I was reviewing the rationale document IEC TR 62368-2 and found a comment
in 5.4.3: "*However, there is no rationale why the creepage distances are
different for printed wiring boards and other isolation material under the
same condition (same PD and same CTI). *"

Apparently some important concepts have been lost over the years. So I am
writing this email to provide a historical perspective that is a detailed
basis of my understanding for this concern.

I have personally studied the IEC 60664-x standards since the days it was
simply known as  IEC 664A.  This, along with the old VDE 0160 which was
eventually the basis for EN 50178. This European Norm was strongly
influenced by the 664 family of standards. EN 50178 has a reference to
reduced creepage distances for printed circuit boards.  Back in the day,
the rationale, as explained to me through DIN VDE 0160, EN 50178, and by
representatives at LGA Landesgewerbeanstalt Bayern, was that printed
circuit material has very well controlled tolerances and is dimensionally
stable. Therefore, if it passes the engineering type tests, then there is
very good confidence that subsequent manufacture will remain so.  When I
asked if the fact that most PCB laminate material is CTI III, and if this
makes any difference, the answer was consistently No.

EN 50178 (1997 & 1998) Table 6 Minimum Creepage Distances, has a note
referring to PCBs for Pollution Degree 1 and 2:

"*These columns apply also to components and parts on PCBs and to other
insulation arrangements with a comparable control of tolerances*"

This was thinking in those days and yes a rationale does exist.  Also, as I
understand it, this rationale came out of a many months long
German-American research project: *Schau, P.v., Middendorf, W.H.: An
International Research Project to Determine New Dimensioning Rules for
Creepage Distances.IEEE Trans. on Electrical Insulations, Vol.EI-18, No.2,
 April 1983; *

I really would like to review the original IEEE paper but unfortunately it
is locked in my subscription.


Douglas E Powell
Laporte, Colorado USA
LinkedIn <>

(UTC -06:00) Mountain Time (US-MDT)

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