I have used the same figures for stability. I would add that for
stability purposes the change in ambient is noted and the measured readings
are adjusted accordingly, up or down. The anticipation is that the ambient
isn't going to change all that much because you are working is a normal
office environment when making these measurements.
If you have something pretty cyclic you might want to refer to the
section on thermal averaging Annex B section B3, if you have the proposed UL
60950 document. I don't know what it is in the 1950 third edition. When I
changed companies I chose to start operating out of the newest standard.
(Not a lot of difference except I feel it is written much better.
As far as the claim that a rise/lowering in ambient is concerned in my
comments above, again I can only give you parenthetical evidence from the
standard. That would be in section 1.4.12 Temperature measurement
conditions. They are allowing for tests at ambient are not exactly the same
as the manufacturers specified ambient. In this section they happily add or
subtract temperatures linearly.
This has just never been a big problem with any of the agencies that I
have dealt with over the years. It should be stated explicitly but it's not.
Gary
-----Original Message-----
From: Kazimier Gawrzyjal [mailto:[email protected]]
Sent: Wednesday, June 28, 2000 10:21 AM
To: 'Dan Mitchell'; [email protected]
Subject: RE: UL1950/UL2601 Thermals
Hi Dan,
The guideline you've mentioned below is one that I've heard from UL and CSA
in the past and have applied within the realm of engineering
judgement/common sense as much as possible without much issue. It is
reasonable to expect that ambient temperature may fluctuate somewhat in pace
with the test lab conditions or EUT design and operation ( EUT may cycle in
operation). The objective of the temperature test is to determine the
operating temperatures of critical and accessible components of the EUT
stabilized over a period of time under conditions that are considered
representative of the normal operation for the EUT to ensure that fire or
shock hazard does not exist.
With that in mind, if your temperatures vary a bit but are cyclical in
nature and offer much margin at the high points, what's the safety issue?
On the other side of the fence, if your temps. are on the edge of "failure"
and the stability question is the defining factor, you'd probably want to
revisit the design anyhow, to alleviate safety and reliability concerns.
To date, the safety engineers in both agencies that I've worked with have
applied engineering judgement in terms of "thermal stability" since the
definition can easily vary with the EUT design, operation and of course with
the test being applied....another good reason for tracking the external
ambient.
Have you requested a written definition from whichever safety agency you're
working with?
My 2 cents and not those of my current employer.
Kaz Gawrzyjal, P. Eng.
Product Safety Engineer
----------------------------------------------
Nortel Networks-Wireless Solutions
Wireless Development Centre
2924 11 Street NE
Calgary, Alberta
Canada, T2E 7L7
tel: 403-232-4805 (ESN 765)
fax: 403-232-4813 (ESN 765)
e-mail: [email protected]
e-,ail: [email protected]
-----Original Message-----
From: Dan Mitchell [ mailto:[email protected]
<mailto:[email protected]> ]
Sent: Wednesday, June 28, 2000 9:32 AM
To: [email protected]
Subject: UL1950/UL2601 Thermals
In UL2601 Clause 42.3.3) Duty Cycle - for Equipment for Continuous
operation it lists 2 ways to conclude the test a) temperature of the
windings stabilize and do not increase by more than 2 deg. C in 1 hr, or b)
2.5 hr, which ever is shorter.
UL1950 only states that; "for continuous operation, until steady conditions
are established". I haven't been able to establish what is meant by
"Steady Conditions". I was told once by a rep. of a large safety company
that it meant "no more than a 1 deg. C rise in 15 minute period". However,
since I can't find this written in the standard, I am a bit skeptical.
If anybody has a good definition of "Steady Conditions" and can point it
out to me in UL1950 or in the PAGs, I would appreciate it.
Daniel W. Mitchell
Product Safety Engineer
Condor DC Power Supplies, Inc.
P: (805) 486-4565 x323
F: (805) 483-4307
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