Re: Meaning of earthed low voltage secondary circuits (SELV)

[georgea]

Dear Huu Dung,

Allow me to express my understanding of the standards, and various
practices.

Section 2.3.1 requires that SELV circuits must be safe to touch, even
after a single fault in basic insulation.  As a result, SELV circuits
must be either double insulated from hazardous votages, as in Class II,
or separated by basic insulation and earth grounded, as in Class I.
These are shown as R1 and B2 respectively in Figure 5A of IEC 60950.

Sections 2.3.3.1 thru 2.3.3.3 discuss three allowable "methods" for
protecting an operator from electric shock from SELV circuits.

However, because many countries do not guarantee reliable earth
grounded outlets, we prefer to double insulate both primary to SELV
and primary to accessible conductive parts.

The secondary of Class II circuits do not truly "float", i.e. drift to
any level voltage.  The secondary circuits are coupled to the primary
via "parasitic" or "virtual" resistance, capacitance, and inductance.
Typically the output of a Class II device will measure about half the
mains voltage when a high impedance voltmeter is used. However, since
the impedance back to the primary is so high, the measured voltage
will disappear if touched, or a low impedance meter is used.

Regards,

George Alspaugh
Lexmark International Inc




huu.dung.dinh%dnv@interlock.lexmark.com on 12/21/2000 02:52:30 AM

Please respond to huu.dung.dinh%dnv@interlock.lexmark.com

To:   emc-pstc%ieee@interlock.lexmark.com
cc:(bcc: George Alspaugh/Lex/Lexmark)
Subject:  Meaning of earthed low voltage secondary circuits (SELV)



Dear All,
Many thanks for this year and all good wishes for a Merry Christmas and a
Happy New Year to all of you and your families.

I would like to ask you a question and will thank for any comments.
Low voltage secondary circuits of class II equipment usually float and have
to be insulated from primary mains by reinforced insulation.
Low voltage secondary circuits of class I equipment most of the time are
directly earthed or referred to earth by some components. My understanding
is that only basic insulation and good earth protection are needed between
these circuits and primary mains if reinforced insulation is provided in the
transformer.
Am I right ?

Best regards to all of you,
Huu Dung Dinh
DET NORSKE VERITAS, RN 413
Testing and Certification of Electrical Equipment
Division Nordic Countries

*+47 67 57 95 91
FAX  +47 67 57 89 60
*Veritasveien 1, N-1322 Høvik, Norway
*huu.dung.d...@dnv.com <mailto:huu.dung.d...@dnv.com>
Web  http://www.dnv.com/eltestlab/ <http://www.dnv.com/eltestlab/>

Meaning of earthed low voltage secondary circuits (SELV)

[Dinh, Huu Dung]

Dear All,
Many thanks for this year and all good wishes for a Merry Christmas and a
Happy New Year to all of you and your families.

I would like to ask you a question and will thank for any comments.
Low voltage secondary circuits of class II equipment usually float and have
to be insulated from primary mains by reinforced insulation.
Low voltage secondary circuits of class I equipment most of the time are
directly earthed or referred to earth by some components. My understanding
is that only basic insulation and good earth protection are needed between
these circuits and primary mains if reinforced insulation is provided in the
transformer.
Am I right ?

Best regards to all of you,
Huu Dung Dinh
DET NORSKE VERITAS, RN 413
Testing and Certification of Electrical Equipment
Division Nordic Countries

*   +47 67 57 95 91
FAX +47 67 57 89 60
*   Veritasveien 1, N-1322 Høvik, Norway
*   huu.dung.d...@dnv.com  
Web http://www.dnv.com/eltestlab/  



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