** Of course there is !!
A Class 2 appliance must not be earthed or it ceases to be one.
You are correct for a transformer based power supply device such as a
plug-pack, but this is not necessarily the case for other products. The
deliniation between Class 1 and Class 2 is becoming very grey. Whereas AS3108
and AS/NZS61558-1 say any transformer that has an earth terminal must be
classified as Class 1 other standards do not.
Definition and references below from AS/NZS60950-1, since I happen to be using
it at the moment. Note the use of earthing is not prohibited, just that the
safety strategy of the design must not rely on the protective earth. This
standard might/is be used for a switch mode PSU plug-pack.
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"COPYRIGHT"
1.2.4.2 CLASS II EQUIPMENT: Equipment in which protection against electric
shock does not
rely on BASIC INSULATION only,but in which additional safety precautions,such
as DOUBLE
INSULATION or REINFORCED INSULATION are provided,there being no reliance on
protective
earthing.
=====
There have been many changes since the old AS3108 and AS3100 type standards
existed as the sole point of reference. None of those standards took into
account EMC filtering or stray currents due to SMPSU, etc. With the more
modern standards you can certainly use Class 2 construction, with functional
(not protective) earthing. Note, connecting an earth does not automatically
make it Class 1 in this standard.
If you want to split hairs on the issue over the Class 2 box-in-box marking,
perhaps I should of said before to scatch off the symbol. It is then
functionally earthed Class 2 construction. It can only be truly Class 1 if the
earthing is protective. There is no marking to define these products (yet).
The mis-wiring of plugs etc is not a factor taken into consideration.
Here is a reference from the same standard:
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"COPYRIGHT"
2.6.2 Functional earthing
If FUNCTIONAL EARTHING of accessible or other conductive parts is necessary,all
of the
following apply to the FUNCTIONAL EARTHING circuit:
- the FUNCTIONAL EARTHING circuit shall be separated from parts at HAZARDOUS
VOLTAGES
in the equipment by either:
..DOUBLE INSULATION or REINFORCED INSULATION or
.. a protectively earthed screen or another protectively earthed conductive
part,
separated from parts at HAZARDOUS VOLTAGES by at least BASIC INSULATION and
- it is permitted to connect the FUNCTIONAL EARTHING circuit to a protective
earth terminal
or to a PROTECTIVE BONDING CONDUCTOR and
** You are just making this crazy drivel up.
The earthing requirements for a class 1 appliance relate to the copper cross
section of the earth wire used, ie must not be not less than 1 sq mm. (
AS3100)
1.0 sq mm for 10A cords up to 2 metres long and 1.5 sq mm for 10A cords longer
than 2 metres.
I did make a mistake with the multiplication factor, it is 1.5 and not 1.6
times. I am not making it up - sometimes I think it would be easier to
understand if I did. For functional earthing there is no size requirement
other than that needed for it to do the intended job. For protective earthing
and bonding you can provide the minimum conductor sizes in the standard (1.0 or
1.5 sq mm), or use the minimum size conductor that meets the following:
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"COPYRIGHT"
2.6.3.3 Size of protective bonding conductors
PROTECTIVE BONDING CONDUCTORS shall comply with one of the following:
- the minimum conductor sizes in table 3B (see 3.2.5);or
- the requirements of 2.6.3.4 and also,if the current rating of the circuit is
more than
16 A,with the minimum conductor sizes in table 2D;or
-for components only,be not smaller than the conductors supplying power to the
component.
The current rating of the circuit used in table 2D and in the test of 2.6.3.4
depends on the
provision and location of overcurrent protective devices and shall be taken as
the smaller of
a)or b)as follows.
a)The rating of an overcurrent protective device specified in the equipment
installation
instructions to be installed in the building installation wiring to protect the
equipment.?47
b)The rating of an overcurrent protective device in the equipment that protects
the circuit
or part required to be earthed.
For PLUGGABLE EQUIPMENT TYPE A and if neither a)nor b)is applicable,the current
rating of
the circuit shall be taken as the RATED CURRENT of the equipment or 16
A,whichever is
greater.
=================
So (b) takes into account the rating of protective device that protects the
circuit or part that needs to be protectively earthed. If a part that could
fail to earth is protected by a 1A fuse then the protective conducter is sized
to pass the following test, otherwise a great many PCB tracks that carry earth
within a small SMPSU would be disallowed.
If a series connected mains socket or device is protected by a (say) 5A fuse
the intermediate bonding conductors to the equipment mounted socket outlet
could be reduced in size accordingly. Fifteen years ago you couldn't do this,
but you can now.
This is the test for bonding conductors.
=================
The test current,duration of the test and test results are as follows:
- if the current rating of the circuit under test is 16 A or less, the test
current is 1,5 times the
current rating of the circuit under test, the current is applied for 60 s and
the resistance of
the PROTECTIVE BONDING CONDUCTOR , calculated from the voltage drop,shall not
exceed
0,1 ?;
=================
There are many cases where AS3100 or AS3108 say a specific thing cannot be done
but a more modern product specific standard may allow it, and AS/NZS3000 is no
longer a prescriptive standard with regard to anything on the equipment side of
the supply interface.
The damn standards change so often it costs us thousands of dollars a year to
keep up to date.