I am making a pcb layout that is required to comply with the IEC 61010-1. In the standard, there is a table for clearances and creepages, which depends on working voltage, pollution degree and dielectric properties (CTI/comparative tracking index). However, I am a bit confused about the two columns: Printed wiring boards CTI >= 100 VS Material Group III CTI >= 100. Since our pcb uses a standard FR-4 material, I found out that FR-4 typical CTI is 175. And since I am making a pcb (so I am applying the spacing on track-track, track-pad, pad-pad, track-copper pour...etc), I figured that the right column I should be looking is at Printed wiring boards CTI >= 100, which tells me creepage should at least be 1.5mm for 250Vrms. Am I correct that this is the column applicable for my project?
Then where is the other column "Material Group III CTI >=100" used (this column tells me creepage is 3mm instead)? Because now that I think about it, I could also look at this column since FR-4 is under material group IIIA. I simply used the other column because it appears to be more specific (printed wiring board).