Overcurrent Devices: Fuses and Circuit Breakers

A tremendous amount of electrical energy is available in almost every electrical power system, so every part of an electrical installation must be protected from excessive current flow. Excessive current flow can be considered in two distinct categories:

1. Instantaneous current from inrush on start-up or from a short circuit

2. Long-time overload current

Overcurrent devices are available in several forms. At low voltage, the most common forms are

1. Non-time-delay fuse

2. Time-delay dual-element fuse

3. Magnetic-only, instantaneous-trip circuit breaker

4. Thermal-magnetic-trip circuit breaker

Figure 12-1 shows the time-current characteristics of the most common of these types of overcurrent devices for a standard 20-ampere (A) device. Note that for instantaneousonly protection, a magnetic-only circuit breaker unlatches and trips (opens the power circuit) immediately on reaching the preset ampere value, as does the thermal-magnetic-trip circuit breaker. However, the instantaneous-trip setting on a thermal-magnetic-trip circuit breaker is normally set at a higher ampere rating than would be a magnetic-only breaker because the thermal element of the thermal-magnetic-trip circuit breaker adequately provides protection within the ampere range of maximum safe operating current. The thermal-magnetic-trip breaker curve and the curve of the time-delay fuse are very similar to each other because the thermal-magnetic-trip breaker curve is designed to mimic the curve of the time-delay fuse.

Figure 12-1: Time-current characteristic curves of typical 20-A overcurrent devices.

Fuses

A fuse heats internally due to I ² R heating, and after enough heat builds up, the thermal element in the fuse simply melts, opening the...