Bullrush charts

[Manator]

Ze=0.35 fault current=230/0.35=632A

eg fig 3A1 bs88-3 sys C 80A

go up from 632A until hit 80A curve

go across to read time 0.6 secs

without a set square or lines, your eyes go all fuzzy.

Edit; my eyes went funny, should be ~1.4 secs, the corner of a sheet of paper works too, but the last decimal point is guesswork as sprocketflup said.

Well I learn something everyday. I have for years been just calculating as second nature, I never suspected that there was a short cut. I will have to try it out next time.

 

[Rob_the_rich]

That's exactly what I'm after,,, but by calculation with formula rather than using the tables so that I only have to enter the protective device and Zs,,, and it'll churn out a CPC size

found http://plotdigitizer.sourceforge.net/ to digitise the curves, would still take time, but looks promising

also found this thread http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=31609 looks like formula is a non-starter.