Walking the Clearance Table Without Getting Lost

The clearance table is the part of IEC 61010-1 that generates the most design arguments. It is not a single table but a family of them, and the value for a given gap depends on the working voltage, the overvoltage category, the pollution degree and the type of insulation.

The inputs, in order

First determine the working voltage across the gap, which is the highest voltage present in normal operation, not the supply voltage. Then the overvoltage category, from the declared installation location. Then the pollution degree at that point of the construction. Then whether the gap provides functional, basic, supplementary or reinforced insulation. Only then is the table consulted.

Interpolation and the next step up

Values are given at discrete voltages, and where a working voltage falls between two rows the higher value applies. A designer who interpolates linearly will produce a gap smaller than required, and the error is small enough to look plausible on a drawing.

The field and the measurement

Clearance is measured through air along the shortest path, including through openings and around barriers. That is why a slot in a barrier does not increase clearance unless the slot is narrow enough to be excluded by the rules, and why a cable that moves can reduce clearance to a value nobody designed. Assess the worst case with the cable in its least favourable position.

Related technical notes

• Creepage Distance and CTI: When the Material Decides the Gap
• The Dielectric Strength Test: Setup Details That Decide the Result
• Why the AC Test Lasts One Minute, and When DC Is Allowed