This is a common method of providing two switches to control one light. It is also an easy way to add a second switch to an existing circuit.
As with the one way switch, the switch cable from the ceiling rose contains two wires, a permanent live and a switched live. The difference is that one wire connects to L1 and the other to L2. This is the cable entering from the top of this diagram.
The second switch is connected to the first switch using three wires. A three core and earth cable is typically used. This contains brown, black and grey wires, but as with other switches, all of these wires are live, so brown sleeving should be placed over the black and grey wires.
This three core cable connects the two switches together. COM on the first switch connects to COM on the second switch, L1 to L1, and L2 to L2.
All of the earth wires connect to a terminal in the backbox, or alternatively to a separate earth connector.
This arrangement is often found in stairways, with one switch upstairs and one switch downstairs. The light can be switched on or off from either switch.
Metal switches are connected in exactly the same way, but the earth wires connect to an earth terminal on the switch plate. This is essential, as without an earth connection, the metal plate could become live in the event of a fault.
Red = Brown
Black = Blue
Red = Brown
Blue = Black
Yellow = Grey
All of these switch wires are live, so red sleeving is used on the black, blue and yellow wires.
The earth wires in the cable are bare copper, and must be covered with green/yellow striped sleeving.
If you have old cables without earth wires, you must NOT install any metal switches or light fittings. The wiring in your home is likely to require replacement, as lighting circuits have been installed with earth wires for over 40 years.
It is possible to have a dimmer switch on a two way circuit, but you can only have ONE dimmer. The other switch must be a standard two way switch, and this will turn the light on and off at the level set by the dimmer switch.
DANGER Dimmer switches should NOT be installed in stairways, as dimly lit stairs are just waiting for people to trip and fall down them.
If you require a dimmer, and also need to control the light level from multiple locations, the only solution is a 'master' electronic dimmer and one or more 'slave' units, which are purchased as a set. The wiring for these will be completely different, and the manufacturer of the units will provide detailed instructions. They are far more expensive than normal switches.