A fuse is a device that protects a circuit from an overcurrent condition only. It has a fusible link directly heated and destroyed by the current passing through it. A fuse contains a currentcarrying element sized so that the heat generated by the flow of normal current through it does not cause it to melt the element; however, when an overcurrent or short-circuit current flows through the fuse, the fusible link will melt and open the circuit. There are several types of fuses in use (Figure 3).

Figure 3 Types of Fuses

The plug fuse is a fuse that consists of a zinc or alloy strip, a fusible element enclosed in porcelain or pyrex housing, and a screw base. This type of fuse is normally used on circuits rated at 125 V or less to ground and has a maximum continuous current-carrying capacity of 30 amps.

The cartridge fuse is constructed with a zinc or alloy fusible element enclosed in a cylindrical fiber tube with the element ends attached to a metallic contact piece at the ends of the tube. This type of fuse is normally used on circuits rated at either 250 volts or 600 volts and has a maximum continuous current-carrying capacity of 600 amps.

Summary

The important information contained in this chapter is summarized below.

System Components and Protection Devices Summary

Single (one-line) diagram - simple and easy to read diagram showing power supplies, loads, and major components in the distribution system

Commercial or utility power - electric power supplied to the facility

Diesel power - economical/practical source of standby power

Failure-free power - power supplied to vital equipment with automatic switching so that interruption of power is minimized

Neutral grounding - helps prevent accidents to personnel and damage to property by fire

Voltage class - high voltage > 15,000 volts, intermediate voltage is 600-15,000 volts, low voltage <_ 600 volts

Protective relays - cause prompt removal of any part of a power system that suffers a short circuit

Overlapping protective zones - created around each element of the power system to prevent element failure from interrupting the whole system operation

Breakers - disconnect component from the power system

Fuse - protects component from overcurrent