Minor comment: Having been the Secretary to a number of BSI committees many years ago, I sympathise with Ralph's last para as the Chairmen can be even worse than the other Members - they "know what they know" and it can take an awful lot of "effort" to "persuade" them that they need to "think again" - once had to refuse to publish a Chairman's version of a new standard because it did not meet the basic BSI guidelines for how a product standard should be written, until I had rewritten a large part of it to make it at least reasonably "testable" for the EMC-related requirements - a little (sometimes a lot!) of subject knowledge is required to set appropriate test and assessment requirements!
Unfortunately, nowadays, a large number of Committee Secretariats appear to employ non-SME staff to run their committees and so they can be lead by the nose by the Chairmen. FWIW, in the context of this thread, I used to use "commonsense" in deciding when the temperatures appeared to have stabilized - -taking into account where the probes were located - particularly when the observed temperatures were substantially below the relevant Insulation Class limits. John E Allen W. London, UK -----Original Message----- From: Ralph McDiarmid [mailto:[email protected]] Sent: 06 January 2017 17:20 To: [email protected] Subject: Re: [PSES] Thermal equilibrium - 10% rule For what it's worth, we measure and log temperatures at 1 minute intervals (sometimes quicker, depending on what is being tested) and graph each data set in Excel and look at the curves. When they go flat (even with some ripple), we call it stable. Good enough for all the agencies we work with. And I like the term steady-state much better, but what it is called isn't terribly important as I see it. The temperature can be stable (no further increase observable) over a period, even if it's oscillating slightly around a mean value. The mean value could be the average of the last 20 or so reading for instance. It requires some judgement. I empathize with John's experience about it taking "many tellings" on a committee to get something right. I have stopped just short of the Makita Khrushchev United Nations technique at times! Ralph McDiarmid Product Compliance Engineering Solar Business Schneider Electric -----Original Message----- From: Ken Javor [mailto:[email protected]] Sent: Friday, January 06, 2017 8:21 AM To: [email protected] Subject: Re: [PSES] Thermal equilibrium - 10% rule I understand that if the system is poorly designed there can be catastrophic results, but the test is written assuming the equipment temperature does equilibrate, right? Ken Javor Phone: (256) 650-5261 > From: John Barnes <[email protected]> > Reply-To: <[email protected]> > Date: Fri, 06 Jan 2017 00:06:24 -0500 > To: Ken Javor <[email protected]> > Cc: <[email protected]> > Subject: Re: [PSES] Thermal equilibrium - 10% rule > > Ken, > Most metals have a positive Temperature Coefficient of Resistance (TCR). > As they get hotter, their resistivity-- and the resistance of a given > conductor-- increases. If you have a current source (or a voltage > source with a large series impedance) driving current through a small > cross-section conductor with poor heat sinking for a long time, the > conductor will go into thermal runaway-- and eventually melt in two-- > when the current through it exceeds a certain critical value, > depending on the ambient temperature. > > A. J. Rainal wrote two papers about this effect on printed circuit > boards (PCB's) about 40 years ago: > * Rainal, A. J., "Current-Carrying Capacity of Fine-Line Printed > Conductors," The Bell System Technical Journal, vol. 60 no. 7, pp. > 1375-1388, September 1981. > * Rainal, A. J., "Temperature Rise at a Constriction in a > Current-Carrying Printed Conductor," The Bell System Technical > Journal, vol. 55 no. 2, pp. 233-269, February 1976. > > Non-resettable fuses use this principle, which is specified by the > I^2t curves in their datasheets. > > You may also want to read up on Preece's Law (Fuse Equations) and > Onderdonk's Fuse Equation. > > > > John Barnes KS4GL (retired) > Lexington, Kentucky > http://www.dbicorporation.com/ > - ---------------------------------------------------------------- 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]> ______________________________________________________________________ This email has been scanned by the Symantec Email Security.cloud service. ______________________________________________________________________ - ---------------------------------------------------------------- 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]> - ---------------------------------------------------------------- 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]>
