The tables in IPC are to isolate different voltages, and if you violate them
by using a smaller gap, current can travel across the surface of the board
to where you don't want it to be. That's why the recommended gaps for
internal layers are so much smaller, no surface to travel across (I think!
Someone correct me if I'm wrong). But when you cut a slit the only path is
across air, which is MUCH harder to cross, so you can have a much smaller
gap of pure air and get the same isolation as a large gap of fiberglass.

BUT WARNING! If you go get the figures for air, you have to take into
account the altitude and maximum humidity of the environment, because not
all air has the same electrical properties! (almost got burned designing
power supplies for submarines once)

Good Luck,
Jack

                -----Original Message-----
                From:   Çeyiz Tekcan (ARGE - VESTMAN)
[mailto:[log in to unmask]]
                Sent:   Tuesday, April 04, 2000 4:00 AM
                Subject:        about using slits for voltage isolation

                Dear all,

                I want to know that, how I can use slit to conductor
isolation for high
                Voltages. I know the formula about conductor clearance (from
IPC-D-330), if
                I use slits, would these clearances be decrease?  How can we
formulate the
                relationship between slits and electrical conductor
clearances ?

                Thanks in advance
                Ceyiz Tekcan
                VESTEL R&D
                PCB Design Engineer
                E-mail:[log in to unmask]
                Phone:+90 236 233 01 31  Ext:2111
                Fax:+90 236 233 38 68