ABB REO 517 Applications Manual page 140

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Breaker failure protection (BFP)
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t : Circuit breaker opening time
br
BFR reset time
The back-up trip delay t2 shall be set:
≥
t2 t1 t margin
+ +
br
(Equation 60)
At the same time it is desired that the back-up trip is done so fast that remote protections
will not trip.
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