LBB Protection Relay: Definition, Function, Importance, Operating Scheme

LBB is a short form of local breaker backup. The LBB protection protects the electrical system by isolating the faulty circuit if the primary breaker fails to operate. Thus, LBB protection is crucial for safeguarding the electrical power system.

What is the LBB Protection Relay?

The LBB protection relay provides backup protection for the primary circuit breaker. When the primary circuit breaker fails to operate during a fault condition, the LBB protection relay clears the fault by operating an upstream breaker. This protection scheme prevents excessive damage and maintains the stability of the power system.

The function of the LBB Protection Relay

The primary function of an LBB protection relay is to detect the failure of a circuit breaker to clear a fault and to initiate the tripping of nearby circuit breakers to isolate the faulted section. The LBB relay monitors the status of the primary breaker and the current flow. If it detects a fault current and the primary breaker does not trip within a predetermined time, the LBB relay sends a trip signal to other breakers.

Importance of LBB Protection Relay

The local breaker backup relays play a vital role in power system protection for several reasons:

  1. Reliability: They enhance the reliability of the power system by ensuring that faults are cleared even if the primary breaker fails.
  2. Prevention of Damage: LBB relays prevent damage to equipment and reduce the risk of widespread outages by isolating faulted sections promptly.
  3. System Stability: Quick fault clearance helps maintain the stability of the power system, avoiding cascading failures.
  4. Redundancy: It provides an additional layer of security, ensuring continuous protection even if the primary protection fails.

Operating Scheme

The operating scheme of the LBB protection relay involves several key steps:

  1. Fault Detection: The LBB relay continuously monitors the circuit for fault conditions, using current transformers (CTs) to detect abnormal current flows.
  2. Breaker Status Monitoring: It monitors the status of the primary circuit breaker, including its tripping mechanism and position.
  3. Time Coordination: The local breaker backup protection relay is coordinated with other protection devices to ensure proper timing. If the primary breaker does not trip within the set time, the LBB relay activates.
  4. Trip Signal Issuance: Upon detecting a breaker failure, the LBB relay sends a trip signal to adjacent breakers to clear the fault.
  5. System Reset: After the fault is cleared and the system is stable, the LBB relay resets and returns to its monitoring state.


LBB protection relays are indispensable in modern power systems. It ensures faults are promptly cleared even when primary breakers fail. By providing an additional layer of protection, they enhance system reliability, prevent extensive damage, and maintain stability. Understanding the definition, function, importance, and operating scheme of LBB protection relays is essential for anyone involved in power system operation and maintenance.

  1. Trip Circuit Supervision Relay
  2. Master Trip Relay
  3. Overcurrent Relay

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