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From: emc-pstc@IEEE.ORG on behalf of Zhang Guoqing
[zhangguoqing@huawei.com]
Sent: Sunday, March 04, 2007 11:03 PM
To: emc-pstc@IEEE.ORG
Cc: sltang@huawei.com
Subject: question on IEC/EN/UL60950-1 clause 1.6.4
Hi All,
I have a question-
Is "1.6.4 neutral conductor" required for AC equipment or DC equipment
or both?
"Line/neutral" is used in a.c. system only, "Line/Mid-point" is used in
both a.c. and d.c. system, is this correct?
Zhang Guoqing
-----
From: emc-pstc@IEEE.ORG on behalf of Richard Nute [rnute@san.rr.com]
Sent: Monday, March 05, 2007 11:02 AM
To: 'Zhang Guoqing'; emc-pstc@IEEE.ORG
Cc: sltang@huawei.com
Subject: RE: question on IEC/EN/UL60950-1 clause 1.6.4
Hello Zhang Guoqing:
While 1.6.4 refers specifically to the neutral
of an AC system, the principle applies to any
supply system, whether AC or DC.
For the purposes of safety, a neutral conductor
is presumed to be connected to earth at the source.
The principle is that, for the purpose of safety,
all supply conductors must be treated as "live"
conductors. That is, the design of the equipment
cannot use a current-carrying earthed conductor,
e.g., a neutral conductor, as a PE conductor.
For AC supply systems, the neutral and line
conductors can be reversed, and the equipment
will operate normally. If earthing is by means
of the neutral conductor, and if a polarity
reversal should occur in the installation, then
those parts of the equipment that are connected
to the neutral would then be live.
For DC supply systems, if the plus and minus
conductors are reversed, the equipment will not
work. Like the AC supply system, those parts
of the equipment connected to the DC line would
be live.
For these reasons, the DC supply conductors
within the equipment should be insulated from
earth just as they are for AC equipment.
Best regards,
Rich
-----
From: emc-pstc@IEEE.ORG on behalf of Tarver, Peter [peter.tarver@sanmina-sci.com]
Sent: Monday, March 05, 2007 11:35 AM
To: emc-pstc@IEEE.ORG
Subject: RE: question on IEC/EN/UL60950-1 clause 1.6.4
It is relevant to point out that, as Bob Johnson indicated, dc power
systems may sometimes be earthed at equipment. This leads to
complications in construction features, markings and installation
instructions (refer to CSA/UL 60950-1, Annex NAB), and requires that the
power system not be earthed elsewhere in the system, following the
single-point earthing model.
Thinking of the "neutral" as the "earthed supply conductor" tends to
help in understanding how (nearly) the same requirements apply to as and
dc power systems.
Regards,
Peter L. Tarver, PE
ptarver@ieee.org
-----
From: emc-pstc@IEEE.ORG on behalf of Robert Johnson [johnson@ITEsafety.com]
Sent: Monday, March 05, 2007 1:05 AM
To: Zhang Guoqing
Cc: emc-pstc@IEEE.ORG; sltang@HUAWEI.COM
Subject: Re: question on IEC/EN/UL60950-1 clause 1.6.4
Zhang Guoqing,
You are correct. The term neutral is associated with AC systems although
that is not clearly indicated in 1.6.4. A definition is probably in
order. It is described further in Annex V.
DC systems sometimes have one side of the supply earthed at or within
the product as mentioned in 3.2.1.2 and therefore not insulated from
earth.
Mid-point is not a commonly used or defined term but more of a
description.
Bob Johnson
ITE Safety <http://www.itesafety.com>
-----
From: emc-pstc@ieee.org on behalf of Robert Johnson [johnson@ITEsafety.com]
Sent: Monday, March 05, 2007 12:59 PM
To: owner-emc-pstc@listserv.ieee.org
Cc: emc-pstc@IEEE.ORG
Subject: Re: question on IEC/EN/UL60950-1 clause 1.6.4
I agree, insulating both DC conductors and the whole input circuit is
good practice and easy to do with switching supplies.
However it is clear the standard does not require it since 3.2.1.2
clearly admits to internal earthing which would violate 1.6.4 if it
applied to DC products.
Some DC products are polarity independent by way of a full wave bridge
on the input. This used to allow a universal AC/DC product as well but
the harmonics requirements now interfere with that.
Bob Johnson
ITE Safety <http://www.itesafety.com>
-----
From: emc-pstc@IEEE.ORG on behalf of Brian O'Connell [oconnellb@tamuracorp.com]
Sent: Monday, March 05, 2007 3:23 PM
To: emc-pstc@IEEE.ORG
Cc: sltang@huawei.com
Subject: RE: question on IEC/EN/UL60950-1 clause 1.6.4
And to emphasize Mr. Nute's point, there is a push to incorporate 350 to
400Vdc power as a standard "mains" for server farms and other IT centers
as part of the current (no pun intended) power-conservation initiatives.
luck,
Brian
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