The Making Capacity of the circuit breaker is the maximum current that a breaker can conduct while making a circuit (at the instant of closing). The making capacity of the breaker is considered as the peak value of current during the first cycle of the current wave after the closure of the circuit breaker.
The making capacity of a circuit breaker is considered to the peak value of the first cycle when we consider an imaginary short circuit between the phases.
Generally, the maximum value of fault current has the possibility of occurrence in the first cycle when the maximum asymmetry occurs in any phase.
Mathematical Formula of Making Capacity
Formula: Making capacity of breaker = 2·55 × Symmetrical breaking capacity
Where 2.55 is obtained by multiplying 1.8 by √ 2
Where √ 2 is used to convert RMS symmetrical to peak and 1.8 is used for doubling effect of maximum symmetry.
Solved Example
A circuit breaker has symmetrical breaking current capacity of 25,000 A (rms). Find the making current.
Solution:
From formula
Making current = 2.55 * Rating symmetrical breaking current
= 2.55 * 25000 = 63750 A