Brian, How is the furnace shut off? If you are using a contactor between the main line filter and the furnace, a phase line might open when it is carrying high current. The inductance of the line filter will try to keep this current flowing, generating a very-high kickback spike at the *output* of the line filter.
Or, since the contacts in the contactor are unlikely to open/close at exactly the same time, a common-mode choke in the line filter can act as a transformer putting noise on the open phase(s) if only 1 or 2 phases are connected to the load. Some years ago, Bill Kimmel and Daryl Gerke wrote about a case where a 3-phase product had a contactor between a line filter and the load, which generated horrendous Conducted Emissions noise every time the contactor opened or closed, because of this transformer action. The solution was to replace the common-mode choke with 3 separate chokes, one for each phase line. John Barnes KS4GL, PE, NCE, NCT, ESDC Eng, ESDC Tech, PSE, Master EMC Design Eng, SM IEEE (retired) Lexington, KY 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]>
