scope:

Includes: This instruction details the procedure for changing the main pull off springs on a Whipp & Bourne MM74 High Speed Circuit Breaker. It also covers the inspection and procedure for changing the offset of the front main contact carrier.

The main pull off springs are located on the main contact mechanism. Their function is to provide sufficient force to open the main contacts, under fault conditions. With the circuit breaker closed, i.e. the contacts making, the springs are under tension. If the springs were to fail there is no force pulling the contacts apart, thus rendering the circuit inoperable.