1. Symbols and Schematic Diagrams
Chapter 2
Symbols and Schematic Diagrams

2. Objectives Discuss symbols used in the drawing of schematic diagrams
Determine the difference among switches that are drawn normally open, normally closed, normally open held closed, and normally closed held open

3. Objectives (cont’d.) Draw standard NEMA control symbols
State rules that apply to schematic or ladder diagrams
Interpret the logic of simple ladder diagrams

4. Introduction
Schematics and wiring diagrams are the written language of motor controls
Symbols vary according to manufacturer and country
Most standardized set in the U.S. by National Electrical Manufacturer’s Association (NEMA)

5. Push Buttons One of the most used symbols in control schematics
Can be shown as normally open or normally closed
Most are momentary contact devices
Contain both movable contacts and stationary contacts

7. Push Buttons (cont’d.) Double acting push buttons contain:
Both normally open and normally closed contacts; four terminal screws

8. Push Buttons (cont’d.) Stacked push buttons
Connect multiple contact units together
Controlled by a single push button
Example: emergency stop button

10. Push Buttons (cont’d.)
Push-pull buttons are both normally open and normally closed contacts

13. Push Buttons (cont’d.)
Lighted push buttons indicate a motor is running, stopped, or tripped on overload

14. Switch Symbols Four basic symbols: Normally open (NO)
Normally closed (NC)
Normally open held closed (NOHC)
Normally closed held open (NCHO)

15. Basic Schematics Diagrams show components in electrical sequence
Always drawn to show components in their de-energized, or off, state
Any contact with the same label or number as a coil is controlled by that coil
When a coil energizes, all contacts controlled by it change position

16. Sensing Devices Determine occurring conditions
Limit, Float, Pressure, Flow, and Temperature Switches

17. Sensing Devices (cont’d.)
Coils, timed contacts, contact and other symbols

19. Selector Switches Common selector switch: the MAN-OFF-AUTO switch
Manual and Automatic
Operated by turning a knob