Hi Chengwee:
> In my years working in power supply industry, I have never done the ground
> continuity test between Earth terminal to the DC output for adaptor.
> Normally I wouldn't trust the PE path with anything less than 18AWG or
> equivalent.
There are two kinds of tests that I have in mind.
First is the type test which is done during the
evaluation of the unit. Second is the production-
line test.
For the type test, the test point can be at the
site where basic insulation is interposed between
the mains and the earthed conductor or part on an
unassembled unit. This test does not subject the
functional earth to the high current.
Many cert houses require production-line ground
continuity tests; some require the test current to
be 25 amps. So, this test must be performed on
the assembled unit and necessarily subjects the
functional earth circuit to the same current as
the PE circuit.
> In addition to that, agency such as UL have ramp up the test current to
40A,
> for 2 min according to the standard 2.6.3.4
> for 20A circuit in U.S.
As previously mentioned, as a type test, 40 A can be
applied only at sites where basic insulation exists
between mains and the earthed conductor or part.
*****
The ability of the circuit to withstand high current
is a function of the various resistances. The
resistances, in turn, are a function of the heating
that results from the current. When small conductors
are used, the conductors must be short (to reduce the
resistance) and heat-sunk.
The construction described by Raymond Li likely uses
short lengths of 18 AWG between the main board and
the EMC shield. The EMC shield provides a good heat-
sink for the wire. The main board also proveds heat-
sinking. So, for a circuit that comprises a
functional earth circuit, it can easily withstand the
25-amp production-line test.
Clearly, a PE circuit requires 18 AWG or bigger wire.
Best regards,
Rich
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