In analyzing circuit breakers in their instantaneous region, which times must be considered to determine selective coordination?

In analyzing circuit breakers in their instantaneous region, which times must be considered to determine selective coordination?

• A. Only the unlatching time
• B. Only the clearing time
• C. Both the unlatching time and the clearing time
• D. Neither time matters

Answer: (C)

When checking selective coordination in the instantaneous region, you must look at the actual time until the fault current is interrupted on the downstream path. That total time includes both how long it takes for the breaker to unlatch after a trip signal (unlatching time) and how long the current remains before being cleared (clearing time). The instantaneous trip mechanism is very fast, but the mechanical delays (unlatching) plus the time to physically interrupt the current (clearing) dominate the response. If you consider only one of these times, you could misjudge which breaker actually clears the fault, risking miscoordination where the upstream trips unnecessarily or too late. So, both unlatching time and clearing time must be accounted for to determine selective coordination.

When checking selective coordination in the instantaneous region, you must look at the actual time until the fault current is interrupted on the downstream path. That total time includes both how long it takes for the breaker to unlatch after a trip signal (unlatching time) and how long the current remains before being cleared (clearing time). The instantaneous trip mechanism is very fast, but the mechanical delays (unlatching) plus the time to physically interrupt the current (clearing) dominate the response. If you consider only one of these times, you could misjudge which breaker actually clears the fault, risking miscoordination where the upstream trips unnecessarily or too late. So, both unlatching time and clearing time must be accounted for to determine selective coordination.