Introduction to Industrial Automation

Figure 3.1  Elementary manual ON-OFF control of a machine. (a) Permanent stop or operation 

of a machine (C), (b) Permanent stop, instantaneous operation of a machine (C), (c) Permanent 

operation, instantaneous stop of a machine (C), and (d) Permanent stop or operation of a 

machine (C).

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Industrial Automation Synthesis 

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corresponding commands from a human operator. In this case, after the event of a power loss, all 

the sequential operations of the machines will be terminated. After the restoration of the power 

loss, and for the case of the circuit shown in Figure 3.1a, all the machines will start operating 

exactly at the same time instant, an issue that could be totally catastrophic for the industrial pro-

cess under consideration. In contrast to this undesirable operation, the automation circuit shown 

in Figure 3.1d does not allow the automatic re-operation of the machine after an impermanent 

power loss. In order for the machine to be able to operate again, the human operator should press 

the START button, which would provide full control of the machine’s operations, while allowing 

the operator to consider specific safety actions in the case of a fault, e.g., the considered power loss.

Like in the circuits shown in Figure 3.1, two horizontal lines will represent the electrical supply 

of the automation circuit. In more detail, the power line marked by R will indicate the wire that is 

carrying the voltage (phase), while the wire marked by N will indicate the neutral phase. Between 

wires R and N we assume a voltage of 230 V, but this can be any kind of standardized AC or DC 

operation voltages for the relay’s coils. For example, in the case that the coils of the relays have a 

nominal operation voltage of 24 V DC, then the upper horizontal power line will be marked as 

the positive pole (+24 V) and the lower horizontal power line as the ground pole.

For all cases of automation circuits, the following remarks should always be considered:

 

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Every automation circuit, simple or complex, is secured by a general safety fuse, especially in 

the cases where the automation includes control panels for human operators.

 

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In the case of control panels for human operators, all the automation circuits should be of 

low voltage in order to ensure the operator’s safety in case of a short circuit, current leakage, 

faulty wires, etc.

 

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The automation circuits are being designed in parallel branches between the R and N lines, 

while there is no restriction in their “length”, meaning the total number of branches.

 

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Every branch of an automation circuit can be simple or complex, which means that it can 

consist of a series, parallel, or a mixed type combination of switching contacts.

 

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Every branch of an automation circuit can contain an unlimited number of switching con-

tacts, but always only one coil should be able to get energized.

 

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It is never permissible to have serial connections of relay coils in the same branch, while it is 

permissible to have a parallel connection of two or multiple coils in the same branch.