Introduction to Industrial Automation

the line (a) and before the T connections of the bus (b).

Lightning protection device for power lines

Lightning protection device for data network

Power Cables

Signal Cables

Network Cables

Concrete-encased

grounding electrode

P

P

P

P

P

P

N

N

N

N

N

N

N

N

Lightning protection device for power lines

Lightning protection device for data network

Power cables

Signal cables

Network cables

Equipotential copper plate

Concrete base

Figure 6.48  Lightning protection through the potential equalization and use of suitable protec-

tive devices for all kinds of cables.

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Basic Operating Principles of PLCs 

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255

the solid-state switch, contained in a digital output module, is connected to a high impedance 

output device. In this case, it is possible to actuate the output device incorrectly without a real 

trigger signal from the CPU.

In both cases, the cause of the error is the fact that the switching solid state elements, presented 

either in the output circuits of the PLC or in the input devices that are connected to the PLC, pro-

duce a leakage current when they are in the OFF state. The leakage current, which may range from 

1 to 20 mA, is capable of causing an inappropriate activation of either the input of the PLC or the 

load that is connected to the output of the PLC. These two cases are presented in Figure 6.51. In 

order to address this problem, a resistive load (or, in general, an impedance) should be inserted in 

parallel to the input circuit impedance (Z) or to the high impedance output device (Z), a process 

that it is called “impedance adjustment of the input or the output”. The calculation of the required 

resistive load is rather simple and, in both cases, will be described in what follows in the form of 

numerical examples.

Ps

C

P

U

Ps

C

P

U

Electrical enclosure no.1

Plc

Plc

Pe

Pe

L

1

Mpi

Mpi

Equipotential conductor

Network

Network

Zone 2 lightning protection

Zone 2 lightning protection

Zone 0 lightning protection

Ps

C

P

U

Ps

C

P

U

Electrical enclosure no. 2

Plc

Plc

Pe

Pe

L

2

Mpi

Mpi

Zone 2 lightning protection

Zone 2 lightning protection

Zone 0 lightning protection

PS

C

P

U

PS

C

P

U

PLC

PLC

PE

PE

L

3

N Pe

MPI

MPI

Zone 1 lightning protection

Zone 2 lightning protection

Zone 2 lightning protection

Figure 6.50  Electric connection pattern of two PLCs inside corresponding enclosures for pro-