35kV Outdoor Bare Busbar Spacing Requirements

For 35kV outdoor bare busbars, typical minimum phase-to-phase and phase-to-ground clearances are around 150–200 mm, with creepage distances adjusted for pollution and environmental conditions.Air Clearance and CreepageAir clearance is the shortest distance through air between two conductive parts of opposite polarity or between a conductor and ground. For 35kV outdoor bare busbars, IEC 60664-1 recommends a minimum air clearance of approximately 150–200 mm, depending on the system's impulse withstand voltage and environmental factors such as altitude and pollution degree . Creepage distance is the shortest path along the surface of an insulating material between two conductive parts. Outdoor installations in pollution degree 3 (industrial or dusty environments) require larger creepage distances to prevent tracking and surface discharge. A common practice is to increase the creepage distance by 20–30% over the minimum IEC values for harsh environments .Environmental ConsiderationsAltitude: At elevations above 2000 meters, air density decreases, reducing dielectric strength. Clearance should be increased by approximately 25% at 3000 meters .Pollution: Higher pollution degrees require longer creepage distances. For outdoor substations exposed to dust, rain, or salt, additional spacing or insulating barriers may be necessary .Insulation: Bare busbars require larger clearances than insulated busbars. If epoxy or heat-shrink insulation is applied, spacing can be reduced based on the insulation's tested performance .Practical Design GuidelinesPhase-to-phase and phase-to-ground spacing are generally the same for ungrounded or impedance-grounded systems, as a ground fault can create full phase-to-phase voltage across unfaulted phases .Busbar supports: Use non-conductive standoffs to maintain spacing and prevent accidental contact.Testing: The design must pass power frequency and impulse withstand tests to ensure safety, as ANSI and IEC standards are performance-based rather than prescribing exact distances .Summary Table (Typical Outdoor 35kV Bare Busbars)ParameterRecommended MinimumPhase-to-phase air clearance150–200 mmPhase-to-ground air clearance150–200 mmCreepage distance (pollution degree 3)250–300 mmAltitude correction+25% at 3000 mThese values are practical engineering guidelines. Final spacing should be verified against IEC 60664-1, ANSI C37.23, and local utility or regulatory requirements, considering site-specific conditions such as humidity, dust, and potential contamination .