Section 3 Fieldbus Topologies Shielding and data line connection
3BDS009029R5001 B 79
Shielding and data line connection
Shielding and grounding rules for MBP transmission technology has the following
aspects to consider:
• Electromagnetic compatibility (EMC)
• Protection against explosion
• Protection of people
IEC 61158-2 specifies grounding as permanently connected to earth through a
sufficiently low impedance and with sufficient carrying capability to prevent voltage
build up which might result in undue hazard to connect equipment or people. Zero
volt lines should be connected to ground, where they are galvanically isolated from
the fieldbus trunk.
Field devices shall be able to function with the mid-point of one terminator or one
inductive coupler directly to ground. Neither of the two conductors of the twisted-
pair may be grounded at any point in the network. Signals shall be applied and
preserved differentially throughout the network.
In addition to the handling of signal conductors particular attention should be paid
to the cable shield and its connection to earth. Fieldbusses using signal frequencies
> 10 kHz should be protected against electromagnetic fields. Therefore the cable
shield and the metallic housing of the field devices and of any auxiliary equipment
(for example connectors) shall make up a common shielding system, avoiding
unnecessary gaps. Ideally the cable shields are connected to the field devices’
housings (or other protective coverings), which are frequently made of metal.
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