3. A surface clearance rule stands in for creepage

Status: accepted, October 2026

Context

KiCad 10's DRC has a creepage constraint, so the first version wrote one into the .kicad_dru. The zone filler ignores it: the ground pour filled up to the 3 mm clearance from the mains side, and the DRC then reported creepage violations against the pour on every mains board. TraceMaker routes to clearance, not creepage, so tracks did the same.

Decision

Two rules are written for the mains side:

• Reinforced isolation: clearance and creepage, on all layers, as the standards say.
• Reinforced isolation, surface copper: on the outer layers, a clearance equal to the creepage distance, except pad to pad.

Creepage is also measured by WholeBoard itself after DRC (mains.check), along the surface and round slots.

Consequences

• Pours and tracks keep the creepage distance from the mains side, so DRC passes and the board is right.
• On outer layers, copper is held further from the mains side than clearance alone requires, even where a slot would have allowed closer. That costs a little board area, on the safe side.
• Pad-to-pad gaps across a barrier part are left to the slot and to WholeBoard's own creepage check.