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Segment Protector
Operation
Failure situations, where the spur current progressively increases, cause the current limiting
electronics to dampen the communication signals due to its increasing impedance. A typical
failure situation is water in the terminal arrangement with increasing conductivity between the
fieldbus wires.
Oscillation sometimes occurs when a fault current is marginally above the current limiting
setpoint and the electronic circuit is just operating. At this point, the voltage to the fault
decreases with a resulting decrease in current that turns off the current limiting circuit. This
cycle continues quite rapidly and can be amplified when a device is transmitting at the same
time. The reaction during this narrow transition point is unpredictable because not every fault
behaves in a repeatable way. For example, the impedance of water vs. voltage or current can
be very non-linear, with the impedance further varying due to the possible impact of
temperature and conductivity.
The following example shows the criticality of such a fault scenario:
A field device with an active backup – a link active scheduler (LAS) – loses communication to a
host in the control room. This situation is due to the low communication signal during a
progressing fault condition. The backup LAS now activates while the host LAS remains active.
At this point, field devices at normally operating spurs still 'see' the backup LAS which is
nearby, as well as the host.
With 2 active LAS on the segment, communication clashes and the segment fails.
Pepperl+Fuchs offers the progressive spur short circuit current limitation that detects slowly
increasing spur current and isolates the faulty spur from the segment to prevent segment
failures.
5.3.4
Device Jabber Protection
Specific software or hardware failures can cause a field device to communicate continuously
(jabber). In this event, the device should disconnect itself from the segment immediately.
The international fieldbus standard IEC 61158-2 requires that a field device contains a 'jabber
Inhibit' circuit or 'watchdog'. Up to this point, not all currently available devices support or
contain the 'jabber inhibit' 'watchdog'.
The Pepperl+Fuchs device couplers have a spur dependent 'jabber inhibit' feature to isolate a
faulty field device from the segment in such events.
5.4
Surge Protection
Optional surge protection modules for the trunk and spur cable are available. They are used for
safeguarding the device coupler electronics against excessive surges and overvoltages.
The SCP-LBF-IA.36.IE* spur modules replace the spur connectors where surge protection at
the spur level is required.
The TCP-LBF-IA1.36.IE* is the trunk module which replaces the T-trunk connector where
surge protection on the trunk is required.
Danger!
Risk of explosion through insufficient type of protection
Using outputs with the insufficient type of protection can cause sparks or other hazards for
potentially explosive atmospheres that can ignite the surrounding atmosphere.
In order to generate intrinsically safe (Ex ic) outputs according to IEC/EN 60079-11 at the
Segment Protector with SCP-LBF* surge protectors on intrinsically safe spur outputs, use the
specified accessories. Accessory: Surge protector TCP-LBF* with an integrated separation
wall at the trunk.
Solid ground rails, ACC-LBF-EB6, are used for routing the ground connections from the
individual surge modules together.
Surge Protectors contain a diagnostic function that predicts their failure, before fieldbus
communication or the segment voltages are adversely affected.
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