Consequenses of a failed breaker:
  1. A Breaker can fail due to Electrical Problems such as Trip Circuit Faulty, Mechanical Problems such as, Breaker Pole getting stuck up while opening.
  2. Now, if the Relay fails, we have a Backup Protection, Similarly, if the Breaker Fails , there should be some Backup Protection.
  3. If there is no Backup Protection for the Breaker Failure, all the Feeders on that Bus will feed the Fault, and there will be Tripping from the Remote End. Because of this, Fault will be sustained for the longer period of duration and as the Tripping is from the Remote end, the restoration time will be more.
Principle of operation:
  1. LBB operates when, the breaker fails to Trip, following the Operation of Master Trip Relay or the energisation of  Trip Coil of the Breaker.
  2. When the LBB operates for a particular Breaker, all the other Circuit Breakers on the concerned Bus are Tripped.
Steps of operation:
  1. Fault occurs and say one of the Protection Relay Operates.
  2. Master Trip Relay Operates.
  3. Breaker Trip is Initiated and at the same time, LBB is initiated.
  4. As soon as the LBB is initiated, the LBB Timer (with time delay of 200ms) starts.
  5. After a time delay of 200 ms, it checks if the Fault is cleared.
  6. If the Fault is cleared within 200ms, the LBB Scheme is Reset, but if the Fault is not cleared, LBB Operates.
  7. When the LBB Operates, it Trips, all the feeders on the concerned Bus with the help of Busbar Tripping Circuit.
LBB Initiation Logic:

After getting Tripping command from the Master Trip Relay, the Breaker does not Trips immediately, but takes an inherent time delay of about 40 ms. During this Time Delay, the LBB relay is not supposed to actuate. Therefore, an intentional time delay of about 200 ms is given.

So, once the Mater Trip relay actuates, a signal is given to the LBB Relay which starts the LBB Timer and is called as LBB Initiation.

LBB Actuation Logic:

After the time delay of 200 ms is completed, the LBB relay checks for the Fault. It uses the following Logic for detecting whether the Fault exists or not.

Master Trip relay has operated and still the Overcurrent as measured by the LBB Relay exists., which implies that the Fault has not been cleared as the Breaker has failed to Trip. 

In this case, it gives a signal to the Busbar relay to Trip all the Feeders on the concerned Bus. 

LBB Relay Settings:

Current Setting, I = 0.8 A

Time Delay, Td = 200 ms  ………As it should be less than Remote Back-up Time (350ms) and greater than the CB Trip Time(30ms).

LBB Testing:
  1. Isolation: Remove the concerned feeder LBB Link
  2. Pickup Test : In this test we check whether, the LBB Relay Picks up and Resets at the set Current.
  3. Timing Test : In this Test we check, whether the LBB Relay operates in the Set Time.

All the above mentioned tests will explained in details in the coming posts and the link to those posts will be mentioned here.