As the appliance is a toaster, could the switch area have been contaminated by carbonized bread particles and/or burnt residue (raisins, glaze drippings)? This might account for the length of arcing, although you would think that good design would include a shield to keep such residue away from the contacts.
Rich, would you talk a little about reforming of the switch contacts by the molten metal? If I were to imagine that I was looking at the center of a contact (along the axis of the arc), the current should set up a circular magnetic pattern. So, are you saying that the molten metal is "swept" around in a circle until it cools to the solid state? And just for fun, how many microseconds might it take for the molten metal to solidify? Ed Price WB6WSN Chula Vista, CA USA -----Original Message----- From: Richard Nute [mailto:[email protected]] Sent: Thursday, October 01, 2015 11:12 AM To: [email protected] Subject: Re: [PSES] Arc flash to earthed parts Hi Scott: When a switch opens (at something other than the zero-crossing), an arc is created and molten metal of the contacts is likely. (In fact, some contact designs use the molten metal to re-form the contacts.) The molten metal is accelerated (at right angles to the current direction) by the magnetic field. What is unusual is the length of the arc -- 1 inch. And, that the arc was from the switch contacts to the earthed parts. The current through the arc to the earthing conductor can be sufficient to trip the RCD. One hypothesis is that the current in the contacts created an excessive amount of molten metal which allowed the arc to also go through the molten metal droplets to the earthed metal. This would explain both the metal on the earthed part and the arc. Excessive contact heating could be caused by contact resistance being high (but not too high). Based on your description, I would hypothesize that the switch contact resistance was high; the switch is worn out, or has inadequate ratings. I don't know of any standard that would address this issue. The situation is analyzed by science and engineering, and appropriate changes to the construction to prevent recurrence. Good luck and best regards, Rich > -----Original Message----- > From: Scott Xe [ <mailto:[email protected]> mailto:[email protected]] > Sent: Wednesday, September 30, 2015 11:22 PM > To: <mailto:[email protected]> [email protected] > Subject: [PSES] Arc flash to earthed parts > > Dear Members, > > I recently encountered a symptom on a toaster. When > turning off by pressing the quit button, there was an arc > flash from the mains switch contacts to earthed parts > where the separation greater than 1 inch apart in air > occasionally. The mains contacts have a sign of carbonisation on the > open/close contact points and > melted in a small area of the contact leaf. The switch > and the earthed parts are inside the plastic case without > the access by the users. It may trip the RCD during arc > flash. In addition, there was vaporised copper layer on > the earthed part. It did not have immediate danger to > the user and definitely not a good product. How should we categorise > this type of fault and is there any > reference standard for it? > > Thanks and regards, > > Scott > - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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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]>
