Switchgear & circuit breakers — LV / MV / HV across air, vacuum, SF6

Reference notes

A circuit breaker is a switching device that carries continuous-load current, interrupts short-circuit current, and isolates circuits when open. Switchgear is the engineered enclosed assembly containing breakers, bus bars, instrument transformers, control wiring, and protection relays. Each breaker has three independent ratings: continuous amps, interrupting kA, and voltage class.

Breaker rating quantities

Arc interruption — the central physics

• When contacts begin to separate while carrying current, an electrical arc forms in the gap — a plasma sustaining current at 5,000-20,000 °C.
• AC current naturally crosses zero every half cycle (8.33 ms at 60 Hz).
• At current zero, the arc momentarily extinguishes. Immediately afterward, the system attempts to recover voltage across the gap → Transient Recovery Voltage (TRV).
• RACE: dielectric strength of the recovering gap vs TRV rise rate.
  • Dielectric wins → arc extinguished, fault current interrupted.
  • TRV wins → reignition, arc continues for another half cycle.
• Modern breakers interrupt AC fault current in 2-5 cycles (33-83 ms) — limited by mechanical contact-opening time plus a few current zeros.

Low-voltage breakers (≤ 1000 V)

• MCB — Miniature Circuit Breaker. Residential / light commercial. 0.5-125 A continuous, 6-25 kA interrupting. DIN-rail mounted. Bimetallic + magnetic trip.
  • Trip curves: B (3-5× In, fast magnetic), C (5-10× In, general), D (10-20× In, motor/transformer inrush), K (8-12× In, motors), Z (2-3× In, electronics).
• MCCB — Molded-Case Circuit Breaker. Industrial. 100-2500 A continuous, 25-200 kA interrupting. Electronic trip units with LSIG functions:
  • L — Long-time (overload thermal).
  • S — Short-time (delayed magnetic for coordination).
  • I — Instantaneous (fastest trip, severe faults).
  • G — Ground-fault (zero-sequence / residual).
• ICCB — Insulated-Case Circuit Breaker. Bridge between MCCB and ACB. Higher endurance than MCCB.
• ACB / LVPCB — Low-Voltage Power Circuit Breaker (Air Circuit Breaker). Open-frame, draw-out construction. 800-6300 A continuous, up to 200 kA interrupting. Used at main switchboards and generator outputs.
• Interrupting medium for all LV breakers: AIR with ARC CHUTES — parallel-plate arc divider that elongates the arc and divides it into many small arcs, raising arc voltage above supply voltage to force extinction. Magnetic blowout coils add Lorentz force to drive the arc into the chutes.

Medium-voltage breakers (5-38 kV) — vacuum dominant

• Vacuum-bottle breakers account for >95% of new MV installations globally.
• Physics: contacts open inside a sealed vacuum bottle at ~10⁻⁵ Pa (essentially no gas). Arc is sustained by sputtered METAL VAPOR from the contact surfaces.
• At current zero, metal vapor cools and condenses back onto the contacts within microseconds — the FASTEST dielectric recovery of any medium.
• Advantages: 2-cycle interruption, sealed for life (no contact maintenance), compact, 30,000-50,000 mechanical operations, no fire risk (unlike oil), no GHG (unlike SF6).
• Concern: chopping current — vacuum can force current to zero before the natural crossing, causing L·di/dt voltage transients. Modern Cu-Cr contact materials minimize this.
• Suppliers: Eaton, ABB, Siemens, Schneider, Hitachi-Mitsubishi, plus Chinese majors.

High-voltage breakers (72-800 kV) — SF6 dominant; phase-out underway

• SF6 (Sulfur Hexafluoride) gas at 0.5-0.8 MPa is the dominant HV interrupting medium since the 1960s.
• Physics: SF6 molecules are ELECTRONEGATIVE — capture free electrons during the arc, accelerating de-ionization at current zero. Dielectric strength 2-3× air at operating pressure.
• Architectures:
  • Puffer breaker — mechanical piston compresses gas around the arc during opening; blasts cooler gas axially through the arc at current zero.
  • Self-blast / auto-puffer — uses the arc's own thermal energy to compress gas in an expansion chamber.
• Modern HV: 50-100 kA interrupting at 245-800 kV, 2-cycle interruption.
• Environmental concern: SF6 has GLOBAL-WARMING POTENTIAL = 23,500× CO2 over 100 years. EU F-gas regulation (revised 2024) phases out SF6 in MV equipment 2026-2030.
• Alternatives being deployed:
  • CO2 / vacuum breakers for 72.5-145 kV (Eaton, Hitachi-ABB, Mitsubishi-Toshiba).
  • G3 gas (fluoronitrile / fluoroketone mixtures, GWP < 1) — GE g3, ABB AirPlus, Hitachi Eco gas.
  • Long-term direction: SF6 displacement across HV range by 2035-2040.
• Oil breakers — historical, fire-risk; phased out from new installations since the 1990s. Still in service in legacy substations.

Switchgear standards & architectures

Metal-clad MV switchgear (IEEE C37.20.2 / IEC 62271-200)

• Each functional compartment SEPARATED by grounded metal barriers:
  • Breaker compartment (draw-out vacuum or SF6 breaker)
  • Bus compartment (main horizontal bus)
  • Cable compartment (outgoing power cables)
  • Secondary / instrument compartment (relays, control wiring, IT secondaries, metering)
• Provides personnel protection: a fault in one compartment is contained; operators can safely access adjacent compartments.
• Draw-out construction allows racking the breaker out for maintenance without de-energizing the bus.

Arc-resistant switchgear (IEEE C37.20.7)

• Designed so an internal arc-flash event vents pressure and ionized gases UPWARD (or through a plenum/duct) AWAY from operators.
• Personnel within 1 m at full open-door operation are protected for the test-rated current and duration (typical: 40-100 kA for 0.1-0.5 s).
• Classes: Type 1 (front access only), Type 2 (front + sides + rear).
• Critical when calculated arc-flash incident energy exceeds Category 4 PPE limits (~40 cal/cm²).
• Combined with remote racking (operating from 30+ ft away) and ZSI (Zone-Selective Interlocking) for faster clearing, has dramatically reduced electrical-worker injuries since the standard was published in 2001.

Gas-Insulated Switchgear (GIS)

• Bus bars, disconnects, and breakers immersed in SF6 (or alternative gas) inside grounded metal enclosures at 0.4-0.5 MPa.
• Compact — 10-20% of the footprint of air-insulated equivalent.
• Used for: urban substations (Manhattan, central Tokyo, central London), offshore wind / oil platforms, indoor installations, heavily polluted environments (salt-fog, industrial pollution).
• Voltage range: 72.5 kV up to 1100 kV (UHV).
• Mandatory SF6 leak monitoring (typical limit 0.5-1% per year), partial-discharge monitoring.
• New direction: CO2-vacuum and g3-gas alternatives — Hitachi-ABB EconiQ, Siemens Blue GIS.

Maintenance & testing (NETA)

• NETA ATS (Acceptance Testing Specification) — pre-energization tests on new installations.
• NETA MTS (Maintenance Testing Specification) — periodic in-service tests.
• Typical MV breaker MTS interval: 3-5 years in normal industrial duty.
• Tests: insulation resistance (megger), contact resistance (micro-ohm), timing (open/close/trip-free), CT/PT ratio + polarity, SF6 gas quality (purity, moisture, SO2 decomposition products), vacuum-bottle integrity test, relay coordination injection test.