FMC Corporation - Talstar 80 - Handbuch (Seite 28)

Wiring Proline Promass 80 PROFIBUS PA
28
4.2 Shielding and grounding
When planning the shielding and grounding for a fieldbus system, there are three important points
to consider:
• Electromagnetic compatibility (EMC)
• Explosion protection
• Safety of the personnel
To ensure the optimum electromagnetic compatibility of systems, it is important that the system
components and above all the cables, which connect the components, are shielded and that no
portion of the system is unshielded. Ideally, the cable shields are connected to the normally metal
housings of the connected field devices. Since these are generally connected to the protective earth,
the shield of the bus cable is grounded many times. Keep the stripped and twisted lengths of cable
shield to the terminals as short as possible.
This approach, which provides the best electromagnetic compatibility and personal safety, can be
used without restriction in systems with good potential matching.
In the case of systems without potential matching, a power supply frequency (50 Hz) equalizing
current can flow between two grounding points which, in unfavorable cases, e.g. when it exceeds
the permissible shield current, may destroy the cable.
To suppress the low frequency equalizing currents on systems without potential equalization, it is
therefore recommended to connect the cable shield directly to the building ground (or protective
earth) at one end only and to use capacitive coupling to connect all other grounding points.
"
Caution!
The legal EMC requirements are fulfilled only when the cable shield is grounded on both sides!