Just dawned on me.
There is another example of class I and class II insulations in
parallel that may be sitting on your desk at the moment. I've seen
electronic systems that had primary wiring from the AC input module that was
routed along grounded SELV circuits. I presume the same thing may happen
for non-grounded SELV circuits. (There are some EMC concerns with this
construction as well).
Gary
From: John Woodgate [mailto:j...@jmwa.demon.co.uk]
Sent: Monday, December 08, 2003 10:21 PM
To: emc-p...@majordomo.ieee.org
Subject: Re: How do I do this test?
I read in !emc-pstc that Rich Nute <ri...@sdd.hp.com> wrote (in
<200312082301.paa09...@epgc264.sdd.hp.com>) about 'How do I do this test?'
on Mon, 8 Dec 2003:
>
>
>
>Hi John:
>
>
>> For professional audio, it's common to design to Class II requirements
>> and then add an earthed screen in the mains or SMPS isolating
>> transformer, and a user option to connect signal common to the PEC or
>> not.
>
>This seems to bring us full circle, back to
>the original topic, that of Class I and Class
>II insulations in parallel.
>
>I would presume that professional audio uses
>Class II requirements between the mains and the
>earthed screen, i.e., the connection to the PEC
>is not a safegurd function?
It varies: the earthed screen has often been regarded as a safeguard, with
basic insulation to the primary circuit. More recently, the realization that
the screen must meet the fault current requirement may have changed the
thinking on this subject. The screen also has a functional role in reducing
mains-borne conducted interference.
>
>IEC 60335 3rd defines Class OI as a basic-
>insulated product with a functional earth
>terminal for optional, non-safeguard connection
>to earth.
Classes O and OI are not permitted now in almost all European countries.
>
>If the product is Class II, and is provided
>with a functional earth terminal for optional
>non-safeguard connection to earth, is it then
>a Class II-I? :-)
>
AFAIK, that term has not been used. Class II is not allowed to have any sort
of earth terminal by some safety standards.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
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