Hi Raymond:
Any product with a PE (ground) connection is, by
definition, a Class I product. The common adapters
you describe, despite being encased in plastic, are
Class I products.
> 1. Function of the grounding plate
> The primary and the secondary is reinforced insulation and withstands over
> 3000Vac. Is this plate to change the whole safety protection system from
> class 2 to class 1? Or the plate is primarily for EMC suppression?
The single-sided ground-plane PCB you describe is
used to control EMC emissions. It may also be used,
as you describe, to electrically ground the dc output.
The ground plane has no safety function, per se.
While the safety standards require a product to be
Class I or Class II, it is physically impossible to
build a purely Class I product. Every Class I product
necessarily includes Class II construction. You have
accurately described the adapter Class II construction
(reinforced insulation, primary-to-secondary).
In other words, the adapter has both Class I construction
and Class II construction.
Safety standards ignore this physical true-ism. Any
product with a PE is Class I, and is evaluated only to
the Class I requirements.
> 2. Earth continuity test
> After the unit is completely assembled, should we conduct the test between
> the earth terminal of the mains plug and the earth of DC output plug?
Yes.
The earth continuity test is required for any accessible
metal part that is susceptible of becoming live in the
event of a fault of basic insulation.
Within the adapter, the Class I part of the construction
has basic insulation between the mains and grounded
conductors. Such grounded conductors must be subject to
the earth continuity test.
Because the dc output is connected to the grounded
conductor, the dc output could become live in the event
of a fault of basic insulation.
So, an earth continuity test must be conducted between
the dc ground and the PE terminal of the mains connector
(because the unit is sealed, the test cannot be made
directly to the conductors where the fault would occur).
> 3. Hipot test
> As the unit is classified as class 1, 1,500 Vac is applied between the
> earth terminal of the mains female connector and the earth of the DC
> output plug. Actually, the primary and secondary can withstand 3000 Vac.
> Is it correct test voltage to apply after the unit is completely
> assembled?
Because the unit is Class I, the hi-pot test voltage is
1500 V rms.
The hi-pot test is always performed on a fully-assembled
unit.
You are correct that the primary-secondary reinforced
insulation must withstand 3000 V rms. Note also that
the primary-foil (wrapped about the outside of the
adapter) must also withstand 3000 V rms (because the
plastic comprises reinforced insulation to accessible
surfaces).
While the unit will probably withstand 3000 V rms, you
should not production-line test to 3000 V rms because
this may overstress the primary-ground insulation.
Best regards,
Rich
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