Introduction to Industrial Automation

240

 

◾

  Introduction to Industrial Automation

Ta

bl

e 6

.3

 

C

al

cula

ti

on o

f t

he R

eq

ui

re

d I

/O D

ig

it

al a

nd A

na

lo

g P

oi

nt

s f

or t

he I

nd

us

tr

ia

l P

ro

du

ct

io

n A

pp

lic

at

io

n S

ho

w

n i

n F

ig

ur

e 6

.3

7

MODULE T

YPE I/O

LOC

AL C

ONNECTION

 

FIRS

T DEP

A

RT

M

EN

T 

NUMBER OF I/O POINTS

PERIPHER

AL C

ONNECTION 

SEC

OND DEP

AR

TMENT 

NUMBER OF I/O POINTS

PERIPHER

AL C

ONNECTION 

THIRD DEP

AR

TMENT 

NUMBER OF I/O POINTS

DIGIT__AL_OUTPUTS_24__V_DC'>DIGIT__AL_INPUTS_230__V__A__C'>DIGIT__AL_OUTPUTS_230__V__A__C'>DIGIT

AL OUTPUTS 230 

V 

A

C

Relays f

or motors DS

20

Relays f

or motors DS

12

Relays f

or motors DS

6

Relays f

or motors IR

10

Relays f

or motors IR

4

Relays f

or motors S/D

18

Relays f

or motors S/D

24

Relays f

or motors S/D

30

Indication lamps

24

Indication lamps

71

Indication lamps

50

TOT

A

L

48

TOT

A

L

125

TOT

A

L

96

DIGIT

AL INPUTS 230 

V 

A

C

Buttons

71

Buttons

50

Buttons

24

Limit switches

20

Limit switches

5

Limit switches

10

Photocells

10

Pro

ximity switches

20

Photocells

6

Pro

ximity switches

5

TOT

A

L

75

Pro

ximity switches

3

TOT

A

L

106

TOT

A

L

43

DIGIT

AL OUTPUTS 24 

V DC

V

alv

e coils

12

V

alv

e coils

20

Indication lamps

8

Signaling devices

2

Signaling devices

10

Indication lamps

20

TOT

A

L

30

TOT

A

L

34

DIGIT

AL INPUTS 24 

V DC

Buttons

6

–

–

AN

ALOG OUTPUTS ±

 1

 V

2

2

–

AN

ALOG INPUTS 4

–

20 mA

4

2

–

---

Basic Operating Principles of PLCs 

◾

 

241

central unit and the peripheral units are placed in industrial-type electrical enclosures. These 

industrial enclosures should be of an appropriate size for the spacious installation of the PLCs’ 

units, and their dimensions should be calculated to meet the maximum temperature criterion in 

their interior. Additionally, it should still be possible to place all the components in such a way to 

provide easy access. Thus, in the event of a fault or maintenance, the replacement of some com-

ponents should not be time consuming. For large PLCs, the cost of installing the equipment in a 

way that meets the manufacturer’s specifications and international regulations is quite important.

With the extension of a central PLC to a peripheral I/O system, it is possible for one portion 

of the controlled production process to shut down for maintenance, conversion, etc., while the 

remainder of the process is working properly. With the peripheral I/O system, the placement 

of multiple PLCs in different parts of an industrial process can be avoided, without implying 

that this is always possible or desirable. Instead, in the case of very large industrial processes or 

multiple small processes under operational coordination, independent PLCs are installed and 

interconnected through a communication network. The relatively smaller PLCs, interconnected 

over an industrial network, are able to replace a large central PLC and have the advantage that 

the likelihood of a simultaneous failure of all the PLCs is very small. On the contrary, a failure 

in the central PLC is quite possible, and this has the potential that the whole process will be out 

of control.

