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4
2
DESCRIPTION
2.1
General
The standard models comprise a stainless steel or GRP , increased safety, Ex e
enclosure, containing a trunk-wiring terminal assembly (TTA), two carrier-mounted
fieldbus barriers and a fieldbus alarm module.
Each barrier module can convert a single, non-intrinsically safe fieldbus trunk into
six, galvanically isolated, intrinsically safe (IS) spur connections for connection to
F
™ fieldbus H1 fieldbus instruments. However, only six spurs are available
oundation
at any one time because they are configured as "redundant pairs" . On power-up the
spurs in one barrier are configured to be in "active" mode, while the spurs in the
other barrier remain in "standby" . In the event of a spur circuit failure in the "active"
barrier module, an automatic switchover ensures that the external spur field circuit
continues to be supported by the corresponding spur in the "standby" module.
Figure 2.1 illustrates an example where Module A - on the left - is the active barrier
and Module B the standby. Spurs 1,2, 4, 5 and 6 of Module A are shown as active
('A') but spur 3 has failed (and effectively gone into standby) so module B has
automatically taken control of spur 3 ('A') to maintain full service to the affected field
circuit.
Fieldbus Trunk
9377-FB-R
Spur Electronics
S
A
A
0V
+
–
+
Spur 1
Carrier
Figure 2.1 - Redundant spurs block diagram + optional alarm module
The spurs are galvanically isolated from the trunk allowing the user to choose the
type of grounding scheme most suitable to their system.
Spur short-circuit protection is provided by the barrier and surge protection can
also be added on individual outgoing spurs by the use of individual Spur Surge
protection modules (part no. FS32).
2.2
Configuration options
There are six spur outputs from each barrier and, as described above, these can
operate in a redundant fashion, supporting each other to maintain continuous
availability for field spur devices. However, if a spur "fails-over" to the other barrier,
the situation might go undetected without frequent inspection of the status of the
barrier LEDs. For this reason, a "failure-reporting" configuration is provided, where
Spur 6 is assigned to an internally located Alarm Module (model 9379-ALM) -
see Figure 2.1.
9377-FB-R
Module
‘A’
A
A
A
S
Alarm
Alarm
–
+
–
+
–
Spur 2
Spur 3
Spur 4
A = ‘Active’
Module
S = ‘Standby’
‘B’
Spur Electronics
S
A
S
S
S
0V
+
–
+
–
Alarm
Module
Spur
Spur 5
6
INM 9370-RD Rev 5
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