[Tempest1981]

Thanks Jacques. So creepage is when current flows/arcs across the surface of an insulator, vs through the air. And it's worse with DC due to its unidirectional nature. Worsens when pollution builds up, or the surface degrades.

[jacquesm]

Indeed. And it's a really nasty thing to properly protect against because that pollution, especially with stuff that is unattended for a long time has a habit of ending up much worse than your worst fantasies. I've taken more than one electrocuted mouse out of the HV section of older color TVs for instance. Up to 250V or so it is manageable, above that you can get the weirdest problems including completely invisible arcing where the only giveaway is the ozone smell and the occasional click. Looking at HV circuitry in the dark or by putting a flame near a suspect spot is a great way to spot these kind of issues.