Measuring Creepage and Clearance Under IEC 61010-1

Clearance is the shortest distance through air. Creepage is the shortest distance along a surface. On a flat, clean board with two pads facing each other, the two numbers happen to be the same, which is why the distinction gets blurred. Add a slot, a rib, a conformal coating or a connector housing, and they diverge sharply.

Start with the four inputs, not the table

IEC 61010-1 selects a required distance from the working voltage, the pollution degree, the overvoltage category (installation category) and the comparative tracking index (CTI) of the insulating material. Get any of those wrong and the whole table lookup is meaningless.

• Working voltage: the highest voltage the insulation sees in normal use, not the nameplate rating of the instrument.
• Pollution degree: most laboratory and office instruments are degree 2; a sealed or potted assembly may justify degree 1, but only with evidence.
• Material group: based on CTI. A higher CTI material permits shorter creepage for the same conditions.
• Altitude: above 2000 m, clearances need a correction factor because the air is thinner.

Where a circuit is protected by a transient-suppression device, the standard allows the overvoltage to be considered reduced, but the documentation has to show that the device is rated for it. "There is a varistor somewhere on the board" is not a basis for a smaller clearance.

How a groove changes the creepage path

Creepage follows the contour of the surface, including down into a groove and back up the other side — but only if the groove is wide enough relative to the pollution degree. Narrow slots below the specified minimum width are bridged by contamination and are treated as if the surface were flat, so the path is measured straight across the opening. This is the single most common mistake on compact boards: a designer cuts a 0.6 mm slot, assumes it doubles the creepage, and the assessor measures straight across it.

Ribs and raised barriers work in the same way. A rib that is tall enough and bonded to the surface so it cannot be bypassed forces the path over its top. A loose insulating washer held in place by a screw does not count, because contamination can get underneath it.

Measuring on a real assembly

Use a calibrated caliper for accessible straight paths and an optical comparator or a sectioned sample for anything inside a connector or under a component. Measure to conductive parts, including solder fillets, screw heads, heatsinks and the metallised layer on a board edge. A heatsink that is floating electrically still counts as a conductive part for clearance purposes unless the standard explicitly permits otherwise.

Check the worst case over the whole assembly, not just the obvious gap. On a mains input board the tightest point is often between a fuse holder's metal clip and a nearby low-voltage trace, or between the primary and secondary windings at the point where the bobbin flange ends.

Documenting it so it survives review

Write down the working voltage, pollution degree, material CTI with the datasheet reference, altitude assumption, table value and the measured value for each critical path, with a photograph marking where the measurement was taken. A report that simply says "creepage 6.4 mm, OK" is not reviewable. When the measured value sits within a few tenths of a millimetre of the requirement, re-measure after the board has been through the assembly process — solder flow, component shift and flux residue can all move the answer.