Brian
Whilst I think I know what you may be getting at, the fact is that an output
considered to be SELV ("Safety Extra Low Voltage", after all) of a correctly
designed and built Class I device which is connected to to a correctly
designed and grounded (earthed) supply building system will be safe, unless
two faults (insulation + grounding) develop - otherwise how can it be called
"SELV"?
That, for years, has been the accepted case for IT equipment where - until
the now widespread use of laptops and other equipment with Class II PSUs -
almost all such equipment was, and is, Class I. Correctly installed and
used, this type of equipment is deemed adequately safe in terms of
leakage/touch currents in (al?)most all countries to (al?)most all codes -
otherwise we have all been living under a severe misconception for those
many years!
However, I do agree with you that a poorly designed Class I PSU, and/or one
connected to a supply system with imperfect or non-existant grounding, is
another matter. Nevertheless, correctly designed Class I equipment will not
generate leakage/touch currents in excess of the accepted limits (given that
the "accepted limits" may or may not be themselves adequately low in some
circumstances, because getting a "belt" from 3.5mA, or even 0.75mA, of
leakage current is NOT a pleasant experience!) unless it is deemed to be a
"high leakage current equipment" per 60950 (etc.).
Therefore, I think you would need to illustrate your statement with some
credible examples to back it up.
John Allen
(The UK one!)
From: [email protected] [mailto:[email protected]] On Behalf Of Brian
O'Connell
Sent: 16 November 2009 17:20
To: 'EMC-PSTC'
Subject: RE: A very basic safety question.
For most IT and AV stuff, Class III equipment has no requirement for
protection from electric shock. Any power connected to a Class III device
must be considered safe to contact.
A power source with Class I construction, with a SELV output will not
necessarily be considered safe to touch (leakage, inherently limited power,
etc). Local code requirements could also effect power source requirements.
Brian
From: [email protected] [mailto:[email protected]]On Behalf Of Piotr Galka
Sent: Saturday, November 14, 2009 2:05 AM
To: EMC-PSTC
Subject: A very basic safety question.
All,
Consider two power supplies, both AC230V to DC12V:
PS1 - with basic insulation and one of its output terminals grounded with
protective conductor, PS2 - with double or reinforced insulation and
floating output.
I understand that PS1 is CLASS I equipment, and PS2 is CLASS II equipment.
Now consider the 12V powered device connected to one of them. Is that device
CLASS III equipment in both cases?
Best Regards
Piotr Galka
MicroMade
Poland
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