I have a copy of MIL-P-55110A, dated 18DEC64 which
has the requirement that insulation resistance be
500 meg. so that value has been around for 34 years.
As to exactly why that number was derived, I don't
know for sure.  I do know that it was based on a
spacing of 26 mils.

There are other requirements for various spaces for
other specifications - such as Bellcore.  And they do
make sense.

To use other sizes of comb patterns, especially closer ones,
we probably need to change that value.

Perhaps some of the design guys might have a clue as
to the origin of the number.

One thing to keep in mind is that on SIR testing for the
assembly documents, there is a visual assessment after
and there can be no corrosion or arcing etc to successfully
pass the test, NO MATTER WHAT RESISTANCE VALUES
YOU OBTAIN.


Susan Mansilla
Robisan Lab

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