Hi John:
Thanks for your comments.
In the end, the “solution” was a different sort of pragmatic approach because
the boards were always enclosed in hermetically sealed high pressure (10,000
psi+) / temperature (180C+) -resistant stainless steel tubes which have very
little free air volume inside them.
That means that there is very little free oxygen for component fires to use,
and calculations proved that ignitions involving all the flammable material
within the enclosures would exhaust that oxygen well before fires could
develop, and also the way the enclosures are built and sealed means that flames
or flammable material could not escape unless there had first also been very
substantial external physical damage.
This is another option. Build a fire inside the equipment and see what
happens. I use a fire-starting tablet or pellet (I’ve forgotten the name). An
enclosure with minimum openings that would allow replenishment of oxygen will
suffocate the fire once the internal oxygen is used up by the fire. The
enclosure does not need to be sealed. Usually, such construction will not have
very much empty space and therefore relatively little initial oxygen to feed
the fire. (I used such testing to prove that a circuit fire would not ignite
an HB enclosure.)
This is another situation where one can show that a fire will not spread very
far beyond the initial fuel.
Whether or not this is accepted as compliance with the standard will depend on
the certification engineer and the policies of the certification house (and
your ability to convince them that the construction is adequate).
Rich
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