Verifying SWA CPC suitability disconnection times / adiabatic equation

[ZEDEZ]

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

 

[tomspark93]

I'm sure you've already covered this but don't forget about equipotential bonding back to a MET located before the supply end if using the armouring as the cpc

 

[Richard Burns]

I assume you used this table not the one to which you linked in Batt cables.

If the conductance is 20% of R1 then the resistance, not resistivity, is 5 times R1.

0.727 * 5 is 3.635, it is not often a multiplication result is less than the original figure, always good to have a reality check on figures!

You should be calculating the PEFC at the origin of the cable not the CU end because a fault may occur in the cable near the fuse and the fault current will be fractionally higher there, but this does not affect your calculation in this case.

Using the adiabatic equation I get a value of 8.93687 mm² for the result not 6.78mm² not sure where the difference occurs, but again no change in the effects of the result.

other than that on a quick overview it seems OK.
Good work, just make sure you get the numbers right.

 

[ZEDEZ]

Appreciate the feedback Richard and thanks for going through my figures - I was debating whether or not it was worth posting here and you've confirmed, it was!

 

[ZEDEZ]

Rich - Just looking through my calcs again now I can see where the difference occurred in our adiabatic results.

I was performing the square root at the wrong point in the calc (after rather than before dividing by k).

Just recalculated doing it the other way round and got the same result as you. Algebra never was my strongest subject

 

[Richard Burns]

Glad you found the error, in this case it did not matter but it can matter a lot in some cases.
Just as a heads up if you check the guidance on 25mm² two core cable you may find references stating the armour is not suitable as a cpc because it does not meet the requirements for selection on the table.
However the armour does meet the requirements for the csa by calculation, as you have found.

 

[ZEDEZ]

Thanks for the heads up Richard. Just out of curiosity, why do such tables state unsuitability? Is it because the tables assume worst case scenario regarding assumed measurements etc.?

 

[Dave OCD]

Well for the sake of 4 metres and long term piece of mind I'd be very tempted to run a seperate 25mm [or even 16mm] main earth. :smiley2:

 

[ian.settle1]

Well for the sake of 4 metres and long term piece of mind I'd be very tempted to run a seperate 25mm [or even 16mm] main earth. :smiley2:

Look better if a 3 core was used rather than running a separate CPC

 

[Dave OCD]

Look better if a 3 core was used rather than running a separate CPC

Yes of course, I was just thinking that the OP had a bit of spare 2 core already that he wanted to make use of. :smiley2: