Siemens 6GK1 901-1GB01 System Manual page 89

Signal connections in existing installations
If unexplained, sporadic disturbances occur in data processing systems or on their
communication connections, it is advisable to check for unwanted shield currents. These can
be measured simply by inserting the cable in question in a clipon ammeter. Currents higher
than approximately 0.1 A indicate problems in the electrical installation, for example in the
TNC system.
If the alternating current power system supplies a large number of electronic devices or
electronically controlled consumers, the highest interference currents can generally be
observed at the third harmonic of the frequency.
Other signs of an unsuitable alternating current power supply are as follows:
● Currents on the PE conductor
● Currents through water pipes and heating pipes
● Progressive corrosion at grounding terminals, on lightning conductors, and water pipes.
Remember that sporadic events such as switching, short circuits, or atmospheric discharge
(lightning strike) can cause current peaks in the system many times higher than the
measured average value.
Troubleshooting
The following measures are suitable for troubleshooting:
● Restructuring the power distribution system (to form a TNS system)
● Replacing the electrical data cabling with fiberoptic cables
● Installing an equipotential bonding conductor parallel to the disturbed data cabling.
Example of installing FO cable in a TNCS system
The graphic below illustrates the relationships between the structure of the alternating
current network, equipotential bonding system, and information technology cabling in a
building.
Three PCs and three S7300 PLCs represent the information technology system. These are
networked using two OSMs. The housing of all the end devices and the OSMs are correctly
connected to the grounding and bonding system of the building. The PCs are connected to
the system via the PE contact of their power supply cable. The housing of the OSMs and the
racks of the S7300 PLCs are connected either directly or via a switching cubicle housing
locally to the CBN. The shields of the twistedpair cables interconnect all the device housings
and are therefore connected to the grounding and bonding systems at both ends.
Passive network components
System Manual, 02/2013, C79000-G8976-C282-01
3.3 Guidelines for setting up networked automation systems in buildings
Note
If shield currents on bus cables cause problems in communication, the safest often
cheapest solution is to replace the disturbed electrical bus connection with a fiberoptic
cable.
Installing network components in cabinets
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