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S7-300 Automation System, Hardware and Installation: CPU 31xC and CPU 31x
A5E00105492-03
5-23
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Low-resistance connections to ground reduce the risk of electric shock in case of a
short-circuit or system fault. Low-impedance connections (large surface, large-
surface contact) reduces the effects of interference on the system or the emission
of interference signals. Here, effective shielding of cables and devices is also a
significant contribution.
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All protection class 1 devices, and all larger metal parts, must be connected to
protective ground. This is the only way to ensure that the system user is protected
from electrical shock. It also deflects interference which is emitted from external
power supply cables and signal cables to cables connected to I/O devices.
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The table below shows an overview of the most important measures for protective
grounding.
Table 5-8 Protective grounding measures
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Cabinet/mounting
structure
Connection to central ground (e.g. equipotential bus line) via
cables with protective conductor quality
Rack / rail Connection to central ground with cable of minimum cross-
section of 10 mm
2
, if the rails are not installed in the cabinet
and not interconnected with larger metallic parts.
Module None
I/O Device Grounding via Schuko plug
Sensors and actuators Grounding in accordance with regulations applying to the
system
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