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

 

[Pete999]

Are you talking Twin and cpc cable, Gigsy?

 

[Gigsy]

Are you talking Twin and cpc cable, Gigsy?

Well i had to find the smallest cable I could use, so I guess I would use 2 core 6mm and single core 1mm

I just don't think my calculations are right, 1mm seems too small

In real life I would use 10mm twin and earth in this situation

 

[Sintra]

Should the time in the adiabatic not be 0.1 instead of 0.01?

 

[Gigsy]

Should the time in the adiabatic not be 0.1 instead of 0.01?

That is exactly what I was thinking, but the table does go down to 0.01 if you can see. Thank you very much for taking the time to look at the calculations.

Is that where I went wrong? Shall I use 0.1 instead of 0.01?

 

[Gigsy]

That is exactly what I was thinking, but the table does go down to 0.01 if you can see. Thank you very much for taking the time to look at the calculations.

Is that where I went wrong? Shall I use 0.1 instead of 0.01?

Have you ever come across a situation where you would use 1mm CPC with 6mm cable?

If I changed it to 0.1 I come up with a 2.5mm CPC, which sounds a bit more suitable.

 

[Gigsy]

It is ok I see my error, I'll try again.

I had to assume a CPC in step 1, I assumed a size of 4mm CPC to get the loop impedance. I based the rest of my calculations on that assumption. Think I'll start again. I have it all wrong I think?

 

[Risteard]

Should the time in the adiabatic not be 0.1 instead of 0.01?

Haven't looked at the calculations but if manufacturers' data is available then depending on fault current a definite minimum operating time of 0.01 seconds may be more realistic than the 0.1 seconds in the BS7671 tables. But the calculation will have needed to be carried out with the cable size used to verify otherwise invalid conclusions could be drawn.

 

[westward10]

There was time when 2.5 T&E incorporated a 1.0 cpc, BS6004 reviewed this and upped the minimum to 1.5 due to possible thermal stress which it may suffer under fault conditions particularly with a 30A, BS3036 fuse. Using this basis a 1.0 cpc with a 6.0 line conductor is almost certainly not going to be suitable with your design current.

 

[Gigsy]

Haven't looked at the calculations but if manufacturers' data is available then depending on fault current a definite minimum operating time of 0.01 seconds may be more realistic than the 0.1 seconds in the BS7671 tables. But the calculation will have needed to be carried out with the cable size used to verify otherwise invalid conclusions could be drawn.

Thank you, so I was right to use 0.01 then

 

[Risteard]

It is ok I see my error, I'll try again.

I had to assume a CPC in step 1, I assumed a size of 4mm CPC to get the loop impedance. I based the rest of my calculations on that assumption. Think I'll start again. I have it all wrong I think?

Then it tells you that 4mm^2 cpc is adequate as it is bigger than the result of your calculation. It does NOT tell you that 1mm^2 cpc is adequate as your calculations weren't based around that.

 

[Gigsy]

There was time when 2.5 T&E incorporated a 1.0 cpc, BS6004 reviewed this and upped the minimum to 1.5 due to possible thermal stress which it may suffer under fault conditions particularly with a 30A, BS3036 fuse. Using this basis a 1.0 cpc with a 6.0 line conductor is almost certainly not going to be suitable with your design current.

Thank you, that is what I thought. My calculations are wrong

 

[Gigsy]

Then it tells you that 4mm^2 cpc is adequate as it is bigger than the result of your calculation. It does NOT tell you that 1mm^2 cpc is adequate as your calculations weren't based around that.

I think I get it. So as the result was lower than 4mm2, so now I can go back and try again factoring in a 2.5mm2 CPC, if the result is higher I cant use 2.5mm2 I will have to use 4mm2?

 

[Risteard]

Exactly. It is an iterative process.

 

[Gigsy]

Exactly. It is an iterative process.

Thank you very much I get it now

I hope you teach, if not you should do.

 

[Dave OCD]

You may be overthinking this as it seems the question is asking for minimum cable size and as clipped direct it will be T&E, therefore having determined that a 6mm T&E is adequate the 2.5mm integral CPC will be adequate also as with the low Ze and type of OCPD deployed automatic disconnection under fault conditions will be achieved well within 'spec'.

 

[Wilko]

I think it was a good pick by Sintra at #4 ... Using the graphic supplied we can only go to 0.1 sec. As Risteard says in #8 it will likely discon quicker, but it varies by manufacturer and mcb model. And using 0.1 sec leads us to the practical cpc of 2.5mm answer too

Reading the report, a couple of suggestions :

• If Ib is 36.96A and In >= Ib, then In can't be 36A ...
• The non impacting correction factors aren't 0, they must be ...

Hope that helps, and good luck Gigsy

 

[Gigsy]

You may be overthinking this as it seems the question is asking for minimum cable size and as clipped direct it will be T&E, therefore having determined that a 6mm T&E is adequate the 2.5mm integral CPC will be adequate also as with the low Ze and type of OCPD deployed automatic disconnection under fault conditions will be achieved well within 'spec'.

You could be right, I may have over thought it. Still it was a good exercise I did learn allot.

 

[Gigsy]

I think it was a good pick by Sintra at #4 ... Using the graphic supplied we can only go to 0.1 sec. As Risteard says in #8 it will likely discon quicker, but it varies by manufacturer and mcb model. And using 0.1 sec leads us to the practical cpc of 2.5mm answer too

Reading the report, a couple of suggestions :

• If Ib is 36.96A and In >= Ib, then In can't be 36A ...
• The non impacting correction factors aren't 0, they must be ...

Hope that helps, and good luck Gigsy

You are right, the protective device must be greater than the current flowing. Ib has to be 37. There is no 37 breaker so I guess it should be 40?

Thank you for pointing this out

 

[Gigsy]

I think it was a good pick by Sintra at #4 ... Using the graphic supplied we can only go to 0.1 sec. As Risteard says in #8 it will likely discon quicker, but it varies by manufacturer and mcb model. And using 0.1 sec leads us to the practical cpc of 2.5mm answer too

Reading the report, a couple of suggestions :

• If Ib is 36.96A and In >= Ib, then In can't be 36A ...
• The non impacting correction factors aren't 0, they must be ...

Hope that helps, and good luck Gigsy

The none impacting factors must be.....
Must be what?

The graphic supplied does goto 0.01, I drew a green line to indicate the value I was using, I may have covered over the writing with the green line.