How to Calculate the Breaking Capacity?

Figure 1: A front view of Schneider molded case circuit breaker with breaking capacity information.

The breaking capacity of the circuit breakers is an important indicator of the circuit breaker, which refers to the ability of the circuit breaker to safely cut off the fault current. There are 36KA, 50KA and other specifications. The breaking capacity of the circuit breaker refers to the ability of the circuit breaker to safely cut off the fault current (often also the determining factor of the price), which is not necessarily related to its rated current. This article will explain some important parameters and provide a possible method of the breaking capacity calculation.

Basics about the Breaking Capacity

The breaking capacity is generally divided into rated breaking capacity (Icu) and service breaking capacity (Ics). Icu is the limit parameter of the breaking capacity of the circuit breaker. After breaking several short-circuit faults, the breaking capacity of the circuit breaker will decrease. Ics is a breaking index of the circuit breaker, that is, after breaking several short-circuit faults, it can still ensure its normal operation.
If Icu=60KA, then when a fault current of 60KA occurs in the line, the circuit breaker can safely cut off the circuit without abnormal conditions such as contact welding and explosion.
If Icu=50KA, then when a 50KA fault current occurs in the circuit, the circuit breaker can safely cut off the circuit without contact bonding, explosion, etc., but the circuit breaker with extreme short-circuit breaking can no longer be used. And if Ics=50KA, when a 50KA fault current occurs, the circuit breaker can safely cut off the circuit, and it can be closed again when the fault is eliminated. Of course, it is best to replace the circuit breaker.

Note that the circuit breaker that has done limit breaking is not allowed to be used again (often it fails) and must be replaced. And if Ics=60KA, after breaking the current, the circuit breaker is allowed to be closed and used again, but it must be replaced after emergency.

Last but not least, the common question is:
Is it true that the higher the breaking capacity is, the better?
Yes, it is, because the greater the breaking capacity, the greater the current and voltage the circuit breaker can carry, which can play a good protective role.

Guide of Calculation

The formular of calculation is shown in figure 2:

Figure 2: Formular of breaking capacity calculation.

The breaking capacity is expressed in mega volt ampere (MVA).
Example: if it is required to break 200 A short circuit current at rated service voltage of 11 kV in a 3 phase system. The solution is:

Figure 3: Solution to the example.

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