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March 9, 2010

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Introduction to Circuit and Motor Protection – CIRCUIT PROTECTION (5)

January 12, 2009
Other Advantages of Dual Elements in Motor Control Circuits
To illustrate the advantages of dual elements, let’s use as an example with both single-element and double-element fuses for a 10-horsepower, 200-volt motor (see Figure 15).

This motor has a full-load amperage rating of 32.2 amps. According to rule 430-52 of the National Electric Code, we could select a single-element, non time-delay fuse of 90 amps to protect the circuit.

The fuse curve shown in Figure16 for this non time-delay fuse should be enough to let the motor start for a short duration. Remember that fuses, according to the curves, will let motors start if the motor start-current curve does not trip the fuse. For instance, this 90-amp fuse does not provide protection for the motor that is under overload as shown by the dotted line of the motor curve in Figure 16. Therefore, it would require overload protection at the starter.

According to article 430-32 of the NEC, if we want to use a dual-element time-delay fuse to protect the motor from overloads and the circuit from short circuits, we would select a 40-amp fuse. This fuse will protect the circuit against short circuits and overload conditions and allow the
motor to start when it pulls the starting inrush current. In fact, if motor overload relays are used in a motor control circuit, the dual-element fuses will serve as motor backup protection just in case the overload relays are the wrong size or fail to operate. The graph in Figure 17 shows the motor damage curve and the protection curve provided by the motor overload heaters. The dual-element curve is shown as backup protection. In addition, the disconnect switch used in the dualelement circuit has a smaller current-carrying capacity than the dual-element circuit in the 90- amp circuit, therefore reducing space and money (see Figure 18). A disconnect switch rating
must be equal to or larger than the amperage rating of the fuse protecting the circuit.