Introduction to Industrial Automation

drop among Z impedence, (b) minimum length of common conductor.

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Basic Operating Principles of PLCs 

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Example 6.1  Voltage Drop Interference

The value of the Z impedance determines the magnitude of the voltage drop. For a cable with an 

impedance of 250 nH/m lead and a switching current of 1.2 A (after the closing of the switch), with 

a rise time of 0.1 

μ

S, the interference voltage will be V

interference

 = L(di/dt)=3 V.

Except for the separation of the equipment in low and high power, special care should be taken to 

ensure that the power lines (cables) are lead into the electrical enclosure in separate plastic channels from 

the corresponding conductors that are carrying the control or the measurement signals. Many industrial 

enclosure manufacturers separate the equipment and the conductors even at their carrying power level. 

For example, they can separate the 230 V AC cables from the 24 V DC conductors that usually feed the 

control units to reduce the probability of electrical interference. In Figure 6.40, there are two examples 

of a right and a wrong separation of equipment within an industrial electrical enclosure. Generally, if 

some simple rules are applied, it can be ensured that the equipment is operating properly within the 

industrial electrical enclosure, and that the causes of electromagnetic interference are avoided as follows:

  1. In the case of lighting inside an industrial enclosure, always use a light with filament tube 

(Linestra).

  2. The cables inside the industrial electrical enclosure are separated into the following three 

categories:

 

a.  Power cables (e.g., power supply cables, motor cables, etc.)

 

b.  Signal and control cables (network cables, digital cables, closed loop cables, etc.)

 

c.  Measurements cables (e.g., analog signal cables from sensors)

  3. The power, control, and data cables should be placed at the largest possible distance between 

them to avoid capacitive and inductive coupling.

  4. Always separate the AC cables from the DC cables.

  5. The distance between the power cables and the cables carrying digital signals should be at 

least 10 cm. Moreover, the distance between the power cable and the cables carrying analog 

signals should be at least 30 cm.

  6. During the installation of individual conductors, the supply and return conductors should 

be placed together in the same plastic channel in order to reduce the interference of the 

electrical fields, based on their different current direction.

  7. In each industrial enclosure that contains a PLC, an electrical socket should also be placed 

so that, in case a programming device is used, the PLC will be protected from currents gen-

erated from uneven electrical fields, and in case the ground cables for the programmer and 

the PLC do not have the same earth potential voltage (are not short circuited).

  8. External conductors, collected by different devices and inserted in the industrial enclosure 

in order to be connected to the PLC, are never connected directly to the terminals of the I/O 

modules, but to additional auxiliary terminals that are typically located at the bottom of the 

industrial enclosure.

Figure 6.41 presents a small industrial enclosure, where there is a modular form PLC in its 

interior, with all the cables well arranged in their respective driving plastic channels.