Chapter 11: Fuses and Miniature Circuit Breakers

11.1 INTRODUCTION

Fuses act as a weak link in a circuit. They reliably rupture and isolate the faulty circuit under overload and short circuit fault conditions so that equipment and personnel are protected. Following fault clearance they must be manually replaced before that circuit may be put back into operation. Striker pins are available on some designs such that remote alarms may be initiated on fuse operation.

Miniature circuit breakers (MCBs) or moulded case circuit breakers (MCCBs) are also overcurrent protection devices often with thermal and magnetic elements for overload and short circuit fault protection. Earth leakage protection, shunt trip coils and undervoltage releases may also be incorporated in the designs. As a switch they allow isolation of the supply from the load. Normally the MCB requires manual resetting after a trip situation but solenoid or motor driven closing is also possible for remote control.

This chapter describes the various types of fuse and MCB together with their different uses and methods of specification. Examples and calculations for correct selection of different applications are also given.

11.2 FUSES