--- title: "Arcing contacts, arc runners and splitters" subject: "Switchgear and Protection" chapter: 6 key: "semester_6/switchgear_and_protection:arcing-contacts-arc-runners" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/arcing-contacts-arc-runners site: BCT Engineering --- # Arcing contacts, arc runners and splitters Switchgear and Protection · Chapter 6: Circuit Breakers - Asked in: 2080 Chaitra, 8 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/arcing-contacts-arc-runners - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-6 ## Question What is the purpose of incorporating arcing contacts alongside main contacts in circuit breakers? Also explain the function of arc runners and arc splitters within an air-break circuit breaker, supported by a clear diagram. ## Short answer Arcing contacts protect main contacts from arc damage by making/breaking the arc. Arc runners (diverging horns) lengthen and cool the arc via thermal and electromagnetic forces. Arc splitters (plates) split the arc into many short arcs, raising voltage above system voltage to extinguish it. ## Answer #### Purpose of arcing contacts In medium and high current breakers each pole has **main contacts** (to carry normal current with very low resistance) and **arcing contacts** in parallel with them. - On **opening**, the main contacts separate first; the current shifts to the arcing contacts, which separate a little later. So the arc forms only on the arcing contacts. - On **closing**, the arcing contacts touch first and the main contacts close later, so any pre-arc also strikes on the arcing contacts. - Arcing contacts are made of hard, heat-resistant material (copper-tungsten, silver-tungsten); main contacts are of high-conductivity silver-plated copper. - Benefits: main contacts are protected from burning and pitting, keep low contact resistance and low heating; only the cheap arcing contacts need replacement. #### Air break circuit breaker An air break CB opens its contacts in air at atmospheric pressure and extinguishes the arc by the **high resistance method**: the arc is lengthened, cooled and split until its voltage exceeds the system voltage. #### Arc runners (arc horns) - Two diverging metal horns extended upward from the arcing contacts. - Once the arc forms, it moves upward along the runners because of (i) the **thermal convection** of hot air and (ii) the **electromagnetic force** of its own current loop (aided by a **blow-out coil** in some designs). - As the runners diverge, the arc becomes longer and thinner; its resistance and voltage increase and it cools. - The arc root is moved off the contacts, protecting them. #### Arc splitters (arc chutes) - A stack of insulated or metal plates placed above the runners inside an **arc chute**. - The rising arc is driven into the plates, where it is **split into many short arcs in series**. - Each short arc needs a minimum cathode-anode voltage drop (about 20–40 V), so the total arc voltage rises sharply above the supply voltage and the current is forced to zero. - The plates also cool the arc and absorb its heat; insulating plates lengthen it in a zig-zag path. ```text ┌─────────────────┐ │ ║ ║ ║ ║ ║ ║ ║ │ arc splitter plates │ \ arc chute / │ (arc split into │ \ ~~~~~~ / │ many short arcs) │ \ ~~~~~~ / │ │ \ ~~~~ / │ arc runners │ \ ~~ / │ (diverging horns) │ ┌──┐ ┌──┐ │ │ │A │ │A │ │ A = arcing contacts │ │M │ │M │ │ M = main contacts └───┴──┴────┴──┴───┘ fixed moving ``` #### Sequence of arc extinction 1. Main contacts open; current transfers to arcing contacts. 2. Arcing contacts open; arc is struck. 3. Arc rises along the runners, getting longer. 4. Arc enters the chute and is split by the splitter plates. 5. Arc voltage exceeds the system voltage; current falls to zero and the arc is extinguished. Air break breakers with these devices are used up to about 11 kV and in DC circuits. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why are arcing contacts provided in parallel with main contacts? A. To carry the normal current B. To make the arc form on them, protecting the main contacts from burning and pitting C. To reduce the noise D. To increase the contact resistance Answer: B. Main contacts separate first, current shifts to the arcing contacts, and the arc strikes there; they are made of heat-resistant material. (Key point: Purpose of arcing contacts) 2. On opening, which contacts separate first? A. Arcing contacts B. Both together C. Main contacts D. Arc runners Answer: C. Main contacts open first, current transfers to arcing contacts, which separate a little later. (Key point: Opening sequence) 3. How do arc runners help extinguish the arc? A. They store the arc energy B. The arc rises along the diverging horns by convection and electromagnetic force, becoming longer and thinner C. They block the air flow D. They short-circuit the arc Answer: B. Lengthening and cooling increases the arc resistance and moves the arc root off the contacts. (Key point: Arc runners) 4. Why do arc splitter plates raise the arc voltage above the supply voltage? A. They heat the arc B. They reduce the supply current C. They shorten the arc D. They split it into many short arcs in series, each with a 20-40 V cathode-anode drop Answer: D. The summed drops exceed the system voltage, forcing the current to zero. 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