Siemens CPU 313C Hardware And Installation Manual page 50

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You should always connect both ends of the cable shielding to ground/functional
ground, since this is the only way to achieve good interference suppression in the
higher frequency range.
If you connect only one end of the shielding (that is, at one or the other end of the
cable) to ground, you will merely achieve an attenuation in the lower frequency
range. One-sided shielding connections could be more favorable in situations
• not allowing the installation of an equipotential bonding conductor,
• if analog signals (some mA or µ A) are transmitted,
• or if foil shielding is used (static shielding).
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Potential differences between two grounding points might cause an
equipotential current flow across shielding connected at both ends. In this case
you should install an additional equipotential bonding conductor..
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Always avoid the flow of operating current to ground.
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are found in the Appendix under the same topic.
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You should always ground the load circuits. This common reference potential
(earth) ensures proper functioning.
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If you want to locate a fault to ground, provide your load power supply (Terminal L-
or M) or the isolating transformer with a removable connection to the protective
conductor (see 2YHUYLHZ *URXQGLQJ Number 4).
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S7-300 Automation System, Hardware and Installation: CPU 31xC and CPU 31x
A5E00105492-03