Help with CPC size

[Gigsy]

Hi I am training to become an electrician. I have a problem to solve to work out the size of the CPC. My calculations say I would use 6mm and 1mm CPC. This can't be right, can it?

Please refer to attached document for details.

View attachment 35629

View attachment 35630

 

[Gigsy]

Think I have corrected the errors

 

[Gigsy]

On page 3 you've circled 2.5 in green mate when working out suitable CPC but quoted the resistance value for 4mm.

Thank you, don't know how that happened, think it happened when I converted from doc to pdf format, thank you

 

[Gigsy]

In is the rated current or current setting of protective device, so that would be 40A BS EN 60898. Therefore your tabulated calculation is slightly off as it uses 37A as opposed to 40A. You could use Ib in this calculation if overload protection was NOT required.

The picture highlighting your first R1+R2 values also show 6mm plus 2mm, however your calculations take the values of 6mm and 4mm.

The final calculation for your S value also does not quite add up either.

I agree the final S value does not seam right, because 6mm cable has a 2.5mm CPC. You should be able to use a smaller CSA conductor than what is recommended, or at least the same. I can't see where I went wrong tho

 

[Gigsy]

Previous posts must be right, I must be using the wrong value for (t)

I read this chart as saying I should use the value 0.01

Other posts say I should use 0.1

I don't see why

The current is well over 200A, at which point the breaker would trip in 0.01s ???????????????

View attachment 35637

 

[Gigsy]

It is ok, i forgot to square root the value of S

 

[Gigsy]

Can anyone confirm if the value of t should be 0.01 or 0.1

 

[DavidSpark]

I believe you'd use the 0.1 value for t, giving a more "worse case scenario" and at those times you really can't split it down anymore from the information you've been given. However having recently done this I was told that you can do a super ultra worse case scenario and plug the value of 200A in as your current in relation to the corresponding t value, if your cpc complies there then you can't go far wrong.

Using 200A and 0.1 for the and 115 for k - you get 0.54mm2 so it proves your choice of cables comply

 

[Chr!s]

Hi

Just use the 0.1, that is what the time current curves in BS7671 state.

 

[Risteard]

I read this chart as saying I should use the value 0.01

You can't use the generic tables for 0.01 second disconnection - as I stated earlier this is the definite minimum operating time but requires the manufacturer's data (which you will note is stated at the bottom of that table). So you need to know what brand of device you are using and consult manufacturer's data before using a disconnection time of 0.01 seconds - although with a sufficient fault current it is likely this will be achieved and therefore 0.1 seconds would be unduly pessimistic.

 

[Risteard]

Hi

Just use the 0.1, that is what the time current curves in BS7671 state.

You don't have to use the BS7671 generic data. If you have the manufacturer's data for the device you are using then that is what you should use for a more realistic result.

 

[Chr!s]

You don't have to use the BS7671 generic data. If you have the manufacturer's data for the device you are using then that is what you should use for a more realistic result.

Hi, yes you are right, but with regards to the op 0.1 is correct. Just a note, you shouldnt use graphs for disconnection times below 0.1, the are not reliable, use the documented KA2S

 

[Risteard]

you shouldnt use graphs for disconnection times below 0.1

I didn't mean to suggest it should be extrapolated from a graph. Apologies if that came across.