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The correct application and installation of corona rings (also called grading rings) has a marked effect on the life expectancy of polymer insulators (also called composite or nonceramic insulators (NCIs)). This visual field guide is intended to assist in the selection, installation, and inspection of corona rings.

Pictures taken by DayCor® cameras indicating corona issues on insulators where corona rings are missing or misplaced.

arcing between the ring and the bushing wrong installation of corona grading ring

missing corona ring missing corona ring on the right bushing


When selecting and installing corona rings, field personnel are faced with the following challenges:

The objective of this guide is to assist field personnel in the selection and installation of corona rings. Also included is a section describing conditions that may exist on in-service corona rings and suggesting what action to consider taking in response to each condition.


This visual guide was developed in collaboration with vendors, researchers, and field personnel. The guide is mainly image-based for ease of use.


Excessively high electric field (E-field) magnitudes on polymer insulators may result in either internal or external discharge activity under either wet or dry conditions. This discharge activity may influence the insulator performance by resulting in either premature degradation or customer complaints due to audible noise or radio interference. Corona rings are often installed to grade the E-field, reducing the likelihood of discharge activity.

Application, Value and Use

This field guide is intended to be used by field personnel when selecting, installing, and inspecting corona rings on overhead transmission line polymer insulators. The guide may also be useful for both design and maintenance engineers in companies that apply polymer insulators. The guide can be used as part of a training program.

The downloadable PDF for this product is optimized for viewing on electronic devices such as tablets. For Android devices, Adobe Reader Version 10.6.1 is required for viewing. A print-resolution PDF is attached to the optimized file. For a copy of this product printed on high-quality paper and ring-bound in a conveniently sized format or a PDF optimized for viewing on electronic devices, please see Related Material.

Available on EPRI Research >>

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