Siemens 7SR224 Technical Manual page 343

Section 5: Supervision Functions
5.1 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.
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).
Figure 5.1-1 - Circuit Breaker Fail
5.1.1 Settings Guidelines
50BF Setting
The current setting must be set below the minimum protection setting current.
50BF DTL1/50BF DTL2
The time delay setting applied to the CB Fail protection must be in excess of the longest CB operate time + relay
reset time + a safety margin.
First Stage (Retrip)
Trip Relay operate time
7SR224 Reset Time
CB Tripping time
Safety Margin
Overall First Stage CBF Time Delay
©2010 Siemens Protection Devices Limited
10ms
20ms
80ms
40ms
150ms
7SR224 Argus Applications
Chapter 7 Page 44 of 50