Introduction to Circuit and Motor Protection - CIRCUIT PROTECTION (3)

Single-element fuses can be subjected to short-circuit currents of 50,000 amps or higher. The response to these currents must be extremely fast to avoid serious damage and run the risk of fire. In fact, if a large ground fault occurs, the response of a fuse must be within a matter of a few
milliseconds.

A single element fuse, when exposed to short circuit conditions, will melt its links simultaneously.
A short-circuit current is cut off in less than 1/2 cycle, or about 8.3 thousands of a second. This is
long before the short-circuit current can reach its full value. Single element fuses are an excellent source of short-circuit protection (see Figure 8). However, temporary and harmless overloads like those created by motors can cause nuisance openings unless the fuses are oversized. These overloads will occur when a fuse is sized to protect a motor circuit and it trips during the inrush current generated when the motor is started (see Figure 8).

However, if a fuse is allowed to withstand the inrush current, it will not provide adequate protection when a motor is running at full load current. For example, if a 10-horsepower motor circuit is fused to the maximum inrush current of 193.2 amps, the motor runs the risk of being burned out if an overcurrent exists at, for instance, double its full load current for a period of time (see curve in Figure 8). The single element fuse must be sized higher than the full load current of a motor in a circuit to allow the motor to start. The proper sizing of non time-delay fuse is specified according to the National Electric Code.
Dual-element time delay fuses are used to protect conductors and circuits from both short circuits and ground faults. Dual-element fuses not only protect against short circuits, but also protect motors from overcurrents caused by stalling, overloads, worn bearings, improper voltage, single phasing, and other possible causes.
The dual-element time-delay fuse has two overload and short-circuit elements built in. If a fuse experiences a short-circuit overcurrent, the fuse will separate the links the same way a single element fuse does. If the fuse experiences an overload condition for 10 seconds, at about 5 times the value of the rated fuse, the overload element will snap out of the connector, disengaging the fuse from the circuit.

This fuse allows motor protection from overloading. For example, if a dual element fuse rated at 40 amps is protecting a circuit of a 10-horsepower motor, we could sustain the overload inrush current of 193 amps for a little over 10 seconds before the fuse would open (see Figure 9).


In normal motor control circuit applications, a dual-element time-delay fuse serves as an invaluable motor-running backup protection to the motor starter overloads (see Figure 10). A dualelement time-delay fuse can be particularly helpful if a phase is lost. In addition, if the magnetic starter’s overload contacts fail to open and the power contacts remain closed because a severe overload has melted the contacts, the backup protection provided by a dual-element fuse will save the motor and circuit.


During troubleshooting, if a fuse is found to be open, it is a good idea to cut it in half and determine if the cause of the opening was a short circuit or an overload. This will help determine the source of a problem.


to be continued..........