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CB ratings: 33 kV, 1500 A, 3000 MVA

Short answer

Rated symmetrical breaking current = 3000/(√3×33) = 52.49 kA rms. Making current = 2.55×52.49 = 133.8 kA peak. Short time rating = 52.49 kA for 1 s.

Determine the rated symmetrical breaking current, making current and short time rating of a circuit breaker with following parameters: 33 kV, 1500 A, 3000 MVA, 1 sec.

The breaking, making and short-time currents of a CB are fixed by its rated MVA and voltage.

Definitions and formulas used

  • Rated symmetrical breaking current I_b = rated breaking MVA / (√3 × rated line kV), rms.
  • Rated making current = 1.8 × √2 × I_b = 2.55 × I_b (peak), since the first major peak of an asymmetrical current can be 1.8 times the peak of the symmetrical current.
  • Short time rating = the current the CB can carry while closed for the stated time without damage; it equals I_b for the given duration.

Given

33 kV, 1500 A, 3000 MVA, 1 s.

I_b = 3000 / (√3 × 33)
    = 52.49 kA (rms)

I_making = 2.55 × 52.49
         = 133.84 kA (peak)

Short time rating = 52.49 kA (rms) for 1 s

Answer: Rated symmetrical breaking current = 52.49 kA rms; making current = 133.8 kA peak; short time rating = 52.49 kA for 1 s.

✓ Answer checked by Claude Opus 5.5 (high effort)

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