--- title: "High resistance and current zero interruption" subject: "Switchgear and Protection" chapter: 6 key: "semester_6/switchgear_and_protection:arc-interruption-methods" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/arc-interruption-methods site: BCT Engineering --- # High resistance and current zero interruption Switchgear and Protection · Chapter 6: Circuit Breakers - Asked in: 2073 Magh, 8 marks - Video: [ARC formation process & ARC interruption methods in Circuit breaker | High and Low resistance method](https://www.youtube.com/watch?v=qMUKVAZlSU8) — Electrical Engineering: Polytechnic special - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/arc-interruption-methods - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6 ## Question Explain the high resistance and current zero interruption method of arc extinction in the circuit breakers. What is the criteria for successful arc interruption? ## Short answer High resistance method raises arc resistance by lengthening, cooling, narrowing, splitting arcs; used in DC/low-voltage AC breakers. Current zero method uses natural AC current zero, preventing restrike by building dielectric strength faster than restriking voltage rises. Successful interruption requires dielectric strength rise rate exceeding RRRV, energy removal exceeding input, and gap withstanding recovery voltage. ## Answer An arc between separating contacts is extinguished either by raising its resistance until the current cannot be maintained (**high resistance method**) or by letting it go out at a natural current zero and preventing it from restriking (**low resistance or current zero method**). #### High resistance method - The arc resistance is increased with time so that the current falls to a value too small to maintain the arc. - Arc resistance is raised by: 1. **Lengthening** the arc (arc runners, separating contacts further). 2. **Cooling** the arc (contact with cold surfaces, air). 3. **Reducing cross-section** (forcing it through narrow slots). 4. **Splitting** it into many short arcs in series (arc splitter plates). - Disadvantage: a lot of energy is dissipated in the arc, so it is used only in **DC breakers** and low/medium voltage **air break** AC breakers. #### Low resistance (current zero) method - Used in all AC high-voltage breakers (oil, air blast, SF6, vacuum). - The arc resistance is kept low until current zero, so arc energy is small. AC current passes through zero 100 times per second (50 Hz); at each zero the arc goes out by itself. - The task is to **prevent restriking** after current zero. Two quantities race against each other: - the **dielectric strength** of the gap, which must rise quickly, and - the **restriking voltage** across the contacts, which also rises quickly. - Gap strength is built up by: 1. **Lengthening the gap** quickly (fast contact separation). 2. **High pressure** in the gap, which increases recombination of ions. 3. **Cooling** (e.g. hydrogen from oil, SF6). 4. **Blast effect:** sweeping out ionised particles with fresh oil, air or SF6. Two theories explain this: the **energy balance theory** (arc restrikes if heat produced exceeds heat removed) and the **voltage race / recovery rate theory** (Slepian's theory: arc does not restrike if the rate of build-up of dielectric strength is faster than the rate of rise of restriking voltage). ```text Voltage │ dielectric strength │ ___________---- │ _-' ___ │ _-' _-' restriking voltage │ / _-' │ / _-' │/_-' └────────────────────── time current zero ``` #### Criteria for successful arc interruption 1. At current zero, the **rate of rise of dielectric strength** of the gap must always exceed the **rate of rise of restriking voltage (RRRV)**; the dielectric strength curve must stay above the restriking voltage curve. 2. The energy removed from the arc space by cooling, deionisation and blast must be greater than the energy supplied by the restriking voltage (energy balance). 3. The gap must withstand the full **recovery voltage** after transients die out. 4. In practice this needs fast contact separation, effective deionisation, and control of RRRV (e.g. resistance switching). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does the high resistance method extinguish the arc? A. By raising arc resistance (lengthening, cooling, narrowing, splitting) until the current cannot be maintained B. By waiting for the natural current zero C. By keeping resistance low until current zero D. By reducing the restriking voltage only Answer: A. Increasing arc resistance reduces the current below the level that sustains the arc; used in DC and LV air break breakers. (Key point: High resistance method) 2. In the current zero method, what must be prevented after current zero? A. Current chopping B. Restriking of the arc C. Contact welding D. Rise of arc resistance Answer: B. AC arcs go out at each current zero; the task is to stop them re-igniting. (Key point: Current zero method) 3. What is the criterion for successful arc interruption (Slepian's theory)? A. RRRV must exceed the rate of rise of dielectric strength B. Energy supplied must exceed energy removed C. Rate of rise of dielectric strength must exceed the RRRV D. Arc voltage must be zero at all times Answer: C. If the gap's dielectric strength builds up faster than the restriking voltage rises, the arc does not restrike. (Key point: Criterion for success) 4. Where is the high resistance method used? A. All HV AC oil breakers B. SF6 breakers C. Vacuum breakers D. DC breakers and LV/MV air break AC breakers Answer: D. It dissipates much energy in the arc, so it is limited to DC and low/medium voltage air break breakers. 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