Normal Inverse Curve Formula of BBC ICM Overcurrent Relay

[GRAEE]

Hello everyone!

What is the formula of a Normal Inverse Curve of BBC ICM overcurrent relays?

Thanks.

 

[sibeen]

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.

 

[juleselec]

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.

 

[GRAEE]

Hello Juleselec & Sibeen,

Thanks for the information. The relay that im referring is the old one, the BBC ICM2.

Thanks very much.

GRAEE