FJH Type Open Corona Grading Ring for Composite Insulators
The FJH type open corona grading ring is an electric-field control fitting used with composite suspension insulators, tension insulators and other high-voltage insulation assemblies. It is normally installed near the energized end of an insulator to improve voltage distribution and reduce excessive electric-field concentration around metal end fittings.
The product consists of a smooth circular or near-circular tubular ring with an open section and a bolted mounting bracket. The open structure allows the ring to be positioned around compatible insulator hardware without passing the complete insulator assembly through a closed ring.
By controlling the local electric field, a correctly designed grading ring can help reduce corona discharge, audible noise, radio interference and electrical stress on the polymeric housing near the end fitting. Its dimensions and installation position must be matched to the insulator design and system operating conditions.
Important: A grading ring is not a universal accessory. Ring diameter, tube diameter, mounting position and bracket geometry must be confirmed through the approved insulator assembly drawing, electrical-field evaluation or type-test configuration.
Key Features
- Open-ring structure for convenient assembly and replacement
- Smooth tubular profile for electric-field control
- Designed for composite insulators and high-voltage insulation assemblies
- Helps reduce electric-field concentration around metal end fittings
- May assist in controlling corona discharge and radio interference
- Bolted mounting bracket provides a secure mechanical connection
- Available in aluminium-alloy or galvanized-steel construction
- Custom ring diameter, opening width and mounting arrangement available
Typical Technical Specifications
| Product Type | Open Corona Ring / Grading Ring |
|---|---|
| Model Series | FJH Type |
| Ring Structure | Open circular or near-circular tubular ring |
| Primary Function | Electric-field grading and corona control |
| Application | Composite insulators, insulator strings and high-voltage line fittings |
| Installation Position | High-voltage end or project-specified end of the insulator assembly |
| Typical Material | Aluminium alloy or carbon steel, subject to the approved model |
| Surface Finish | Natural aluminium, galvanized or project-specific protective finish |
| Mounting Method | Bolted bracket connection |
| System Voltage | Project-specific; confirm from the insulation-coordination design |
| Customization | Ring diameter, tube diameter, opening width, bracket and mounting dimensions |
Final dimensions, material and electrical performance must be confirmed using the approved drawing and insulator assembly configuration.
Operating Principle
The metal end fitting of a high-voltage insulator can create concentrated electric fields, particularly near the junction between the end fitting, polymer housing and surrounding air. The grading ring changes the geometry of the energized metal assembly and distributes the electric field over a larger, smoother surface.
Ring effectiveness depends on its diameter, tube size, axial position, distance from the insulator housing and relationship with nearby energized and grounded components. Installation in an unapproved position may reduce its effectiveness or create new field-concentration points.
Materials
Aluminium-Alloy Grading Ring
Aluminium alloy offers low weight, good conductivity and natural corrosion resistance. Welded and formed areas should be smooth and free from sharp projections that could increase electric-field concentration.
Galvanized-Steel Grading Ring
Steel versions provide high mechanical rigidity and may be hot-dip galvanized for outdoor corrosion protection. The coating requirement must be stated in the purchase specification.
The photograph alone cannot conclusively identify the base material. Material grade, wall thickness and surface treatment should therefore be confirmed before publication and ordering.
Applications
- Composite suspension-insulator strings
- Composite tension-insulator assemblies
- High-voltage overhead transmission lines
- Substation insulation assemblies
- Line-end fittings requiring electric-field control
- Insulator configurations requiring corona mitigation
Installation Guidance
- Confirm the grading-ring model against the approved insulator assembly drawing.
- Identify the specified high-voltage or grounded-end installation position.
- Inspect the ring, welds and mounting bracket for deformation or sharp edges.
- Position the open ring around the corresponding insulator end fitting.
- Install the bracket using the specified bolts, washers and locking components.
- Align the ring concentrically with the insulator axis where required.
- Check the specified axial distance between the ring and insulator housing.
- Tighten all fasteners to the manufacturer’s specified torque.
- Verify electrical clearances from conductors, towers and adjacent hardware.
Installation must follow the approved line design. Do not change the ring orientation or mounting distance without engineering confirmation.
Quality Inspection
- Overall ring diameter and opening-width inspection
- Tube diameter and wall-thickness inspection
- Roundness and dimensional-tolerance inspection
- Bracket dimensions and mounting-hole inspection
- Weld appearance and connection-strength inspection
- Surface smoothness and burr inspection
- Coating appearance and thickness inspection when applicable
- Trial assembly with the specified insulator fitting
- Corona or radio-interference testing when required by the project
Reference Standards
- IEC 61284:1997 – Overhead lines – Requirements and tests for fittings
- IEC 61109:2025 – Composite suspension and tension insulators for overhead lines
- IEC 62217:2025 – Polymeric high-voltage insulators for indoor and outdoor use
- ISO 1461:2022 – Hot-dip galvanized coatings, when applicable to a steel version
- Applicable utility drawings and insulation-coordination specifications
These standards are references only. Compliance or certification must be supported by reports covering the exact grading-ring and insulator assembly.
Ordering Information
Please provide the following information when requesting a quotation:
- FJH model or approved product drawing
- AC or DC system and nominal system voltage
- Insulator type, model and end-fitting dimensions
- Required ring diameter and tube diameter
- Opening width and mounting-bracket dimensions
- Required material and surface treatment
- Installation position and orientation
- Required mechanical or electrical test reports
- Order quantity, marking and packaging requirements
Frequently Asked Questions
What is the purpose of a grading ring?
It modifies the electric-field distribution around the end of a high-voltage insulator and helps reduce excessive electrical stress and corona activity.
Is a grading ring required on every composite insulator?
No. The requirement depends on system voltage, insulator length, end-fitting geometry and electric-field performance. The approved insulator or line design must be followed.
Can the same ring be used on different insulator models?
Only when the mounting dimensions and electrical-field requirements have been verified. Similar-looking insulators may require different ring diameters or installation positions.
Is this product IEC certified?
IEC standards provide relevant requirements and test references, but the exact product must have supporting test documentation before an IEC compliance claim is made.
Can the ring dimensions be customized?
Yes. Ring diameter, tube size, opening width and mounting bracket can be manufactured according to the approved project drawing.
