minimum cpc sizes

[SafetyEICR]

Just running thro some design calcs for minimum cpc size and im getting some unrealistic readings. Example 1.5mm c.s.a live and 4.mm cpc on a lighting circuit?? circuit is Zdb = 0.083


Circuit 1L1


Assuming we can use 1.5mm cpc



R1 + R2 = (12.10 + 12.10) x 1.2 x 3.57 / 1000 = 0.10ohms


ZS = 0.083 + 0.10 = 0.183


Ia = 230 / 0.183 = 1,256a


S = 1,256 squared x 0.1 = 157753 square root this figure / 115 = 3.45mm cannot be right surely??

 

[SafetyEICR]

All done, just don't know how to put symbols on here

 

[SafetyEICR]

Oh... Is that right? Ive got a lot of calcs to change then!!

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Ive been doing the square root of the answer to 2090 squared x 0.1

 

[Baker1988]

Oh... Is that right? Ive got a lot of calcs to change then!!

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Ive been doing the square root of the answer to 2090 squared x 0.1

Yeah that is what you do mate if that is what you are doing then it is correct

 

[Geoffsd]

No. You have the right answers.

You are just noting the sums in a sort of shorthand.

 

[SafetyEICR]

So the answer would be 1.81mm and not 5.74mm. Which would make more sense.

 

[Geoffsd]

S = √(2090² x 0.1) / 115 = 5.74mm

 

[Guest55]

agree with geoff , your maths was correct first time around , 5.7mm is the minimum cpc size.

 

[Geoffsd]

Briggsy -

As this is just a theoretical exercise with unrealistic numbers,
are you supposed to work out the time by first using the equation in 434.5.2 and then the equation in 543.1.3?

I have just done that with the above values and it comes to 1.498.

I did it earlier with the values in your first post and it came to a similar figure 0.989 (I think)

 

[SafetyEICR]

Hi Geoff, I've just done the calc the way set out in 434.5.2 and it gives a much lower cpc size. Wish this was pointed out on the course!! Don't think ill have time to go back over all calcs with this method, ill just have to add a statement onto the end of the project. Thanks for your help.

 

[Risteard]

You would need to redo the calculation with your new cpc size to confirm whether it can withstand as don't forget that the larger cpc will reduce the Zs and therefore further increase the prospective earth fault current!

Heres one of the calcs. assuming we can use 1.5mmR1 + R2 = (12.10 + 12.10) x 1.2 x 3.57 / 1000 = 0.10ohms


ZS = 0.083 + 0.10 = 0.186ohms


Ia = 230 / 0.186 = 1,236a


S = 1236 x 0.1 / 115 = 3.40mm

 

[bartkusal]

Thats it, it all seems correct, as Biff said you have extremly small Ze plus low R1+R2 by having very short circuits, where did you seen a ring 7.15 meters long and then lighting circuit 3.57meters long at the furthest point, its just unrealistic knowns used in calcs.