35kV Cabinet Busbar Spacing

For 35 kV distribution cabinets, typical busbar spacing ranges from 7 to 8 inches between bushings, with phase-to-phase and phase-to-ground clearances designed according to ANSI, IEC, and manufacturer guidelines.

ANSI and Manufacturer Guidelines

ANSI standards for switchgear are performance-based, meaning they specify dielectric and impulse withstand tests rather than exact spacing dimensions. Manufacturers design busbar spacing to meet these tests safely. For 35 kV switchgear, Eaton's Cooper Power series specifies minimum bushing centerline spacing of 8 inches, except for the C-phase bushings, which may be 7 inches. Standoff brackets or parking stands are typically mounted 4 inches from the bushing centerline to support the busbar mechanically and maintain clearance .

IEC Clearance Considerations

IEC standards provide guidelines for phase-to-phase and phase-to-earth spacing, which vary depending on pollution degree, insulation type, and environmental conditions. For pollution degree 3, larger clearances or additional insulation barriers are recommended. Designers must also account for altitude, as air pressure decreases at higher elevations, reducing dielectric strength. For example, at 3000 meters, clearances should be increased by approximately 25% .

Practical Design Factors

• Insulation Type: Bare busbars require larger clearances, while insulated busbars (heat shrink or epoxy-coated) can use smaller spacing, provided insulation is tested for thermal, mechanical, and electrical stress .
• Mechanical Support: Busbar supports must prevent sagging or vibration, which could reduce phase-to-phase gaps during short-circuit events .
• Phase-to-Ground and Phase-to-Phase: In metal-enclosed switchgear, phase-to-phase and phase-to-ground distances are usually equal, as ungrounded or impedance-grounded systems can experience full phase-to-ground voltage during a fault .

Summary

For a 35 kV distribution cabinet:

• Bushing centerline spacing: 7–8 inches (manufacturer-specific)
• Phase-to-phase and phase-to-ground clearances: Determined by dielectric withstand requirements, insulation type, and environmental factors
• Design considerations: Include standoff supports, insulation performance, altitude correction, and mechanical stability Following these guidelines ensures safe operation, compliance with standards, and long-term reliability of the medium-voltage distribution cabinet.