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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.
As many modern types of electrical accessories (e.g., home automation control elements from non-UK manufacturers) are not available in BS 4662 format, other standard mounting boxes are increasingly used as well, such as those defined in DIN 49073-1 (60 mm diameter, 45 mm deep, fixing screws 60 mm apart) or, less commonly in the UK, ANSI/NEMA OS-1.
List of the mainly electro mechanical switching systems from Hasler AG Bern, which were used in public telephone network in Switzerland for many decades. Hasler AG finally merged into Ascom in 1987. HS 25 (modified from the Ericsson OL-100 system with double relays and 25-point selector (Ericsson license), mainly used for small villages and ...
An example is a pushbutton switch. SPST-NC Form B [4] Single pole, single throw, normally closed A simple on-off switch. The two terminals are normally connected together (closed) and are open when the switch is activated. An example is a pushbutton switch. SPDT Form C [4] Single pole, double throw: Two-way: Three-way
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The No. 4 Electronic Switching System (4ESS) is a class 4 telephone electronic switching system that was the first digital electronic toll switch introduced by Western Electric for long-distance switching. It was introduced in Chicago in January 1976, to replace the 4A crossbar switch. [1]