Why are circuit breakers rated in apparent power

Circuit breakers are rated in apparent power (S) because it's a more comprehensive and practical way to describe their interrupting capability. Here's why:

1. Apparent power includes both real and reactive power: Apparent power (S) is the vector sum of real power (P) and reactive power (Q). Real power is the actual power consumed by the load, while reactive power is the power that is stored in the magnetic field of the inductor or the electric field of the capacitor. Apparent power takes into account both the real and reactive components of the load current.
2. Circuit breakers need to interrupt both real and reactive currents: When a fault occurs, the circuit breaker needs to interrupt not only the real current (I) but also the reactive current (Iq) associated with the inductive or capacitive load. Apparent power rating ensures that the breaker can interrupt the total current, including both real and reactive components.
3. Reactive power can be significant in certain applications: In power systems with significant amounts of inductive or capacitive loads, such as motors, transformers, or power factor correction (PFC) circuits, the reactive power component can be substantial. A circuit breaker rated in apparent power can handle these high reactive currents, ensuring reliable operation and fault clearance.
4. Apparent power rating is more conservative than current rating: A circuit breaker rated in apparent power is designed to interrupt a current that is higher than the actual current flowing through the load. This is because the apparent power rating takes into account the reactive component, which can be higher than the real current. This conservative approach ensures that the breaker can safely interrupt the fault current, even in cases where the reactive power is significant.
5. Easier to design and test: Rating circuit breakers in apparent power simplifies the design and testing process. It eliminates the need to consider the complex interactions between real and reactive currents, making it easier to ensure that the breaker can interrupt the fault current reliably.

In summary, circuit breakers are rated in apparent power because it's a more comprehensive and practical way to describe their interrupting capability, taking into account both real and reactive power components. This rating ensures that the breaker can safely interrupt fault currents, even in applications with significant reactive power components.