Hi Brian:
> I am attempting to comply with both the letter and spirit of 60950:2000, cl
> 5.2. Note that 5.2.2 allows for separate testing, according the type of
> insulation required. When testing a (class 1) power supply, the withstand
> level for primary to chassis is Basic; and for primary to secondary is
> Reinforced. Typically, I will apply approx 2500vdc for Basic and 4300vdc for
> reinforced. But to pass primary to secondary test, 60950 says that I can
> allow for following:
>
> "care is taken that the voltage applied to the reinforced insulation does
> not overstress basic...".
Good requirement and good advice. But, very
difficult to apply in practice.
> Also
>
> "to avoid damage to components or insulation which are not involved in the
> test, disconnection of ICs or the like and the use of equipotential bonding
> are permitted."
Again, good advice but very difficult to apply
in practice.
> For Pri/Sec testing, the screw connecting the Y-caps to the chassis is
> removed, insulation is inserted between the screw insert and chassis.
Well... I would just lift the cap and let it
hang in air for the test.
Inserting solid insulation between the Y-cap
terminal and the chassis means that you must
also consider:
the creepage and clearance around the solid
insulation, and
that most of the hi-pot voltage will appear
across the solid insulation.
> A (new) agency engineer says that inserting a piece of insulator defeats the
> purpose of the test. For class 1 construction, it is just not possible for
> me to pass 4300vdc test levels without inserting my little piece of valox,
> unless I physically remove all Y-caps (and the unit will not operate
> reliably with y-caps). I have inspected units from several other companies
> and have determined that it was not possible to have ever passed pri-sec
> test levels without isolating these circuits.
Inserting the solid insulator DOES NOT defeat
the purpose of the test. While it is true that
the inserted solid insulation gets tested in
parallel with the pri-sec reinforced insulation,
the test results apply to the pri-sec insulation.
If a failure should occur, then it would be
necessary to sort out whether the failure was
your inserted solid insulation or the pri-sec
insulation.
If your secondary is not grounded, then you can
open the protective ground and test pri-sec
without lifting the Y-cap. (Put the unit on an
insulating surface, and be careful not to touch
the unit during this test!)
Our products are designed in the same way. We
don't lift the Y-caps or anything else when we
hi-pot pri-sec (secondary is functionally grounded).
Y-caps have enough margin to easily withstand
the 4300 V dc without damage. In other words,
our pri-gnd meets 4300 V dc.
In general, solid insulations have electric
strengths in the neighborhood of 10 kV or more.
The required spacings have electric strengths
in the neighborhood of 5 kV. Y-cap electric
strength usually is greater than the lead
spacing.
You should expect a system electric strength
approaching 5 kV rms.
During design, we always hi-pot test to failure
or max voltage of the hi-pot tester, whichever
is lower. In our designs, the weakest point
(4500-5000 V rms) in the power supply is the pwb
terminals of the Y-cap on the back side of the
board (where the sharp points are located).
The hi-pot test is a pass-fail test, and gives
no data. Hi-pot to failure identifies and
measures the weakest link. This is useful data
for future evaluation of production-line hi-pot
failures.
Good luck, and best regards,
Rich
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