Construction and Theory of Operation

The inner construction of the Fuse Link is ilustrated in the cut-away view above. There are two distinct current-responsive elements: one slow, one fast.

The slow current-responsive element is made up of a number of components. The heater coil and the soldered junction are the two primary components. The insulated strain pin serves to carry the tension exerted when the fuse link is installed in a fuse cutout, and as a heat conductor to the soldered junction. The ceramic tube serves as a heat absorber.

The slow current-responsive element functions in the following manner: The heater coil generates heat at a rate which is proportional to the square of the current. This heat is absorbed by the ceramic material and transmitted to the soldered junction via the metallic strain pin. When a certain value of current flows for a specific length of time, sufficient heat is generated and transmitted to the soldered junction to cause melting of the solder, separation of the fuse link, and interruption of the circuit. The time-current curve of the slow current-responsive element is the portion above the “knee” (4 seconds to 5 minutes on the time axis) in the graph on the right.

The fast current-responsive element is constructed like the single element in a conventional fuse link. Operation of the fuse link in time periods of less than 4 seconds is conventional. The time-current curve of the fast current- responsive element is the portion below the “knee” in the time- current graph on the right.