Safety Distance Between Phases of Outdoor 35kV Busbars

For outdoor 35kV busbars, the recommended phase-to-phase air clearance is typically around 200–250 mm, depending on insulation coordination, pollution degree, and environmental conditions.

Phase-to-Phase Clearance

The phase-to-phase clearance is the shortest distance through air between two live conductors of different phases. For a 35kV outdoor system, IEC standards and practical engineering guidelines suggest a minimum air clearance of approximately 200 mm, which may be increased to 250 mm in areas with high pollution, humidity, or dust to prevent flashover and arcing . This distance ensures that the system can withstand overvoltages and maintains dielectric strength under normal and abnormal conditions .

Phase-to-Ground Clearance

The phase-to-ground clearance is generally similar to the phase-to-phase clearance for outdoor busbars, as a ground fault can subject the remaining phases to full line-to-line voltage. For 35kV systems, a minimum of 200 mm is recommended, with adjustments for environmental factors and insulation coordination .

Creepage Distance

In addition to air clearance, creepage distance along insulating surfaces must be considered. Outdoor busbars are exposed to pollution and moisture, so the creepage distance should be at least 25–30 mm per kV, resulting in approximately 875–1050 mm for 35kV systems in heavily polluted areas . Proper creepage prevents surface tracking and flashover.

Design Considerations

• Environmental factors: Dust, humidity, and industrial pollution increase the risk of flashover, requiring larger clearances.
• Busbar configuration: Flat, horizontal, or vertical arrangements affect the required spacing.
• Insulation coordination: The clearance should match the system's impulse withstand voltage and overvoltage category.
• Safety margins: Engineers often apply a 10–20% safety margin above the minimum IEC values for outdoor installations .

Summary

For 35kV outdoor busbars, a practical design guideline is:

• Phase-to-phase air clearance: 200–250 mm
• Phase-to-ground clearance: 200 mm
• Creepage distance: 875–1050 mm (depending on pollution degree) These values ensure safe operation, reduce the risk of arcing, and comply with IEC standards and international best practices .