Multi-way switching
 This article deals with the possible wiring circuits when several switches are wired together with the purpose of being able to switch a load (usually light) on and off from various locations.
 There are three possible wiring schemes. For convenience they are named A, B and C in this article. Only the A- and B-type wiring schemes are used in ordinary lighting circuits. The C-type wiring scheme might be used in low voltage circuits.
The electrical switches used for two-way switching (2 locations where to turn the light on and off) are called two-way switches (UK) or three-way switches (US). These types of switches might be marked with a C for the common terminal and L1 and L2 at the other terminals. In the US the screw at the common terminal might be marked with the colour black.
When more than two locations where to turn the light on and off are necessary the extra switches must be of another type called intermediate switches (UK) or four-way switches (US). It will be seen in the following diagrams that these switches are placed in between the before-mentioned two-way switches in all wiring circuits (A-, B- and C-type).
In the A-type multi-way switching circuit the live wire is connected to the common terminal of one of the two-way switches. The lighting fixture is connected to the common terminal of the other two-way switch. The wires in between the electrical switches are called 'strappers' (UK) or 'travellers' (US).
Two-way switching, A-type
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Wiring diagram of the A-type 2-way switching circuit showing the wire connections between two 2-way switches and a lamp. With the present positions of the switches the light is off - there is no current path from the live wire to the neutral.
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Three-way switching, A-type
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Wiring diagram of the A-type 3-way switching circuit. The only difference from the above mentioned 2-way switching circuit is an intermediate switch situated in between the two 2-way switches.
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With further intermediate switches situated in series in between the two 2-way switches it is possible to turn the light on and off at as many locations as might be desired. This is also true with the B- and C-type wiring schemes that follows.
In the B-type multi-way switching circuit a wire is connected in between the common terminals of the two-way switches.
It might be an advantage in comparison with the A-type that the live wire is present at two locations at the B-type wiring scheme in case of a socket outlet.
Two-way switching, B-type
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Wiring diagram of the B-type 2-way switching circuit. This wiring scheme is characterized by a wire in between the common terminals of the 2-way switches.
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Three-way switching, B-type
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Wiring diagram of the B-type 3-way switching circuit. It is important that the live wire and the wire to the lamp are connected to the same side (here at the left side). Otherwise the light will not be alternately turned on and off in all combinations.
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As it was mentioned at the A-type section more than one intermediate switch might be installed in between the two-way switches.
The C-type wiring circuit was used in the old days with the purpose of saving some lengths of wire but it is not used today for two good reasons. Firstly both the neutral and the live wires are connected to the switch making a short circuit possible when switching off because of the electric arc generated. Secondly a lamp might be turned off and still have live potential. In one of the combinations of the C-type wiring circuit a lamp is turned off when both terminals of the lamp are 'hot'.
In the C-type wiring circuit the lighting fixture is connected in between the two common terminals of the two-way switches.
Two-way switching, C-type
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Wiring diagram of the C-type 2-way switching circuit. The circuit is only used with low voltage for reasons stated above.
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Three-way switching, C-type
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Wiring diagram of the C-type 3-way switching circuit. If the left 2-way switch changes its position the lamp will go off. But the lampholder will still be connected to live wires on both terminals. That is one of the reasons why this wiring scheme should not be used in ordinary lighting circuits.
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When on and off possibilities are desired at many locations it is usually preferred to use push buttons and bi-stable or impulse relays or a system of intelligent house control.
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