--- title: "CB terms: making, breaking, operating duty" subject: "Switchgear and Protection" chapter: 6 key: "semester_6/switchgear_and_protection:cb-terms-making-breaking-duty" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/cb-terms-making-breaking-duty site: BCT Engineering --- # CB terms: making, breaking, operating duty Switchgear and Protection · Chapter 6: Circuit Breakers - Asked in: 2075 Baisakh, 4 marks - Video: [SGP210 Circuit Breakers Ratings](https://www.youtube.com/watch?v=OJenX-r3uSw) — Lectures in Electrical Engineering - 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-terms-making-breaking-duty - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6 ## Question Explain the terms related to Circuit Breaker: i) Making Capacity ii) Breaking Capacity iii) Operation Duty iv) Rated Current. ## Short answer Making capacity is the peak closing current including DC offset, equal to 2.55 times the symmetrical breaking current. Breaking capacity is the rms symmetrical current or √3×V×I in MVA. Operating duty is the specified O–t–CO–t′–CO sequence; rated current is the continuous rms current without exceeding temperature limits. ## Answer These terms define the ratings of a circuit breaker as per IEC 62271-100 / IS 13118. #### i) Making capacity The **peak value of current** that the CB can make (close on to) at rated voltage when closing on a short circuit, including the DC offset of the first cycle. The contacts must withstand the large electromagnetic force at this first peak. ```text Making current = 1.8 × √2 × I_b = 2.55 × I_b (peak) ``` #### ii) Breaking capacity The largest current the CB can break at rated voltage under specified conditions. Expressed as the **rms symmetrical breaking current** *I_b* (kA) or as MVA: ```text Breaking capacity (MVA) = √3 × V(kV) × I_b(kA) ``` Asymmetrical breaking current includes the DC component at contact separation. #### iii) Operating duty The **sequence of operations** the CB must perform at rated breaking current, with given time intervals. Standard duties: - **O – t – CO – t′ – CO**, e.g. O – 3 min – CO – 3 min – CO (non-auto-reclose duty) - **O – 0.3 s – CO – 3 min – CO** (rapid auto-reclosing duty) Here O = open, CO = close followed immediately by open, *t* = dead time. The CB must keep its rated breaking capacity throughout this sequence. #### iv) Rated current The **rms value of normal current** the CB can carry continuously at rated frequency without the temperature of any part exceeding its permitted limit. Standard values: 400, 630, 800, 1250, 1600, 2000, 2500, 3150 A, etc. **Example:** a 33 kV CB rated 1250 A, 25 kA, duty O–0.3 s–CO–3 min–CO has breaking capacity √3 × 33 × 25 = 1429 MVA and making capacity 2.55 × 25 = 63.75 kA peak. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Making capacity of a CB is A. the rms value of the normal current B. the peak current including DC offset in the first cycle that it can close onto a short circuit C. the time to close the contacts D. the MVA it can interrupt Answer: B. It is a peak value equal to 2.55 x I_b, because the contacts face large electromagnetic force at the first peak. (Key point: Making capacity) 2. Breaking capacity can be expressed as A. rms symmetrical breaking current I_b or MVA = sqrt(3) x kV x I_b B. peak current x 2.55 C. normal current x voltage D. short time current x time Answer: A. It is the largest current broken at rated voltage, given in kA rms or MVA. (Key point: Breaking capacity) 3. In the duty O - 0.3 s - CO - 3 min - CO, what does CO mean? A. Close, followed immediately by open B. Continuous operation C. Current off D. Circuit open for long time Answer: A. O is open and CO is a close operation followed immediately by an open; the CB must keep its rated breaking capacity through the sequence. (Key point: Operating duty) 4. The rated current of a CB is A. the maximum fault current it can break B. the rms normal current it carries continuously without exceeding temperature limits C. the peak making current D. the current it can carry for 3 s Answer: B. It is the continuous normal current at rated frequency, e.g. 400, 630, 800, 1250 A. 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