Introduction to Industrial Automation

3.3   The Meaning of the Electrical and Mechanical Latch

Many times, the operation of a motor prevents the operation of another one, mainly due to the fact 

that the executed physical operations can be in contrast. For example, in the case that a motor creates 

the insertion of a robotic arm in the interior of a car during a car assembly production line, then the 

operation of the corresponding motor for the movement of the cars in the production line should be 

prevented. In these cases, the automation circuit should mutually exclude the operation of the second 

motor, even if such an operation is commanded by mistake. This functionality is achieved through 

the insertion of an NC contact of a power relay in the activation branch of the other power relay, in 

which the mutual exclusion should be achieved, as indicated in Figure 3.15. This action is called “elec-

trical latch”. Through the electrical latch, the activation of the relay is prevented based on an electrical 

approach, without determining that it is impossible to have a mechanical activation of both relays.

N

C

50 Hz 230 V

R

 

 

Stop

Start

e

C

 

 

Stop

Start

 

 

Stop

Start

 

 

Stop

Start

Control

panel 1

3 wires 

Control

panel 2

Control

panel 3

Control

panel n