A new approach to low-voltage circuit breaker short-circuit selective coordination

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 low- voltage circuit breakers is introduced, indicating that higher levels of coordination can be achieved than those shown by the use of time-current curves alone.