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Guidelines for Electrical Installation
D301087 1211 - BL20 CANopen 7-8
7.4 Shielding of cables
Shielding is used to prevent interference from voltages and the radiation of interference fields by
cables. Therefore, use only shielded cables with shielding braids made from good conducting materials
(copper or aluminum) with a minimum degree of coverage of 80 %.
The cable shield should always be connected to both sides of the respective reference potential (if no
exception is made, for example, such as high-resistant, symmetrical, analog signal cables). Only then
can the cable shield attain the best results possible against electrical and magnetic fields.
A one-sided shield connection merely achieves an isolation against electrical fields.
The insulation of the shielded data-cable should be stripped and connected to the shield rail when the
system is not in operation. The connection and securing of the shield should be made using metal
shield clamps. The shield clamps must enclose the shielding braid and in so doing create a large surface
contact area. The shield rail must have a low impedance (for example, fixing points of 10 to 20 cm apart)
and be connected to a reference potential area.
The cable shield should not be severed, but routed further within the system (for example, to the
switchgear cabinet), right up to the interface connection.
Attention
When installing, please pay attention to the following...
– the shield should be connected immediately when entering the
– system,
– the shield connection to the shield rail should be of low
– impedance,
– the stripped cable-ends are to be kept as short as possible,
– the cable shield is not to be used as a bonding conductor.
Note
Should it not be possible to ground the shield on both sides due to switching arrangements
or device specific reasons, then it is possible to route the second cable shield side to the local
reference potential via a capacitor (short connection distances). If necessary, a varistor or
resistor can be connected parallel to the capacitor, to prevent disruptive discharges when
interference pulses occur.
A further possibility is a double-shielded cable (galvanically separated), whereby the
innermost shield is connected on one side and the outermost shield is connected on both
sides.