When a circuit breaker's contacts separate, an arc burns between them, and the breaker's whole job is to kill that arc. There are two fundamentally different ways to do it: raise the arc's resistance until the current dies away, or keep the resistance low and wait for a natural current zero. This post explains both methods and where each one is used.
Figure 3.1: High-resistance vs low-resistance arc interruption. (a) Arc current falls as Rarc rises. (b)
Dielectric recovery must outrun the restriking voltage (RRRV). (Schematic, not to scale — curve shapes only.)
1. High-resistance interruption (DC and low/medium AC)
The arc resistance Rarc is progressively increased over time so that the arc current decreases continuously to zero — without causing destructive overvoltages from L·di/dt. Since Rarc = ρ·l / A, resistance is increased by:
- (i) Arc lengthening — via magnetic blowout coils, which stretch the arc;
- (ii) Arc cooling — removing heat from the arc column;
- (iii) Arc splitting — into series sub-arcs using metal splitter plates;
- (iv) Constricting the arc — reducing its cross-section A.
Because a large amount of energy is dissipated inside the chute, this method is limited to DC and low/medium-capacity AC circuit breakers — for example MCBs and air circuit breakers (ACBs). It cannot handle the enormous energy of a high-voltage fault.
2. Low-resistance (current-zero) interruption (high-power AC)
This method is used exclusively in high-power AC circuit breakers. During the half-cycle conduction period the arc length and arc resistance are kept intentionally low, so minimal arc energy (∫ varc·i dt) is generated. The breaker then waits for the natural current zero — which occurs 100 times per second in a 50 Hz system — and at that instant (i = 0) the deionised medium rapidly rebuilds its dielectric strength.
Success is a race, described by two theories:
- Slepian's Dielectric Race Theory — the dielectric strength of the gap must rebuild faster than the Rate of Rise of Restriking Voltage (RRRV). If the recovery voltage overtakes the gap strength, the arc restrikes.
- Cassie–Mayr Energy Balance Theory — the medium must remove heat faster than the arc puts power in. If heat removal wins, the arc channel cools and dies; if not, it re-ignites.
Key points to remember
- High-resistance method: raise Rarc = ρ·l/A by lengthening (magnetic blowout), cooling, splitting and constricting — DC and LV/MV AC only.
- Low-resistance method: keep arc energy low, then use the natural current zero — for high-power AC breakers.
- At current zero, dielectric recovery must outrun RRRV (Slepian) and heat removal must outrun power input (Cassie–Mayr).
- The high-resistance method wastes energy in the chute, so it cannot scale to high-voltage faults.
Based on Power System Protection & Switchgear, Complete Examination Solution, Exam 07 May 2026, Q3(a).