Functions
2.6 Circuit-Breaker Failure Protection 50BF
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
101
2.6
Circuit-Breaker Failure Protection 50BF
The circuit-breaker failure protection provides rapid backup fault clearance in the event that the circuit breaker
fails to respond to a trip command from a protection function of the local circuit breaker.
2.6.1
Description
General
Each time a fault protection relay of a feeder issues a trip command to the circuit breaker, it is repeated to the
breaker failure protection (Figure 2-34). A timer T–BF in the breaker failure protection is started. The timer runs
as long as a trip command is present and current continues to flow through the breaker poles.
Figure 2-34
Simplified function diagram of circuit-breaker failure protection with current flow monitoring
Normally, the breaker will open and interrupt the fault current. The current monitoring element quickly resets
(typical 10 ms) and stops the timer T–BF.
If the trip command is not carried out (breaker failure case), current continues to flow and the timer runs to its
set limit. The breaker failure protection then issues a command to trip the backup breakers and interrupt the
fault current.
The reset time of the feeder protection is not relevant because the breaker failure protection itself recognizes
the interruption of the current.
For protection functions where the tripping criterion is not dependent on current (e.g. Buchholz protection),
current flow is not a reliable criterion for proper operation of the breaker. In such cases, the circuit-breaker po-
sition can be derived from the auxiliary contacts of the breaker. Therefore, instead of monitoring the current,
the condition of the auxiliary contacts is monitored (see Figure 2-35). For this purpose, the outputs from the
auxiliary contacts must be fed to binary inputs on the relay (refer also to Subsection 2.16).
