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]] 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 ------------------------------------------- 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]
