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Fig. 3-2. Shielding and one-sided grounding of the fieldbus cable shielding
1 Supply unit
2 Distribution box (T-box)
3 Bus terminator
4 Grounding point for fieldbus cable shielding
5 Optional grounding of the field device, isolated from cable shielding.
Handling Precautions:
If the shielding of the cable is grounded at more than one point in systems without
potential matching, power supply frequency equalizing currents can occur that
damage the bus cable or shielding or have serious effect on signal transmission.
f
In such cases the shielding of the fieldbus cable is to be grounded on only
one side, i.e. it must not be connected to the ground terminal of the housing
(terminal head, field housing). The shield that is not connected should be
insulated!
3-3-6. Bus termination
The start and end of each fieldbus segment are always to be terminated with a bus terminator.
With various junction boxes (non-Ex), the bus termination can be activated via a switch. If
this is not the case, a separate bus terminator must be installed. Note the following points in
addition:
In the case of a branched bus segment, the device furthest from the segment coupler
represents the end of the bus.
If the fieldbus is extended with a repeater, then the extension must also be terminated at
both ends.
3-3-7. Further information
General information and further pointers on wiring can be found on www.fieldbus.org, the
Web site of the Fieldbus Foundation.