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Fundamentals of Electricity - Switches

The flow of electric current is turned on or off using a switch. In the open position, the electrical path is broken and no current flows. When the switch is closed, the electrical path through the switch and line is complete and current can flow in the circuit. The number of circuits controlled by a switch depends upon its number of poles and throws.

Types of Switches

Single Pole - Single Throw Switch (SPST)
Controls current in one conductor only and current is either flowing or not flowing in the conductor.

Single Pole - Double Throw Switch (SPDT)
Controls current flow in two conductors in a circuit. When current flows in one conductor, no current flows in the other.

Double Pole - Single Throw Switch (DPST)
Controls current flow in two separate conductors. Current is either flowing in both conductors or not flowing in either.

Double Pole - Double Throw Switch (DPDT)
Equivalent to using two single pole-double throw switches that move together and can be used to control current flow in two separate conductors or circuits.

Methods of Actuating Switches

Manual Switches
Manual switches, such as the home light switch, or pushbutton or toggle-type switches, that are activated by hand.

Mechanically Actuated Switches
Switches which are activated by some mechanically applied force, such as range oven or clothes dryer door switches that are actuated by the door pressing against a pushbutton.

Pressure Actuated Switches
Pressure (or vacuum) switches may contain a diaphragm which moves to close or open the switch contacts when gas pressure is present or absent on the diaphragm.

Temperature Actuated Switches
Range oven and automatic pilot devices are examples of temperature actuated switches. These switches contain an element with an enclosed fluid. As the heat from the oven or pilot flame heats the element, the fluid expands, creating pressure in the element and exerting pressure on a diaphragm that moves to open or close the switch.

Electromagnetically Actuated Switches (Relays)
Activates the switch when an electromagnet attracts a movable armature that carries one of the two switch contacts, thus opening or closing the switch, depending on the switch arrangement.