Hi Don:
Questions: 1. What is the physical mechanism that causes the arc with the faster pulse rate but not with the slower rate (or 60 Hz AC)? The phenomenon is nicely explained in IEC 60664-4, Edition 1. Here is an excerpt from Clause 3, Clearances: "Up to now, the effect of the ions (which are usually positive) which are generated during inception of breakdown has not been considered. These ions are generated at the crest of the sine-wave and there is usually enough time for them to travel to the electrodes during the remaining part of that halfwave. However, in large clearances or at high frequency, the polarity may be reversed before the ions have been extracted from the clearance. This will result in a distortion of the electrostatic field and will reduce the breakdown voltage. The average velocity of the ions is approximately 6 ´ 102 m/s [1]*. At 50 Hz, the time interval between the crest and the zero crossing of the sine-wave is 5 ms, resulting in the ions moving approximately 300 cm. Therefore, at power frequency, this aspect will only be relevant for very large clearances. However, if the frequency is increased to the kHz range, this phenomenon will also be relevant for small clearances." 2. It appears that the creepage (and clearance) distance that we used (based on UL standards for pollution degree 2) are not adequate for 100 kHz FTB. Are there any spacing guidelines available that will allow us to pass the FTB test reliably? Yes. See IEC 60664-4, Edition 1, Insulation coordination for equipment within low-voltage systems – Part 4: Considerations of high-frequency voltage stress – The voltages you quote suggest the shape of the electric field is inhomogeneous, meaning the conductors tend to be pointed rather than spherical. A homogeneous field (sphere-to-sphere) has significantly greater electric strength than does an inhomogeneous field. See IEC 60664-1, Table A.1 for a comparison of clearance distances for homogeneous and inhomogeneous fields. Best regards, Richard Nute Product Safety Consultant San Diego - 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 ______________________________________________________________________
