Introduction to Industrial Automation

6.10.2   Electromagnetic  Interference

Recently, electromagnetic compatibility (EMC) has become more and more important, due to the 

variety of electrical and electronic devices in both industrial environments and everyday environ-

ments. Electromagnetic compatibility is denoted as the ability of electrical equipment to operate 

---

246

 

◾

  Introduction to Industrial Automation

PLC

Fuses

Bad separation of equipment and wires

Correct separation of equipment and wires

PLC

Power supply

Modem

Frequency

inverter 

Fuses

PS

C

P

U

C

P

Power section

Control section

Power supply

Modem

Frequency

inverter 

Industrial enclosure

PS

C

P

U

C

P

Wire terminals

Industrial enclosure

Power terminals

Control terminals

Figure 6.40  Power equipment and wire separation in an industrial enclosure including a PLC.

---

Basic Operating Principles of PLCs 

◾

 

247

without to create electrical interferences in the operation of other devices, and also without being 

influenced from the electrical interference produced by other devices.

Modern automation and control systems are able to combine a multitude of such devices that 

contain power electronics, classical electrical devices and components, digital circuits, and digital 

data processing units. In all these devices, there is a wide variety of currents, voltages, and field 

densities that cause various malfunctions that are responsible for the downtime of machines or 

even of entire systems. Due to the high cost of a potential loss of production, nowadays, electro-

magnetic compatibility issues are taken indispensably into account in the design of new industrial 

plants and are a quality criterion for both the individual devices as well as the whole automation 

system.* Two more definitions complement a basic approach to this issue:

 

◾

Electromagnetic interference (EMI) is called the operational influence of an electric circuit, 

an electrical device, or a living organism from the electromagnetic environment.

 

◾

Electromagnetic emission (EME) is called the phenomenon of electromagnetic energy 

production-emission from a source.

Electromagnetic interference may occur in the form of inductive or electrostatic interference, 

and generally interference due to the existence of an electromagnetic field. Figure 6.42 illustrates 

the production of EMI in the form of galvanic, inductive, capacitive, and radiative coupling inside 

an enclosure containing a PLC and various kinds of conductors. PLCs, as well as the PCs for 

industrial use, should operate reliably in an environment that is generally considered to be full of 

generated and transmitted electromagnetic interference signals. They should also be designed so 

that they do not affect other devices. The latter issue, however, is a matter for the manufacturers 

of PLCs, and cannot be influenced by the design engineer. The design engineer can only reject 

a PLC that does not meet the EMI levels or provide technical information about itself. In a pro-

grammable automation system, there are several paths between the PLC and its electromagnetic 

environment, through which interfering signals can be propagated, as depicted in Figure 6.43. The 

power supplies, the I/O modules, the communication interfaces, and even the metallic environ-

ment of the industrial enclosures, all can act as coupling paths for all of the above interference 

*

 

(Norms and guidelines on electromagnetic compatibility, DIN VDE 0870 and European 

Directive 89/336/EEC).