Introduction to Industrial Automation

3.3.1   Sequential Start—Latch of Machines (Chain Latch)

In industrial production lines where multiple machines are lined up one after another and are 

functionally cooperative for the completion of a physical production procedure, it is common to 

cancel the operation of a machine just in case the previous machine has not been set in operation. 

With this exception, it is ideal that the machines should start progressively (like a chain), with 

only one operational command. The desired automation circuit for four machines is presented in 

Figure 3.16, and the number of utilized machines can be generalized independently.

The circuit of Figure 3.16 presents a special functionality, which is desirable in many cases. 

Let’s assume that during the operation of the four machines, a fault takes place and the thermal 

relay e

2

 is energized, and thus machines C

2

, C

3

, and C

4

 stop operating, which is likely to happen 

(the breaking of the operational chain). If machine C

1

 continues to operate, the thermal relay is 

de-energized, and then the three machines will start operating again, a situation that could be 

forbidden for some specific physical processes. In these cases, after the de-energizing of the ther-

mal relay, we don’t want the machines to restart, instead we need a new command (the press of a 

START button). This functionality can also be considered as an indirect way of forcing the opera-

tor to notice the break in operation in the machines. The characteristics of this desired automation 

are achieved through the automation circuit shown in Figure 3.17.

N

C

1

50 Hz 230 V

R

Stop

Start

e

1

C

1

e

2

C

2

C

1

e

3

C

2

e

4

C

3

C

3 

C

4