Replacing a one way light switch with 2 two way light switches allows one light to be switched on and off independently of the other switch. An example of such a system is in a hallway where the lights in the hall upstairs can be switched on and off from a switch downstairs. Other examples are a room with has two doorways, with a light switch at each door, or a bedroom, with a main light switch and another switch within reach of the bed.
Firstly decide where to locate the second switch. Then cut a route from the existing switch to the desired location of the new switch. Prepare this location for the mounting box of the new switch, see Installing a electrical mounting box for more details. The cable which will run from the old switch to the new switch needs to be three core (red/yellow/blue conductors) and earth 1.0mm2 cable.
SWITCH OFF ELECTRICITY AT THE MAINS - Switch off the mains supply at the consumer unit and remove the fuse for the circuit you are working on. Channel a route for the cable in the wall going up from the existing switch to the ceiling then across and down to the location of the new switch. Use a cable detector to check for any existing cables in your planned route. The cable in a length of conduit should then be fitted in the channels so that the cable runs from one switch to the other. Feed both ends of the cable through the mounting boxes so that a length of cable is available at each end. The cable channels and the surround of the new mounting box can now be plastered over.
At the original switch, prepare the new cable for connection (see Preparing cable for connection). If the original switch was a one way switch then a two way switch will have to replace it. You can tell if it is a two way switch, because the switch will have three terminals usually marked C, L1 and L2 - Note that metal cased switches have a terminal on the case for an earth wire and should only be used for an earth wire.
To connect the new two way switch, fit the red conductor from the original cable together with the yellow conductor from the new cable to the screw terminal marked L1 or 1. Connect the black conductor from the original cable with the blue conductor into the terminal marked L2 or 2. Next connect the red conductor from the new cable to the terminal marked C or Common. Finally secure the earth conductors (with a green/yellow sleeving over it) to the terminal on the mounting box. This switch is now complete and can be secured in place.
At the new switch, prepare the new cable for connection.
Connect the yellow conductor to the screw terminal marked L1 or 1. Connect the blue conductor into the terminal marked L2 or 2. Next connect the red conductor to the terminal marked C or Common. Finally secure the earth conductor (with a green/yellow sleeving over it) to the terminal on the mounting box. This switch is now complete and can be secured in place.
The circuit is complete, replace the fuse in the consumer unit and restore power.