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 <emc-p...@ieee.org> 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 <sdoug...@ieee.org> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
