---
title: "CB ratings: 33 kV, 1500 A, 3000 MVA"
subject: "Switchgear and Protection"
chapter: 6
key: "semester_6/switchgear_and_protection:cb-ratings-1500a-3000mva"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/cb-ratings-1500a-3000mva
site: BCT Engineering
---

# CB ratings: 33 kV, 1500 A, 3000 MVA

Switchgear and Protection · Chapter 6: Circuit Breakers

- Asked in: 2077 Chaitra, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/cb-ratings-1500a-3000mva
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6

## Question

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.

## 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.

## Answer

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.

```text
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.**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. To find the rated breaking current of the 33 kV, 3000 MVA breaker you should use
   A. 3000 / (sqrt(3) x 33)
   B. 3000 / 33
   C. 3000 x sqrt(3) x 33
   D. 1500 x sqrt(3)
   Answer: A. I_b = MVA / (sqrt(3) x line kV); the 1500 A normal current is not used. (Key point: Breaking current method)

2. The rated symmetrical breaking current is
   A. 90.9 kA
   B. 26.24 kA
   C. 52.49 kA
   D. 133.8 kA
   Answer: C. 3000 / (1.732 x 33) = 52.49 kA rms. (Key point: Breaking current value)

3. The making current of this breaker is about
   A. 133.8 kA peak
   B. 52.49 kA peak
   C. 94.5 kA peak
   D. 68.2 kA peak
   Answer: A. 2.55 x 52.49 = 133.8 kA peak. (Key point: Making current value)

4. The short time rating for 1 s is
   A. 133.8 kA for 1 s
   B. 1500 A for 1 s
   C. 52.49 kA for 1 s
   D. 52.49 kA for 3 s
   Answer: C. Short time rating equals I_b for the stated duration, here 1 s. (Key point: Short time rating with duration)

## Related in Switchgear and Protection

- [CB ratings: 2500 A, 1500 MVA, 33 kV](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acb-ratings-2500a-1500mva) — Switchgear and Protection, chapter 6
- [Breaking, making, short-time ratings (500 MVA)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acb-ratings-500a-500mva) — Switchgear and Protection, chapter 6
- [Breaking MVA and making kA (800 A, 33 kV)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acb-breaking-making-800a) — Switchgear and Protection, chapter 6
- [CB terms: making, breaking, operating duty](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acb-terms-making-breaking-duty) — Switchgear and Protection, chapter 6
- [Selecting CB for 6.6 kV, 650 MVA](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aselect-cb-6-6kv-650mva) — Switchgear and Protection, chapter 6

## Related in other subjects

- [Short circuit MVA of feeder circuit breaker](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Afeeder-cb-short-circuit-mva) — Power System Analysis II, chapter 3, asked 2×