INOR PROFIPAQ-L - Manual (Page 34)

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7 Appendix B. PROFIBUS Cabling
7.1 Shielding and grounding
Proper electrical installation is necessary to prevent errors due to sensor lead resistance
and electrical noise. Shielded, twisted cable should be used for best results in electrically
noisy environments. All power to the transmitter is supplied over the signal wiring. Signal
wiring should be shielded, twisted pair for best results. Do not run unshielded signal wiring
in conduit or open trays with power wiring or near heavy electrical equipment.
The standard EN 61158-2 states that all bus interfaces must be isolated from local
ground. To provide optimal system electromagnetic compatibility, it is very important that
system components, and particularly the lines connecting the components, are shielded and
that these shields provide as complete an electric protective envelope as possible.
The importance of this requirement increases with the signal frequencies processed in
the system. With regard to the fieldbus, this means that ideally the cable shields are
connected to the field devices' housings (or other protective coverings) which are frequently
made of metal. Since these are usually connected with local ground or with the protective
conductor, the shield of the bus cable is grounded several times. This method provides
optimum electromagnetic compatibility and protection of personnel (Figure 30), and can be
used as desired in systems with optimum equipotential bonding.
1 Power supply
2 Main cable / Trunk cable
3 Junction box
4 Stub cable / Spur
5 Field device
6 Equipotential Bonding
Figure 30 Ideal combination of shielding and grounding (principle guidelines).
Cable parameters for Ex-use
Rc (loop cable resistance): 15 – 150 Ω/km
Lc (loop cable inductance): 0.4 – 1 mH/km
Cc (loop cable capacitance): 45 – 200 nF/km
The maximum cable length of each spur (from junction box to field device, see figure 30)
in a FISCO/FNICO system is 60 m in IIC and IIB according to IEC 60079-27.
In the same way the maximum length of each trunk cable (from Power Supply to
Junction box, see figure 30), including the length of all connected spurs, is 1 km in Gas
group IIC and 5 km in Gas group IIB.