Tetrapolar plug
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A standard tetrapolar telephone plug in electronics and telecommunications, has four round metal pins and one plastic pin. The design is only used in Belgium for telephone wiring.[1] It is similar to the tripolar telephone plug of Italy and also the Swedish telephone plug.


The plastic pin adds a presence function. When not inserted into a jack, the jack itself (mechanically) connects the incoming line to the next socket. The article on the Swedish telephone plug details its functional principles.
References
[edit]- ^ Manley, Myron (2002). Global Connect!: Telecommunications, Cell Communications, Internet Connectivity, Mobile Connectivity : 161 Countries. World Trade Press. pp. 100–101. ISBN 978-1-885073-92-1.