Revision 3.1 - 81 - March 2010
Junction Box
+
-
Transmitter
+
-
SG
+
-
SG
+
-
SG
+-
FF Host
Interface
FF IS PWR
Supply
24 vdc
GND Reference
Host Enclosure
Process Tank
Pump
Structural Steel
Structural Steel
Safety Earthing system including bonding, earth bars, earth rods
both in the earth and above ground level. (Equal potential
Earthing system) This meets all international codes and standards
for proper personnel safety.
Cable Tray/Conduit
Cable Tray/Conduit
Heater
Process Piping
T
Control Center Area
Average Distance 500
meters
Average Distance
50 meters
Shield Earthing and Safety Bonding
+
-
SG
+
-
SG
+
-
SG
+
-
SG
FF Terminal Block
Case
GND
+
-
SG
+
-
SG
+
-
SG
+
-
SG
FF Terminal Block
Case
GND
+
-
Transmitter
Typical FF shielded twisted
pair cable
+
+
--
SH
SH
One of many different Fieldbus power
systems that can be utilized for segment
power
Note: The FF Shield ground
connection can be made at any one
of the different points, marshalling
panel, power source and Host
termination.
Shield grounded at the instrument end and
capacitively coupled at the power source end.
Class D
Figure 7-9. Class D: Multi-point Shielding Using Capacitive Coupling.
Multi-point shielding using capacitive coupling (Class D) is a variation of multi-point
shielding (Class B) except than an adequate equipotential ground does not exists throughout
the plant site. Similar to Class B, this topology requires the shield to be connected to ground at
several points, including the instruments and field junction boxes. However, at the control
center area, the shield is connected to ground through a coupling capacitor. The coupling
capacitor is used to block DC ground loop currents that would result from a poor equipotential
ground.
Similar to Class B, this topology offer better EMC susceptibility at high frequencies and blocks
low frequency currents that would be carried by the shield in a multi-point shielding.
However, a fault condition, such as a lighting strike, could result in a high voltage being
present at the Host system side. Class A, B or C is preferred topology over Class D.
7.3.5 Segment Isolation Segregation and Surge Protection
Segment isolation is a basic requirement for F
OUNDATION fieldbus design. Where devices are
installed in areas that are considered to be at a high risk of lightning strikes, segment
segregation shall be considered as an additional measure of protection. High-risk applications
include tank farms where transmitters are located on top of tanks, remote areas in open
spaces, devices requiring more than two wires (temperature), unprotected structures with little
or no surrounding steel, and transmitters that are located on the top of columns or in the open
on top of structures.
7.3.5.1 Isolation