Siemens S7-400 Installation Manual page 287

Operation under Industrial Conditions
Where the monitor and programmable controller are operated under harsh
industrial conditions or there is a great distance between monitor and
programmable controller, the apparatus may be at different ground potentials; this,
in turn, can result in interference and disturbances caused by ground loops.
In these cases, double-shielded coaxial cable (triaxial cable) must be used to
transmit the video signals. The inner braided shield of this cable serves as the
return conductor and must not be connected to the shield bus. The outer braided
shield serves to discharge interference currents and must be incorporated in
shielding and grounding measures.
To avoid ground loops, the electronics ground and housing ground of the monitor
must be separate. This requirement is considered to be met when one of the
following conditions is fulfilled:
• Electronic and housing grounds of the monitor are isolated from each other.
• Electronic and housing grounds of the monitor are interconnected via a
voltage-dependent resistor (VDR) fitted by the manufacturer of the monitor.
Shielding and Grounding under Industrial Conditions
If the monitor and programmable controller are operated under harsh industrial
conditions, you must observe the following:
On the side of the programmable controller:
• Connect the cable shields in the cabinet to the shield bus immediately after
cabinet entry. The following points are important:
– Strip the video cables without damaging the conductors.
– Secure the outer braided shield with the largest possible area to the shield
bus of the programmable controller (for example, with metal hose clamps
which surround the shield, or with cable clamps).
• Provide large-area contact between the shield buses and the frame or cabinet
wall.
• Connect the shield bus to the ground point of the cabinet.
On the side of the monitor:
• Isolate the electronic ground and housing ground from each other. Proceed as
follows:
– Remove the jumper on the monitor to separate the two grounds.
– Fit touch protection to the video sockets, because when the grounds have
been separated, a dangerous touch voltage of more than 40 V may be
present at the sockets.
S7-400 and M7-400 Programmable Controllers Hardware and Installation
A5E00069481-04
Assembling and Installing Systems
A-35