Shielding And Grounding - Endress+Hauser PROline prowirl 72 PROFIBUS PA Documentation Technique

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Shielding and grounding

When planning the shielding and grounding for a field bus 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 will be connected to the field devices' housings, which are
usually metal. Since these housings are generally connected to the protective ground
conductor, the shield of the bus cable will thus be grounded many times.
This approach, which provides the best electromagnetic compatibility, can be used
without restriction in plants with good potential equalisation.
In the case of plants without potential equalisation, a mains frequency (50 Hz) equali-
sing 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 equalising currents on systems lacking potential equa-
lisation, it is therefore advisable to connect the cable shield directly to the building (or
protective ground conductor) at only one end and to use capacitive coupling to connect
it to all other grounding points.
Potential equalisation with shielding grounded at both ends
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R
C
Fig. 3: Examples for connection of potential matching cables
1 = distributor/T-box
2 = Prowirl 72 resp. Prowirl 73 PROFIBUS PA bus devices for hazardous area
3 = bus termination: R = 90...100 Ω , C = 0...2.2 µ F
4 = bus power supply or process control system variant 4a
4a = shielding connected via capacitor
5 = bus power supply or process control system variant 5a
5a = potential equalisation line led out
Variant 4/4a: With capacitive grounding of the shielding in the safe area the potential
equalisation line does not need to be led out of the safe area.
Use small capacitors (e.g. 1 nF, 1500 V dielectric strength, ceramic).
The total capacitance connected at the shielding may not exceed 10 nF.
Variant 5/5a: Potential equalisation line is led out of the safe area.
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1
2
PROline Prowirl 72/73 PROFIBUS PA
E X E X
4a
2
5a
F06-72PBxxZZ-04-xx-xx-xx-001
Endress+Hauser
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