Revision 3.1 - 77 - March 2010
7.3.4 Shielding
Shielding is a very important aspect of segment design. Various options (Classes A – D) may
be used, depending on local codes, standards and practices. These classes are illustrated in
this section and larger versions of these illustrations are given in Appendix 3 for clarity.
Class A design shown in Figure 7-6 is recommended for most regions of the world. The
instrument shield is terminated at the Host (fieldbus power supply) end of the network and is
not connected to ground at any other place.
Commentary:
Some device I/O shields, such as grounded thermocouples on temperature multiplexers, may
require grounding separately from the fieldbus shield. Some regions, such as Europe, prefer
using Class B design in areas where there is equipotential grounding assured.
Class C design is an alternate recommendation shown in Figure 7-8. The instrument shield is
terminated at the Host (fieldbus power supply) end of the network in the marshalling cabinet
and is connected to ground at field devices with isolating device couplers. It is commonly used
for IS spurs on a high-power trunk.
Class D design is not recommended.
If a multiple homerun cable goes to a field junction box, do not attach the cable shield wires
from different networks together. This creates ground loops and noise onto the network.
The approaches to the grounding concepts (e.g., construction) may vary due to national
standards, but the same basic concepts (equipotential ground) are consistent worldwide.
Grounding concepts can vary depending on national standards. In Great Britain, for example,
installation with central grounding is common practice whereas in other European countries,
such as Germany, a plantwide potential matching line is usually laid. In North America, on the
other hand, the cables are often routed in pipes (conduit) that lead to the central cabinets.
Every concept has advantages and restrictions, which are set off against the plant topology
and external conditions such as the extension of the fieldbus, the grounding concept of the
plant, interference frequencies to be expected, existing Ex areas and the need to protect the
fieldbus against lightning.
Generally speaking, in all the shielding concepts, instruments are connected to a potential
matching cable with a large cross-section, which is necessary from the point of view of
explosion protection alone. This potential matching cable runs through the plant and is
grounded at one point. Depending on the building type, a separate cable does not always have
to be routed. In concrete and steel constructions, the reinforcements and additional connection
points of the constructions assume the task of potential matching. Ultimately, the guidelines
with regard to installing in hazardous areas and the applicable national regulations also have
to be observed.