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Siemens SCALANCE MM900 Compact Operating Instructions Page 56

Industrial ethernet switches.
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A.2 Equipotential bonding
When and why is equipotential bonding necessary?
Several good reasons for equipotential bonding are listed below:
● Devices with a grounded interface can be damaged by potential differences.
● The shield of the PROFIBUS cable must not be used for equipotential bonding. This is,
however, the case if parts of the system connected by the cable shield are connected to
different grounding points.
● Equipotential bonding is a requirement for lightning protection.
Rule for equipotential bonding
Remember the following points about equipotential bonding systems:
● The lower the impedance of the equipotential bonding cable, the greater the effectiveness
of the equipotential bonding.
● The impedance of the additional bonding conductor must not exceed 10% of the shield
impedance of the bus cable.
● Make largearea contact between the bonding conductor and the PE conductor.
● Protect the bonding conductor from corrosion.
● Install the bonding conductor so that the area enclosed by the bonding conductor and
signal cables is as small as possible.
● Use copper or galvanized steel for the bonding conductor
● Include metal, conductive cable channels/racks in the equipotential bonding of the
building and between the individual parts of the system. The individual segments of the
channels/racks must be connected together with low inductance and low resistance and
connected to the building ground system as often as possible. Expansion joints and
angled connections should be bridged by additional flexible grounding bands.
● The connections between the individual segments of channels must be protected from
corrosion (longterm stability)
● If there are connections between sections of buildings (for example separated by
expansion joints) with their own reference point for the building ground network, a
bonding conductor (equivalent copper crosssectional area ≥10 mm
parallel to the cables. This bonding conductor is not necessary if metal, conducting cable
channels/racks are used.
Bonding conductors are unnecessary if the sections of a system are connected
exclusively using fiberoptic cable (FO).
Notes on systems in which no equipotential bonding is possible
To ensure greater immunity to interference, cables for SIMATIC NET PROFINET and
PROFIBUS are always shielded. Due to the defined shielding property, the shield needs to
make contact at both ends.
Compact Operating Instructions, 05/2016, A5E02630805-12
) must be installed


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