Introduction to Circuit and Motor Protection – CIRCUIT PROTECTION (4)

– Fuse Curves
In the National Electric Code, non time-delay and dual-element time-delay fuses are rated under normal conditions in the range of 300 and 175 percent of the full load current of motors.

This gives, for example, a non time-delay fuse only three times its rating to allow for the surge of
current created by starting a motor (see Figure 11). A motor may pull at least six times the full load current when it is first started. Fuses, however, do not open instantly when current is just above the fuse rating.

The graph in Figure 12 represents single-element and dual-element time-delay fuses, rated at 100 amps. It takes 10 seconds (see Figure 13) for the time-delay fuse to open at a current of 500 amps. However, it only takes 2 tenths of a second for the non time-delay fuse to open at a current of 500 amps (see Figure 14).

A non time-delay fuse can be used to protect a circuit if the motor inrush current can be sustained by a fuse during the inrush time. If the motor current passes this limit, the fuse will open. If this is the case, a slightly larger fuse, up to 4 times the full load current according to code, can be used to protect the circuit. Otherwise, it is best to select a dual-element time-delay fuse that will provide adequate time for the inrush current.

to be continued………

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