Appendix
A.2 Protection against electromagnetic interference
S7-300, CPU 31xC and CPU 31x: Installation
Operating Instructions, Edition 08/2004, A5E00105492-05
A-7
A.2.2.5
5. Basic rule for ensuring EMC
Rule 5: Homogeneous reference potential
Create a homogeneous reference potential and ground electrical equipment whenever
possible (refer to the section on Equipotential bonding).
•
Route your equipotential conductors over a wide area if potential differences exist or are
expected between your system components.
•
Make sure you carefully direct your grounding measures. Grounding measures protect
the controller and its functions.
•
Form a star circuit to connect the equipment in your system and the cabinets containing
central/expansion units to the grounding/protective conductor system. This prevents the
formation of ground loops.
See also
Equipotential bonding (Page A-14)
A.2.3
EMC-compliant installation of PLCs
Introduction
Quite often it is the case that interference suppression measures are not taken until
corruption of user signals is detected after the controller is actually in operation.
Frequently, the causes of such interference are found in inadequate reference potentials as
a result of faulty installation. This section shows you how to avoid such errors.
Inactive metal parts
Inactive parts are referred to as electrically conductive elements, separated from active
elements by a basic insulating and only subject to electrical potential if an error occurs.
Installation and ground bonding of inactive metal parts
Bond all inactive metal parts to a large-surface ground when you install the S7-300. Proper
ground bonding ensures a homogeneous reference potential for the controller and reduces
the effect of interference coupling.
The ground connection establishes an electrically conductive interconnection of all inactive
parts. The sum of all interconnected inactive parts is referred to as chassis ground.
This chassis ground must never develop a hazardous potential even if a fault occurs.
Therefore, chassis ground must be connected to the protective conductor using cables with
an adequate conductor cross-section. To avoid ground loops, physically separate chassis
ground elements (cabinets, parts of the building construction or machine) must be bonded to
the protective conductor system in a star circuit.
