Shielding And Grounding - Siemens S5-155U System Manual

3.6.2

Shielding and Grounding

System Manual
C79000-B8576-C199-03
Interference-Free Connection of Monitors
In these cases, double-shielded coaxial cables (triaxial cables) must be
used to transmit the video signals. The inner braided shield of the
coaxial cable serves as the return conductor and must not be
connected to the shield bar. The outer braided shield provides a path to
ground for interference currents and must be incorporated in the
shielding and grounding measures.
To avoid ground loops, the electronics ground and housing ground of
the monitor must be independent. This requirement is met when:
• • The electronics and housing grounds of the monitor are
metallically isolated,
• • or the electronics and housing grounds are connected via a
voltage-dependent resistor (VDR) fitted by the manufacturer of the
monitor.
If the monitor and PLC are used under harsh industrial conditions, you
must ensure the following at the PLC end:
• • Cables shields should be connected to the shield bar in the cabinet,
immediately after the cable inlet;
- the video cables should be fully stripped;
- the outer braided shield should be secured to the shield bar of the
PLC over as large an area as possible (e.g. with metal ties which
enclose the shield or with cable clamps).
• • Shield bars should be connected to the frame or cabinet panel over
a large area.
• • Shield bars should be connected to the ground point of the cabinet.
You must ensure the following at the monitor end:
• • Separate the electronics ground and housing ground.
- Remove the jumper at the monitor to separate the two grounds.
- Fit touch protection at the video sockets because hazardous touch
voltages of more than 40 V may be present at the sockets after
separation of the grounds.
• • Connect the ground clamp of the monitor to the protective
conductor.
• • Connect the cable shields to the ground clamp of the monitor;
- fully strip the video cables;
- secure the outer braided shield to the ground clamp of the
monitor over a large area.
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