Re: [PSES] IEC/ANSI/UL/CSA 60950-1 ed.2 Mains Vrms vs Vpeak

Fri, 31 Mar 2017 23:06:24 -0700

"Unlike all other engineering disciplines, safety engineering is consensus driven, not research driven.  It is almost devoid of physical laws to guide its practitioners.”
 
Yikes! This sounds rather subjective and flies in the face of one of my arguments to design engineers who say these rules are just opinions and therfore invalid.  My answer to them usually is, while it is true that many requirements are a result of litigation and CYA, much of it is also based in real science.  I usually point to IEC 60664-1.  Now I am hearing that the 60950 committee did not use the document so much as I had originally thought.  

I'm not sure I'm happy with this idea. Maybe those design engineers have a point after all.


-- Doug

Douglas E Powell
doug...@gmail.com




From: ri...@ieee.org
Sent: March 31, 2017 5:11 PM
To: doug...@gmail.com; EMC-PSTC@LISTSERV.IEEE.ORG
Reply-to: ri...@ieee.org
Subject: RE: [PSES] IEC/ANSI/UL/CSA 60950-1 ed.2 Mains Vrms vs Vpeak

 

 

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

 

 

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