2.8 Circuit Breaker Failure Protection
103
7ST6 Manual
E50417-G1176-C251-A3
2.8
Circuit Breaker Failure Protection
This function is optional, i.e. not included in all device variants.
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 protective func-
tion of the local circuit breaker.
2.8.1
Functional Description
General
Whenever a protection function issues a trip command to the circuit breaker, this is si-
multaneous sent to the breaker failure protection (Figure 2-26). The breaker failure
protection consists of two stages. Two time stages
tBF1
(address
3904
) and
tBF2
(address
3905
) are started in it. The timers run as long as a trip command is present
and current continues to flow through the breaker poles.
Figure 2-26
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
stage quickly resets (typically 1/2 cycle of the power system frequency) and stops the
timers
tBF1
(address
3904
) and
tBF2
(address
3905
).
If the trip command is not carried out (breaker failure case), current continues to flow.
When the delay time for the backup trip element
tBF1
(address
3904
) has elapsed,
a TRIP command for the backup trip element is generated. If this TRIP command,
also, fails to interrupt the short-circuit current, the breaker failure protection issues a
further TRIP command to the higher-level circuit breakers after the delay time
tBF2
(address
3905
) has elapsed.
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