Why 0.1s for Main Earthing?

LearnerDave · Nov 15, 2011

Hello,

I wonder if anybody could point me in the right direction to find out why 0.1s is used as the time in the adiabatic equation with regard to the main earthing conductor?

I can find the table 41.1 for final circuits not exceeding 32A, and 411.3.2.3 - TN = 5s and 411.3.2.4 - TT = 1s, but I am struggling to find any reference to the 0.1s!

Any pointers would be much appreciated.

Cheers Dave

telectrix · Nov 15, 2011

look at the time/current curves in brb. say for example you have a 80A BS1361 fuse and your pefc is 1100A. p244 will show that that fuse will break in 0.1 sec. with that fault current. say your fault current was 800A, then the value for t would be 0.4sec.

LearnerDave · Nov 15, 2011

Thank you very much Telectrix. The penny's dropped - You use the on site measured PEFC to determine the time the protection device will discconect in, and also use it for I2 of the adiabatic equation. Phew, thats been bugging me for days.

Thanks again. Dave