12 kV and 24 kV Epoxy Resin Bushing

Introduction

Epoxy resin bushings are widely used in medium-voltage switchgear, transformers, ring main units (RMUs), and solid insulated equipment. Their primary function is to safely pass a live conductor through a grounded enclosure while maintaining adequate electrical insulation.

Two key insulation parameters define bushing performance:

• Clearance (air insulation distance)

• Creepage distance (surface insulation distance)

Correct design of these distances is critical for preventing flashover, partial discharge, and long-term insulation failure, especially under polluted or humid conditions.

Key Definitions

Clearance (Insulation Distance)

Clearance is the shortest distance through air between live parts and earth or between phases.

It determines the ability to withstand:

• Power-frequency voltage

• Lightning impulse overvoltage

Creepage Distance

Creepage distance is the shortest distance along the surface of the insulation between live parts and earth.

It is mainly affected by:

• Pollution level

• Moisture

• Surface contamination

• Insulation material

For epoxy resin bushings, creepage distance is often the design-limiting factor.

Applicable IEC Standards

Epoxy resin bushing insulation design typically follows:

• IEC 62271-1 – Common specifications

• IEC 62271-200 – Metal-enclosed switchgear

• IEC 60071 – Insulation coordination

• IEC 60815 – Creepage distance in polluted environments

Rated Voltage and Insulation Levels

These insulation levels directly influence minimum clearance requirements.

Clearance Requirements for Epoxy Resin Bushings

Typical Phase-to-Earth Clearance

Notes:

• Values assume indoor, controlled environment

• Outdoor or high-altitude installations require increased clearance

• Solid insulation can reduce air clearance compared with air-insulated bushings, but safety margins are still applied

Phase-to-Phase Clearance

Phase-phase clearance is usually slightly larger than phase-earth.

Creepage Distance Requirements (Epoxy Resin)

Creepage distance depends strongly on pollution severity, as defined in IEC 60815.

Typical Creepage Distance per kV

For epoxy resin insulation:

12 kV Epoxy Resin Bushing Creepage

Most indoor switchgear uses medium pollution as the design basis.

24 kV Epoxy Resin Bushing Creepage

For outdoor or coastal environments, heavy or very heavy pollution values are mandatory.

Why Epoxy Resin Allows Compact Design

Compared with porcelain or air-insulated bushings, epoxy resin offers:

• Higher dielectric strength

• Uniform insulation structure

• Better resistance to moisture ingress

• Improved partial discharge performance

This allows shorter clearances, but creepage distance cannot be reduced excessively, because surface pollution still governs flashover risk.

Design Features Used to Achieve Creepage Distance

Epoxy resin bushings typically use:

• Multiple sheds or ribs

• Optimized shed profile

• Hydrophobic surface treatment

• Smooth, defect-free molding

These features increase effective creepage without excessive height.

all type epoxy resin bushing

Indoor vs Outdoor Requirements

Indoor Switchgear Bushings

• Lower pollution level

• Smaller creepage acceptable

• Focus on clearance and impulse withstand

Outdoor Bushings

• High pollution exposure

• Longer creepage mandatory

• UV and weather resistance required

Outdoor epoxy bushings are often oversized compared with indoor types.

Altitude Correction

At altitudes above 1000 m, air density decreases.

Impact:

• Clearance must be increased

• Creepage remains unchanged

Typical correction:

• Increase clearance by 1% per 100 m above 1000 m

Common Design Mistakes

• Using indoor creepage values for outdoor installations

• Ignoring pollution severity

• Over-reducing clearance due to solid insulation

• Poor shed profile causing water bridging

• Inadequate quality control of epoxy casting

These mistakes significantly increase flashover risk.

Practical Selection Guidelines

When specifying epoxy resin bushings:

• Confirm system highest voltage

• Define installation environment

• Apply correct pollution level

• Check IEC compliance

• Coordinate with switchgear insulation design

Never rely solely on “rated voltage” without creepage verification.

Summary Table

Conclusion

For 12 kV and 24 kV epoxy resin bushings, correct insulation design requires careful coordination of clearance and creepage distance. While epoxy resin enables compact, solid-insulated solutions, creepage distance remains governed by pollution severity and IEC standards.

Properly designed bushings ensure:

• High dielectric reliability

• Resistance to pollution and moisture

• Long service life

• Compliance with utility and international standards

Correct insulation distance design is therefore a foundation of safe and durable medium-voltage equipment.