Hans Turck BL20-ECO, Instructions For Use Manual

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2020/09
8.4 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 mate -
rials (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 used in stationary 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.
NOTE
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 potential compensation.
NOTE
Should it not be possible to ground the shield on both sides due to switching arrange -
ments 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 dis -
charges 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.