--- title: "Breaking, making, short-time ratings (500 MVA)" subject: "Switchgear and Protection" chapter: 6 key: "semester_6/switchgear_and_protection:cb-ratings-500a-500mva" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/cb-ratings-500a-500mva site: BCT Engineering --- # Breaking, making, short-time ratings (500 MVA) Switchgear and Protection · Chapter 6: Circuit Breakers - Asked in: 2073 Bhadra, 3+5 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-500a-500mva - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6 ## Question Explain what do you understand by: i) Rated symmetrical breaking current ii) Making capacity iii) Short time current rating. Obtain their value for a three phase CB rated as 500 A, 500 MVA, 11 kV, 3 sec. ## Short answer Rated symmetrical breaking current is the rms AC component the CB can interrupt; making capacity is the peak withstand (2.55×I_b); short time rating is the rms carry current for specified time. For 500 MVA, 11 kV: I_b = 26.24 kA rms, making = 66.92 kA peak, short time = 26.24 kA for 3 s. ## Answer #### 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: ```text 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: ```text 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 ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the rated symmetrical breaking current? A. The peak current the breaker can close onto B. The rms value of the AC component of the short-circuit current the CB can interrupt at rated voltage C. The current it can carry continuously D. The current for 3 s without damage while open Answer: B. It is the rms of the symmetrical AC component interrupted at contact separation, and it fixes the breaking capacity. (Key point: Meaning of symmetrical breaking current) 2. Making capacity is best defined as A. the rms current carried continuously B. the peak current (with DC offset) in the first cycle that the CB can close onto a short circuit C. the MVA the CB can break D. the voltage it can withstand across open contacts Answer: B. Contacts must withstand large electromagnetic forces at the first major peak, so making capacity is a peak value. (Key point: Making capacity meaning) 3. What does the short time current rating describe? A. Current the closed CB can carry for a stated time (1 s or 3 s) without damage while a downstream breaker clears the fault B. Time taken by the arc to extinguish C. Minimum current for tripping D. Current that can be made in the first cycle Answer: A. It is a thermal and mechanical withstand rating for the closed position, usually equal to I_b. (Key point: Short time rating meaning) 4. For 500 MVA at 11 kV, the breaking current and making current are A. 45.5 kA and 115.9 kA B. 26.24 kA and 66.92 kA C. 26.24 kA and 47.2 kA D. 500 A and 1275 A Answer: B. I_b = 500/(1.732 x 11) = 26.24 kA; making = 2.55 x 26.24 = 66.92 kA peak. 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