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Discuss swa/cpc in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

benji

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Arms
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Ok you can use the armour as a cpc if when the calcs. are done it is not acceptable then you can use an additional earth cable to supplement the armour .If we had a 50mm 4core and the calcs.say it has to have an additional cpc and that a 6.0mm would be ok ,then what happens in an earth fault situation?does say 95% of fault go down the armour and the other 5% go down the 6.0mm .Any comments welcome (i don't no the answer)

yours benji
 
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S

Spudnik

Fault current would flow greater in the armour as the CSA of this would be greater than the 6mm2.

What percentage would flow down each would depend on the manufacturers specs of the cable as to what the CSA is of the armour.
 

benji

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Arms
  • Thread Starter Thread Starter
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yeah ,thats about where i'me coming from i keep saying if your going to pull a separate cpc in then you might as well pull one big enough to carry all the fault current
yours benji
 
G

Guest123

Hey.

I can see your argument but, you obviously wouldn't run a 6mm cpc as the cpc for a circuit wired in 50mm 4core.

Personally I would be running a 25mm cpc with the armoured.

Obviously any current choses the path of least resistance so depending on the value of "k" for the armouring of the 50mm and installation factors of the 25mm copper cpc this would influence the amount of fault current in both.

Hope this makes sense.:D
 

ian.settle1

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Mentor
Arms
Hey.

I can see your argument but, you obviously wouldn't run a 6mm cpc as the cpc for a circuit wired in 50mm 4core.

Personally I would be running a 25mm cpc with the armoured.

Obviously any current choses the path of least resistance so depending on the value of "k" for the armouring of the 50mm and installation factors of the 25mm copper cpc this would influence the amount of fault current in both.

Hope this makes sense.:D

I agree would run 25mm if running a seperate cpc.

As to which one the fault current would flow down is nothing to do with which is the greater in size but which one has the lowest resistance path to earth that the current fault current will flow in.
 
G

Guest123

I agree would run 25mm if running a seperate cpc.

As to which one the fault current would flow down is nothing to do with which is the greater in size but which one has the lowest resistance path to earth that the current fault current will flow in.
Agreed, as I said path of least resistance, but surely the bigger the csa of the conductor the lower the resistance in opposition?
 
S

Spudnik

And also the construction, for instance copper cable or steel (or whatever they throw in the mix) of the SWA.
 
M

Mikeru

My understanding from reading the guidance notes, is that in cases where the csa of the armour in not large enough, increasing the size of the cable to make it compliant is preferable to running a separate CPC.And also, that where a separate CPC is run, it must be rated to meet the requirements of the whole fault current, rather than used in addition to the armour because it can be unpredictable how the fault current will split between them. :)
 
G

Guest123

My understanding from reading the guidance notes, is that in cases where the csa of the armour in not large enough, increasing the size of the cable to make it compliant is preferable to running a separate CPC.And also, that where a separate CPC is run, it must be rated to meet the requirements of the whole fault current, rather than used in addition to the armour because it can be unpredictable how the fault current will split between them. :)

Which I'm sure everyone does. CPC's are selected in accordance with the result of the adiabatic equation or table 54.7 in "ol red" not just on a whim.

As for upgrading an entire swa from 50mm to 70mm for instance, purely for the sake of a greater csa of armouring, I would disagree there as the cost difference between upgrading swa or installing an additional cpc is massive.

Indeed all design drawings I've seen, the swa is selected in accordance with the demand of the installation it's supplying, not for csa of armouring and it's ability to cope with the fault current.
 
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