A symbol is a simple graphical representation of some type of component, device, and load that can be drawn easily on a two-dimensional scale. On electrical or electronic diagrams, symbols are used to represent electrical components.
Electrical symbols used in industry today are what engineers use to identify parts of an equipment or make sense of a manufacturing process or operation or the sequence of an operation. Electrical symbols used today in wiring and ladder diagrams come from National Electrical Manufacturer Association (NEMA) or the International Electrical Coalition (IEC). The organization standardized the electrical symbols to make drawings simpler to read and easier to communicate.
There are a few things to remember when interpreting an electrical symbol.
1.First thing is to study or become familiar with the symbol name and what it represents. For instance, a three phase motor is a circle with three lines connected to it, shown in figure 1. The circle represents the shape of the motor while the lines represent the number of conductors connected to it.
Fig.1: Three phase motor
2.The next process to analyzing a symbol is to observe the natural position of the device, shown in figure 2.
Fig.2: Normally Open (NO) switch
When observing a control device symbol such as a push button, look carefully at the lines and how they are situated to the terminals. If the lines of the push button are not touching it is considered to be normally open (NO), shown in figure 3. If the line of the push button is making contact then it is normally closed (NC), shown in figure 4.
Fig.3: Normally Open (NO) push button switch
Fig.4: Normally Closed (NC) Push Button Switch
Symbols that represent loads never show if they are on or off so it is important to observe the control device to see if the load such as a motor, control relay, or pilot light is supposed to be on when power is applied to the circuit.
3.Finally, when interpreting a symbol it is important to remember that the symbol is representing a device in its rested state and never showing the device as being activated by someone or something.
4.Lastly, if there is a symbol hard to recognize or understand its function, always consult the manufacturer.
Below are some standard symbols commonly used in electrical drawings by engineers.
Fig.5: Electrical Drawing Symbols