IEC 60664-1 - Insulation Coordination for Low-Voltage Equipment

Creepage vs clearance

Key facts

Full title

Insulation coordination for equipment within low-voltage supply systems - Part 1: Principles, requirements and tests

Published by

IEC (TC 109, Insulation co-ordination for low-voltage equipment)

Current edition

3rd edition, IEC 60664-1:2020, with Amendment 1:2025 (published 6 May 2025)

Scope

Equipment rated up to AC 1000 V or DC 1500 V, at frequencies up to 30 kHz, altitude up to 2000 m (correction factors above)

Sets

Clearance (through air), creepage (along a surface) and solid-insulation requirements

Key inputs

Working voltage, overvoltage category, pollution degree and material group (CTI)

Role

A horizontal base standard; product safety standards derive their spacing tables from it

Clearance and creepage: two different failures

The standard sizes two distances that fail in different ways. Clearance is the shortest distance through air between two conductive parts. It is set by the peak or impulse voltage the insulation must survive, ranked by the overvoltage category, and it guards against a fast breakdown of the air gap.

Creepage is the shortest path along the surface of the insulation between the same two parts. It is set by the RMS or DC working voltage together with the pollution degree and the material's tracking resistance, and it guards against a slow build-up of a conductive track across the surface over the product's life. Because they answer different questions, clearance and creepage are calculated separately and one is often larger than the other.

Pollution degree

The pollution degree classifies how much conductive contamination and condensation the insulation is exposed to, from Pollution Degree 1 (clean and dry, no pollution) to Pollution Degree 4 (persistent conductive pollution). Most enclosed office and laboratory equipment is treated as Pollution Degree 2.

Pollution degree affects creepage, not clearance. A dirtier or damper environment lets a surface track form more easily, so a higher pollution degree requires a longer creepage distance for the same voltage.

Material group and CTI

The insulating material is classified into one of four groups by its comparative tracking index (CTI), a measured value of how well the surface resists forming a conductive track under contamination and voltage.

A material with a lower CTI tracks more easily and therefore needs a longer creepage distance for the same working voltage and pollution degree. The CTI itself is measured by the method in IEC 60112.

Overvoltage category

The overvoltage category ranks equipment by its position in the supply and the transient overvoltage it must withstand, from Category I (protected, lowest transient) up to Category IV (at the origin of the installation, highest transient). Each category corresponds to a rated impulse withstand voltage.

That impulse withstand voltage is what sizes the clearance. The overvoltage categories here run in parallel with the measurement categories used for test and measurement equipment: both rank a point by how much transient energy is available where it sits in the installation.

How product standards use it

IEC 60664-1 is a horizontal base standard: it defines the principles, the method and the reference tables, but it does not decide the final number for any particular product.

Product safety standards apply it. IEC 62368-1 and IEC 61010-1 take the working voltage, pollution degree, material group and overvoltage category for the equipment they cover, run them through the IEC 60664-1 method, and set the clearance and creepage that the product must meet. In practice you start from the product standard, which points back to this one.

Material groups by comparative tracking index (CTI)

Tools that use this standard

Related standards

Frequently asked questions