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]
