Appendix
A.2 Protection against electromagnetic interference
S7-300, CPU 31xC and CPU 31x: Installation
A-14 Operating Instructions, Edition 08/2004, A5E00105492-05
A.2.7 Equipotential bonding
Potential differences
Potential differences can occur between separate system elements. This can result in high
equipotential currents, e.g. if the cable shielding is terminated at both ends and grounded to
different system components.
The cause of potential difference can be differences in the power supplies.
Warning
Cable shielding is not suitable for equipotential bonding. Always use the prescribed cables
(e.g. with a cross-section of 16 mm
2
). When installing MPI/DP networks, provide a sufficient
conductor cross-section. Otherwise, interface hardware might get damaged or even be
destroyed.
Equipotential bonding conductor
To reduce potential differences and ensure proper functioning of your electronic equipment,
you must install equipotential bonding conductors.
Note the following points on the use of equipotential bonding conductors:
• The lower the impedance of an equipotential bonding conductor, the more effective is
equipotential bonding.
• When shielded signal cables interconnect two system components and the shielding is
connected on both ends to ground/protective conductors, the impedance of the additional
equipotential bonding conductor must not exceed 10% of the shielding impedance.
• Determine the cross-section of your equipotential bonding conductor on the basis of the
maximum equalizing current that will flow through it. The equipotential bonding conductor
cross-section that has proven best in practice is 16 mm
2
.
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