Operating instructions

Guidelines for Installing Networked Automation Systems in Buildings
7-9
SIMATIC NET Twisted-Pair and Fiber-Optic Networks
C79000-G8976-C125-02
7.3.3 Requirements of the Power Distribution System
General
HD 384.3 S2 (IEC 60364-3:1993, modified, /22/) describes various power
distribution systems (TN-S, TN-C,S, TN-C, TT and IT systems). Additional national
or local regulations stipulate the measures required for protection from electric
shock and stipulate the requirements for a grounding system (see also section 7.2
protection from electric shock).
The outer surfaces of switching cubicles, device casings, connectors and bus
cables are conductive to provide shielding and must be connected to the grounding
system to ensure safety. To ensure that the EMC shield effect is achieved, they
make further requirements of the grounding system and grounding of the power
distribution system. These result in an alternating power distribution system with
non-current carrying grounding conductors, for example as in the TN-S system.
Cable shields are part of the equipotential bonding network of a system.
Since the shields of twisted-pair cables are included in the bonding system, all the
currents coupled into the bonding system of a building or plant flow through them.
Depending on the intensity and frequency range, these shield currents can cause
disturbances in data communication. Measures must therefore be taken to avoid
the alternating power distribution system of a plant including the bonding system in
the power return cabling. A TN–S system with separate cables or N and PE, for
example, meets these requirements. The EN 50310:2000 /21/ standard provides
detailed guidelines for installing a network system for supplying information
technology equipment.
Note
DTEs and /or network components connected over shielded twisted-pair cables
must only be supplied by alternating power distribution systems whose grounding
conductors cannot contribute to the transmission of energy. There must be no
PEN cable within the entire system. This condition is met, for example, by a TN-S
system.