Introduction to Industrial Automation

Table 3.1 (Continued)  Symbols of Switching Contacts and Automation Devices

R

S

T

a) Multi-line symbol of 3-phase 

power supply

b) Single-line symbol of three 

conductors

3-pole switch and fuses 

combination

Table 3.2  Meaning of Letters in Automation Circuits

Device

Letter

Example

Power switch

a

Load isolation switch, motor protection switch, 

etc.

Auxiliary switch

b

rs

Selector switch, button, limit switch

Rotary switch

Power relay

c

Power relay for motor supply

Auxiliary relay

d

Relay used as logical component

Protection or safety devices

e

Melt fuses, automatic fuses, thermal overload 

relay, etc.

Measurement transformers

f

Current and voltage transformers

Measuring instruments

g

Ampere meter, voltage meter, power meter, etc.

Indication devices

h

Lamp, siren, horn, etc.

Capacitors

k

Capacitor for power factor correction

Machines and power transformers

m

Motors of any type, power transformers

Rectifiers and batteries

n

Bridge rectifiers, batteries 24 V DC for 

automation circuits supply

Resistors

r

Heat resistors, induction motor resistors, etc.

Special type devices

u

Any device not belonging to the above 

categories

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Industrial Automation Synthesis 

◾

 

105

  3. In every column, we denote the number of the branch where a closed or an open contact 

exists, respectively, and this is happening for all the existing contacts of the relay.

  4. We repeat step (3) for all of the remaining relays.

As an example, in the table of the C

3

 relay and in the column O, there are the numbers 2 and 

3, which indicate that one open contact of C

3

 exists in branch 2 and one in branch 3. With this 

approach, the numbering of all utilized NO and NC contacts that are needed for the correspond-

ing relay takes place. This procedure is extremely important, especially in the case of selecting the 

necessary relays and implementing a specific automation circuit.