29.5 : 断路器和保险丝的选择

A circuit breaker interrupts fault currents by extinguishing the arc, which is elongated and cooled to stop the flow of electricity.

Breakers are classified based on their operating voltage and the medium used to extinguish the arc.

High-voltage breakers automatically reclose 15 to 50 cycles post-interruption and lock out if the fault persists, requiring operator intervention.

Low-voltage applications use molded-case circuit breakers with magnetic trips for large fault currents and thermal trips for smaller, sustained overloads.

The E/X simplified method aids breaker selection by calculating the maximum symmetrical short-circuit current using pre-fault voltage and system reactance, neglecting other factors.

For generators, two-cycle breakers are selected based on sub-transient fault current, calculated using sub-transient reactances. For synchronous motors, sub-transient or transient reactances apply, depending on breaker speed.

Fuses, overcurrent devices with a fusible link in a sand-filled tube, act as conductors under normal operation. Overload current raises the link temperature, causing it to melt and form an arc.