Section 9: Supervision Functions; Circuit-Breaker Fail (50Bf); Settings Guidelines - Siemens 7SR18 Solkor Manual

7SR18 Applications Guide

Section 9: Supervision Functions

9.1

Circuit-Breaker Fail (50BF)

Where a circuit breaker fails to operate to clear fault current the power system will remain in a hazardous state
until the fault is cleared by remote or back-up protections. To minimise any delay, CB Failure protection provides
a signal to either re-trip the local CB or back-trip 'adjacent' CBs.
The function is initiated by the operation of user selectable protection functions or from a binary input. Current
flow is monitored after a tripping signal has been issued, if any of the 50BF current check elements have not reset
before the timers have expired, an output is given. For CB trips where the fault is not current related an additional
input is provided (50BF Mech Trip) which monitors the CB closed input and provides an output if the circuit
breaker has not opened before the timers expire.
The relay incorporates a two-stage circuit breaker fail feature. For some systems, only the first will be used and
the CB Failure output will be used to back-trip the adjacent CB(s). On other systems, however, this output will be
used to re-trip the local CB to minimise potential disruption to the system; if possible via a secondary trip coil and
wiring. The second CB Failure stage will then be used to back-trip the adjacent CB(s).
If the CB is faulty and unable to open, a faulty contact can be wired to the CB faulty input of the relay and if a trip
occurs while this input is raised the CB fail delay timers may be by-passed to allow back tripping to occur without
delay.
Figure 9.1-1 Circuit Breaker Fail
9.1.1

Settings Guidelines

50BF Setting
The phase current setting must be set below the minimum protection setting current.
50BF Setting-I4
The EF current setting must be set below the minimum protection setting current.
50BF Ext Trig
Any binary input can be mapped to this input to trigger the circuit breaker fail function. Note current must be
above setting for the function to operate.
50BF Mech Trip
Any binary input can be mapped to this input to trigger the circuit breaker fail function. Note for the function to
operate the circuit breaker closed input is used to detect a failure, not the current.
50BF CB Faulty
Any binary input can be mapped to this input, if it is energised when a trip initiation is received an output will be
given immediately (the timers are by passed).
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