Hi Vic:
> I have a Class 1 product. It has a double insulation barrier between
Primary
> and earthed SELV, and basic between primary and the metal enclosure.
>
> How can I perform an electrical strength test across the primary/SELV
> barrier?
The construction you describe has two insulations in
parallel:
1) basic insulation between mains and earth, and
2) double insulation between mains and earthed SELV.
Since the insulations are in parallel, the basic
insulation must be disconnected in order to test the
double insulation.
One could simply test the double insulation together
with the basic insulation and accept the consequences of
potential failure of the basic insulation during the test.
This is not as horrific as it might sound.
Typical solid insulations have electric strengths in the
order of 10 kV and more. Air insulation is the "weakest"
insulation in the system, but is renewable so no damage
is done to the air (or product) if it should break down.
In performing an electric strength test, one can only
damage solid insulation. So, the issue is whether or not
the solid insulation employed as basic insulation is good
enough to withstand the double insulation electric
strength test. Almost any solid insulation should readily
withstand the double-insulation electric strength test
voltage without degradation.
In our products, the only solid basic insulations are:
1) the IEC 320 appliance inlet;
2) the printed wiring board;
3) Y-caps.
We use Y1 caps, which are equivalent to doube insulation.
The only solid insulations that would be subject to over-
voltage are the appliance inlet and the PWB. We've not
seen any indication that these insulations are degraded
by the double-insulation electric strength test.
In my experience, the weakest insulation is the air between
the pads of the Y-caps on the back side of the PWB.
Typical breakdown voltage is 4500-5500 V rms.
The minimum clearances required by most safety standards
have a great deal of margin between withstand voltage and
breakdown voltage. For example, a 2 mm purely
inhomogeneous-field clearance (basic insulation) breaks
down at 1.68 kV rms. Most constructions have a field
that is much better than purely inhomogeneous.
For the production-line, we use 4300 V dc, mains-to-earth.
Dc tends not to ionize the air as quickly as ac, and
therefore is less stressful to solid insulation than ac.
Best regards,
Rich
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