Medium Voltage Switchgear Selection Guide: Key Parameters and Standards

Air-insulated switchgear (AIS): Traditional, cheaper, needs more space, requires more maintenance.

Gas-insulated switchgear (GIS): Compact, sealed, low maintenance, expensive, suitable for urban areas.

Hybrid switchgear: Combination of AIS and GIS, balance of cost and space.

RMU (Ring Main Unit): For distribution networks, compact, sealed, maintenance-free.

Rated voltage: 12kV, 24kV, 36kV (IEC) or 15kV, 27kV, 38kV (IEEE).

Rated current: 630A, 1250A, 1600A, 2000A, 2500A, 3150A.

Short-circuit withstand: 20kA, 25kA, 31.5kA, 40kA for 1-3 seconds.

Insulation level: Power frequency withstand (28kV, 50kV, 70kV), lightning impulse (75kV, 125kV, 170kV).

IEC 62271-200: AC metal-enclosed switchgear for rated voltages above 1kV and up to 52kV.

IEC 62271-203: Gas-insulated switchgear for rated voltages above 52kV.

IEEE C37.20: Switchgear assemblies including metal-enclosed interrupter switches.

Buyers should specify which standard to comply with in RFQ.

Vacuum circuit breaker (VCB): Most common for 12-36kV, maintenance-free, long life.

SF6 circuit breaker: For higher voltages (>36kV), but environmental concerns.

Air circuit breaker (ACB): For low voltage, not medium voltage.

Recommendation: Use VCB for 12-36kV applications.

Clearance: 1.5m in front, 1m behind (IEC 62271-200).

Ventilation: For indoor installation, 10-15 air changes/hour.

Cable termination: Top or bottom entry, prefabricated cable terminations preferred.

Protection: IP4X minimum for indoor, IP54 for outdoor or dusty environment.

1. Undersizing short-circuit withstand (breaker fails during fault)

2. Wrong insulation level (fails dielectric test)

3. No maintenance access (cannot maintain circuit breaker)

4. Ignoring ambient temperature (breaker derates at high temperature)

5. No CT/PT provision (cannot add metering later)