While it is admirable to strive for excellance in
ones products it must be recognised that perfection is
not achievable. Very few threats to the performance
and reliability of a product have a hard cut-off value,
Nearly all show a statistical drop in probability as the
intensity of the threat increases. If this is so then all
test levels will only provide a statistical level of
protection. Most of the standard levels are set after
consideration of statistical surveys I do not think
corona will provide an absolute limit at 25kV. I
remember physics experiments at school where a
combination of frictionally induced charge and
voltage amplification by conductor separation produced
sparks of an inch and a half, implying a voltage of about
40kV. If you are going to apply ESD test limits way above
those those judged by the standards bodies to give
adequate protection in the environment concerned there
is no logical reason not to do so to all the other aspects
of EMC. There is absolute limit to the size of RF field
that may be encountered in rare circumstances and you
may occasionally encounter extremely sensitive
equipment that may be interferred with by emissions
way below those of the standards. Unless you have
reason to believe that the environment is markedly
worse that that envisioned in the standard or the
results of a failure are particularly catastrophic I
would not inflate the test levels above the standards
and the most severe of the European product and
gereric standards call 8kV air discharge.
Nick Rouse
