Journeyman Electrician: A Simple Guide To Switches

A toggle switch is actuated by the movement of an actuator that extends from the body of the switch.  Mercury toggle switches don’t wear out as quickly as the standard type and they operate silently.  Instead of a snap as the contacts are made, liquid mercury flows onto the contacts and makes the electrical connection.
A rocker switch is similar to a toggle switch but operates by rocking the actuator, where one side of the switch is raised while the other side is depressed.
Slide switches are actuated by moving an actuator from side to side or up and down.

Rotary switches are actuated through the rotation of a shaft that moves various contacts to make the connections.  A key switch is a special case of a rotary switch with a key being required to change the position.

A leaf switch is composed of metal leaves.  The action of moving one leaf causes it to bend and make contact with the other leaf.
Pushbutton switches are actuated by pressing in on a button.  Some ON-OFF types automatically change between being extended and depressed each time the switch is pressed.
The knife switch is so named because it has a thin metal blade that is hinged and rotates into and out of position to make or break contact.

Common Uses of Switch Designs
Almost all switch types may be found mounted in panels, whether they are electrical distribution panels or control panels for equipment and appliances.  Key switches are used for added security, on machines and computers for example.  Toggle and rocker switches are commonly used for control of power to loads such as lights. 

The common wall switch is usually either a toggle or rocker switch. 

Knife switches are used for the application of high current where arcing might occur.  The simple design of the knife switch actuator limits the effects from this arcing and extends the life of the switch. 

Leaf switches are often used as limit switches to control mechanical devices.  A limit switch is usually a leaf switch that is used on machinery to open a circuit when travel has reached a preset limit.

Configurations of Poles and Throws in Common Switches
The number of parallel independent connections (poles) is usually one or two, but can range much higher.  The number of positions (throws) possible for each pole is also usually one or two but can also be higher.  A rotary switch, for example, may have dozens of possible positions for each of several poles that are ganged together.  The following abbreviations are used for the most common combinations of poles and throws:

 
Switches with higher numbers of poles and throws are abbreviated using numbers such as 3PDT for a three pole double throw switch.

Considerations in Operating a Knife Switch
Knife switches are used in high current applications where arcing might occur.  To minimize arcing, the switch must be opened and closed quickly.  When closing the switch the operator must push the knife actuator firmly and rapidly into the contacts so there is no intermittent contact.  When opening the switch the operator should jerk the actuator out and away from the contacts quickly to avoid or limit any arc.
Single throw knife switches that are mounted vertically must have their blade hinge at the bottom.  The load side of a knife switch is always wired to the blade so that the piece that is closer to the operator’s hand is not live when the switch is open.
Knife switches have open frames that expose the electrical contacts so their use is limited.  Often there is an insulated handle that is mechanically coupled to the switch contacts which are concealed inside an enclosure.  An enclosed knife switches that require the switch to be closed before the housing door can be opened is known as a safety switch.

Configurations for Wiring Wall Switches
Simple wall switches to turn lights on and off are usually
SPST toggle or rocker switches.  They switch the ungrounded (black) conductor, while the grounded (white) conductor is connected directly to the load.
SPDT switches can be used for three-way connections where two switches can be used to control a single light.  This is shown below:


The light will be energized only when both switches are in the same position, either up or down.  If the switches are in opposite positions as shown the light will be off.  The wires that connect the two switches together are called traveler wires.

Operation and Use of a 4-way Switch
The four-way switch has two common inputs that can be switched to either of two outputs as shown:


Four-way switches are used in combination with three way switches to provide more than two control points for a circuit.  Shown below is the configuration for three switches to control a load.  For higher numbers of control points, additional 4-way switches are added between the 3-way switches at each end of the string.