Circuit Breaker and Fuse Selection

A circuit breaker is a device engineered to interrupt fault currents and sometimes reclose automatically. When a fault current is detected, the breaker separates the electrical contacts, which generates an arc. This arc is extinguished by methods such as elongation, cooling, or splitting, depending on the breaker's design. Breakers are categorized based on the voltage they operate at and the medium used for arc extinction, such as air, oil, SF6 gas, or vacuum.

In high-voltage systems, circuit breakers typically feature automatic reclosing mechanisms. These breakers reclose within 15 to 50 cycles after an interruption. If the fault persists, they lock out, necessitating manual intervention by an operator. In low-voltage systems, dual-trip molded case breakers are common. These breakers utilize magnetic instantaneous trips to handle large faults and thermal trips to handle smaller faults.

The E/X method simplifies breaker selection by calculating the maximum symmetrical short-circuit current. This approach uses the prefault voltage and system reactance characteristics, ignoring other factors. For generators, two-cycle breakers are used to calculate subtransient fault current, relying on subtransient machine reactances. For synchronous motors, subtransient or transient reactances are considered based on the breaker's operating speed.

Fuses serve as overcurrent protection devices, containing a fusible link within a sand-filled tube. Under normal conditions, the fusible link conducts electricity. When an overload current occurs, the link's temperature rises, causing it to melt and form an arc. Fuses are specified based on four key factors: current rating, voltage rating, interrupting rating, and time-delay characteristics. Fuses must be replaced manually once they have melted.

Both circuit breakers and fuses play critical roles in protecting electrical systems from fault currents and overcurrent conditions. Circuit breakers, with their ability to automatically reclose and handle different types of faults, are essential in high and low-voltage applications. Fuses, while simpler and requiring manual replacement, provide reliable protection by interrupting overcurrents. Understanding the specific requirements and characteristics of the electrical system is crucial in selecting the appropriate protective devices to ensure safety and operational continuity.