There are two terminals in a one way light switch. When the switch is on, both terminals are connected together.
Typically, these terminals will be marked COM and L1 (sometimes L1 and L2). Although, technically, it doesn’t matter which way round you connect the wires, it is best to stick to convention and connect the permanemt live (from the supply) to COM and the switched live (to the lamp) to L1.
This is the most common type of switch, and is used where a light is controlled from a single switch (although you will often see a two way switch used, with one terminal left unused).
This is fairly obvious, but when the switch is ‘on’ the COM and L1 terminals are connected together. When ‘off’ they are not.
2 way light switch wiring (3 wire system, new cable colours)
2 way light switching (3 wire system, old cable colours)
2 way light switching (2 wire system)
3 way light switch wiring (3 wire system, old colours)
3 way light switch wiring (3 wire system, new cable colours)
3 way light switch wiring (2 wire system)
A two way light switch has three terminals and is quite like a sigle way switch but now the ‘off’ position can be utilised also.
Some brands of switch may have slightly different labeling on the terminals (such as C L1 L2 or even L1 L2 and L3).
This type of switch is used where two switches control the same light, such as at the top and bottom of a stairway. You can use it as a one way switch, by using the COM and L1 terminals only.
In position 1 (when the switch is down or ‘on’), COM and L1 are connected together (just like the one way switch). In position 2 (when the switch is up or ‘off’), COM and L2 are connected together.
There is never a connection betweeen L1 and L2.