Siemens Simatic S7-300, Hardware And Installation Manual, Page: 50 / 242

Configuring
S7-300 Programmable Controller Hardware and Installation
4-22
A5E00105492-01
Rule: Ground the cable shielding
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 conductor,
•
if analog signals (some mA or
µ
A) are transmitted,
•
or if foil shielding is used (static shielding).
Note
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 conductor..
Caution
Always avoid the flow of operating current to ground.
Details on cable shielding and equipotential bonding ...
are found in the Appendix under the same topic.
Rule: Ground the load circuits
You should always ground the load circuits. This common reference potential
(earth) ensures proper functioning.
Tip:
If you want to localize a short to ground, provide your load power supply (Terminal
L– or M) or the isolating transformer with a removable connection to the protective
conductor (see Overview: Grounding Number 4).
This tip does not apply to CPUs 31xC and 312 IFM, because these cannot be
operated ungrounded.