Specifications and dimensions of bushings for primary distribution boxes

Primary distribution box bushings are designed to insulate conductors from the transformer tank, with dimensions and specifications standardized according to IEC 60137 and IEEE C57. 19, and are available in various voltage and current ratings.

Voltage and Current Ratings

Primary distribution bushings typically cover voltage ratings from 600 V up to 35 kV or higher, depending on the application, with current ratings commonly ranging from 200 A to 600 A for medium-voltage distribution transformers and apparatus. For example, Eaton's 600 A, 35 kV deadbreak bushings comply with IEEE Std 386-2006 and are suitable for sidewall mounting in transformers or switches.

Insulation and Materials

Bushings are constructed with oil-impregnated paper (OIP), epoxy, or high-temperature nylon (HTN) resin, providing electrical insulation between the conductor and the transformer tank. High-voltage bushings often use conical porcelain shells for transformer-end insulation, while low-voltage bushings may use polyester or glass-reinforced nylon resin. The insulation design ensures low partial discharge levels and reliable operation under heavy pollution or wet conditions.

Creepage Distance and Pollution Class

The creepage distance is critical for outdoor bushings and is defined according to IEC 60815-3. Typical values are 25 mm per kV for air-end porcelain in heavily polluted atmospheres.

AB

B specifies very heavy pollution class with 54 mm/kV for high-voltage bushings. Polymeric insulators may allow reduced creepage distances compared to ceramic types.

Mounting and Mechanical Specifications

Bushings are designed for vertical or inclined mounting up to 30° from vertical. They include robust sealing systems to prevent moisture ingress and ensure a reliable ground connection throughout the service life. Cantilever load ratings are specified in IEC 60137, with Level 1 for normal applications and Level 2 for severe mechanical loads.

Termination and Compatibility

Primary bushings provide termination points for both air-side conductors and transformer leads. Low-voltage bushings often include spade terminals for internal connections to fuse holders such as Type KTM or DO-III, while high-voltage bushings are compatible with separable connectors like BOL-T or T-OP II systems. Bushings are designed to mate with inserts conforming to IEEE 386-2006 standards.

Thermal and Electrical Performance

Bushings are rated for continuous AC voltage and maximum current at specified ambient and oil temperatures, with hot spot limits typically 105°C for standard bushings and 120°C for RIP types. They undergo routine and type tests for lightning impulse, switching impulse, and power factor to ensure compliance with IEC and ANSI standards.

Summary

Primary distribution box bushings are engineered to provide electrical insulation, mechanical support, and reliable termination for medium-voltage distribution transformers. Key specifications include voltage/current ratings, insulation type, creepage distance, mounting angle, and compliance with IEC 60137 and IEEE C57. 19 standards.

Material choices, sealing systems, and compatibility with fuse holders or separable connectors ensure safe and durable operation in various environmental conditions.