Introduction to Industrial Automation

Figure 2.63  Example of reed switches used as position sensors.

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  Introduction to Industrial Automation

the opening of the circuit breaker. In order to enable this action, a solenoid-produced force with 

the capacity to open a circuit breaker is needed. Such a solenoid is called a “shunt trip coil” and is 

shown in Figure 2.64a. It is mounted inside a circuit breaker to perform the trip operation. There 

are many reasons why a trip operation or disconnection is desirable. Any abnormal occurrence or 

critical fault may cause more destruction if the circuit breaker remains closed. For example, in the 

case of a factory fire, the automation circuit detecting fire via a corresponding sensor must activate 

the trip coil in order for a general power interruption to take place. In such a case, the automation 

circuit should be powered by a remote separate source, such as a battery.

The under-voltage release coil, shown in Figure 2.64b, is also a solenoid actuator that performs 

a similar trip to the circuit breaker, due to the low voltage of the supply network. It is known 

that a low voltage may destroy all electrical devices (mainly electric motors) if operated for a long 

period of time. The under-voltage release coil monitors the voltage of the supplying power line 

and automatically trips the circuit breaker when the voltage falls below 70% of its nominal range. 

If the under-voltage coil mechanism is not energized, it is impossible to close the circuit breaker, 

either manually or electrically. Both trip coils may have an additional break-make contact for 

signaling purposes in the automation circuit. In Chapters 3 and 4 dealing with the automation 

circuits’ design, how the automation circuit of a power transformer activates the trip coil of a cir-

cuit breaker is demonstrated.