Since my 23 June message, I may have found the answer to my quest as to
where the air insulation (clearances) distances came from: IEC 664, Edition
1, 1980, Appendix Table AI, withstand voltages, and Table AII, breakdown
voltages.
According to IEC 664, Table AII is "experimental data" by
Prof. Dr. Ing. W. Pfeiffer, convenor of IEC TC109/MT3,
elektrotechnische zeitschrift Ausg. B, 1976.
Dr. Hermstein, elektrotechnische zeitschrift Ausg. A, 1969.
These are German electrotechnical journals issue A, 1969, and issue B, 1976.
I could not find copies of these. Perhaps our German subscribers can find
these.
I surmise from the tables that these two people tested air breakdown voltage
as a function of distance. I did find that Dr. Hermstein did some
experimental work on electrical performance of gasses that has been
discredited.
Table AI (IEC 664) is withstand voltages based on the breakdown voltages in
Table AII (IEC 664). This is the source of IEC 60664-1 clearance distance
tables which have been used by a number of IEC standards committees.
I've attached a plot of both the breakdown voltage per distance and the
withstand voltage per distance through air. These are linear axes while the
IEC 664 and IEC 60664-1 plots are logarithmic axes. I've included trend
lines (dotted) and their equations. (The voltage-distance tables are not in
IEC 60664-1.)
I suspect the non-linearity of the breakdown (red) line is due to
measurement problems. I would expect the line to be straight except for the
small dimensions that approach the Paschen voltage limit for air, 327 volts
peak. (Paschen studied gas breakdowns at very small gaps and found that
various gasses do not break down at very small gap dimensions.)
Best regards,
Rich
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