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Creepage Distance and CTI: When the Material Decides the Gap
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Clearance is the distance through air; creepage is the distance along the surface of the insulating material. They are often confused because on a clean dry assembly they are nearly the same, and they diverge exactly where it matters: at moulded ribs, slots, and joints between materials.
CTI
The comparative tracking index describes how resistant a material is to forming a conductive track when a voltage is applied across a wet contaminated surface. Materials are grouped into bands, and the group determines the creepage multiplier. A material with a high CTI allows shorter creepage for the same working voltage, which is why enclosure material selection is a safety decision and not only a cost decision.
Where the surface path is longer than it looks
A rib or a groove adds surface distance, and the standard gives rules for how much of a groove counts. A joint between two moulded parts is a surface path even when the parts are pressed together, because contamination and moisture enter the joint. Potting or conformal coating can remove the path from consideration only if the coating is assessed as an insulation, which requires its own evidence.
The practical check
Measure creepage with a thin tape or a piece of wire laid along the surface, following every contour, not with calipers across the straight line. The difference between the two measurements on a ribbed moulding is often a factor of two, and it is the difference between a pass and a non-conformance.
