Siemens SIMATIC S7-400 Hardware And Installation Manual page 244

Assembling and Installing Systems
Five Basic Rules for Ensuring Electromagnetic Compatibility
In many cases, you can ensure electromagnetic compatibility by observing the
following five basic rules:
Rule 1
When installing the programmable controllers, provide large-area good quality
grounding of the inactive metal parts (see Section A.2).
Make a large-area low-impedance connection of all inactive metal parts to
chassis ground.
For screw connections on painted or anodized metal parts, either use special
contact washers or remove the insulating protective layers from the contact
points.
If possible, do not use aluminum parts for grounding. Aluminum oxidizes easily
and is therefore less suitable for grounding.
Make a central connection between chassis ground and the ground/protective
ground conductor system.
Rule 2
Ensure proper routing of lines when wiring (see Sections A.3 and A.4).
Arrange the cabling in line groups (AC power cables, power supply cables,
signal lines, data lines).
Always install AC power cables and signal or data lines in separate ducts or
bunches.
Route the signal and data lines as closely as possible to grounded surfaces
such as cabinet elements, metal bars and cabinet panels.
Rule 3
Ensure that cable shields are properly secured (see Section 4.9).
Only use shielded data lines. The shield must have a large-area connection to
ground at each end.
Analog lines must always be shielded. For the transfer of signals with low
amplitudes, it may be advisable to connect the shield to ground at only one end.
Provide the line shields with a large-area connection to a shield/protective
ground bar immediately after the cabinet inlet, and secure the shields with cable
clamps. Route the grounded shield as far as the module without interruption,
but do not ground it there again.
There must be a low-impedance connection between shield/protective ground
bar and cabinet/housing.
Use metal or metallized connector cases for shielded data lines.
A-4
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S7-400 and M7-400 Programmable Controllers, Installation Manual
C79000-G7076-C424-01