# Why was circuit breaker capacity rated in kva not in ka and kv?

Why was circuit breaker capacity rated in kva not in ka and kv?

## Breaking Capacity in MVA

Earlier the breaking capacity of a circuit breaker was expressed in MVA which depended on the rated breaking current and the rated operating voltage of the CB.

It was calculated as:

Breaking Capacity = √3×V×I×10−6 MVA

Where:

V - Rated line voltage

I - Rated line current

Ex: a 100 MVA breaking capacity circuit breaker with 11 kV rated voltage would have a breaking current of:

## Why Moved to kA and kV?

Expressing breaking capacity in MVA has some drawbacks:

• Breaking capacity depends on both voltage and current ratings therefore breaking capacity cannot be determined from either of them alone. It is much more convenient to express it in kA alone.
• This is actually war that the formula used to determine MVA is not as straightforward as just declaring the breaking current in kA.
• To say that ‘MVA does not define the breaking current capability of the breaker’, is a valid argument.
• Thus, circuit breaker manufacturers switched to declaring the breaking capacity in terms of kilo amperes at the said voltage in kilovolts. This is more reasonable and easy for selection. For instance, anyone can understand what is being meant by a circuit breaker that is rated to handle 50 kA at 11 kV.

That is why, as it was mentioned before, historically the term MVA was used, however nowadays the common practice is to indicate the breaking capacity in kA at the rated voltage in kV for the circuit breaker. It offer a new and convenient, all-encompassing and easy-to-understand instrument in defining and choosing circuit breakers.