Breaking Current Capacity Of circuit Breaker

What is the Breaking capacity of a circuit breaker?

Breaking current capacity or interrupting the capacity of the circuit breaker is a very important parameter for the selection of the breaker for the electrical network. The short circuit fault current of the electrical installation must be first known in order to select the suitable rating of the breaker for electrical installation.  At the time of a short circuit fault in the electrical network, a very high current of magnitude 30 KA to 50 KA, depending on the size of the transformers in the electrical network, flows in the circuit.

The electrical circuit interrupting device must be able to interrupt high short circuit current, without being damaged. The technical term to show the fault interrupting the current capacity of the breaker is known as the breaking capacity of the circuit breaker.

Why Breaking Capacity of Circuit breaker Important?

When a fault occurs in the electrical network, a large amount of current flows through the breaker poles. After tripping of the breaker at fault, if the breaker is again switched on under fault conditions, a large current flows through the breaker. If the breaker does not interrupt that fault current. then it may cause severe damage to a breaker. It also creates heavy stress on the electrical network.

For interrupting the fault current when the breaker is switched on under fault conditions, the other important parameter termed making current capacity must be checked.

It is very important to understand why the making current of the breaker is more than the breaking current. Under normal conditions, the power system operates under balanced conditions with all equipment carrying normal current. At the time of the fault, the power system gets unbalanced and the system voltage and the current get disrupted. The fault current has sub transient, transients, and DC components.

The making current of a circuit breaker is the peak value of the maximum current loop during sub-transient conditions including DC components when the breaker is closed under fault conditions. The circuit breaker should meet all the requirements of nominal breaker capacity, breaking capacity, and making capacity.  

Relationship between Making Current and breaking current

Let symmetrical breaking current = I

Peak Value of symmetrical current = 1.414 I

The DC component current is almost equal to the peak value of current during sub transient period.

Making current= 1.414 I *1.8 = 2.55 I

The making current of the breaker is 2.55 times the breaking current. If the system fault current is 35 KA, the breaking capacity of the breaker should be 40 KA for 3 Sec, and making current capacity of the breaker shall be 100 KA

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