Local Breaker Backup Protection LBB 50Z or 50BF

Local Breaker Backup Protection LBB 50Z or 50BF:

Local Breaker Backup Protection is used to protect the generator when the generator’s circuit breaker failed to trip the generator during the fault condition. It is a backup protection of a generator.

Principle of LBB protection:

Mostly any fault on the generator, the trip command is directly given to the generator circuit breaker. But most of the time the circuit breaker works fine but due to some external fault on the circuit breaker such as trip circuit lose contact, delayed circuit breaker operation, circuit breaker’s internal mechanism issue, etc. the circuit breaker does not isolate the generator under fault condition. Failure of the breaker leads to loss of protection, single phasing, negative phase sequence currents inducted on the rotor etc. To avoid such a condition, we use LBB protection.

Local Breaker Back Protection LBB 50Z or 50BF
Local Breaker Back Protection LBB 50Z or 50BF


Local Breaker Backup Protection Operation:

LBB protection Monitors both trip and closing position of the generator’s breaker. The LBB protection operates when the breaker trip is initiated after a suitable time interval if confirmation of the confirmation of breaker tripping from three poles is not received. LBB energize the all grid 110kV circuit breaker associated with the generator breaker. Once the LBB protection opted, then the entire station is called as blackout.

Learn More:   Classification Generator tripping Scheme Class A, Class B & Class C

Relay acted:

  • 50Z or 50BF relay opted
  • 86H opted, Turbine trip, generator trip and grid circuit breaker trip

Recovery from LBB:

Slowly charge the field circuit breaker and Never attempt to close the faulty generator’s circuit breaker without proper investigation. Neglecting, it may causes permanent damage to the generator and transformer.


  1. Hi, you have given a very concise explanation of breaker failure protection. Thank you. Please help me understand the ; recovery from LBB where you state “slowly charge the failed circuit breaker”. What do you mean by slowly charge, how can it be done & why should it be slow? Thanks.


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