One more valuable link: Although the subject of this thread was was voltage separation in the z-axis of a PCB, I did find a very useful ONLINE CALCULATOR for Creepage and Clearance per IEC/UL http://www.creepage.com/ It saves having to wade through the specifications and various tables, and if you are calculating for a PCB construction, select the "IIIa/IIIb" Material Group simple. (big thanks to Dave Larusso!) Jack