How it works
Arc extinction requires either high dielectric recovery of the gap (voltage withstand must exceed recovery voltage) or forcing the current to zero and preventing reignition. Minimum oil circuit breakers (MOCB) use transformer oil for arc quenching and insulation; the arc decomposes oil into hydrogen (good arc-cooling gas) and carbon. SF6 circuit breakers use sulphur hexafluoride gas at 3–4 bar: SF6 has dielectric strength 2.5× air and excellent arc-quenching properties, making it dominant at 132 kV and above. Vacuum circuit breakers (VCB) — standard for 11 kV and 33 kV — quench the arc in a near-perfect vacuum where metal vapour condenses rapidly on the contact surfaces after current zero.
Key points to remember
Circuit breaker ratings: rated voltage, rated current (continuous), rated short-circuit breaking current (symmetrical kA), rated short-circuit making current (peak kA = 2.55 × symmetrical for DC offset), and rated operating sequence (O−0.3s−CO−3min−CO). Breaking capacity in MVA = √3 × V × Isc. For a 11 kV bus with Isc = 25 kA, breaking capacity = √3 × 11 × 25 = 476 MVA. Restriking voltage (transient recovery voltage, TRV) immediately after contact separation must not exceed the breaker's rated TRV or reignition occurs. Air blast CBs are used at 220 kV and above in some older installations.
Exam tip
The examiner always asks you to compare SF6, vacuum, and MOCB circuit breakers — focus on voltage level, arc quenching medium, maintenance requirement, and environmental considerations (SF6 is a potent greenhouse gas).