I don't have much to say about flicker, but maybe I can reduce your skepticism about harmonics. I have had three experiences regarding failures caused by third harmonic currents.
The first was with mercury vapor lighting fixtures. An array of 1000w 277v single phase ballasts in hibay lighting for warehouses were connected as a delta load on a three phase 277/480 volt supply. The third harmonic current circulating in the loads overheated the ballasts to the point of boiling the potting compound. The ballast (about ten pounds) would burst the aluminum container and fall from the ceiling, which caught people's attention. The solution was to replace them with a better ballast and fuse each fixture. I participated in this investigation in 1968 as a co-op student. This also resulted in electrical code changes. The second incident (1980s) was the burnout of a three phase distribution transformer from third harmonic currents circulating in the delta primary (where it is hidden from overcurrent protection). This was at a manufacturing location of a computer company where the loads were racks of computers preparing for shipment. The solution was to regularly audit load currents at the facility and assure sufficient margin to protect transformers and neutrals. The third incident (1980s) was the overheating of neutral conductors in office modular partitions. Partitions carried two of three phases of 120/208v supply to single phase outlets for desktop computers. The combination of the unbalanced neutral current (which normally in a balanced system without harmonics would equal the phase current that would typically go back the third phase) plus the third harmonic current (which adds from each phase in the neutral) overheated the neutral conductor (which has no overcurrent protection). Insulation damage resulted. The solution was to work with the partition manufacturers to design a three phase model so the neutral (when single phase loads were reasonably balanced across three phases) carried mostly just harmonic currents. This was all in the days before standards for harmonics were established. Desktop minicomputers at that time also consumed much more power. A solution of using four pole breakers with overcurrent in the neutral and series overcurrent in delta loads would have also solved the electrical distribution risks, but the solution chosen worldwide was to regulate harmonic content of load currents. So, as you see, there were some real risks and failures in harmonics, but the problem does not appear regularly today due to standards compliance. Bob Johnson -----Original Message----- From: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org] On Behalf Of Pettit, Ghery Sent: Tuesday, January 22, 2002 12:25 PM To: 'cherryclo...@aol.com'; Pettit, Ghery Cc: emc-p...@majordomo.ieee.org Subject: RE: EN 61000-3-3 compliant heater controller Keith, Many times the neutral cable problems result from using a smaller gauge wire for the neutral in the mistaken assumption that the neutral doesn't carry any significant current (true if the 3 phase load is balanced - when did you last see that?). Harmonics may make that problem worse, but the fundamental frequency current can be a problem, as well. The solution, used by many building designers, is to use the same size wire for the neutral as for the phase conductors. Do you expect me to believe that personal computers cause significant harmonics problems? There is no evidence of this in the U.S. If there is in Europe, then you are admitting that your power distribution system design is deficient. Lousy design on the part of the power providers is fixed by saddling the manufacturers of the load with the full expense of the solution? Sorry. I don't buy that approach. A facilities solution is usually more economical than fixing the 'problem' at each individual load. Quoting a few consultants is nice, but doesn't prove a case. I remain a total skeptic on this matter. IEC 61000-3-2 needs (and is getting) a complete overhaul. IEC 61000-3-3 should be withdrawn. Flicker is real, no doubt, but this issue gets more attention than it deserves in the home environment. Ghery Pettit
