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5.2
Sustained Fault, CB Fail & CB Position Monitoring
The Sustained Fault Monitor (87/50SFM) feature can be used where either dedicated CB fail protection is not
fitted and/or where the location of the protection CTs may introduce operational constraints.
If after operation of the differential protection the fault remains uncleared this is due to either a CB fail condition or
because the fault has occurred in a protection blind spot created by the position of the CTs. This function can
identify and differentiate between a failed CB and a blind spot fault – see Chapter 7, Applications Guide.
When selected to 'Enabled' the 87SFM Delay starts after operation of the 87/50-1 protection, when the DTL
elapses it indicates that the fault remains un-cleared.
87SFM functionality is enhanced where CB status information is available either over the 61850 bus or from CB
auxiliary switches wired to binary inputs of the relay. When enabled by the 87/50BF (differential current CB fail )
feature a CB fail condition can be determined and the faulted circuit breaker(s) can be identified.
The 87/50SFM feature will pick up where 87/50 is Enabled, 87/50SFM is Enabled, an 87/50-n element has
operated and both the 'Inhibit 87/50SFM and CT50 have not operated.
Outputs can be mapped to any combination of output contacts or LEDs, outputs are available from:
87/50SFM where the 87SFM DTL has elapsed.
87/50BF output is issued where CB status monitoring is available and one of the monitored CBs does
not indicate open i.e. a CB fail condition has been detected.
CBn Cct Close Alarm output is issued to identify the failed CB where CB status monitoring is available
and the CB fail condition above is detected.
87/50BF Blind Spot output is issued where CB status is available and all CBs are open after a
sustained differential current is detected.
Operation of the Sustained Fault Monitor function can be inhibited from:
Inhibit 87SFM
CT50
87/50SFM
Disabled
Enabled
87/50-1 Operate
1
87/50-2 Operate
Inhibit 87/50SFM
CT50
87/50 Enabled
87/50BF
Enabled
Disabled
CB1 Cct Close
CB2 Cct Close
CB3 Cct Close
CBn Cct Close
©2013 Siemens Protection Devices Limited
A binary or virtual input.
Operation of the CT Supervision elements
&
87/50SFM Delay
&
Figure 5-2
Logic Diagram: 87/50SFM and 87/50BF
Chapter 1) 7SR23 DAD Description Of Operation
&
&
1
&
&
&
&
87/50SFM Pickup
87/50SFM
87/50BF Blind Spot
87/50BF
CB1 Cct Close Alarm
CB2 Cct Close Alarm
CB3 Cct Close Alarm
CBn Cct Close Alarm
Chapter 1 Page 27 of 36
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