What Is Insulation Coordination?

Published 27 Aug 2026Updated 27 Aug 20266 min readAdvancedStandards

Quick answer — Insulation coordination is the selection of the insulation levels of equipment so they withstand the system and surge voltages with the right margin, and so surge arresters and protection operate to protect the most expensive equipment. It matches Basic Insulation Level (BIL) to the expected overvoltage, using arresters to clamp surges and choosing BIL so a flashover (if any) occurs at a cheap, replaceable point rather than at a transformer.

Contents

The idea
How it works
Applied and induced tests
Why it matters

What is insulation coordination in a power system?

The idea

You choose the insulation strength (BIL) of equipment so it survives the normal and switching voltage, and the clearances, so that any surge is diverted by arrester protection before it threatens the asset. The cost rises with BIL, so the design balances cost and risk.

How it works

Arresters limit the surge voltage to a value below the equipment BIL, with a margin; the BIL increases with system voltage. Coordination ensures the BIL of the transformer (or bushing) is above the protective level of the arrester. A transformer at a given system voltage has a defined BIL and test levels. It is given as e.g. 110 kV system, 550 kV BIL.

Applied and induced tests

The applied (power-frequency) test proves the insulation to earth and between windings; the induced test proves the inter-turn insulation. Both are specified with the insulation level.

Why it matters

It prevents insulation failure from lightning or switching surges and ensures that, if a failure does occur, it is at a location and component that is cheaper and safer to replace.

References & standards

IEC 60071 (insulation coordination)
IEC 60076-3
IEEE C57.12.00

Book series Academy

Related manufacturers

Acme Electric (Hubbell) · Central Moloney · CORE Transformers · Daelim Transformer (US) · Delta Star · Eaton

Related components

Transformer bushings & terminations Insulation materials

Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.

Source transformers for this application

Submit an RFQ or browse manufacturers and components.

Submit an RFQ Get Verified