Tim, You have a number of things to consider here:
Equipment of this power level is likely to be installed very close to the supply substation and hence be subject to quite large transients in the event of lightning strike. This is what IEC 61010 used to call Installation Category III (see also IEC 60664). Tufnol (correct spelling) is not necessarily the best of insulators as it comprises a phenolic resin-bonded fabric, which will char in the presence of an arc forming a carbon track that will reduce the effective clearance. The remaining air gap may subsequently fail to extinguish an arc. That said, with the correct creepage and clearance distances, you should not be able to establish an arc and hence should not have a problem. However, humidity could be a problem that meant that the Tufnol was not acting as an effective insulator. Is there any possibility that the Tufnol had absorbed moisture such that only the air gap was really effective? Knowing your line of business, could your installation be subject to direct lightning strike placing it in Installation Category IV? You would likely need much greater creepage and clearance distances for such an installation. Best regards Neil R. Barker CEng MIET FSEE MIEEE Manager Quality Engineering e2v technologies (uk) ltd 106 Waterhouse Lane Chelmsford Essex CM1 2QU UK Tel: (+44) 1245 453616 Fax: (+44) 1245 453571 Mob: (+44) 7801 723735 P Please consider the environment before printing this email. From: Haynes, Tim (SELEX) (UK Capability Green) [mailto:[email protected]] Sent: 27 February 2007 15:56 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 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 ______________________________________________________________________
