In IEC 60664-1:2021 table F.5 (former F.4): Creepage distances to avoid failure due to tracking or 60950-1:2005 Table 2N – Minimum creepage distances the second and third columns are applicable for "printed boards" or "printed wiring materials". For voltages below 1000V and a pollution degree of 2 or 1 the creepage distances are far shorter then the "general" distances (see Creepage distance for PCBs handling line voltage AC? for the table extracts of IEC 60664-1).
- What is the reason to allow shorter creepage distances for PCBs than for other materials?
- Are there any restrictions to allow the application of the shorter creepage distances?
- Are the shorter distances only applicable for bare PCBs or also for assembled ones?
I was not able to find a satisfying answer to my questions in the standard or in the application guide IEC 60664-2-1 or in the internet. The only phrase that may give a hint was the following (but I find it rather ambiguous):
For creepage distances on printed wiring material only used under pollution degree 1 and 2, a reduced dimensioning is applicable according to IEC 60664-1. Attention is drawn on the possible reduction or other path of creepage distances due to the components.
But the main question for me is my first one: Why for PCBs it is allowed to use shorter creepage distances? I really can't see a reason.