Comparative Tracking Index

[Rick Busche]

        IEC 950 defines CIT (comparative tracking index) in terms of
material groups for the determination of creapage distance. Specifically
groups I, II, IIIa and IIIb. are mentioned. In paragraph 2.9.3 (note 3)
the actual index (CTI) is referred to as <600 or <400. My questions are
as follows:

        1. Is this CTI (as assigned by UL in the yellow book) actually a
voltage level?
        2. The FR4 laminate we use has been rated at 205. As such the
maximum creapage distance is mandated. Is it common to find better
material? Is this 205 level typical of common FR4 materials?

        Thanks

        Rick Busche
        Evans & Sutherland
        rbus...@es.com