Figure 3-10.Switch schematic symbols.
OTHER TYPES OF SWITCHES
You have learned that switches are classified by the number of poles, throws, and breaks. There are
other factors used to describe a switch such as the type of actuator and the number of positions. In
addition, switches are classified by whether the switch has momentary contacts or is locked into or out of
position and whether or not the switch is snap-acting.
Type of Actuator
In addition to the pushbutton, toggle, and knife actuated switches already described, switches can
have other actuators. There are rocker switches, paddle switches, keyboard switches and mercury
switches (in which a small amount of mercury makes the electrical contact between two conductors).
Number of Positions
Switches are also classified by the number of positions of the actuating device. Figure 3-11 shows
three toggle switches, the toggle positions, and schematic diagrams of the switch. Figure 3-11(A) is a
single-pole, single-throw, two-position switch. The switch is marked to indicate the ON position (when
the switch is closed) and the OFF position (when the switch is open). Figure 3-11(B) is a single-pole,
double-throw, three-position switch. The switch markings show two ON positions and an OFF position.
When this switch is OFF, no connection is made between any of the terminals. In either of the ON
positions, the center terminal is connected to one of the outside terminals. (The outside terminals are not
connected together in any position of the switch.) Figure 3-11(C) is a single-pole, double-throw,
two-position switch. There is no OFF position. In either position of this switch, the center terminal is
connected to one of the outside terminals.