Talk:2 Way Switching

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Live or Line

Every DIYer knows what live is. Some know what line is, some don't. NT 08:48, 1 July 2010 (BST)

FAQ material

By Matthew Marks 15/9/1998

Many people have learnt the wiring of a two-way switch at school. How it is done in practice, though, is slightly different, to avoid having to put a connector block inside a switch box. This is the layout:

Line -----------------o------------------------------------
                      |                                    |
                      O        (triple and earth cable)    O
          Switch A      \________________________________/     Switch B
                      O                                    O
                      |                                    |
Lamp -----------------o------------------------------------

All connections between cables are made at switch contacts. The colour coding of the triple and earth is arbitrary, although I connect red to line (as the incoming core will be red), yellow to the switch commons (as it is the "middle" colour) and blue to the lamp (as it is nearest in colour to the outgoing black core).

Adding more switches is easily done: use a special "intermediate switch" which has four terminals. This breaks into the red and blue cores (if following the suggested colour code) and either connects them as normal, or connects incoming red to outgoing blue, and vice versa. With care, the yellow core can be left untouched, so a connector block does not need to be used here either.

As many intermediate switches as you desire can be fitted, although as far as I am aware, they are not available as doubles or triples on the same wall-plate.

A few tips:

  • Arrange the wiring so that the lamp is off if all switches are up. This makes it easy to be sure that the lampholder is not live when changing bulbs
  • As with all wiring, use green and yellow sleeving around bare earth conductors
  • As with all wiring, put a ring of red tape around exposed cable cores which are not red, but are nevertheless live: yellow and blue cores of triple and earth cable, and black cores of twin and earth switch feeds. (Twin and earth with two red cores is also available, but with a price premium.)