Hi Raymond:
> >If the dc output is connected to the PE terminal,
> >then the dc output could become live in the event
> >of a fault in the basic insulation between mains
> >and the PE terminal.
>
> I think it may not be the case in practice. Even the dc output is
> connected to the PE, if the basic insulation in the earth terminal should
> fail, the dc output still safe. It is because the leakage current will go
> to the earth path (low impedance) rather than the dc output + human body
> path where has much higher impedance.
Agreed.
The situation is that the PE must be able to carry the
fault current in the event of a failure of basic
insulation. Consequently, the PE circuit from the
furthest point where a failure of basic insulation may
occur must be capable of carrying the fault current --
up to 25 amperes (according to the standard) until the
mains circuit overcurrent device operates.
If the PE is connected to earth, then the dc output will
not rise to the mains voltage. This is the principle of
protection in the event of a fault.
My reference to "live in the event of a fault" is the
principle behind the determination of what conductors
must be connected to the PE terminal and must be
capable of carrying 25 amperes for 1 minute.
> >If the adapter is sealed, then the only way to
> >test for earth continuity is to check from the
> >dc output side to the appliance coupler earth
> >terminal.
>
> There is a comment from the supplier that the secondary components are not
> supposed to handle 25A current and unforeseen damage (early failure) may
> happen. Any comments?
The supplier is correct.
However, the issue is that of testing the PE circuit
after the unit is assembled. This can only be done by
testing between the dc output and the PE terminal.
This means that the secondary circuit path must be
capable of 25-amperes for 1 minute in order to test the
PE circuit.
In practice, the construction you describe can indeed
pass this test. The single-sided PWB earth plane has
sufficient cross-sectional area to carry the 25-ampere
current. The leads to the PWB from the dc output to
the PWB and from the PWB to the PE terminal must be
reasonably robust (e.g., 22 AWG) and short length.
This prevents these conductors form overheating during
the test.
I've dealt with a number of these adapters and have had
no problems with the 25-amp test, dc output to PE
terminal.
Best regards,
Rich
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