Side-by-side comparison
| Parameter | Air Blast CB | SF6 Circuit Breaker |
|---|---|---|
| Arc Quenching Medium | Compressed air at 20–30 bar | SF6 gas at 3–6 bar (self-blast or puffer type) |
| Dielectric Strength | Low relative to SF6; needs multi-break design | 2–3× air at same pressure; single-break up to 245 kV |
| Number of Breaks (420 kV) | 4–8 series breaks required | 1–2 breaks sufficient |
| Arc Interruption Time | 2–3 cycles (40–60 ms at 50 Hz) | 1–2 cycles (20–40 ms) |
| Operating Noise | Very loud — 130–140 dB; compressed air exhaust | Near silent — suitable for urban GIS substations |
| Maintenance Interval | Frequent — compressor, valves, nozzles wear | Long — 20–25 years before major overhaul |
| GHG Concern | None — air is inert | SF6 GWP = 23,500; strict IEC 62271-4 handling required |
| Moisture Sensitivity | Moisture reduces dielectric strength | Moisture forms HF acid with SF6 decomposition — critical |
| Typical Manufacturers | GEC, older BHEL designs | ABB HD4, Siemens 3AP, BHEL SF6 CB |
| Application Trend | Obsolete for new installations | Standard for all new AIS/GIS above 33 kV |
Key differences
Air-blast breakers need 20–30 bar compressors and multi-break heads because air's dielectric strength is low; a 420 kV air-blast CB may need six series interrupters. SF6 at 5 bar interrupts the same arc in one or two breaks, cutting height from 8 m to under 3 m and enabling GIS (Gas Insulated Switchgear). The critical SF6 drawback is its global warming potential of 23,500 (CO₂ equivalent), driving IEC 62271-4 to mandate leak detection and gas recovery. PGCIL has committed to phasing out new SF6 purchases by 2030 and evaluating g3 (clean air) and vacuum alternatives above 145 kV.
When to use Air Blast CB
Use air-blast circuit breakers only when maintaining legacy installations where replacement of existing equipment is not yet scheduled. Example: a RSEB 220 kV substation installed in 1978 still operates its original BHEL air-blast CBs; spare parts availability, not performance, justifies keeping them.
When to use SF6 Circuit Breaker
Use SF6 circuit breakers for all new 33 kV and above switchgear installations, particularly in GIS substations with space constraints. Example: the new 220 kV GIS substation at Bangalore's Whitefield IT corridor uses ABB ELK-14 SF6 breakers; the entire bay fits in a 400 m² footprint vs 2500 m² for equivalent AIS.
Recommendation
For any new substation above 33 kV, choose SF6 — interrupting performance, size, and maintenance intervals leave no competition. Keep air-blast only as a legacy obligation, never specify it new. If asked about environmental alternatives, mention vacuum CBs (proven to 145 kV) and g3 gas technology for future projects.
Exam tip: Examiners ask students to compare the dielectric strength of SF6 and air at the same pressure and state the number of series breaks required for each type at 420 kV — SF6 is 2–3× stronger, needing 1–2 breaks vs 6–8 for air-blast.
Interview tip: An interviewer at ABB, Siemens, or PGCIL will ask about SF6's environmental problem and what alternatives exist — cite GWP of 23,500, IEC 62271-4 gas handling, and mention vacuum CB (up to 145 kV) and g3 dry air technology as successors.