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The Mysteries of Switches

Multiple-switch circuits can be mysterious things. Controlling a light from both the top and bottom of a stairway is accepted without question. Even controlling a single light from three, four or more locations is considered unremarkable by most homeowners. But how many of us understand how this is accomplished? Could you troubleshoot or replace one of the multiple switches in such a circuit? Understanding how three- and four-way switches work will allow you to design complex lighting circuits, as well as repair them when they go bad. Illustration 1 shows a house plan with three types of lighting circuits:

A dining room light controlled by a single switch.

A living room light controlled by two switches.

A hall light controlled by three switches. Illustration 2 shows how switches may be distinguished:

Single-Pole: Two same-colored terminals plus green ground. Toggle says ON/OFF.

Three-Way: Two light terminals, one dark terminal, plus green ground. No ON/OFF.

Four-Way: Four same-colored terminals plus green ground. No ON/OFF. So, how do multiple-switch circuits work? Illustration 3 demonstrates the clever way in which three- and four-way switches control the flow of electricity so that a single light (or a bank of lights) may be turned on or off at a single location, regardless of the position of the other switches. In all of the switches, red shows the path with the toggle up; green shows the path with the toggle down. The lamp will light if and only if the black current-carrying wire is uninterrupted. The simplest circuit is that with the single-pole switch. With the toggle up (labeled ON), the current flows from the input terminal to the output terminal along the red path. When the toggle is switched down (OFF), the green path leads nowhere, and the flow of electricity is interrupted. The two-switch circuit uses two three-way switches. Remember that the toggles of three-way switches are not labeled ON and OFF. Such switches may be on in either position. The illustration shows both switch toggles in the UP (red line) position, and the current flow is seen to be continuous. Switch either of the toggles DOWN (green line), and you will see that the flow is interrupted. Now, with the first toggle still down, switch the other toggle down, and you will see that the current is reestablished and the lamp lights up. Even more clever is the four-way switch, which allows three or more switch circuits. Trace the current in the three-switch circuit. The illustration shows all three toggles in the UP (red line) position, and the current flow is seen to be continuous. Switch the right toggle DOWN (green line) and the flow is interrupted. Now, with the right toggle still down, switch the left toggle down, and you will see that the current is reestablished. Alternatively, leave the left toggle up, but switch the center toggle down, and the path is completed, lighting the lamp. No matter what the position of the three toggles, you will find that switching any one changes the lamp's status. Remarkably, any number of switches can be added to the circuit simply by sandwiching more four-way switches between the pair of three-way switches. Amazing! Locating the lamp at the end of a string of switches makes the wiring of multiple-switch circuits conceptually simple (provided you have followed the logic so far!). The use of three-conductor (white, black and red, plus bare ground) cable, however, allows the lamp to be placed anywhere among the switches. Space doesn't allow display of all possibilities, but Illustration 4 shows three common examples of how these circuits are actually wired. Most homeowners don't get the chance to wire multiple-switch circuits from scratch, but it is inevitable that you will have the opportunity - or burden, depending on your outlook - to replace a defective switch. Following two simple rules will make the job simple and safe: 1. Turn off the power at the circuit breaker or fuse before starting your repair. 2. Label each wire, then sketch or photograph the switch terminals before removing any of the wires. Make sure the wires go back to their proper terminals exactly as numbered.