Table 6.4  Calculation of the required I/O modules for a preselected density of I/O points 

per module

Module Type I/O

Local System 

FIRST Department 

Number of Modules

Peripheral System 

SECOND Department 

Number of Modules

Peripheral System 

THIRD Department 

Number of Modules

MODULES OF 16 DIGITAL 

OUTPUTS 230 V AC

8

3

6

0

3

0

MODULES OF 16 DIGITAL 

INPUTS 230 V AC

7

6

5

5

3

5

MODULES OF 16 DIGITAL 

OUTPUTS 24 V DC

3

14

2

2

1

8

MODULES OF 8 DIGITAL 

INPUTS 24 V DC

1

2

–

–

MODULES OF 4 ANALOG 

OUTPUTS ±1 V

1

2

1

2

–

MODULES OF 4 ANALOG 

INPUTS 4-20 mA

1

0

1

2

–

TOTAL NUMBER OF 

MODULES

21

15

7

NUMBER OF EXTENSION 

RUCKS

3+1

4

3

5

2

6

Note:

  The numbers in narrow columns express the redundant inputs or outputs which are avail-

able for future use, except for the last row where empty places in the ruck express the 

future installation of modules.

---

242

 

◾

  Introduction to Industrial Automation

P

1

Local I/O system

Peripheral I/O system

PLC central unit

1st local extension unit 

2nd local extension unit 

3rd local extension unit 

Peripheral extension unit

for 3rd department   

Peripheral extension unit

for 2nd department  

C

C

P

2

C

P

1

PS

C

P

U

DO

220 V

16

DO

220 V

16

DO

220 V

16

DO

220 V

16

DO

220 V

16

DO

220 V

16

DO

220 V

16

DO

220 V

16

DI

220 V

16

DI

220 V

16

DI

220 V

16

DI

220 V

16

DI

220 V

16

DI

220 V

16

DI

220 V

16

DO

16

24 V =

DO

16

24 V =

DO

16

24 V =

C

P

1

AO

±1 V

4

AI

4-20mA

4

C

P

1

DI

24 V =

8

DO

16

24 V =

DI

220 V

16

DI

220 V

16

DI

220 V

16

PS

C

P

2

DO

220 V

16

DO

220 V

16

DO

220 V

16

C

P

1

C

P

1

PS

C

P

2

DO

220 V

16

DO

220 V

16

C

P

1

DI

220 V

16

DI

220 V

16

DI

220 V

16

C

P

1

C

P

1

DO

220 V

16

DO

220 V

16

DO

220 V

16

DO

220 V

16

DI

220 V

16

DI

220 V

16

DO

16

24V =

DO

16

AO

4

AI

4

±1V

4-20mA

24 V =

Figure 6.38  I/O hardware system configuration for the industrial production application 

shown in Figure 6.37.

---

Basic Operating Principles of PLCs 

◾

 

243

6.10   On the Installation of PLCs

The installation and operation of PLCs should follow certain rules and standards in order to avoid 

operational problems as much as possible. In this case, particular attention should be paid to the 

installation of the PLCs in harsh industrial environments, since more causes for malfunctions 

exist. The primary source of installation instructions for a PLC is always the manufacturer of the 

PLC the directions of which should be followed closely. In addition to the installation instruc-

tions from a particular PLC manufacturer, there are some more general issues regarding the proper 

functionality of the PLCs that are addressed in the following subsections.

6.10.1   Electrical Enclosure for the PLC Installation

International standards and standardization associations* have adopted a number of specifications 

that industrial electrical enclosures should meet. These regulations are related first to the environ-

ment where the PLCs will be installed and second to the degree of protection required by the 

contained electrical equipment. For example, the National Electrical Manufacturers Association 

(NEMA) defines the enclosure type 12 as suitable for electronic control devices. More analytically, 

enclosures of this degree of protection are constructed without knockouts for indoor use to provide 

protection to personnel against access to hazardous parts, against ingress of solid foreign objects 

(falling dirt and circulating dust, lint, fibers, and flying), and to provide protection with respect to 

harmful effects on the equipment due to the ingress of water (dripping and light splashing). The 

International Protection Rating, often referred to as an Ingress Protection (IP) rating, is a set of 

codes used to define specific levels of enclosure protection. These codes consist of the prefix IP, fol-

lowed by two numbers that express classifications used to measure levels of protection. The overall 

IP ratings according to IEC 60529 standard are displayed in Table 6.5.

