Hi Sam:
> Correct me if I'm wrong, but the point of the type-level hipot test is not
> to simulate a real-life condition (like surge protection), but to simulate
> years of insulation degradation, which happens with or without surge
> protection.
I disagree.
Primary circuits are subject to overvoltages due
to switching of inductive loads such as motors.
These overvoltages are NORMAL and, to some extent,
predictable. Many studies have been published on
the magnitude, duration, and repetition of these
overvoltages.
Safety insulations in primary circuits must be
able to withstand these NORMAL overvoltages.
Hence, the hi-pot type test is one measure of the
adequacy of the primary circuit safety insulations
to withstand these NORMAL overvoltages.
The hi-pot type test does not in any way imply
reliability of the solid insulation. Failure of
solid insulation due to impressed voltage starts
with partial discharges in voids in the insulation.
Reliability is a function of the magnitude of
voltage across any void. The smaller the void,
the more reliable the insulation. A perfectly
homogeneous solid insulation is very reliable
with respect to the impressed voltage. (Many
solid insulations approach perfect homogeniety.)
An overvoltage limiting device, i.e., surge
protector, can limit the magnitude of voltage
across a safety insulation. This in turn limits
the magnitude of voltage across any void in the
insulation. The lower the voltage across any
void, the lower the magnitude of partial
discharge and the longer the life of the solid
insulation.
In practice, however, for typical mains over-
voltages, the magnitudes, durations, and
repetition rates together with the homogeniety
of the insulation are such that damage due to
partial discharge almost never causes
catastrophic solid insulation failure in the
lifetime of the product.
Best regards,
Rich
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