CPC size help

[pencilpusher]

Lenny, I have read this previously but I am grateful for you posting it. However, it does not help in telling which answer is correct in my original question. The fact is I have still been given different answers.

To determine the CSA required for a CPC, you need to furnish the type and rating of the overcurrent protective device.
If for instance you intend using a 32A type B MCB, the time/current graphs in Appendix 3 of BS7671, indicates that the PEFC required to provide disconnection between 0.1 and 5s is only 160A.
Your calculated value of 800A will be more than sufficient to to allow the device to operate, and according to the adiabatic equation, will require a minimum CPC CSA of 0.44mm².

 

[telectrix]

i get 2.2sq.mm, so 1.5 is undersize

 

[pencilpusher]

So, I have had 0.44mm2, 2.2mm2 and my 5mm2. Telectrix, what values did you use?

If you use the 160 amps Spin used, it is 0.44mm2. If you use 800 amps I make it 5mm2????

 

[Guest123]

Using your actual fault current, the result is 2.2mm.

 

[telectrix]

i used 800A and t=0.1sec. k = 115. so S = sq.rt. ( 800sq. x 0.1 ) /115 = 2.2

 

[pencilpusher]

Thanks all. Using the armour of the SWA in parallel with the CPC core of the SWA, would that give K as 115 or 51? I suspect 115 because the core is being used too. If it was just the armour used I suppose it would be 51 and the min CSA of CPC would be even larger?

 

[pencilpusher]

So if I need 2.2mm2 and the T&E part of the install is only 1.5mm2, I need to upgrade it. Now, replacing the 2.5/1.5 T&E for say 6mm2/2.5mm2 I could still have problems becuase the run is quite short, I will lower R1+R2 again and at the moment I have a ZS of 0.28 ohms. Increase the line conductor CSA and my R1+R2 could lower to say 0.22 ohms and that then gives a fault current of 1045 amps. The adiabatic then gives a minimum CSA of 2.8mm2 and again does not comply and so on. My thoughts are that I could use a single 2.5mm2 CPC conductor and not use the T&E CPC at all. Is that OK? That would leave all conductors at 2.5mm and complies.

The problem is that becuase a TNCS earthing system gives a low Ze, I am always going to struggle with PEFC's such as in this example and I am racking my brain trying to work it out!

Thanks

PP

 

[malcolmsanford]

So if I need 2.2mm2 and the T&E part of the install is only 1.5mm2, I need to upgrade it. Now, replacing the 2.5/1.5 T&E for say 6mm2/2.5mm2 I could still have problems becuase the run is quite short, I will lower R1+R2 again and at the moment I have a ZS of 0.28 ohms. Increase the line conductor CSA and my R1+R2 could lower to say 0.22 ohms and that then gives a fault current of 1045 amps. The adiabatic then gives a minimum CSA of 2.8mm2 and again does not comply and so on. My thoughts are that I could use a single 2.5mm2 CPC conductor and not use the T&E CPC at all. Is that OK? That would leave all conductors at 2.5mm and complies.

The problem is that becuase a TNCS earthing system gives a low Ze, I am always going to struggle with PEFC's such as in this example and I am racking my brain trying to work it out!

Thanks

PP

Welcome to the wonderful world of electrical design, everyday is another step on that road to the perfect install ........not.

Because of the limitations imposed on you with T + E and it's CPC size, you are infact going to have to upsize it on such a low Zs. Unlike SWA where you can now run a parallel CPC with in , reg 521.5.1, you shouldn't with T+E.

Your only other course of action is to look at another erection methods, SWA for instance, SY/YY 3 core cable, MICC, singles in trunking/conduit.

 

[pencilpusher]

What an absolute pain in the a**e. I suppose the best method would be to run SWA from CU all the way to the garage CU and lose the T&E. I do wonder though how many people actually check the CPC sizes before they install it. Electrical design......ppfff

 

[malcolmsanford]

Proper sparks like you look at this mate, and so they should

 

[Guest123]

There is another way of doing this.

On my 2400 course, I had the same problem in that the adiabatic equation always came out with the next size up CPC that I currently had.

However, increasing the CPC size also decreased the resistance which in turn increased the PFC for use as (I) in the equation and so it went on.

In the end, I went to the manufacturer and obtained their I2T values for the protective device I was using and used this value in the equation instead.

This gave a minimum size that remained constant regardless of any other circuit factors as it was device specific.

I have the MEM values on file still but I dont know what make/model of device you have.

 

[pencilpusher]

Lenny thanks. The OCPD protecting the sub main is a Hager 20amp Type B RCBO.

 

[Guest123]

Well using the info I have, the minimum CPC size for a hager type B, 20A, 6KA device would be 1.2mm. Obviously as this is below the 1.5mm we have, the circuit complies.

 

[pencilpusher]

Lenny what a rollercoaster!! Thank you. Is there any chance you could send me those figures please as I tend to use Hager products. If not, where can I get them?

 

[Guest123]

The catalogue is an old one i have in my 'dad drawer':sifone:

I'm sure google would turn up an online PDF copy.

To help you understand a bit more, have a read of 434.5.2, essentially if K(squared) x S(squared) is greater than than the let through energy (I2t) from the manufacturer, the circuit complies.

The value of (I2t) I have is 19,500. So, K(squared) = 115 x 115 = 13225. S(squared) = 1.5 x 1.5 = 2.25. 13225 x 2.25 = 29.756.25.

29.756.25 is greater than 19,500 so it also complies this way.