What is the structure and principle of the sealed pole?

What is a solid-sealed terminal?

Simply put, a solid-sealed terminal consists of a vacuum interrupter (VI) and its connected conductive components (such as the terminal block) cast into a single, solid component using a special epoxy resin and an advanced automatic pressure gelation (APG) process.

This technology represents a significant upgrade to traditional air-insulated and composite-insulated circuit breakers, significantly improving their performance and reliability.

 

Structure of the Encapsulated Pole

The encapsulated pole has a typical three-layer structure, resembling a "sandwich":

Core Layer: Vacuum Interrupter

This is the "heart" of the encapsulated pole, responsible for interrupting the current. It consists of a pair of contacts (stationary and moving) and a ceramic or glass outer shell, with a high vacuum interior (above 10^-4 Pa).

The moving contact connects to the operating mechanism via a bellows, enabling opening and closing operations.

Middle Layer: Epoxy Resin Insulation

This is the core of the "encapsulation." Made of a high-performance epoxy resin mixture, it offers exceptional electrical and mechanical strength, as well as heat and weather resistance.

Using the APG process, it completely encapsulates the vacuum interrupter and conductive components, forming a seamless, integrated structure.

Outer layer: Embedded conductive components

Upper outlet holder: Typically cast together with the static end of the vacuum interrupter, serving as the upper connection point of the circuit breaker.

Lower outlet holder/flexible connector: The movable end of the vacuum interrupter is connected to the lower outlet holder via a flexible connector or directly, serving as the lower connection point of the circuit breaker. This part is sometimes partially encased.

Structural Features:

Integrated design: The vacuum interrupter is no longer a separately removable component, but is integrated into the insulating housing.

No air gaps: The epoxy resin adheres tightly to the internal components, eliminating air gaps.

 

Working Principle of the Embedded Pole

The principle of the embedded pole can be understood from two perspectives: insulation and arc extinguishing.

1. Insulation (the core advantage of embedded technology)
This is the biggest improvement of the embedded pole compared to traditional structures. Its core lies in the use of a solid insulating medium (epoxy resin) to significantly improve electric field distribution.

Pain point of traditional circuit breakers: In air-insulated circuit breakers, the vacuum interrupter is exposed to air. Dust, condensation, and chemical corrosion can easily shorten the creepage distance on its surface, leading to electric field distortion, partial discharge, and even insulation flashover in harsh environments.

Solution of the embedded pole:

Equalizing the electric field: Epoxy resin has a dielectric constant much higher than air. When tightly wrapped around the vacuum interrupter and conductive parts, it effectively evens out the electric field distribution, particularly eliminating the electric field concentration point at the junction of the vacuum interrupter's ceramic shell and the metal end cap.

Eliminating External Environmental Influences: Critical insulating components (vacuum interrupters) are completely sealed in an inert epoxy resin, completely isolating them from external moisture, dust, contaminants, and chemical gases. This significantly enhances the circuit breaker's environmental resistance (e.g., condensation and pollution levels).

Increasing Creepage Distance: Through design, a shed-like structure can be directly formed on the epoxy resin housing, further increasing the creepage distance of the external surface and improving pollution flashover protection.

2. Arc Extinguishing Principle
The arc extinguishing function is completely performed by the internal vacuum interrupter, similar to traditional vacuum circuit breakers.

Closed State: The moving and static contacts are in contact, and current is flowing normally.

Open State: When the operating mechanism drives the moving contacts apart, a high-temperature metal vapor arc (vacuum arc) is generated between the contacts. Because there are very few gas molecules in a vacuum, the arc is sustained by the metal vapor generated by the contact material.

Arc Extinguishing at Current Zero Crossing: When the AC current naturally crosses zero, the metal vapor rapidly diffuses and condenses on the contacts and shield. The dielectric strength of the contact gap is instantly restored, the arc is extinguished, and the current is successfully interrupted.

 

Main Advantages of Sealed Pole Breakers

High Reliability and Insulation Performance: This fundamentally eliminates the effects of environmental factors (such as condensation and contamination) on insulation, preventing insulation failures in the vacuum interrupter caused by external factors.

Compact Size: Due to its excellent insulation performance, the phase-to-phase and ground distances of sealed pole breakers can be designed to be smaller at the same voltage level, making the entire circuit breaker structure more compact and facilitating the miniaturization of switchgear (such as the KYN28A center-mounted switchgear).

Harsh Environment Resistance: This system is particularly suitable for demanding environments such as high humidity, high altitude, and high pollution.

Maintenance-Free: After the vacuum interrupter is sealed, its external insulation requires virtually no maintenance.

High Mechanical Strength: The epoxy resin casing provides solid protection for the vacuum interrupter, enabling it to better withstand shock, vibration, and the significant electrodynamic forces generated by short-circuit currents.

 

Sealed Pole vs Traditional Insulation

 

Features	Air-insulated circuit breakers	Composite Insulated Circuit Breaker	Sealed Pole Circuit Breaker
Insulation medium	Mainly rely on air gaps	Air Gap + Silicone Rubber or Heat Shrink Tubing	Epoxy resin solid insulation
Vacuum interrupter	Exposed to air	Enclosed in silicone/heat shrink tubing, but not completely tight	Completely encapsulated with epoxy resin, no air gaps
Environmental impact	Large, susceptible to condensation and contamination	Minor, Improved	Extremely small, virtually unaffected
Dimensions	Large	Medium	Small, most compact
Reliability	Low	High	Highest

 

Summary

Solid-sealed pole assemblies represent a significant advancement in medium-voltage vacuum circuit breaker technology. By encapsulating the vacuum arc extinguishing chamber within epoxy resin, they achieve optimised electric field distribution and environmental isolation. With high reliability, compact dimensions, and exceptional environmental adaptability as their core advantages, they have become the mainstream and standard design solution in the medium-voltage sector.

Shaanxi Huadian is a professional manufacturer of high-quality sealed poles. The company has extensive expertise in the manufacturing of core components for vacuum circuit breakers, insulation processes (such as the APG epoxy resin pressure gel process), and quality control.If you would like to learn more,please contact us at:

Email:pannie@hdswitchgear.com.

Whatsapp/Wechat:+8618789455087