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A "4-way" (intermediate) switch is a purpose built double pole, double throw (DPDT) switch, internally wired in manufacture to reverse the connections between the input and output and having only four external terminals. This switch has two pairs of "traveler" terminals that it connects either straight through, or crossed over (transposed, or ...
Initially 4-wire was used and many older installations still use it, then the 6-wire became the new standard, but the 4-wire has latterly been reissued to all Openreach technicians as part of cost savings. Modern 4-wire however is the same diameter as 6-wire to allow technicians to retain existing tacking guns and cable clips.
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The circuit diagrams in this article follow the usual conventions in electronics; [1] lines represent conductors, filled small circles represent junctions of conductors, and open small circles represent terminals for connection to the outside world. In most cases, impedances are represented by rectangles.
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Most automated telephone exchanges use digital switching rather than mechanical or analog switching. The trunks connecting the exchanges are also digital, called circuits or channels. However analog two-wire circuits are still used to connect the last mile from the exchange to the telephone in the home (also called the local loop).
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