Hell Duncan:
> - Would they use S1 when leakage testing our type of equipment?
Your equipment is cord-connected equipment, not
direct-plug-in equipment. Therefore, S1 is not
used when testing leakage current from your
equipment.
> - What is the definition of 'direct plug in' (it is not mentioned in the
definitions section)
Direct-plug-in equipment is the "wall wart" type
of product where the product is integrated into
the plug. Examples are:
Small plug-in power supplies.
CO detectors
Some electronic room deodorizers
Night lights
Some electronic bug killers
> - For 'direct plug in' equipment what is the reason for operating this
> switch
i.e what is the rationale behind the provision of S1.
UL is the instigator of the S1 (neutral switch)
when measuring leakage current.
The story, as I know it, is simply a convenience
switch during product testing. For products with
a single-pole power switch, it simulates the
situation of a two-wire product being connected
to the supply where the polarity is such that the
single-pole switch is in the neutral pole of the
supply.
UL designed a test fixture for their technicians
such that during the leakage current test, the
test technician need not reverse the plug and
operate the power switch, but instead operate
two switches on the his leakage current test
fixture, one to reverse the supply and the other,
S1, to open the neutral.
The situation of a single-pole power switch and
reverse polarity (i.e., the power switch in the
neutral conductor) has the effect of doubling the
leakage current.
When USA manufacturers started moving into the
international market where polarity was not
assured, they started using double-pole power
switches. These were hi-tech products with line
filters that caused relatively high leakage
currents. When tested with S1 open, the leakage
current exceeded the 3.5 mA limit. So, the
manufacturers lobbied UL to change the standard
such that S1 was not operated if the product had
a double-pole switch.
This was accepted for the high-tech equipment.
As for why the test applies to direct-plug-in
units and not for cord-connected units... I can
only guess. My guess is that the standard
presumes an overcurrent failure of the unit, in
which case the overcurrent device can be in either
pole of the supply, and thus the leakage current
can be double the normal leakage current.
Of course, this also applies to cord-connected
products, so my guess must be wrong.
Unless all products that are not direct-plug-in
are two-wire plus ground, in which case both a
fuse operation and an open ground is a double-
fault condition and would not be tested.
Best regards,
Rich
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