29.5
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 breake…
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.
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Q1: How does a circuit breaker interrupt fault currents?
A circuit breaker interrupts fault currents by separating electrical contacts, which generates an arc. This arc is extinguished through elongation, cooling, or splitting depending on the breaker's design. The extinguishing process stops the flow of electricity and protects the system from damage caused by excessive current.
Q2: What is the difference between magnetic and thermal trips in low-voltage breakers?
Low-voltage molded-case breakers use magnetic instantaneous trips to handle large fault currents and thermal trips for smaller, sustained overloads. Magnetic trips respond quickly to sudden high currents, while thermal trips protect against prolonged overcurrent conditions by monitoring temperature rise in the conductor.
Q3: How do high-voltage circuit breakers respond after interrupting a fault?
High-voltage circuit breakers automatically reclose within 15 to 50 cycles after an interruption. If the fault persists after reclosing, the breaker locks out and requires manual operator intervention to restore service and investigate the underlying fault condition affecting the system.
Q4: What is the E/X method used for in breaker selection?
The E/X simplified method aids breaker selection by calculating the maximum symmetrical short-circuit current using pre-fault voltage and system reactance while neglecting other factors. This approach streamlines the selection process for appropriate breaker ratings in power system three phase short circuits applications.
Q5: How do fuses operate as overcurrent protection devices?
Fuses contain a fusible link within a sand-filled tube that conducts electricity under normal operation. When overload current occurs, the link's temperature rises, causing it to melt and form an arc, interrupting the circuit. Fuses must be manually replaced once they have melted to restore protection.
Q6: Why are different reactances used for generator and motor breaker selection?
For generators, two-cycle breakers are selected based on sub-transient fault current using sub-transient reactances. For synchronous motors, sub-transient or transient reactances apply depending on breaker speed. These distinctions account for different machine response characteristics during fault conditions in rotating equipment.
Q7: What key factors determine fuse specifications?
Fuses are specified based on four key factors: current rating, voltage rating, interrupting rating, and time-delay characteristics. These specifications ensure the fuse provides appropriate protection for the specific electrical system and load conditions while maintaining safety and operational continuity throughout normal and fault operation.