Regarding the cause, consider arc tracking. I wouldn't count on arcs self extinguishing as the more current that is available the harder it will be to quench the arc. Arcs can sustain themselves in idea conditions across long distances. From a table on page 542 of the Ignition Handbook by Babrauskas at 230V and 400A an arc (between copper electrodes) may be as long as about 100mm. The clearance tables only take voltage into account, so they can only consider preventing arcs from forming.
Gary From: [email protected] [mailto:[email protected]] On Behalf Of Haynes, Tim (SELEX) (UK Capability Green) Sent: Tuesday, February 27, 2007 7:56 AM To: emc-pstc Subject: Creepy Clearance distances.... Hi All, I have a problem (not that one!) A piece of high current 230V 400A apparatus has failed with an obvious breakdown to ground that has resulted in an arc forming and burning through 2mm steel before the current caused the over current protection (400A MCB) to operate. Separation distance from conductor to earth is 10mm - part air, part Tufnell bush (spelling?). The Tufnell is burnt and the hole through the 2mm steel is right at the edge of the Tufnell bush. The only way I can see this failing, is to have had a large (lightning) transient on the supply causing a flashover that did not extinguish. (Comments on the above would be appreciated) However, the real question is - in the clearance distances (as from creepage and clearance) are they set so that when the over voltage (that causes breakdown) is removed the arc self extinguishes? Regards Tim 6239 desk A1S77 P Please consider the environment before printing this email. ******************************************************************** This email and any attachments are confidential to the intended recipient and may also be privileged. If you are not the intended recipient please delete it from your system and notify the sender. You should not copy it or use it for any purpose nor disclose or distribute its contents to any other person. ******************************************************************** - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html 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] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html 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] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
