In message <20140513062547.5939345.78916.6...@gmail.com>, dated Tue, 13
May 2014, Doug Powell <doug...@gmail.com> writes:
There was a short term breakdown (hours) of common nylon material at a
frequency of 13.56 MHz. In this case a white nylon Tywrap disintegrated
and wire routing was compromised.
This is neither clearance or creepage but a *bulk* effect of the
electric field on the nylon. It may simply be due to heating caused by
dielectric loss, but nylon is not usually considered a poor insulator at
tens of MHz. However, that applies to *dry* nylon, and some grades of
nylon absorb water rather too well. Also, the magnetic field is probably
accompanied by a significant electric field, whose strength at some
points in the enclosure could be very high, enough to cause ionization
in any air-filled voids in the nylon. An examination of the debris might
give a clue to the failure mechanism.
The solution was Tefzel Tywraps which were unaffected by that
frequency. The point is, with thousands of formulations for polymers,
it is impossible to catalog all the RF frequencies which may cause
breakdown materials.
Even so, there are principles, as outlined above, that must be taken
into consideration. Wherever plastic materials are exposed to high
electric and/or magnetic field strengths, DC to gamma rays, exposure
tests are essential if failure is critical. (OK, DC magnetic fields can
be disregarded.)
--
OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk
Nondum ex silvis sumus
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
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