Introduction to Circuit and Motor Protection – CIRCUIT PROTECTION (2)

Fuses—Single Element & Dual Element
Fuses are used to protect a circuit in which a short-circuit fault can occur.

This protection is supplied by the design built into the fuse, which quickly disconnects the power provided to a system. Fuses are represented by the symbol shown in Figure 3.

Inductive loads, such as squirrel-cage motors, will pull between 6 to 10 times the amount of fullload, normal current when they are first started. For example, a 200-volt, 10-horsepower motor will pull 193.2 amps for a short duration before it reaches its full-load amperage current (see Figure 4). In this case, it pulls 6 times its full-load current. When this circuit is protected with fuses against short circuits and ground faults (see Figure 5), the fuse must be able to allow this overcurrent situation to occur without breaking or disconnecting power to the motor circuit.

Single element and dual element fuses are used in different applications. A single element fuse, also called a non time-delay fuse, is made of one conducting element that has one or more links enclosed in a tube, or cartridge, surrounded by arc-quenching filler material.
Under normal operation, the fuse conducts current simply by acting as a conductor (see Figure 6). If an overload occurs and persists for a short interval, the temperature created by the overcurrent will reach a level that melts the link forming a gap and breaking the circuit.

While overload currents are normally 6 to 10 times the normal current, short-circuit currents are much higher than the normal current (see Figure 7).


to be continued………..

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