Wiring
RID14
18
Hauser
5.3.5
Shielding and grounding
NOTICE
Equalizing current can damage the bus cable or bus shield
‣
If the shielding of the cable is grounded at more than one point in systems without
potential matching, mains frequency equalizing currents can occur that damage the bus
cable or the bus shield or have a serious effect on signal transmission. In such cases the
shielding of the fieldbus cable is to be grounded on one side only, i.e. it must not be
connected to the ground terminal of the housing. The shield that is not connected
should be insulated!
Optimum electromagnetic compatibility (EMC) of the fieldbus system can only be
guaranteed if the system components and, in particular, the lines are shielded and the
shield forms as complete a cover as possible. A shield coverage of 90% is ideal.
• To ensure an optimum EMC protective effect, connect the shield as often as possible to
the reference ground.
• For reasons of explosion protection, you should refrain from grounding however.
To comply with both requirements, the fieldbus system allows three different types of
shielding:
• Shielding at both ends
• Shielding at one end on the feed side with capacitance termination at the field device
• Shielding at one end on the feed side
Experience shows that the best results with regard to EMC are achieved in most cases in
installations with shielding at one end. Appropriate measures with regard to input wiring
must be taken to allow unrestricted operation when EMC interference is present. These
measures have been taken into account for this device. Operation in the event of
disturbance variables as per NAMUR NE21 is possible with shielding at one end.
Where applicable, national installation regulations and guidelines must be observed
during the installation!
one point of the shielding is connected directly with the reference ground. In systems
without potential equalization, therefore, cable shielding of fieldbus systems should only
be grounded on one side, for example at the fieldbus supply unit or at safety barriers.
1
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5
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50
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A0012570
9
Shielding and grounding of the fieldbus cable shield at one end
1
Supply unit
2
Distribution box (T-box)
3
Bus terminator
4
Grounding point for fieldbus cable shield
5
Optional grounding of the field device, isolated from cable shielding.
5.3.6
Bus termination
The start and end of each fieldbus segment must always be terminated by a bus
terminator. With various junction boxes (non-Ex), the bus termination can be activated via
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