SWA and separate conductor as CPC

[fatboyslim]

Evening all,

Having a debate with a colleague about using the SWA armouring and a separate conductor both as CPC's for a circuit. So in theory bringing down the Zs of a circuit due to using both as CPC's.

I'm swaying to thinking this is wrong however, I'm struggling to find any regs to express my thinking.

How would you preform an R1 + R2 when you have two R2's in theory?! Would you link both R2's together?

Anyone have any thoughts? Perhaps I am wrong and my colleague is correct.

 

[davesparks]

If the armour is not compliant as a CPC then it's a lot easier to just calculate the size of copper CPC required as if there wasn't cable armour than to try and calculate the required size taking in to account the effect of the armour.

Obviously the armour is still connect and will operate to lower Zs in the real world.

 

[ipf]

Evening all,

Having a debate with a colleague about using the SWA armouring and a separate conductor both as CPC's for a circuit. So in theory bringing down the Zs of a circuit due to using both as CPC's.
How would you preform an R1 + R2 when you have two R2's in theory?! Would you link both R2's together?

All it's doing is upping the earth conductor size and as a result reducing the R1+R2
value and thus Zs. The earths will be connected together at both ends anyway.

 

[fatboyslim]

If the armour is not compliant as a CPC then it's a lot easier to just calculate the size of copper CPC required as if there wasn't cable armour than to try and calculate the required size taking in to account the effect of the armour.

Obviously the armour is still connect and will operate to lower Zs in the real world.

Thanks for the reply.
So my colleague is correct, you can use the SWA armouring as the main CPC (ensuring it's correctly sized) then using a separate conductor as a CPC to lower the Zs even if this conductor is undersized, as the SWA armouring is sufficient to take the fault current while the separate CPC is just to lower the Zs to meet disconnection times.

 

[timhoward]

Thanks for the reply.
So my colleague is correct, you can use the SWA armouring as the main CPC (ensuring it's correctly sized) then using a separate conductor as a CPC to lower the Zs even if this conductor is undersized, as the SWA armouring is sufficient to take the fault current while the separate CPC is just to lower the Zs to meet disconnection times.

It's early but I think what you are meaning is that the SWA's armouring meets the adiabatic equation for thermal constraints but the disconnection time for the protective device isn't adequate.
If you then add a separate CPC, the R1+R2 lowers, so the fault current increases.
So the collective CPC's need to pass both tests.
As stated above, it's usually simpler to ensure the separate CPC passes both tests, and then anything the actual armouring offers is a 'bonus'.

If it turns out I'm still asleep I'm sure someone will correct me soon!

 

[Dustydazzler]

It's early but I think what you are meaning is that the SWA's armouring meets the adiabatic equation for thermal constraints but the disconnection time for the protective device isn't adequate.
If you then add a separate CPC, the R1+R2 lowers, so the fault current increases.
So the collective CPC's need to pass both tests.
As stated above, it's usually simpler to ensure the separate CPC passes both tests, and then anything the actual armouring offers is a 'bonus'.

If it turns out I'm still asleep I'm sure someone will correct me soon!

This is how I have always done it and how everyone I have worked with has done it. in effect the armour of the cable just becomes and bit of earthed metal work. I never install 2 core cable anymore

 

[fatboyslim]

It's early but I think what you are meaning is that the SWA's armouring meets the adiabatic equation for thermal constraints but the disconnection time for the protective device isn't adequate.
If you then add a separate CPC, the R1+R2 lowers, so the fault current increases.
So the collective CPC's need to pass both tests.
As stated above, it's usually simpler to ensure the separate CPC passes both tests, and then anything the actual armouring offers is a 'bonus'.

If it turns out I'm still asleep I'm sure someone will correct me soon!

So my colleague is right if all CPC's meet the requirements however, it's just a long winded way compared to sizing the correct separate CPC or extra core as a CPC in the first place!!

 

[ipf]

This is how I have always done it and how everyone I have worked with has done it. in effect the armour of the cable just becomes and bit of earthed metal work. I never install 2 core cable anymore

Me neither for single phase, as a rule....... but I regularly use 4core swa for TP+N, if it measures up.