The expanded requirements in the 2005 NEC for selective coordination have prompted many engineers to question if selective coordination can be achieved with low-voltage circuit breakers. This is a valid question as the time-current curves of low-voltage circuit breakers often suggest that coordination can only be achieved in the overload and low level fault current zones. Therefore, how time-current curves are developed and their limitations are discussed. A new way for system designers to evaluate the level of selective coordination between lowvoltage circuit breakers is introduced, indicating that higher levels of coordination can be achieved than those shown by the use of time-current curves alone.

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