--- title: "CB ratings: 2500 A, 1500 MVA, 33 kV" subject: "Switchgear and Protection" chapter: 6 key: "semester_6/switchgear_and_protection:cb-ratings-2500a-1500mva" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/cb-ratings-2500a-1500mva site: BCT Engineering --- # CB ratings: 2500 A, 1500 MVA, 33 kV Switchgear and Protection · Chapter 6: Circuit Breakers - Asked in: 2070 Bhadra, 2+2+2+2 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-2500a-1500mva - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6 ## Question A circuit breaker is rated as 2500 A, 1500 MVA, 33 KV, 3 sec, 3 phase OCB. Determine (i) the rated symmetrical breaking current (ii) rated making current (iii) short time rating and (iv) rated service voltage. ## Short answer Rated symmetrical breaking current = 1500/(√3×33) = 26.24 kA rms. Making current = 2.55×26.24 = 66.92 kA peak. Short time rating = 26.24 kA for 3 s. Rated service voltage = 33 kV. ## Answer The ratings of a CB follow from its breaking MVA and rated 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 2500 A, 1500 MVA, 33 kV, 3 s, 3-phase oil CB. #### (i) Rated symmetrical breaking current ```text I_b = MVA / (√3 × kV) = 1500 / (√3 × 33) = 26.24 kA (rms) ``` #### (ii) Rated making current ```text I_m = 2.55 × I_b = 2.55 × 26.24 = 66.92 kA (peak) ``` #### (iii) Short time rating ```text = 26.24 kA (rms) for 3 seconds ``` #### (iv) Rated service voltage ```text = 33 kV (rms, line to line) ``` **Answer: (i) 26.24 kA rms; (ii) 66.92 kA peak; (iii) 26.24 kA for 3 s; (iv) 33 kV rms (line). Rated normal current is 2500 A.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which formula gives the rated symmetrical breaking current of a 3-phase CB from its MVA rating? A. I_b = MVA / kV B. I_b = MVA x kV / sqrt(3) C. I_b = MVA / (sqrt(3) x line kV) D. I_b = MVA x sqrt(3) / kV Answer: C. Breaking MVA = sqrt(3) x line kV x I_b, so I_b = MVA / (sqrt(3) x kV). (Key point: Breaking current formula) 2. For 1500 MVA at 33 kV, what is the rated symmetrical breaking current? A. 45.5 kA B. 26.24 kA C. 75.8 kA D. 2.5 kA Answer: B. 1500 / (1.732 x 33) = 26.24 kA rms. The 2500 A is only the normal rated current. (Key point: Breaking current value) 3. Why is the rated making current taken as 2.55 x I_b? A. Because the breaker must carry 2.55 times its normal current B. Because the first major peak of an asymmetrical current can be 1.8 times the symmetrical peak, i.e. 1.8 x sqrt(2) x I_b C. Because the voltage rises 2.55 times at closing D. Because of the 3 s time rating Answer: B. Including the DC offset, the first peak is 1.8 x sqrt(2) x I_b = 2.55 I_b (peak). (Key point: Making current) 4. What is the short time rating of this breaker? A. 2500 A for 3 s B. 66.92 kA for 3 s C. 33 kA for 3 s D. 26.24 kA rms for 3 s Answer: D. The short time rating equals the symmetrical breaking current for the stated duration: 26.24 kA for 3 s. (Key point: Short time rating) ## Related in Switchgear and Protection - [CB ratings: 33 kV, 1500 A, 3000 MVA](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acb-ratings-1500a-3000mva) — 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× - [Initial symmetrical current, load short circuit](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Ainitial-current-load-short-circuit) — Power System Analysis II, chapter 3