Help with formula please.

allan.vaughan4 · Jan 23, 2016

Hi All,

I am trying to understand the formula Ze x Ia is less than or equal to Uo x Cmin.

I need to identify Ia. (Value in amps causing operation of protective device).

I believe you use ohms law. (I = V / R) . V being either 230 V / 400 V.

i am struggling to identify where you get the resistance figure from, I have carried out calculations using Ze as the figure for R but this does not work in the equation as you end up with 230 V or 400 V Less than or equal to Uo x Cmin (218.5 V).

So the figures I have pulled up are say Ze = 25 Ohms. V is 230 OR 400. And I have been using R as the Ze figure again = 25 Ohms. This is obviously where I am going wrong.

Can anybody explain where the proper place to get R is from please?

Your help in this matter would be very much appreciated!!

Many thanks,

Allan.

Rob_the_rich · Jan 23, 2016

To get Ia you look up the time/current characteristics of the relevant OCPD, then decide the permitted disconnection time. This gives you a figure in Amps. Multiply that by your measured Ze (or by enquiry, for the type of system). Then see whether this figure is greater than Uo x Cmin, if greater, it does not comply with 411.4.5

Eg BS 88-3 type C 100A 5seconds gives 580A x 0.25 (TNCS) gives 145, which complies (<218.5V)

Rob_the_rich · Jan 23, 2016

Actually, the above post is rubbish, its been a long day.

The formula in 411.4.5 is Zs x Ia <or=to Uo x Cmin

Not Ze which is the external part of Zs.

Rob_the_rich · Jan 23, 2016

So the max Zs for the above fuse from table 41.4 is 0.38ohms x 580 =220.4 which is slightly greater than 218.5, so it shouldn't comply.

My brain is fried.

allan.vaughan4 · Jan 23, 2016

Great, finally got it!!!!

I have just done the working out for a 32A type D 60898 mcb.

Zs table 41.3 for 0.4 sec is 0.34 ohms

I delta n fig 3A6 640A

0.34 X 640 = 217.6 which is lower than 218.5 so allowable.

Many thanks for taking the time to explain it to me,.

Thanks, Allan.