IEEE C57.166-2026

This guide provides acceptance and maintenance criteria for insulating liquids used in transformers, on-load tap changers, step-voltage regulators, and reactors.

The purpose of this guide is to assist the user of the equipment in evaluating insulating liquids as follows: This guide also discusses the following information on insulating liquids: This guide does not cover dissolved gas analysis of insulating liquids, which is covered by other IEEE standards.

New IEEE Standard - Active. Recommendations regarding tests and evaluation procedures of insulating liquids are made in this guide. References are made in this guide to methods of reconditioning and reclaiming insulating liquids and the levels at which these methods become necessary. Assisting the power equipment operator in evaluating the serviceability of liquid insulation received in equipment, as received from the supplier for filling new equipment at the installation site and as processed into such equipment, and assisting the operator in maintaining the insulating liquid in serviceable condition are intended by this guide. Insulating liquids used in transformers, reactors, on-load tap changers, and step-voltage regulators are covered in this guide.