Circuit-Breaker Fail (50Bf) - Siemens 7SR105 Rho User Manual

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).
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
7SR105 or 7SR17 Disengaging Time
CB Trip time
Safety Margin
50BF-1 Delay
Stage 2 (Back Trip)
First BF Time Delay
Trip Relay operate time
7SR105 or 7SR17 Disengaging Time
CB Trip time
Margin
50BF-2 Delay
©2018 Siemens Protection Devices
BACK-TRIP
50BF-2 Delay
S/Stn Busbar
RE-TRIP
50BF-1 Delay
M
Figure 4.7 Circuit Breaker Fail
7SR105 Rho and 7SR17 Rho Applications Guide
50
BF
10 ms
50 ms
80 ms
60 ms
200 ms
200 ms
10 ms
50 ms
80 ms
60 ms
400 ms
Chapter 7 Page 27 of 31