Specifications

7SR242 Duobias Applications Guide
5.3 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 the next ‘upstream’ CB.
Figure 5-1 - Circuit Breaker Fail
The CBF function is initiated by the operation of either:
Protection functions that operate binary outputs mapped as OUTPUT CONFIG>BINARY OUTPUT
CONFIG>CBn Trip Contacts, or
A binary input mapped as INPUT CONFIG>INPUT MATRIX >C50BF-n Ext Trip
Each 50BF uses phase segregated current check elements and two timers.
Current in each phase is monitored and if any of the 50BF current check elements have not reset before the
timers have expired an output is given. Typically a single stage scheme is used, DTL1 is wired to back-trip the
adjacent CBs e.g. via the busbar protection system. Alternatively the first timer output can be wired to re-trip the
failed CB through a different trip coil, and the second timer output is wired to trip the adjacent CBs.
Practical time sequences for single and two stage 50BF applications are illustrated below.
CB Backtrip
Sucessful
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340
System
Fault
ms from fault
occuring
Relay
Operation
and CBF
Timer
Started
Main
Trip
Relay
Operation
Failure of
CB to trip
Reset of
CBF elements
does not occur
Backtrip
Operation
Backtrip
Trip Relay
CB Operate Time
Stage 1 CBF Timer (Backtrip) = 120ms
Figure 5-2 - Single Stage Circuit Breaker Fail Timing
©2010 Siemens Protection Devices Limited Chapter 7 Page 44 of 56