What are "contact box" and "wall bushing"? What is the relationship between them?

"Contact box" and "wall bushing" are two crucial and functionally related insulating components in switchgear (especially medium-voltage switchgear such as the KYN28A type). Together, they constitute the electrical connection and insulation barrier between fixed components (such as busbar compartments and cable compartments) and movable components (such as circuit breaker trolleys) in switchgear.

Contact Box

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Overview
The contact box, also known as the stationary contact box or primary stationary contact box, is an insulating container installed on a fixed partition in the circuit breaker compartment of the switchgear to house and fix the stationary contacts. When the circuit breaker trolley is pushed into the working position, the moving contacts (cloverleaf contacts) on the trolley will insert into it, making reliable contact with the stationary contacts to complete the circuit connection.

Main Functions

♦Insulation:

Ground Insulation: Isolates the stationary contacts (high voltage) from the grounded metal partition and cabinet.

Phase-to-Phase Insulation: Isolates stationary contacts of adjacent phases, preventing phase-to-phase short circuits.

Surface Insulation: Its insulation structure ensures sufficient creepage distance even under conditions of condensation and contamination.

♦Support and Fixation: Provides robust mechanical support for the stationary contacts, ensuring stability during moving contact insertion, separation, and short-circuit current flow, preventing displacement or deformation.

♦Arc Extinguishing and Protection:

Arc Protection Zone: When the circuit breaker is in the "test position" for opening and closing operations, an arc may occur. The insulating walls of the contact box effectively confine the arc within the box, preventing it from spreading to other phases or grounded parts, making it a crucial arc fault protection component.

♦Safety Protection: Prevents accidental contact with live stationary contacts by personnel or foreign objects.

Guiding and positioning: Its internal structure is usually designed with guide grooves or flared mouths to facilitate the smooth and accurate alignment and insertion of the moving contact on the circuit breaker trolley into the stationary contact.

Structure and Materials

Structure: Typically a box-shaped structure made of insulating material, with embedded metal stationary contacts. Holes are located on the bottom or sides for connecting busbars or cables.

Materials: Mainly manufactured using epoxy resin via the APG process. Epoxy resin possesses excellent electrical properties, mechanical strength, and arc resistance, while the APG process ensures a dense, bubble-free, and stable product. Some products also use DMC/SMC or engineering plastics.

Through-wall bushings

Overview: Through-wall bushings are devices that guide conductive rods (such as busbars) through metal partitions or walls, maintaining insulation between the conductive rods and the partitions. In switchgear, they are commonly used for electrical connections between cabinets or between different compartments within a cabinet.

Main Functions
♦Insulation and Sealing:

Main Insulation: Isolates the conductive rod (high voltage) passing through the partition from the grounded metal partition.

Sealing: When needed (e.g., in gas-insulated switchgear), it ensures the airtightness of the gas chamber, preventing leakage of insulating gases such as SF6.

♦Mechanical Support and Fixing: Fixes and supports the busbars or cable terminals passing through, bearing their weight and electrodynamic forces.

♦Compartment Isolation: In switchgear, it is a key component for achieving physical isolation and electrical connection between the busbar compartment, circuit breaker compartment, and cable compartment, meeting the standard requirements of "metal enclosure, compartment isolation".

Structure and Materials

Structure: Typically a cylindrical or plate-like structure with a central conductive rod (or mounting holes) and an outer layer of insulation. Flanges at both ends are used for fixing to the partition.

Materials:

Air-insulated switchgear: Commonly uses epoxy resin (APG process), porcelain insulators, or SMC.

Gas-insulated switchgear: Must use epoxy resin casting sleeves that ensure long-term airtightness and achieve a perfect seal with metal flanges.

Relationship and Differences Between the Two

In a typical KYN-type trolley switchgear, the contact box and the through-wall bushing often work together to perform a crucial function: connecting the busbar compartment and the circuit breaker trolley.

A typical workflow is as follows:

The main busbar is located in the busbar compartment.

The main busbar passes through the partition via the through-wall bushing and enters the circuit breaker compartment.

Inside the circuit breaker compartment, the stationary contacts of the contact box are connected to the conductive end of the through-wall bushing.

When the circuit breaker trolley is pushed in, its moving contacts insert into the contact box, thus completing the electrical connection from the busbar to the circuit breaker.

Summary

The contact box is the "movable socket" facing the circuit breaker trolley within the switchgear, the core of enabling the "plug-in" connection of the trolley, and undertakes the critical task of arc protection.

The wall bushing is the "insulating bridge" between the various fixed compartments within the switchgear, ensuring the conductors safely pass through the grounding barrier and maintaining the compartmental structure of the cabinet.

Both are indispensable basic insulation components in the primary circuit of the switchgear, and their quality and reliability directly affect the safe and stable operation of the entire switchgear. In modern switchgear design, they are often combined with solid-sealed pole technology to jointly improve the insulation level and maintenance-free performance of the equipment.

Shaanxi Huadian is professional Insulation Components for Vacuum Switchgear manufacturers and suppliers in China,End-to-end quality control, long lifespan, and high reliability.Please contact us if needed.

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