Two way switching means having two or more switches in different locations to control one lamp. They are wired so that operation of either switch will control the light(s).

Standard two way switching circuit

To achieve two way control the usual single switch is replaced with one of the following circuits:

A typical application is that of being able to switch the hall and landing lights on and off from both upstairs and downstairs. An implementation that allows both lights to be controlled from both locations could look like this:

(in the above example the landing light would be switched off, and the hall light would be on)

Two way switches usually have terminals marked COM, L1 and L2. In the above diagram, the black core of the 3&E has been connected to the COM terminal, and the Brown to L1 and the Grey to L2.

Alternative method of Two Way Switching

        o------------------------------------o             L1
       /                    
a ----O                                        O----- b    COM
                                              /
        o------------------------------------o             L2 

    switch           cable run            switch

Three Way (or more!) Switching

The addition of an "intermediate" cross over switch allows a two way switched lamp to be controlled from a third position. In fact any number of intermediate switches can be added, to add as many extra switching positions as required.

a ------o---------------o   o----------------o             L1
         \               \ /
          O--------->     X      <---------O              COM (not connected at X)
                         / \                \
b ------o---------------o   o----------------o             L2

    switch   cable   crossover sw   cable   switch

An application of above using multiple switching positions to switch multiple lamps:

Alternative method of Three Way Switching

        o---------------o   o----------------o             L1
       /                 \ /
a ----O                   X                    O----- b    COM
                         / \                  /
        o---------------o   o----------------o             L2

    switch   cable   crossover sw   cable   switch

a & b are the two connections that are used in place of the usual single switch (i.e. one connected to line, and the other is the switched line return to the ceiling rose or lamp fitting)
the switches at each end are 1 pole 2 way switches
Any number of intermediate crossover switches may be added to give three or more switching positions.
the cable runs are from one switch location to another
3 core & earth cable is usually used for 2 way switch circuits.

Notes

The first example given in each case is the preferred connection method, as it ensures the light is powered from the expected circuit (i.e. that feeding other lights on the same storey). It is also less likely to cause interference problems with electronic systems (the example given in the electrical regulations is inductive hearing loop systems but similar considerations apply to computer networks and other wired systems). Caution: Two way light switches in this switching arrangement will carry live wires from two independent circuits.

The alternative circuits appear to require one less wire but in reality it is usually necessary to return the COM connection at the second switch position to the first switch or the rose or light fitting which typically needs a wire. They are included here as they are often encountered in older wiring. Caution: Something to watch out for on circuits wired in this way, is lamps being powered from unexpected circuits (e.g. upstairs lamp powered from the downstairs circuit and vice versa), also the not uncommon practice of "borrowing" a neutral connection from a circuit other than the one providing the line connection. Needless to say this practice will cause unexpected trips when a modern consumer unit incorporating multiple RCDs or RCBOs is installed.

Sometimes two way switched circuits are more likely to give rise to occasional flashing problems when used with low power CFL lamps.

Carter system

Another two way switching method exists and is occasionally encountered. Its called the Carter system (other names are also used).

The lights run off wires A and B. A switch at one end switches wire A between line and neutral. The switch at the other end switches wire B between line and neutral. Thus the lights can see 2 neutrals, 2 lines or one of each.

This approach leaves the supply to the holder live when switched off, creating a very slight risk. It also feeds line and neutral to the poles of a switch, and switches do mechanically fail, creating the risk of a short. For these reasons this method has long been non-compliant, and should not be used.

Two way switching with PIRs

Another common requirement is to a single lamp to be switched by multiple PIR sensors (or for that matter multiple lamps to be switched by one). This can be achieved like this:

Note the light switch in the above diagram can force the lights "on", but will leave them in automatic mode when in its "off" position.

Keywords

Two way, two way switching, two way switch, intermediate switch, multiple light switches, switching a light from two locations, twin switches, two way wiring, hearing aid compatibility, remote switching

2 Way Switching

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