Dan - The requirement is to avoid exceeding the approved temperature ratings of the various insulation systems within the product. The necessary test time "is what it is" in order to determine that the limits have not been exceeded. It may be 15 hours - I've certainly evaluated some systems that required nearly a day to stabilize. Some systems have significant thermal impedances between various sub-modules, and these require time to stabilize. Small systems (e.g., modems) tend to stabilize quickly, whereas large cabinetized products, with subcompartments and significant thermal impedances, require longer. If your product line is similar to the latter, then longer thermal tests are just a normal part of the business.
Regards, Mark Gill, P.E. EMC/Safety/NEBS Design Nortel Networks - RTP, NC, USA > -----Original Message----- > From: Dan Mitchell [SMTP:[email protected]] > Sent: Wednesday, June 28, 2000 6:52 PM > To: Tarver, Peter [SC1:4N02:EXCH] > Cc: [email protected] > Subject: RE: UL1950/UL2601 Thermals > > > > > Fine and dandy, but when do you draw the line. If you have a product that > slowly increases at less than the 1 deg C in 15 minutes after say 2 1/2 > hours, do you keep on testing it say 5 hrs, or 10 hrs, 15 hrs? This > could > cause excessively long test times if you are a very zealous person who > follows the letter of the standard (because there is no time limit, or > temp > vs. time limit). > > Daniel Mitchell > Product Safety Engineer > Condor DC Power Supplies, Inc. > > > > > > > "Peter Tarver" <[email protected]> on 06/28/2000 01:15:16 PM > > To: [email protected] > cc: (bcc: Dan Mitchell/CondorDC) > > Subject: RE: UL1950/UL2601 Thermals > > > > > That's really just a rule of thumb. A working premise that provides > reasonable results in a relatively short time. As Kaz mentioned, it's not > uncommon for some circuits/parts to exhibit a cycling of temperature, > either > by cyclical loads presented to them or some form of (generic) limiting > circuitry. > > Even using that rule of thumb, it doesn't account for minor increases in > ambient temperature over the relevant time frame. One might find that > some > circuits/parts continue to increase in temperature, while others in the > same > product have reached thermal equilibrium (within the somewhat loose > definition that applies here). > > Best answer: use engineering judgment. If you clearly have increasing > temperatures that aren't resulting from unintentional outside influences, > the test is not completed. > > Regards, > > Peter L. Tarver, PE > Homologation Engineering > Nortel Networks > [email protected] > > > -----Original Message----- > From: Dan Mitchell [mailto:[email protected]] > > > 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 > > > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Jim Bacher: [email protected] > Michael Garretson: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > > > << File: Internet HTML >>
