If the available fault currents exceed the selective coordination values shown for circuit-breaker-to-circuit-breaker pairs, selective coordination can be achieved.

If the available fault currents exceed the selective coordination values shown for circuit-breaker-to-circuit-breaker pairs, selective coordination can be achieved.

• A. True
• B. False
• C. Not specified
• D. Sometimes

Answer: (B)

Selective coordination relies on a staged, near-fault tripping sequence: the local downstream device must trip first, and upstream devices only trip if the fault is so large that coordination can no longer be maintained. The values shown for circuit-breaker-to-circuit-breaker pairs set the maximum fault current that still allows this orderly sequence. If the actual fault current exceeds those coordination values, the upstream device can trip in its instantaneous region before the downstream device, disrupting the intended selectivity. So you cannot achieve selective coordination when available fault currents are higher than those coordination values. In practice, exceeding the coordination values means you’d need to adjust protections or reduce fault current to restore selective coordination.

Selective coordination relies on a staged, near-fault tripping sequence: the local downstream device must trip first, and upstream devices only trip if the fault is so large that coordination can no longer be maintained. The values shown for circuit-breaker-to-circuit-breaker pairs set the maximum fault current that still allows this orderly sequence. If the actual fault current exceeds those coordination values, the upstream device can trip in its instantaneous region before the downstream device, disrupting the intended selectivity. So you cannot achieve selective coordination when available fault currents are higher than those coordination values. In practice, exceeding the coordination values means you’d need to adjust protections or reduce fault current to restore selective coordination.