The value of short circuit breaking current is expressed in RMS. During short circuit, the CB is not only subjected to thermal stress, it also suffers seriously from mechanical stresses. So during determining short circuit capacity, the mechanical strength of the CB is also considered.

Explanation:

Introduction
In electrical circuits, a circuit breaker is a device that automatically interrupts the flow of current to protect the circuit from damage caused by overcurrent or short circuit conditions. When a fault occurs, the circuit breaker trips to disconnect the circuit and prevent further damage.

Current Specification
The current specification of a circuit breaker refers to the maximum amount of current that the breaker can safely interrupt without causing damage. This specification is typically given in terms of the rated current or the breaking capacity of the circuit breaker.

Types of Current
When specifying the making current of a circuit breaker, it is important to consider the type of current being referred to. There are several ways to represent current, including RMS value, peak value, and average value.

RMS Value
The RMS (Root Mean Square) value of an AC (Alternating Current) waveform is a measure of its effective value. It is the square root of the average of the squares of the instantaneous values of the waveform over one cycle. The RMS value is commonly used to specify the magnitude of AC currents and voltages.

Peak Value
The peak value of an AC waveform is the maximum value that it reaches during one cycle. It is equal to the RMS value multiplied by the square root of 2 (approximately 1.414). The peak value is often used to specify the magnitude of AC currents and voltages.

Average Value
The average value of an AC waveform is the average of its instantaneous values over one cycle. It is calculated by integrating the waveform over one cycle and dividing by the period of the waveform. The average value is rarely used to specify the magnitude of AC currents and voltages.

Circuit Breaker Specification
When specifying the making current of a circuit breaker, none of the above values (RMS, peak, or average) are used. The making current of a circuit breaker is typically specified in terms of the maximum fault current that the breaker can interrupt without causing damage. This value is determined based on the specific application and electrical system requirements.

Conclusion
In conclusion, the normal practice to specify the making current of a circuit breaker is none of the options provided (RMS, peak, or average value). The making current is typically specified in terms of the maximum fault current that the breaker can safely interrupt. This specification is determined based on the specific application and electrical system requirements.