Hello from San Diego:
One of our colleagues has asked me to discuss further
the reason behind the hi-pot test.
The insulation between the mains and accessible
conductive parts (including low-voltage-energized
accessible conductive parts) is critical to the
scheme of protection from electric shock.
Because the insulation is critical to protection
from electric shock, the design of the mains
insulation must be such as to maximize the
preservation (reliablity) of the insulation. Or,
to put it another way, the product design must
minimize the deteriorating influences on the mains
insulation.
One of the deteriorating influences on insulation
is voltage.
Mains insulation is subject to two different voltages.
The first is the normal mains voltage itself. The
second is the normal overvoltages.
Note that I said that overvoltages are normal.
Mains overvoltages are generated by the normal
operation of inductive devices connected to the mains
such as motors and transformers and ignitors and
similar devices. These inductances in their normal
operation create back-EMFs that are imposed on the
mains circuits and propagated throughout the system.
Under standard conditions, these overvoltages can
be predicted at the point where they are generated.
But, their amplitude at any point in the system is
a variable depending on the local distribution
system and its parameters.
These overvoltages have been studied and measured,
and the results have been published in numerous
technical articles. The conclusions, time and
again, show that the maximum overvoltages are in
the 900 to 1200 volts peak. From these studies,
a standard test waveform, representing these
overvoltages, has been established, the 1.2 x 50
impulse.
So, the mains insulation is subject to both the
normal mains operating voltage and the normal mains
overvoltage.
Over the years, we have found that designing the
mains insulation for 1500 V rms provides adequate
preservation (reliability). Simply by selecting
certified mains components insures adequate
insulation to withstand the normal mains
overvoltages. Certified mains components such
as appliance couplers, fuseholders, and wire rated
at 120 V or 250 V are subjected to 1500-volt
dielectric withstand tests.
We test the design with the hi-pot test. This test
confirms that we have selected and controlled the
two forms of insulation, solid and air, to have an
electric strength of at least 1500 V rms.
We also test the production process with the hi-pot
test. This test confirms that our production
process has assembled the unit properly, and that no
manufacturing defects are in the product that would
cause the insulation to fail in the event of a
normal overvoltage.
Best regards,
Rich
ps: Note that a creepage is not an insulation; it
is the interface between solid and air
insulations. A creepage is critical to safety
because the solid insulation at the interface
is subject to the long-term deteriorating
influences of the normal mains voltage, not the
normal overvoltage.
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