Electrical Symbols

Control circuits can be represented pictorially in various ways. One of the more common approaches is to use control logic diagrams which use common symbols to represent control components. Although control symbols vary throughout the world, the symbols used in this course are common in the United States and many other countries.

Contact Symbols                           Various devices incorporate contacts to control the flow of current to other control components. When in operation, a contact my be either open, a condition which blocks current flow, or closed, a condition which allows current flow. Control logic diagrams, however, cannot show the dynamic operation of contacts. Instead, these diagrams show contacts as either normally open (NO) or normally closed (NC).

                                                        

 

The standard method of showing contacts is to indicate the circuit condition produced when the actuating device is in the de-energized (off) state.

 

For example, in the following illustration, the contacts are part of a relay. The contacts are shown as normally open to indicate that, when there is no power applied to the relay’s coil, the contacts are open. With the contacts open, there is no current flow to light.

 

 

Symbols on a control logic diagram are usually not shown in their energized (on) state. However, in this course, contacts and switches are sometimes shown in their energized state for explanation purposes. In such cases, the symbol is highlighted.