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From: emc-pstc@ieee.org on behalf of Steve Thomas -
T1 Technology ]
Sent: Tuesday, November 28, 2006 7:25 AM
To:
Subject: Creepage requirements for PCB's in EN 60950:2006
Hello Group, Just reviewing EN 60950:2006 table
2N…. Am I right in thinking that the creepage distances for non-coated
PCB’s has been relaxed?
Ie Reinforced creepage distance is now 2mm for a
working voltage of 250V, material group IIIa? Whereas before it was 5mm
(ref Table 2L EN 60950-1:2001)?
Or am I missing something, such as additional
requirements for ‘Printed boards’?
Thanks in advance,
Steve Thomas
Design manager
T1 Technology (UK)
From: emc-pstc@ieee.org on behalf of Joe Randolph
]
Sent: Wednesday, December 13, 2006 9:58 PM
To: Steve Thomas - T1 Technology;
Subject: Re: Creepage requirements for PCB's in EN 60950:2006
Fellow List Members:
Steve's question prompted me to purchase the 2006 edition of EN
60950-1 so I could look at the requirements in question. I must admit
that I am perplexed by what I see.
I am hoping that some of the safety experts on this list can explain
the rationale behind the changes.
In all the previous editions of EN 60950, the creepage distance for
reinforced insulation, 250 VRMS working voltage, in a typical ITE
product was 5 mm. For example, in Table 2L of the 2001 edition of EN
60950-1, if you use a working voltage of 250 VRMS, pollution degree 2,
and material group IIIa, the entry for basic/supplementary insulation is
2.5 mm, which is then doubled to 5.0 mm for reinforced insulation. This
requirement is the same in the 2000 and 1992 editions of EN 60950 that I
have on file.
In the 2006 edition of EN 60950-1, Table 2N has replaced Table 2L of
the 2001 edition. In Table 2N of the 2006 edition, two new columns have
been added for "Printed Boards." In these new columns, the minimum
creepage distance for basic/supplementary insulation, working voltage
250 VRMS, pollution degree 2, material group IIIa is only 1.0 mm, which
doubles to 2.0 mm for reinforced insulation.
In other words, for the exact same type of reinforced insulation, the
minimum creepage distance is 2.0 mm on a printed board and 5.0 mm
everywhere else. This makes no sense to me.
I note that the practical limitation for the printed board
application is actually 4.0 mm, since the applicable creepage distance
can not be less than the applicable clearance distance. All the more
reason to question why these new columns have been added to table 2N in
the 2006 edition.
I have looked to see whether the term "printed board" is defined
somewhere as having a minimum coating or some other special conditions,
but I can not find anything. It appears that for some reason, Table 2N
in the 2006 edition treats bare printed boards very differently from
other materials, even if their tracking index is the same.
For the time being I am inclined to simply ignore these new columns
because they make no sense to me. However, I would very much appreciate
an explanation of why these changes were made and what the rationale
was.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
From: emc-pstc@IEEE.ORG on behalf of Tarver, Peter
]
Sent: Thursday, December 14, 2006 12:21 PM
To:
Subject: RE: Creepage requirements for PCB's in EN 60950:2006
Joe and Steve -
I was a bit surprised by the relaxations for Creepage Distances, when
I first saw the IEC document, but the Creepage Distance table has also
been expanded beyond just adding two columns for printed boards. I don't
know precisely what the rationale was for the relaxations, but some
things might be inferred or guessed at through closer study of old Table
2L and the new table 2N. Some observations:
1) The two new columns in Table 2N do not include Material Group IIIb.
If you refer to the major column heading, "Other materials," you'll see
IIIb there, and for PD2, IIIb, 250V, the 2.5 mm FI/BI/SI value remains.
2) Table 2L had no Creepage Distances indicated for PD1, but Table 2N
does.
3) Table 2L had no voltages less than 50 V nor higher than 1 kV, but
Table 2N does.
4) The "resolution" for voltage demarcations is much higher in Table
2N than in Table 2L.
It seems reasonable to conclude that:
a) the writers were giving additional consideration for material
groups and PD, including separating MG IIIa and MG IIIb for printed
boards.
b) a revisit of the IEC 60664 standards and their revisions more
recent than the textual appearance of Table 2L (it's survived relatively
unchanged through many 60950 revision cycles) probably came into play.
(I don't have copies of the latter revisions of the 60664-x standards,
so I don't really know.)
c) technologies in ITE and telecom equipment are incorporating both
lower and higher voltages than once was the case and the expansion of
Table 2N reduces the need to refer to IEC 60664-x, when outside the
scope of Table 2L.
I'm confident that for the lower voltage ranges, industry lobbied to
have a resolution conducive to the level in circuit integration and
component pin densities in modern integrated circuits, where voltages
are shrinking. Inclusion of the higher voltages made evaluations easier
all around for SMPS and the like.
d) historical data likely also came into play.
What I don't get is why, for printed boards, MGs I, II and IIIa are
suddenly lumped together. It makes some sense to allow this when a board
is suitably coated, but not across the board, but even when coated I
still would have expected some differences between MG treatments.
Regards,
Peter L. Tarver, PE
From: emc-pstc@ieee.org on behalf of Joe Randolph
]
Sent: Thursday, December 14, 2006 4:23 PM
To: Tarver, Peter;
Subject: RE: Creepage requirements for PCB's in EN 60950:2006
Hi Peter:
I was hoping that you would comment on this change. Thanks for your
thoughts.
One thing I'm still perplexed by is the fact that in the new columns
for printed boards, the applicable creepage distances are dramatically
less than the applicable clearance distances. In the example I cited in
my earlier posting, reinforced insulation for a working voltage of 250
VRMS, pollution degree 2, material group IIIa requires a minimum
creepage distance of only 2.0 mm for a printed board. However, the
applicable clearance distance in Table 2K is 4.0 mm, unchanged from the
earlier editions of EN 60950.
The text of clause 2.10.4.3 states that when the specified creepage
distance is less than the specified clearance distance, the minimum
clearance distance shall be applied as the minimum creepage distance.
This suggests that most (perhaps all) of the reduced distances in the
new columns for printed boards could not actually be used in a given
board design. Right now I can't envision a situation where the values in
the new columns in Table 2N could ever be used. Am I missing something?
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
http://www.randolph-telecom.com
From: emc-pstc@ieee.org on behalf of Ralph McDiarmid
]
Sent: Thursday, December 14, 2006 4:50 PM
To:
Subject: RE: Creepage requirements for PCB's in EN 60950:2006
The over-voltage category (sometimes "installation category")
generally determines the clearance distance. In circuits where the
transient-voltage levels are low, the clearances may be less than the
creepage, depending on the Working Voltage.
Ralph McDiarmid, AScT
Compliance Engineering Group
Xantrex Technology Inc.
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