There are different curves that can be set on certain circuit breakers with the equation for the curve defined by IEC 60255-151 (previosly 60255-3). This document is similar to IEEE C37.112 .
The equation is t = TMS(((k/((I/Is)^a) -1))+ C))))
Where:
t = operating time for a constant current
I = energising current
Is = overcurrent setting
TMS = time multiplying setting
k, a and c = constants defining the curve
For nor4mal Inverse, defined by the IEC, K = 0.14, a = 0.02 and c = 0.
Sibeen is more or less right. The old BBC ICMs are made to BS142, which has the same equations as IEC 255. Attached is a scanned copy of the BBC ICM2 manual.