VIPA SSM-BG41, Руководство пользователя

Страница: 257 / 286

Manual BG41/BG42/BG43 Startup
Rev. 99/49 5-33
5.7.4 Screening of cables
Electric, magnetic or electromagnetic interference fields are weakened by a screening; here one talks
about damping.
Interfering currents are conducted to earth on cable screens through screen rails conductively
connected to the case. Here one must insure, that the connection to the protective earth conductor is
impedance weak, or else the interference currents would become interference sources.
When screening cables the following is to be observed:
•
=
Use where possible only cables with a braided shield. .
•
=
The congruent thickness of the screens should be more than 80%.
•
=
Avoid cables with membrane screens, as the foil can be easily damaged by tensile and
compression loads: the result is a reduction in the screen effect.
•
=
Usually the screens should always be put on both sides. A good interference suppression in a
high frequency area can only be achieved by a connection of the screens on both sides.
In the case of an exception the screen can be put on one side. Thereby one only achieves a
damping of the lower frequencies. A one-sided screen connection can be better, if:
- the cable laying of a equipotential bonding conductor can not be carried out
- analogue signals (a few mV or µA) are transmitted.
- membrane screens (static screens) are used.
•
=
Always use metallic or metallized plugs by data cables for serial couplings. Fasten the data
cable’s screen to the plugs case. Do not put the screen onto pin 1 of the connector strip!
•
=
It is advisable by stationary operation to strip the isolation of the screened cable continuosly and
to put it on the screen/protective earth conductor rail.
By potential differences between the earthing points, a compensatory
current can flow over the screen connected on both sides.
Remedial action: potential compensation cable
•
=
Use metal cable clamps to secure the braided shields. The clamps must surround the screen
extensively and have good contact.
•
=
Place the screen in the cabinet on a screen rail directly after the cable has entered. Direct the
screen to the building block , do not put it on again!