Arcing contacts, arc runners and splitters

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.

         ┌─────────────────┐
         │ ║ ║ ║ ║ ║ ║ ║   │  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.