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Answer Posted / satya
None of these answers is precise, except ans 2 to large extent.
OK. LBB is Local Breaker Backup. (This is opposed to Remote Back up.)
What is LBB?
First of all here is the philospohy that must be met for LBB.
1)There must be a fault. ( Or more precisely, a protective relay has issued a command to trip the breaker)
2) But that breaker has failed to trip.
Then LBB scheme disconnects other breakers at that station ( Local).
If LBB fails to isolate ( assuming an elemnt is still connected while others have opened) , the remote end stations breakers connected to this station will trip ( Remote Backup)
An LBB scheme thus ensures
1) The relay commanded trip on fault. ( NO contacts of main trip relay used)
2) That there was a current of rasonable amount after some reasonable time. ( Breaker did not open for that much time)
eg we set 200ms time. If so it means that the breaker still has current flowing after 200ms means it failed.
Then the LBB relay will issue a trip signal to other breakers at that bus.
OK Explain well in interview.
OK just a supplement. Should LBB operate if the breaker fails to open on manual command ( not fault)? Think about it.
| Is This Answer Correct ? | 9 Yes | 4 No |
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