7SR105 Rho and 7SR17 Rho Applications Guide
©2018 Siemens Protection Devices Chapter 7 Page 27 of 31
4.7 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.
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) e.g. the busbar incomer. 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).
50
BF
BACK-TRIP
50BF-2 Delay
M
RE-TRIP
50BF-1 Delay
S/Stn Busbar
Figure 4.7 Circuit Breaker Fail
50BF Setting
The current setting must be set below the minimum protection setting current.
50BF-1 Delay/50BF-2 Delay
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.
Stage 1 (Retrip)
Trip Relay operate time 10 ms
7SR105 or 7SR17 Disengaging Time 50 ms
CB Trip time 80 ms
Safety Margin 60 ms
50BF-1 Delay 200 ms
Stage 2 (Back Trip)
First BF Time Delay 200 ms
Trip Relay operate time 10 ms
7SR105 or 7SR17 Disengaging Time 50 ms
CB Trip time 80 ms
Margin 60 ms
50BF-2 Delay 400 ms
To Next Page
Loading...
