As you may know looking at some of my posts I've been thrown back in the deep end recently – and don’t have much experience with complicated calculations. As this is the first time I've used SWA to go from the main supply to the primary CCU, and as I'm using the steel armour as my CPC, rather than rely a single table to verify suitability, I'd like to calculate everything for peace of mind and am wondering if someone here wouldn't mind checking over my calcs.
So what we have is a TN-S single phase supply, with 4m of 25mm² 2 core SWA running from the supply to the CCU. I'll be using an 80a switch with BS-88 fuse to protect the SWA.
The Ze as measured at supply is 0.18 Ω, 240v.
To get the resistance of R1 for the SWA I've used the table in Appendix I of the on-site guide, which tells me my line conductor resistance for 25mm² copper is 0.727mΩ per metre.
The table I found below tells me the conductivity of the steel armour is 20% that of the phase conductor.
http://www.batt.co.uk/upload/files/currentratingsandelectricaldatabs5467_1352477469.pdf
So that must mean the resistivity will be 5 times higher for the armour - 0.727 x 5, R2 = 0.3635. So @1 metre R1 + R2 = 1.0905mΩ.
Over 4m this is 1.0905 x 4 / 1000 = 0.00436Ω.
So using the steel armour as CPC, the Zs at the CCU should be 0.18436 Ω. This gives me a PFC of 1.3kA.
What about disconnection times?
So using Fig 3A1 in BS7671, I can see my disconnection time is <0.1s which is well within limits.
Let’s use the adiabatic equation to see if the SWA insulation can handle this …
S= √(I²×t)/k
Where S = CPC²
For steel armour Table 54.4 in BS7671 tells me the value for k is 46.
So √(1300² x 0.1)/46 = 6.78mm²
Looking at the below table I can see that the steel armour of 25mm² SWA has a CSA of 42mm² (13.51mm² copper equivalent), well above 6.78mm² as calculated above.
SWA, armour equivalent copper - Ted
If you've followed me this far, thank you. If you can see any glaring errors please let me know – otherwise any other advice / information would be much appreciated.
And just as I'm proof reading the above, I found the below link which seems to answer all questions anyone might have regarding SWA and everything I've been asking about:
SWA as CPC
EZ
So what we have is a TN-S single phase supply, with 4m of 25mm² 2 core SWA running from the supply to the CCU. I'll be using an 80a switch with BS-88 fuse to protect the SWA.
The Ze as measured at supply is 0.18 Ω, 240v.
To get the resistance of R1 for the SWA I've used the table in Appendix I of the on-site guide, which tells me my line conductor resistance for 25mm² copper is 0.727mΩ per metre.
The table I found below tells me the conductivity of the steel armour is 20% that of the phase conductor.
http://www.batt.co.uk/upload/files/currentratingsandelectricaldatabs5467_1352477469.pdf
So that must mean the resistivity will be 5 times higher for the armour - 0.727 x 5, R2 = 0.3635. So @1 metre R1 + R2 = 1.0905mΩ.
Over 4m this is 1.0905 x 4 / 1000 = 0.00436Ω.
So using the steel armour as CPC, the Zs at the CCU should be 0.18436 Ω. This gives me a PFC of 1.3kA.
What about disconnection times?
So using Fig 3A1 in BS7671, I can see my disconnection time is <0.1s which is well within limits.
Let’s use the adiabatic equation to see if the SWA insulation can handle this …
S= √(I²×t)/k
Where S = CPC²
For steel armour Table 54.4 in BS7671 tells me the value for k is 46.
So √(1300² x 0.1)/46 = 6.78mm²
Looking at the below table I can see that the steel armour of 25mm² SWA has a CSA of 42mm² (13.51mm² copper equivalent), well above 6.78mm² as calculated above.
SWA, armour equivalent copper - Ted
If you've followed me this far, thank you. If you can see any glaring errors please let me know – otherwise any other advice / information would be much appreciated.
And just as I'm proof reading the above, I found the below link which seems to answer all questions anyone might have regarding SWA and everything I've been asking about:
SWA as CPC
EZ
