Functions
2.22 Circuit Breaker Failure Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
396
2.22
Circuit Breaker Failure Protection
The circuit breaker failure protection provides rapid back-up 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.
2.22.1
Functional Description
General
Whenever e.g. a short-circuit protection relay of a feeder issues a trip command to the circuit breaker, this is
repeated to the circuit breaker failure protection (Figure 2-183). A timer T–BF in the circuit breaker failure pro-
tection is started. The timer runs as long as a trip command is present and current continues to flow through
the circuit breaker poles.
Figure 2-183
Simplified function diagram of circuit breaker failure protection with current flow monitoring
Normally, the circuit breaker will open and interrupt the fault current. The current monitoring stage quickly resets
(typical 10 ms) and stops the timer T–BF.
If the trip command is not carried out (circuit breaker failure case), current continues to flow and the timer runs
to its set limit. The circuit breaker failure protection then issues a command to trip the backup circuit breakers
and interrupt the fault current.
The reset time of the feeder protection is not relevant because the circuit breaker failure protection itself rec-
ognizes the interruption of the current.
For protection functions where the tripping criteria are 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 position of the auxiliary contacts is monitored (see Figure 2-184). For this purpose, the outputs from the
auxiliary contacts must be fed to binary inputs of the relay (refer also to Section 2.25.1).
