Comparative Tracking Index (CTI) of insulating material — why pollution degree determines the required creepage distance

The guide on forms of separation of low-voltage switchgear assemblies covers the internal separation between busbars, functional units and terminals within a switchgear assembly. This article covers another, less visible aspect of the same IEC 61439 series: the property of the insulating material itself that determines how close live parts may lie next to each other along a surface — the Comparative Tracking Index.

What tracking is

Under polluted and moist conditions, a thin, conductive, carbonized path can form on the surface of an insulating material between two live parts — tracking. This happens because a moist, contaminated layer on the surface allows local leakage current to flow; the resulting heat carbonizes the material on a microscopic scale, creating a permanent conductive path that can extend further with repetition, eventually leading to breakdown or even fire.

The CTI test per IEC 60112

IEC 60112 describes the test method: drops of a dilute ammonium chloride solution are applied to the surface of the material under test between two platinum electrodes across which a voltage is applied. The CTI is the highest voltage (in volts) at which the material withstands a specified number of drops without a breakdown tracking path forming.

Material groups per IEC 61439-1

Based on the CTI value, IEC 61439-1 classifies insulating materials into four material groups:

Material group I: CTI ≥ 600 — best resistance to tracking.
Material group II: 400 ≤ CTI < 600.
Material group IIIa: 175 ≤ CTI < 400.
Material group IIIb: 100 ≤ CTI < 175 — lowest tracking resistance among still-permitted materials.

The higher the material group, the smaller the creepage distance required for the same working voltage and pollution degree — with a higher-grade material, a manufacturer can therefore build more compactly without compromising safety.

Pollution degree: the other factor in the equation

The required creepage distance does not depend on the material alone, but also on the pollution degree (PD) of the environment in which the switchgear assembly is installed:

PD1: no pollution, or only dry, non-conductive pollution with no influence — for example a sealed, fully enclosed housing.
PD2: only non-conductive pollution, with occasional temporary conductivity due to condensation expected — the common situation for a normal, indoor-installed switchgear assembly.
PD3: conductive pollution, or dry non-conductive pollution that becomes conductive due to expected condensation — typical of industrial environments and of spaces with a lot of dust, moisture, or (in a horticultural environment) fertilizer residue.
PD4: persistent conductivity, for example due to conductive dust or precipitation — the most severe category.

At a higher pollution degree, a larger creepage distance is required for the same working voltage and the same material, because the likelihood of a conductive contamination layer on the surface is greater.

Why this matters

A switchgear assembly that functions without issue in a clean office environment (PD2) can, when installed in a dusty, moist or chemically loaded space (PD3) — for example a livestock building, a car wash, or a horticultural technical room with fertilizer vapor — require a creepage distance larger than the one the enclosure was originally designed for, even if the IP rating of that same enclosure is identical for both environments. IP describes protection against the ingress of dust and moisture from outside; CTI and pollution degree describe what happens when contamination nonetheless ends up on the insulating surface inside the enclosure, or forms there through condensation.

Note: the exact minimum creepage distances prescribed per voltage level and pollution degree are set out in tables in IEC 60664-1 and IEC 61439-1; this article covers the underlying principle, not the full table values.

Practical relevance

When specifying a switchgear assembly for an environment with elevated pollution — for example a horticultural technical room with fertilizer vapor and high humidity — it is worthwhile not only to check the IP rating of the enclosure, but also to ask for the material group of the insulating materials used (terminal blocks, busbar supports, PCBs) and whether the manufacturer has dimensioned the creepage distances for the actual pollution degree of the intended site, rather than a default PD2 assumption.

Common mistakes

Assuming a high IP rating rules out tracking — IP protects against pollution from outside, not against tracking on material surfaces inside an otherwise properly sealed enclosure where condensation or contamination nonetheless occurs.
Applying a switchgear assembly sized for a clean (PD2) environment in a polluted (PD3/PD4) environment without reassessment.
Using material group IIIb (low CTI) in an environment with high pollution degree without verifying the correspondingly larger creepage distance that is required.
Confusing CTI with dielectric breakdown voltage — these are two separate material properties: breakdown voltage describes resistance to breakdown through the material, CTI describes resistance to tracking along the surface.