Endress+Hauser Cubemass DCI - Shielding and Grounding

Wiring Cubemass DCI MODBUS RS485
20 Endress+Hauser
4.1.1 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
• Employee safety
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
ground, the shield of the bus cable is grounded many times. Make sure that the stripped and twisted
lengths of cable shield to the terminals are as short as possible.
This approach, which provides the best electromagnetic compatibility and employee safety, can be
used without restriction in systems with optimum potential equalization.
In the case of systems without potential equalization, a mains frequency (50 Hz) equalizing current
can flow between two grounding points which can destroy the cable in unfavorable cases, e.g. when
it exceeds the permissible shield current.
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
ground) at one end only and to use capacitive coupling to connect all other grounding points.
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Caution!
The legal EMC requirements are fulfilled only when the cable shield is grounded on both sides!