From Electrical Equipment Handbook: Troubleshooting and Maintenance


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:

  1. The fuse combines the sensing and interrupting elements in one unit.

  2. 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.

  3. 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.

  4. 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.

  5. 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...

Products & Services
Fuses protect electrical devices and components from overcurrents and short circuits that occur in improperly operating circuits.
Disconnect Switches
Disconnect switches rapidly disconnect circuits from power supplies in the event of an emergency.
Power Strips
Power strips are used in applications where multiple AC outlets are needed, often without surge or line noise protection.
Fuse Holders
Fuse holders are devices for containing, protecting and mounting fuses. Blown fuses can be changed quickly and easily when used in conjunction with a fuse holder.
Power Distribution Units (PDU)
Power distribution units (PDUs) have an electrical input and several outputs, often as electrical outlets, for powering multiple devices. 

Topics of Interest

The function of bearings is to keep the shaft or rotor properly aligned with the stationary parts under axial and radial loads. Bearings that provide radial positioning to rotors are known as line or...

5.1 INTRODUCTION Fuses (fusible wire links) are one of the oldest and most universally used overload protection methods. However, because the function of the fuse is thought to be elementary, it...

Current limiting fuses can reduce both the magnitude and duration of a fault current. A UL Listed, current limiting fuse must clear a short circuit current in less than one half cycle in its current...

5.6 Co-ordination with Fuses When co-ordinating overcurrent relays it may be necessary to consider the time/current characteristics of fuses which are used to protect MV/LV substation transformers.

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...