Hi Mike:
> I've been asked why a routine hipot test is required on an end assembly
> computer when it uses a Listed power supply that has already been hipot
> tested. So far I don't like the way I've worded my response. Basically
> what I've said is that a power supply is approved as a component. The end
> safety of the device depends on the installation.
There are two answers...
First, certification houses require testing of the
end-product in its final configuration. This
requirement is based on the idea that the end-product
manufacturer can affect the primary circut insulation
until the enclosure is installed.
In some cases, where the power supply includes all of
the primary circuit including the input connector, and
is fully enclosed in its own enclosure that meets end-
product specs, the certifier will waive the end-product
test. Note that this waiver is based on the idea that
the construction of the power supply precludes the end-
product manufacturer from affecting the primary circuit
insulation.
Second, it is appropriate to re-test due to the effects
of time and transportation on the power supply. Others
have already provided anecdotes on these effects. I'll
add one more: A primary wire was pinched, but passed
the power supply manufacturer's hi-pot test. After
crossing the ocean in a ship, the power supply failed
the hi-pot test. During the crossing, the wire
insulation cold-flowed until the remaining insulation
was too thin to pass the hi-pot test.
Best regards,
Rich
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