Breaking, making, short-time ratings (500 MVA)

i) Rated symmetrical breaking current

The rms value of the AC (symmetrical) component of the short-circuit current that the CB can interrupt at its rated voltage, at the instant of contact separation. It is the basis of the breaking capacity:

Breaking capacity (MVA) = √3 × V(kV) × I_b(kA)

ii) Making capacity

The peak value of current (including the DC offset) during the first cycle that the CB can safely make when it closes on a short circuit. The contacts must withstand the large electromagnetic forces at the first major peak, which can be 1.8 times the symmetrical peak:

I_making = 1.8 × √2 × I_b = 2.55 × I_b (peak)

iii) Short time current rating

The rms current the CB can carry in the closed position for a stated short time (1 s or 3 s) without thermal or mechanical damage, while a downstream breaker clears the fault. It is usually equal to the rated symmetrical breaking current for the given time.

Calculation for 500 A, 500 MVA, 11 kV, 3 s CB

I_b = 500 / (√3 × 11)
    = 26.24 kA (rms)

I_making = 2.55 × 26.24
         = 66.92 kA (peak)

Short time rating = 26.24 kA (rms) for 3 s

Answer: Rated symmetrical breaking current = 26.24 kA rms; making capacity = 66.92 kA peak; short time rating = 26.24 kA for 3 s. (Rated normal current = 500 A, rated voltage = 11 kV.)

Note that the 33 kV, 1500 MVA breaker and this 11 kV, 500 MVA breaker have the same breaking current, because MVA and voltage are in the same ratio.