Mounting and connecting
3.3 Connecting the HMI device
Basic Panels
38 Operating Instructions, 04/2012, A5E02421799-03
3.3.2 Connecting the equipotential bonding circuit
Differences in electrical potential
Differences in electrical potential can develop between spatially separated plant
components. Such electrical potential differences can lead to high equalizing currents over
the data cables and therefore to the destruction of their interfaces. Equalizing currents can
develop if the cable shielding is terminated at both ends and grounded to different plant
parts.
Differences in potential may develop when a system is connected to different mains
supplies.
General requirements for equipotential bonding
Differences in potential must be reduced by means of equipotential bonding in order to
ensure trouble-free operation of the relevant components of the electronic system. The
following must therefore be observed when installing the equipotential bonding circuit:
● The effectiveness of equipotential bonding increases as the impedance of the
equipotential bonding conductor decreases or as its cross-section increases.
● If two plant parts are interconnected by means of shielded data cables and their shielding
is bonded at both ends to the grounding/protective conductor, the impedance of the
additionally installed equipotential bonding cable must not exceed 10% of the shielding
impedance.
● The cross-section of an equipotential bonding conductor must be capable of handling the
maximum equalizing current. The best practical results for equipotential bonding between
two cabinets have been achieved with a minimum conductor cross-section of 16 mm².
● Use equipotential bonding conductors made of copper or galvanized steel. Establish a
large surface contact between the equipotential bonding conductors and the
grounding/protective conductor and protect these from corrosion.
● Clamp the shielding of the data cable on the HMI device flush and near the equipotential
rail using suitable cable clamps.
● Route the equipotential bonding conductor and data cables in parallel and with minimum
clearance between these.
NOTICE
Equipotential bonding cable
Cable shielding is not suitable for equipotential bonding. Always use the prescribed
equipotential bonding conductors. The cross-section of the equipotential bonding
conductor must not be less than 16 mm². Always use cables with an adequate cross-
section when installing MPI and PROFIBUS DP networks. The interface modules may
otherwise be damaged or destroyed.
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