High resistance and current zero interruption

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).

 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).