The double throw element is provided by the two traveler wires that pass onward from the four way switch to the next switch. For example the diagram below shows the inside of each switch and their onoff positions.
Light Switch Wikipedia
How does a double light switch work. Understanding exactly how a four way switch and two three way switches work together to control a light fixture from all three locations can be tricky to visualize. This means that at any time either switch may be upside down so that its on in the down position or off in the up position. The third wire creates repetitive switching at either end. A two way light switch produces light when two of three wires transport a current to either end of the installation. A double pole double throw switch dpdt manages two circuits and turns both on and off together. A two way light switch requires a three way cable three wires plus ground four conductors in all to be run in order for the light to be switched from any station.
A double switch allows you to operate two lights or appliances from the same location. A double pole switch is what you need to control a 240 volt device because 240 volt circuits have two hot wires. In regular single pole switches you can attach the hot wires to either screw terminal. Not only that but this also allows you to turn on the light from one switch and turn it off from the other switch. Double switches sometimes called double pole allow you to separately control the power being sent to multiple places from the same switch. The red wire is the traveler that carries the current from one switch to another.
For example you might want to turn on a bathroom light separately from the ceiling fan. The difference between a single and double pole switch is that the latter controls two circuits at the same time. Most manufacturers use the abbreviations dpst and dpdt to indicate switch types. It has four terminals. A double pole switch has two connectors poles that can control two circuits. A double pole single throw switch dpst turns two circuits on or off.
This wiring setup uses the two traveler wires to allow for either light switch to control the light fixture. In a double switch you want the feed wire from the service panel to be attached to the screws on the side. Two for incoming hots and two for outgoing hots.
Gallery of How Does A Double Light Switch Work
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