II - DCE FEL ČVUT v Praze

Mechanical Configuration of an Automation System with SIMATIC S7 Ex Modules
1.9.2 Distributed Arrangement of Systems with S7-300, M7-300 and
ET 200M
I/O Modules with Intrinsically-Safe Signals
C79000-G7076-C152-04
The process engineering of a plant, such as gas supply, requires a
wide-ranging exchange of information between the systems with the
distributed Ex I/O devices and the central, electrical or electronic
measurement and control system. This necessitates a great number of cable
connections, sometimes extending over several hundred meters - in the case
of gas storage systems, over several thousands of meters. In the event of a
lightning strike, therefore, extensive voltage pick-up occurs.
A distributed arrangement of instrumentation and control equipment with
relatively short cables to the plant, and the connection of distributed I/O
stations to each other and to the central controller via a bus (PROFIBUS-DP)
or fiber-optic cable, are an important measure for reducing overvoltages
between sections of the plant.
You will find more detailed information on this arrangement in the manuals
specified in the foreword.
1.9.3 Shielding of Cables and Buildings
Overvoltages between separate plant sections or buildings cannot be avoided
in practice by meshing. In the event of a lightning strike, a circulating current
will flow over the path created by metal connections between the buildings
or between a building and I/O device. Cable cores are ideal for this purpose.
The lightning or partial lightning current must therefore be offered other
conductive connections. Shielding which can be implemented in different
ways is particularly suitable, for example:
A helical current-rated metal strip or metal braid as the cable shield, e.g.
NYCY or A2Y(K)Y.
By installing the cables in continuously connected metal conduits which
are grounded at both ends.
By installing the cables in reinforced concrete ducts with throughconnected
reinforcement or on closed cable racks made of metal.
By laying conductors (shield conductors) in parallel with cables. This
measure, however, only relieves the cables of partial lightning currents.
or
By laying fiber-optic cables.
Overvoltage-sensitive equipment must also be shielded to ensure the currents
at the cable ends cannot destroy this equipment. This is achieved with metal
housings or by installing the equipment in metal cabinets which are
connected to the ground conductor.
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