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The dual-element fuse provides the same short-circuit current protection as the single-element fuse along with time-delay protection against slow over-currents. Figure 1 shows cut-away pictures of a dual-element fuse. From these pictures notice that the dual-element fuse provides a short-circuit element and an overload element. Short-circuit elements (neck-down sections) are located on each end of the fuse, while the overcurrent element is located between them. The buildup won't damage the motor, and large overloads will be cleared quickly, since they will cause heat buildup that will permanently damage the motor. This design allows the motor to draw high inrush current, up to 500% of the full-load current for 10 seconds, which is adequate time to allow the motor to start. It will also allow the motor to develop extra horsepower to meet an increased load demand for several minutes, since the extra current that is drawn won't cause the fuse to open.
Figure 1a shows a normal dual-element fuse and notice the neck-down sections on each end and the time-delay element in the middle of the fuse. Figure 1b shows the excess heat of a slow overcurrent as it begins to loosen the spring- loaded time-delay element. Figure 1c shows the spring-loaded time-delay element has caused an open in the fuse.
Figure 1d shows the dual-element fuse has experienced a short-circuit current and an arc is established in the neck-down section. Figure 1e shows the neck-down section has opened and interrupted current flowing through the fuse.