Rich,
Thanks for the expanded discussion of electric strength testing purposes.
Some words regarding typical hi-pot failures may help illustrate the
points you made. The following observations are based solely on ITE for
the home/office environment.
RANDOM FAILS--Assuming a good job was done in the design of a product,
random fails are usually few and far between. Examples might be (1) a
loose screw laying on the power supply PCB, (2) pinhole defect in
insulation, (3) a defective component, (4) an enlarged solder "footprint"
that decreases or bridges the creepage distance, etc. Although there is
no such thing as "zero defects", today's manufacturing technologies have
dramatically reduced the rate of random defects in components, and the
number of random hi-pot failures. Since primary voltages are often
confined within the incoming power supply, a unit failing hi-pot can
usually be corrected with a replacement of the power supply.
PROCESS FAILS--In my opinion, the detection of PROCESS failures is
the primary value of the hi-pot test. These come in bunches. Examples
we have seen are:
1. Power supply PCB cut such that edge traces are less than prescribed
creepage/clearance from grounded pads or metal enclosure. [drift in
PCB cutting process]
2. Transformers wound in a way that decreases creepage/clearance. [New
operator(s) at xfmr supplier.]
3. PCB traces too close. [New board layout by vendor.]
4. Bad solder joints. [These are often found only with the arc detect
option available on some hi-pots.]
Hi-pot failures due to unforeseen process changes require immediate
analysis and recovery to avoid the product pipeline from running dry.
PASSING UNITS--A unit that passes hi-pot has demonstrated only one thing,
that it had no insulation defects during the 1 sec or so test. The unit
could develop an insulation defect at any time after the test. For
example, a loose part may move from a "safe" position to a position that
bridges insulation. Or, the first line transient that the unit sees in
the field may arc through a weak spot in insulation. This is why the
detection of process changes is the primary value of this test.
I hope this aids the overall hi-pot discussion.
Regards, George
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