Rich: As always I appreciated your eloquent explanation of the concept of the creapage requirement. Unfortunately experience has taught me long before your email.
My question was based on the fact the standard does not SEEM to address the larger creapage distance >1000 volts. My reasoning could only conclude those writing the standard concluded the clearance issue would be just as critical as creapage >1000 volts. My recommendations have always ignored the minimum REQUIREMENT in this area because my nose recalls too many less then conservative designs burning up. I also recall an incident way back when I was wet behind the ears and still servicing TV (I saw the first PCB show up in TVs, How old I feel now). I opened the newspaper and saw a home of a customer burned due to the TV. This home was one I had used the vacuum cleaner in their TV every time I visited. Apparently I was too successful at my last visit and it was a long time since I had been there, no cleaning and I could picture the creapage disaster that happened. I was never absolutely sure of this since I was not a part of the post mortem. I seem to remember some one saying `All safety standards come from bad experiences.' or something like that. Right Rich? As always I enjoyed your detailed understanding. Best regards, Terry >>> Rich Nute <[email protected]> 07/31/01 01:59PM >>> Hi Terry: > I thought the lack of creapage spec. at >1000 V is that the dielectric > strength of air would be less then the insulating material and surface > accumulated contaminates. As a result the clearance distance would be the > first to break down >1000 Volts. In general, the electric strength of air is orders of magnitude less than that of solid insulation. The distance between conductors on the surface of an insulator (e.g., a printed wiring board), must be based on the electric strength of air, not the solid insulation. In some applications, the surface of a solid insulation is subject to deposition of an unknown foreign matter (referred to in the standards as a pollution). Think of this "pollution" as a bread- crumb trail between the two conductors, that is, small pieces of matter separated by air. Consider the worst-case where the "crumbs" are metallic. These pieces of metal short out some of the air, thus redistributing the electric field (equipotential lines). At some point in the accumulation of foreign matter, the electric field between two adjacent "crumbs" becomes so great as to break down the air between the two crumbs. (An alternative theory is that the crumbs themselves dissipate power, glow, and change into a gas.) This micro-arc (or glowing) has a very high temperature, in the thousands of degrees C. While the energy is very small, the thermal energy can do microscopic damage to the surface of the solid insulation. For organic insulators, the damaged surface degrades to a microscopic carbon dot. Carbon, being the stuff resistors are made of, contributes to further redistribution of the electric field. And the process continues. Over a long period of time, a "tree" of carbon paths will form on the surface of the solid insulation. Eventually, the resistance tree will connect the two conductors, and a continuous leakage current will result. The resistive path dissipates power in the form of heat. This creates still more carbon, reducing the value of the resistance, and the leakage current goes up. And the power (and heat) disspated in the carbon path goes up. The process continues until a final catastrophic event destroys the solid insulation (and, hopefully, causes the circuit protection to operate). According to the researchers, this surface- insulation failure mechanism is mainly due to the working voltage across the insulation. (On the other hand, the through-insulation failure of solid and air insulation is mainly due to overvoltages, not the working voltage.) The values of distance along the surface of solid insulation (creepage distance) are based on working voltage. The values of distance through solid insulation and of distance through air insulation are based on expected overvoltages. The values of distance along the surface of solid insulation are not related to the values of distance for air insulation. As a general rule, the values for creepage distance exceed the value for clearance. When both are subjected to an overvoltage test, the clearance usually will break down rather than the creepage distance. However, this is not the intent of the requirements (because the clearance distances are minimums and could be much larger than the creepage distances). I'm afraid I cannot comment as to why there are no values for creepage above 1000 V rms. Best regards, Rich ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.rcic.com/ click on "Virtual Conference Hall," ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.rcic.com/ click on "Virtual Conference Hall,"
