System Protection
When a heavy fault occurs within the transformer primary bushings, a link clears the transformer from the system before damage can occur, and before any other protective device can operate and cause an unnecessary interruption to any other segment of the system.
Application Data
A comparison of the time-current curves of the fuse link with those of conventional fuse links and the safe-loading time current characteristics for distribution transformers ilustrates the application potential of the fuse links.
The rather unusual current rating assigned to fuse links is an aid in their application since the current rating assigned is identical to the continuous current rating of the transformers which they were specifically designed to protect.
If the link is used in place of ordinary links, the full overload capacity of the transformer is made available, but at the same time the transformer is protected from faults and overloads wwhich could either destroy or shorten its life expectancy. Fuse link is the perfect match for transformer protection.
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.
