Brian, I think that my Email of 30/4/2003 should answer your questions. In regard to your second substantive para, you will see that it is not always required to test at the highest ambient temperature. An extract is below... Richard Hughes
Are you REQUIRED to conduct heating tests at the highest rated ambient temperature - in your example 30 degC? Well, that depends on whether your equipment has a temperature controlled fan or similar. If your equipment has no such fan or control mechanism then it is permitted to run the test at lab ambient and extrapolate, see 1.4.12.3. For such equipment therefore you will be able to run the tests just the way you seem to have done in the past. However, if your equipment does have such a such fan or control mechanism then it could/would be unfair to run the test at (say) 20 degC and then add 10 degC (or 10 K if you prefer) to your results before comparing the 'corrected' results against the limit values: after all, in such equipment the fan could possibly run twice as fast at 30 degC as at 20 degC. Hence the requirement was added in part to make the test fairer. It was also added because in the future it is expected that there will be a Part 22 that will cover Equipment Installed Outdoors - and such equipment will obviously work at temperatures in excess of 25 degC in the summer (well, in many parts of the world). For manufacturers running their own tests there can also be the. practical advantage of combining product safety tests with product functionality tests (i.e. making sure the equipment doesn't fail to function when run at its max/min rated temperatures). Why change from limits based on a temperature RISE to ABSOLUTE temperature? As indicated above, there are reasons why you may need to run tests at the maximum temperature specified by the manufacturer rather than at lab ambient and then convert. When non-safety engineers used IEC 60950 etc. it was always previously necessary to explain to them that the figures given were RISES above an ambient contained elsewhere in the standard rather than absolute figures. In addition, if the equipment was not designed for a maximum temperature of 25 degC but 32 degC or 40 deg C (to pick a couple of common examples) then it was also necessary to adjust the RISE figures by subtracting 7 K or 15 K (respectively for the examples given). What a complicated way of doing things! In addition, why specify the limits for Normal conditions as a RISE but those for Fault conditions (e.g. see Table B.1) as an ABSOLUTE temperature? All the best, Richard Hughes From: [email protected] [mailto:[email protected]] Sent: Monday, June 02, 2003 6:24 PM To: [email protected] Subject: 60950-1 temperatures Good People Clause 1.4.12.3 of this standard specifies that "..amount of heating or cooling is not designed to be dependent on ambient temperature..." does not seem relevant, because the standard requires you to find the worst-case test conditions for evaluation of the component. And because of the requirement for worst-case test conditions, why is the term Tamb - Tma, as used in the equation to the temperature limit criteria? As my test conditions must comply by testing at rated temperature, this term will always evaluate to zero. So I interpret the standard so that clause 1.4.12 is the only criteria required. What nuance or associated clause am I missing? TIA. luck, Brian This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
