TABLE OF CONTENTS A fuse is an overcurrent protective device with a circuit-opening fusible part that is heated and severed by the passage of current through it. Fuses can meet most of the protection requirements for good system operation.
The fuse is a thermal device. Heat will melt the fuse element regardless of its source.
The fundamental features of fuses are as follows:
The fuse combines the sensing and interrupting elements in one unit.
It is a single-phase device. Only the fuse in the affected phase will melt to isolate the fault.
Three-phase motors will continue to run on single-phase power for extended periods. This may result in overheating and damage to the motors.
The fuse response is a function of I 2 T, where I is the current and T is the time the current exists. It has an inverse-time characteristic the higher the current, the faster the fuse blows.
Most fuses require considerably more current than their amperage rating to operate. For example, NEMA standards require that E-rated fuses of 100E and below melt in 300 s at 200 to 240 percent of their rating. Fuses above 100E must melt in 600 s at 220 to 264 percent of their rating. These durations are considered extremely long for short-circuit protection.
Fuses should be coordinated with downstream devices to ensure faults are cleared within reasonable times (1 to 5 s or faster). A fault magnitude of 5 or more times the current rating of the fuse is required...
