Introduction to Industrial Automation

Figure 6.44  RS 485 connection between two PLCs.

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  Introduction to Industrial Automation

engineering, further analysis on this issue will not be provided here; however, the most important 

general rules and design guidelines are as follows:*

  1. All PLCs should be firmly grounded in accordance with the manufacturer’s specifica-

tions. Many PLCs have individual grounding points that should all be grounded; for 

example, a ground point on the power supply and a second ground point on the PLC’s 

mounting rack. All connections of the ground conductors should be screwed and never 

soldered.

  2. The industrial electrical enclosure within which the PLC will be installed should be evalu-

ated for good grounding.

  3.  Every ground conductor should have a resistance less than 0.1 

Ω

, and resistance between 

the earth bus-bar of the industrial enclosure and the ground must be less than 0.1 

Ω

. 

The grounding conductors should indicate a current flow only during the event of a 

failure.

  4. The entire grounding “chain” must be continuous and there should not be a discontinuity 

point, which means that a part of the equipment is substantially ungrounded, although will 

appear to have grounding connections. In Figure 6.45, an indicative grounding chain is 

*

 

DIN VDE 0160.

Earth

Shielded cable for

analog inputs/outputs

C

P

U

PS

C

P

C

P

C

P

PLC

Ι/O

Ι/O

Ι/O

Door

grounding

Grounding

copper bar