Pollution Degree and How It Changes the Creepage Requirement

Clearance and creepage are pulled from tables in IEC 61010-1, and most of the arguments about those tables are really arguments about inputs that were guessed rather than measured. Working voltage is one input; pollution degree is the one people get wrong most often, because it is a judgement about the environment, not a number on the nameplate.

The three degrees

Pollution degree 1 is a clean, dry, typically enclosed environment where conductive dust and moisture are excluded. Degree 2, the default for most laboratory and office instruments, assumes only non-conductive pollution that may become conductive if humidity rises. Degree 3 covers industrial or outdoor conditions where conductive dust, rain or moisture is expected. The degree sets how much margin the creepage table demands for the same voltage.

What counts as clean

Claiming degree 1 is not free. It needs evidence that the assembly is actually sealed or potted so conductive contamination cannot reach the critical spacing. A plastic case with a loose lid is not degree 1 by wishful thinking. Most products sit honestly at degree 2, and that is fine; the mistake is assuming degree 1 to shave a millimetre off a board and then shipping it into a dusty panel.

Effect on creepage

For a given working voltage and material group, moving from degree 2 to degree 3 can nearly double the required creepage. On a compact board that is the difference between a routable layout and an impossible one. The material group, derived from the comparative tracking index (CTI) of the insulating material, also shifts the requirement, so the board house must know the actual CTI of the laminate, not an assumed value.

• State the pollution degree and the material group on the report.
• Do not assume degree 1 without a sealed or potted assembly.
• Check the worst-case path, not an easy gap.
• Mind narrow slots: below the minimum width they are ignored by the creepage path.

Coating and sealing

A conformal coating or a properly potted section can move the effective pollution degree down, but only with evidence and only if the coating is continuous and qualified for the purpose. A thin lacquer brushed over a corner does not earn degree 1. If a coating is relied on to reduce creepage, the assessment must say so and the process must actually deliver a continuous film.

Documenting it

Write down the working voltage, the pollution degree with its justification, the material CTI with the datasheet reference, and the measured path for each critical gap, with a photograph marking where the measurement was taken. A report that simply states "creepage 5 mm, OK" cannot be reviewed, and the first question a certifier asks is almost always "what pollution degree did you assume?"