#LBB or local breaker backup relay is used to clear a faulty section of power system during failure of a circuit breaker. without #LBB protection any uncleared fault due to faulty CB will trip by upstream distance or overcurrent relay which will take considerable delay, which may damage power system stability. generators/transmission lines may isolate themselves from grid due to unstable power swing caused by delay fault clearing. So, #LBB relay is a must in power system protection.
