--- title: "Fault clearing process with trip circuit" subject: "Switchgear and Protection" chapter: 1 key: "semester_6/switchgear_and_protection:fault-clearing-trip-circuit" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/fault-clearing-trip-circuit site: BCT Engineering --- # Fault clearing process with trip circuit Switchgear and Protection · Chapter 1: Principle of Power System Protection - Asked in: 2075 Baisakh, 6 marks - Video: [Trip circuit of Circuit Breaker Explained with Animation_Switchgear & protection](https://www.youtube.com/watch?v=LyVmB2Ca_AI) — ELECTRICAL PERSPECTIVE (ELECTRICAL PERSPECTIVE) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/fault-clearing-trip-circuit - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-1 ## Question Explain the fault clearing process with the help of trip circuit. ## Short answer The fault current via CT energizes the relay coil; relay contacts close, allowing DC from battery through trip coil to release the breaker latch. The breaker opens, arc extinguishes, and the auxiliary switch resets the trip circuit. Total fault clearing time = relay time + opening time + arcing time. ## Answer Fault clearing is the sequence of events from the instant the fault occurs to the final interruption of fault current by the circuit breaker. It uses the **trip circuit**, which links the relay to the breaker. #### Trip circuit ```text Line ═══[ CB main contacts ]═══════ │ ▲ CT │ opens │ │ ┌─────▼─────┐ ┌──────────┴──┐ │ Relay coil│ │ Trip coil │ └─────┬─────┘ └──┬──────────┘ │ closes │ (+)───┤ Relay contact ├──[52a]──(−) DC battery aux. switch ``` - **CT/VT:** reduce line current and voltage to relay level. - **Relay operating coil:** energised by the CT secondary. - **Relay contact:** in the DC trip circuit. - **Trip coil:** solenoid that releases the breaker mechanism. - **Auxiliary switch (52a):** closed when breaker is closed; opens with the breaker and breaks trip-coil current. - **Battery (station DC supply):** independent of the AC system, which may collapse during the fault. #### Fault clearing process 1. **Fault occurs:** current in the line rises sharply. 2. **Sensing:** the CT secondary current rises proportionally and flows through the relay coil. 3. **Relay operation:** when current exceeds the pick-up setting, the relay (after its set time) closes its contacts. This time is the **relay time**. 4. **Trip coil energised:** DC current flows from the battery through relay contacts, trip coil and the auxiliary switch. 5. **Breaker opens:** the trip coil plunger releases the latch; stored energy (spring/pneumatic) separates the main contacts. This is the **opening time**. 6. **Arc extinction:** an arc forms between the contacts and is extinguished at a current zero by the arc-quenching medium (SF₆, vacuum, oil, air). This is the **arcing time**. 7. **Trip circuit reset:** the auxiliary switch opens, de-energising the trip coil to prevent it burning. **Total fault clearing time = relay time + breaker opening time + arcing time**, e.g. 1 cycle + 1.5 cycles + 0.5–1 cycle ≈ 3–4 cycles (60–80 ms at 50 Hz) for a modern EHV breaker. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is the trip coil supplied from a station DC battery? A. DC gives higher fault current B. It is independent of the AC system, which may collapse during the fault C. The relay needs DC to sense current D. AC cannot energise any coil Answer: B. Battery supply stays available when AC voltage collapses. (Key point: DC supply) 2. What is the role of auxiliary switch 52a? A. It opens with the breaker and breaks trip-coil current, preventing burn-out B. It closes the relay contact C. It measures fault current D. It extinguishes the arc Answer: A. After the breaker opens, 52a de-energises the trip coil. (Key point: Auxiliary switch 52a) 3. What is total fault clearing time? A. Relay time only B. Opening time minus arcing time C. Arcing time + CT time only D. Relay time + breaker opening time + arcing time Answer: D. The three intervals add up, e.g. 3-4 cycles for modern EHV breakers. (Key point: Clearing time) 4. When is the arc extinguished? A. At the peak of current B. When the relay resets C. At a current zero by the arc-quenching medium D. When the CT saturates Answer: C. Quenching media like SF6 or vacuum interrupt at a current zero. (Key point: Arc extinction) ## Related in Switchgear and Protection - [Minimum TMS of relay A (16000 A fault)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aminimum-tms-relay-a) — Switchgear and Protection, chapter 7 - [RRRV from short circuit test (132 kV)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Arrrv-132kv-test) — Switchgear and Protection, chapter 6, asked 2× - [RRRV from short circuit test (220 kV)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Arrrv-220kv-test) — Switchgear and Protection, chapter 6, asked 2× - [Arc extinction in vacuum circuit breakers](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Avacuum-cb-arc-extinction) — Switchgear and Protection, chapter 6 - [Zones and backup for breaker 7 failure](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Azones-backup-breaker-7) — Switchgear and Protection, chapter 1 ## Related in other subjects - [Critical clearing angle/time, fault at infinite bus](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Acritical-clearing-infinite-bus-h5) — Power System Analysis II, chapter 6 - [Critical clearing time and angle, fault at P](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Acritical-clearing-time-point-p-h5) — Power System Analysis II, chapter 6 - [Critical clearing, fault at infinite bus, H = 6](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Acritical-clearing-fault-infinite-bus-h6) — Power System Analysis II, chapter 6 - [Critical clearing for temporary sending-end fault](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Acritical-clearing-temporary-fault-new) — Power System Analysis II, chapter 6