A key issue in the installation of a PLC that should not be overlooked is the heat dissipation 

within the electrical enclosure in which the PLC will be installed. In this case, the temperature 

inside the electrical enclosure must not, for any reason, exceed the maximum operating tempera-

ture set by the manufacturer of the PLC, which in most cases is about 50°C. For this reason, the 

type of electrical enclosure and its dimensions should be calculated according to the equivalent 

thermal load of installed power devices, where the possible ventilation will be examined if it is 

required.

To avoid electromagnetic interference within the PLC’s enclosure, the high-power electrical 

equipment should be completely separated from the low-power electronic or control equipment. 

The power equipment includes:

 

◾

Power relays

 

◾

Transformers

 

◾

Frequency converters

 

◾

DC power supplies

 

◾

Any other power device

*

 

International Organizations: IEC: International Electrotechnical Commission; CENELEC: European 

Committee for Electrotechnical Standardization; National Organizations: VDE Germany; BSI England; UTE 

France; NEMA USA.

---

244

 

◾

  Introduction to Industrial Automation

The control equipment (low power) mainly includes the PLC, but also any other special purpose 

electronic device, e.g., an electronic stepper motor drive card. A galvanic link is created when two 

or more electrical circuits share a common part of an electrical conductor, as shown in Figure 6.39, 

which is usually the case for earth and chassis connections. In the circuit of Figure 6.39a, a voltage 

drop will be created in the common conductor with Z impedance when the relay C is energized. 

This voltage drop interferes with the signal in the second circuit containing the PLC, which has 

undesirable side effects. The simplest solution to this problem is to aim for a short a length as possi-

ble of the common part of the conductor, a property that is able to reduce the interference due to the 

galvanic coupling. This situation is explained in Figure 6.39b as well as in the following example.

Table 6.5  Ingress Protection (IP) Ratings of Electrical Enclosures according to IEC 60529

Ingress Protection (IP) Two-Number Ratings

1st Number Protection against SOLIDS

2nd Number Protection against LIQUIDS

0

No protection

0

No protection

1

Protection against objects over 50 mm 

(e.g. hands, large tools)

1

Protection against vertically falling 

drops of water or condensation

2

Protection against objects over 12 mm 

(e.g. fingers)

2

Protection against falling drops of water 

up to 15° from vertical

3

Protection against objects over 2.5 mm 

(e.g. wires, small tools)

3

Protection against water spray up to 60° 

from vertical

4

Protection against objects over 1 mm 

(e.g. wires, specific fine tools)

4

Protection against water spray from all 

directions

5

Limited protection against dust

5

Protection against low pressure water 

jets from all directions

6

Complete protection against dust

6

Protection against high pressure water 

jets from all directions

7

Protection against temporary immersion 

in water

8

Complete protection against long periods 

of immersion in water under pressure

Example: IP 67 Enclosure means totally protected against dust and immersion

V

1

V

1

V

2

V

2

PLC

INPUTS

OUTPUTS

PLC

INPUTS

OUTPUTS

I

1

+I

2

I

1

I

2

I

1

I

2

Z

V

1

V

2

PLC

INPUTS

OUTPUTS

Z

V

1

V

2

PLC

INPUTS

OUTPUTS

Z

V

1

V

1

V

2

V

2

PLC

INPUTS

OUTPUTS

PLC

INPUTS

OUTPUTS

V

1

V

2

PLC

INPUTS

OUTPUTS

V

1

V

2

PLC

INPUTS

OUTPUTS

PLC

Inputs

Outputs

Z

C

C

PLC

(b)

(a)

V

1

V

2

V

2

V

1

Inputs

Outputs

Figure 6.39  Galvanic coupling in electric circuits with a common return conductor. (a) voltage