Hi Doug:
>From your message, I don’t know if your question is about the concept of
>“significant digits” or the standard itself. Perhaps I can answer both.
The number 300 can have one, two, or three significant digits. One, for sure.
If we multiply by the square root of 2 to get the peak, and if we follow the
significant digits rules, we can only say 400, not 420 or 424 without knowing
the significant digits in the value of 300. 0.300 x 103 would be three
significant units, in which case the peak value would be 424.
(Standards committees have little understanding or patience for significant
digits.)
In the standard, the 300 volts was chosen because, throughout the world, mains
voltages are either comfortably below or above that value, e.g., 220, 230, 240,
including tolerances. 240 +10% would be 264, which is comfortably below 300.
Working backwards, 420 divided by the square root of 2 is 296, still
comfortably above 250. This bit of discrepancy doesn’t get in the way of
determining the clearance for almost any of the world’s mains voltages,
including tolerances.
If you look at the “rms” values for Table K, they are:
50.2 rms for 71 peak (for 50 rms)
148 rms for 210 peak (for 150 rms)
296 rms for 420 peak (for 300 rms)
593 rms for 840 peak (for 600 rms)
Etc.
Maybe you can round to the nearest 10 when multiplying by square root of 2.
But, it may not always work since committee judgement was used to arrive at the
values. You may have to resort to something like “If more than 150 and not
more than 300, then Table K.”
Table L only applies when the “working voltage” exceeds the mains voltage.
This was to account for the switching voltage of a SMPS exceeding the mains
voltage.
Back in the days of IEC 60950-1, 2nd, little attention was paid to IEC 60664-1.
One of my colleagues said: “Unlike all other engineering disciplines, safety
engineering is consensus driven, not research driven. It is almost devoid of
physical laws to guide its practitioners.”
Best regards,
Rich
From: Doug Powell [mailto:doug...@gmail.com]
Sent: Wednesday, March 29, 2017 5:49 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] IEC/ANSI/UL/CSA 60950-1 ed.2 Mains Vrms vs Vpeak
All,
I am reviewing the requirements of "2.10.3.3 Clearances in primary circuits"
and it seems interesting to me that the math is off by just a little bit. In
particular "For an AC MAINS SUPPLY not exceeding 300 V r.m.s. (420 V peak)".
When in actuality the calculated peak of 300 Vrms is 424 Vpeak. Using three
significant digits instead of two. This value is important to spacings
determination in that it invokes using tables 2K plus 2L instead of table 2K
alone. I checked the Edition 2 of IEC, ANSI/UL and CSA standards and they all
have the same statement. Edition 1 of IEC 60950-1 does not include this value
within parenthesis which tells me it was probably added as a clarification by
the committee in Edition 2.
I am one to build excel-based spacings calculators and this has changed the
math somewhat, rounding up to the nearest 10 Volts
=IF(10*ROUNDUP(Vrms*SQRT(2)/10,0)>420,"Tables 2K + 2L","Table 2K")
instead of using
=IF(Vrms*SQRT(2))>424,"Tables 2K & 2L","Table 2K")
(Note: use of =MROUND() could potentially round down and not up)
In several other sections of the standard another pair of voltages appear
together in at least 8 locations and are rendered "42,4 V peak, or 60 V d.c";
which is correct for three significant digits. I know the consequences are
probably minimal and it has raised my curiosity as to why this happened. Was
anyone in this forum present during this part of the revision discussions and
can shed some light?
thanks Doug
--
Douglas E Powell
doug...@gmail.com <mailto:doug...@gmail.com>
http://www.linkedin.com/in/dougp01
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