Whatever the reason, may have to go back 100 years. Go to books.google and read "Practical testing of electrical machines" by Leonard Oulto. Remember that the periods for some test sequences are similar. Same time period also found in other standards for some types of mold stress-relief tests, min pre-conditioning, and other non-electrical stuff.
So your theory of the work-day period seems more than coincidental. But we still need the insight from those older and wiser as to why this time period seems to be common. Note that most of the transformer standards have a (convoluted) test flow where the 7 hour over-load test is just the 'gateway' for other longer-term stuff. Brian From: Doug Powell [mailto:[email protected]] Sent: Monday, February 02, 2015 4:35 PM To: [email protected] Subject: [PSES] Historical question, 7 hours All, Today I was reviewing an output overload requirement and once again there is that ubiquitous number of "7 hours" in the test duration. I have seen this in a wide variety of tests such as power supply output overloads and short circuits, locked rotor tests, battery reversal tests and so on. Of course, there are many other tests with different durations, but this #7 seems to be very common. For a number of years I have wondered what is the magic behind this number and so today I am asking if anyone actually knows the history behind this choice. My suspicion is that this number is based the need of a 1/2 hour to set up the test and a 1/2 hour to record the results, 7 hours to stabilize the results, making for an 8 hour day. Apparently, back in the mists of time, someone came up with this scheme and thereafter everyone else copied it (plagiarism being the sincerest form of flattery). Could it really be as simple as that or is there no other scientific basis to prove all tests lasting at more than 7 hours will not fail... say 7 hours and 5 minutes? On a side note, I have worked with products having enough thermal capacity that one thermal time constant is on the order of hours. On some of those products, thermal stability is not achievable in less than 10 hours. Thanks, :Doug Douglas E Powell [email protected] http://www.linkedin.com/in/dougp01 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
