Understanding Circuit Breaker Breaking Capacity

A circuit breaker is a critical protection device in electrical power systems. It is designed to interrupt fault currents safely and quickly to prevent damage to equipment and ensure system stability. A circuit breaker's capability is defined by several parameters in its rating.

The given rating for the 3-phase oil circuit breaker includes the following information:

• Rated continuous current: 1450 A
• Rated MVA capacity: 1,200 MVA
• Rated voltage: 33 kV
• Rated short-time duration or operating time: 3 s
• Number of phases: 3-phase

The question specifically asks for the breaking capacity.

Identifying Breaking Capacity from Rating

The breaking capacity of a circuit breaker is its ability to interrupt or "break" the circuit under fault conditions. It is the maximum fault level the breaker can safely clear. This capacity is typically expressed in either MVA (Mega Volt-Amperes) or kA (kiloAmperes) at the rated voltage.

In the given rating:

• The 1450 A is the rated continuous current the breaker can carry under normal load.
• The 33 kV is the rated voltage of the system the breaker is designed for.
• The 3 s is related to the short-time withstand current or the time it takes for the breaker to operate and clear a fault.
• The 1,200 MVA value is explicitly given as a capacity rating in Mega Volt-Amperes. This value directly represents the maximum apparent power the breaker can interrupt during a short circuit at its rated voltage.

Therefore, the 1,200 MVA value provided in the rating corresponds to the circuit breaker's breaking capacity.

Comparing with Options

Let's look at the provided options:

1. 1,200 MVA: This value is listed in the circuit breaker's rating and represents its breaking capacity.
2. 33 kV: This is the rated voltage, not the breaking capacity.
3. 3 s: This is the rated operating time or short-time duration, not the breaking capacity.
4. 1,450 A: This is the rated continuous current, not the breaking capacity.

Based on the standard definitions of circuit breaker ratings, the breaking capacity is given as 1,200 MVA.