) i had lots of 3 core spare, b) it was going to be installed in an environment where if the sheath were damaged, excessive corrosion would cause failure iof the Armour as CPC, C) in a TNC-S scenario where i could see the armour carrying diverted neutral currents, i would assess this ti the best of my knowledge and then decide.

 

[KAS1]

Nothing wrong with using the SWA as the cpc, although most companies I've worked for over the years always use a three core for single phase and four core for motor cables so as to use the spare core as earth together with the armour.

 

[aleksandar]

Yes, you can use SWA armour as a CPC. If you use the adiabatic formula you can check if the armouring is suitable or not.

 

[MDJ]

Of course it can be used and will probably be adequate, have a look at the table engineer put up recently, however If I was doing the job I would run a 3 core rather than 2 core and use both the armoured and the third core together, and banjo the ends as well with fly leads to the earth terminals, I really cannot see the problem with this sytem and providing it is allowed for in the fixed price quotation why wouldn't it be a good idea not to use a 3rd core as well as the sheath?

Mike

 

[Engineer54]

Never a good idea to rely on the armour to make up any deficiencey in the size of a 3rd core CPC. Being as there is a big difference in K values and therefore will not divide equally to carry a fault current to operate a protective device. Where a 3rd core is employed as the CPC, it should be of a sufficient size to provide the Zs required, to trip the protective device and carry the full fault current of that circuit.

 

[MDJ]

The third core would probably be adequate in most circumstances of course, the sheath would be connected to it via a fly lead to the banjo for professional reasons, I can't imagine running in an armoured cable and not connecting it to an earthed point, so having both of them connected together at an earthed point would be normal and not to ensure a correct sized cable, as you said the cpc should sufficient in size, but they are bound to be connected together.

 

[SLSpark]

ARMOUR FOR EARTH OF COURSE THATS WHAT ITS THERE FOR(and underground protection etc) IF TERMINATED TO THE CONTAINMENT (trunking) and that should be earthed anyway then you have just made the armour part of the earthing system common practice to run seperate earth along with armour , if actually seen this done with a 5 core armour which gives a 3rd earth overkill maybe but the earth is the most important. ever heard the saying "ITS JUST AN EARTH" without a earth it shouldn't be connected