What is a two-way light switch and how does it work?

Landings, hallways and long rooms need a light controlled from two places, and the arrangement that does it is one of the neatest bits of everyday electrical design. Understanding it explains why replacing a two-way switch has more wires than you expect, and why a two-gang switch on a landing is not the same thing at all.

Step by step

1. Understand a one-way switch first.It has two terminals and simply breaks the line conductor going to the lamp. Off means the circuit is open.
2. Understand the changeover switch.A two-way switch has three terminals: common, L1 and L2. It always connects common to one of the other two — it never disconnects, it selects. That is the whole mechanism.
3. See how two of them make a circuit.The permanent switched supply arrives at the common of switch A. L1 and L2 of switch A connect to L1 and L2 of switch B — those two wires are the strappers. The common of switch B goes on to the lamp. Current flows only when both switches have selected the same strapper.
4. See why either switch changes the state.Flipping either switch changes which strapper it has selected, so a circuit that was complete becomes broken and vice versa. Neither switch is the master.
5. Understand the three-core-and-earth cable.Between the two switches runs a cable with brown, black and grey cores plus an earth (older: red, yellow and blue). Two are the strappers, one is the common link. Any core used as a line conductor should be flagged brown.
6. Understand the intermediate switch.For three or more control points, the extra switches sit between the two-way switches and simply swap the pair of strappers over. An intermediate switch has four terminals and looks like a two-gang switch to the uninitiated.
7. Know why only one dimmer is allowed.A dimmer works by chopping the waveform, and two dimmers in series on the same lamp fight each other, producing flicker, buzz and premature failure. In a two-way arrangement, fit a two-way dimmer at one position and an ordinary two-way switch at the other.
8. Do not confuse two-way with two-gang.Two-gang means two separate switches on one plate controlling two different lights. Two-way means one light controlled from two places. A plate can be both — a two-gang two-way switch — which is where most confusion comes from.

Tips

• When replacing a two-way switch, photograph and label the cores before removal. The colours between the two switches are not a reliable guide to function.
• Retrofit wireless and smart two-way solutions exist for cases where pulling a three-core cable is impractical, and they avoid disturbing plaster.
• If a light behaves oddly — on in three of the four switch combinations — that usually means the strappers and common have been crossed at one switch.

Common mistakes

• Fitting dimmers at both ends of a two-way circuit — Two phase-control dimmers in series distort each other's waveform. The symptoms are buzzing, flicker, dead zones in the range and shortened LED driver life.
• Assuming the black or grey core is neutral — In a three-core-and-earth switch cable there is no neutral at all in a traditional installation. Every core is a line conductor and should be flagged brown.
• Replacing a two-way switch with a one-way switch — The circuit needs a changeover at that position. A one-way switch has nowhere for the third core, and the light will only work from one place, unpredictably.

If it doesn't work

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