Operating instructions

Guidelines for Installing Networked Automation Systems in Buildings
7-19
SIMATIC NET Twisted-Pair and Fiber-Optic Networks
C79000-G8976-C125-02
7.4.2 Cable Categories and Clearances
Fiber-Optic Cables
When using fiber-optic cables, mechanical protection is necessary, however the
EMC rules do not apply.
Cable Groups
It is useful to group wires and cables into various categories according to the
signals they carry, possible interference signals, and their sensitivity to
interference. Minimum clearances can be specified for these categories so that
interference-free operation can be expected under normal operating conditions if
the clearance is adhered to.
Conditions
Grouping cables according to voltage classes assumes that the interference
voltages relate directly to the power supply voltage conducted (the lower the
supply voltage, the lower the interference voltage). Remember, however, that DC
or 50 Hz power supply voltages do not represent any danger to Industrial Ethernet
bus cables. The critical interference voltages in the kHz to MHz frequency range
are created by the “consumer” connected to the cable. A 24 V DC cable with which
a relay is switched regularly has a far more critical interference range than a 230 V
cable supplying a light-bulb.
In the information shown below, it is assumed that all the components within an
automation system and all the plant components controlled by the system (for
example machines, robots etc.) at least meet the requirements of the European
standards for electromagnetic compatibility in an industrial environment. If devices
are defective or incorrectly installed, higher interference voltages must be
expected!
The following is assumed:
S The cables for analog signals, data signals and process signals are always
shielded.
S The distance from the cables to the chassis surface of the system (cabinet wall,
grounded and conducting cable channel, ...) is not more than 10 cm.
Note
In general, the greater the distance between cables and the shorter the distances
over which the cables run parallel to each other, the less the danger of
interference.