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

[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

 

[MDJ]

Not quite sure what your trying to say Fella, I am basically agreeing with engineer but pointing out I never meant to suggest the sheath and third core should be considered as the overall conductor size, just stating the third core is probably adequate but it will inevitably be connected with the shealth at an earth point somewhere in the circuit.

 

[Engineer54]

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 separate 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

Only for those that know no different, ...only where the SWA CSA doesn't meet compliance, or where the distance starts to have an effect on the steel wire armour CPC. If your using a 3rd core as your CPC, then it will probably be the same size as the circuit conductors, so will probably already be over kill.

In other words under most circumstances, the armour of SWA cables upto 95mm will always comply as a sufficient CPC means...

 

[Dan]

Just bumping some threads in the Electrical Wiring Theories and Regulations forum category here on our Electricians Forums. If this specific topic isn't current, just ignore it, it'll drop off the list in no time. However, if it's something you'd like to contribute to, feel free to reply and bring it back into current discussion.