Introduction to Industrial Automation

rack of a PLC (Siemens).

PS

C

P

U

AI

4

4

PS

C

P

U

PLC

AI

4-20 mA

4

Sensing

element

Shielded twisted-pair cable

Braided shield

To central grounding system

Figure 6.46  Grounding of a shielded cable connecting an analog sensor to a PLC.

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Basic Operating Principles of PLCs 

◾

 

253

The lightning protection and the corresponding measurements for the evaluation of the 

technical parameters are divided into two parts, external and internal lightning protection in 

accordance with the regulations VDE 0185 and IEC 62305. This protection is further clas-

sified into four zones of lightning protection with the lightning protection of types “Room” 

and “Device” that correspond to zones two and three. Lightning protection is generally an 

important issue in electrical installations, and thus it is not possible to present it in this book 

in full detail.

Internal lightning protection is aimed at reducing the effects of lightning current and the 

resulting electric and magnetic fields on metal installations on all kinds of electric appliances 

within a closed space. Internal lightning protection must exclude the generation of danger-

ous sparks between electrical or telecommunication installations and metal structures, and 

include:

 

◾

The equipotential bonding through the grounding system

 

◾

The shielding and overall isolation between the installation sections

 

◾

The use of devices for protection against overvoltages caused by lightning (surge arresters, 

surge protectors, and surge suppressors are terms used for such devices)

The equipotential procedure reduces potential differences across devices and conductors 

induced by the lightning current, and ideally means that an identical state of electrical poten-

tial for all equipment items is valid. This is achieved by bridging the metallic structures with 

the electrical and telecommunication installations through connecting conductors, as well as 

with the external conductive parts. All the incoming and outgoing cables in the space that 

should be protected should be equipped with an equipotential system. This rule includes power 

cables and the low-power signal cables, as well as the metallic channel cable trays shown in 

Figure 6.48. Also, all the cables between two buildings must be shielded, and subsequently, to 

bridge the corresponding shielding. The closed metallic cable trays facilitate the shielding and 

the equipotential connection. In Figure 6.49, it is presented as a way of edging two or more 

PLCs of lightning protection devices in a communication network, either directly to the bus 

cable or at the T-branch.

At this point, it should be noted that there are a plethora of lightning protection devices for 

communication network cables in the related market. Manufacturers of these devices generally 

provide the information required for the correct connection method and for their appropriate 

location along the communication interface cable. In Figure 6.50, a detailed approach of how 

to interface two PLCs with lightning protection is shown, located in adjacent metallic industrial 

enclosures in the same interior space. Lightning protection in the power supply section of the 

whole installation is also depicted.