What points do you need to measure in PCB according to IEC 601? Is it the closest distance between any conductive parts? For example minimum air clearance for means of operator protection in secondary circuits, so I measure the closest distance between any conductive part in secondary circuit? I know air clearance and creepage distance principle, but don't know how to use in practice.
Creepage is along the surface, "as the ant walks", and clearance is shortest distance, "as the bird flys."
In practice, creepage and clearance on a PCB are the same initially. The minimum clearance creates a no man's land between the two sides of the isolation barrier. Nothing conductive can be in that area. If more creepage is required, a slot can be milled in the PCB at the appropriate point. It's fairly common to see slots under isolating components that cross the isolation barrier.