Switches

Objectives

  • Describe the function of a switch.
  • Illustrate the fundamental concepts of "poles", "throws", "normally open", and "normally closed."
  • Identify common switch components.

What Is a Switch?

Electric circuits usually need a control device. That control device simply "switches" the load into or out of the circuit.

Operate the switch by clicking the "Switch" button below the graphic, and watch the circuit respond.


Shapes, Sizes & Applications

Electrical control switches are available in a variety of shapes and sizes.

They can be operated as toggles, selectors, push buttons, or switched by mechanical devices. You also can find pressure, flow, level and temperature switches in instrumentation applications and on mobile equipment.

Normally Closed or Normally Open?

Switch contacts can be normally closed (N.C.) or normally opened (N.O.).

Normally closed and normally open identify a switch's state when there is no power or external force operating on it.

Normally Closed Switches

N.C. switches have closed contacts with a resistance close to zero ohms. The four symbols below are a few examples of N.C. switches. Note that each symbol illustrates that there is continuity across the two sides of the switch.

Normally Open Switches

N.O. switches have open contacts with very high resistance (several hundreds of megohms). The same four switch symbols are shown below, but this time, they are drawn as N.O., without contact between the two sides of the switch.

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