In the late 1980s, HP Labs was having wiring failures and fires in the cubicle wiring. Someone visited from MIT Project Athena and made a side comment about their wiring problems with computers at every desk. For a while, HP Labs had extension cords strung everywhere to reduce the demand on the wiring. Those went away when Steelcase replaced all the cubicle wiring with stuff that could handle the triplen current.
wunder K6WRU Walter Underwood CM87wj http://observer.wunderwood.org/ (my blog) > On Mar 4, 2020, at 10:44 PM, Jim Brown <[email protected]> wrote: > > On 3/4/2020 10:15 PM, Jim Brown wrote: >> Power factor is a quantity associated with mains power (and similar power >> distribution systems). In the old days (when I went through EE), it was, >> indeed, related to the phase angle. > > I meant to add that power factor now includes the triplen harmonics resulting > from non-sinusoidal current with power systems that are mostly electronic > loads. Current flows mostly in short peaks at the positive and negative peaks > of the mains waveform to charge the filter capacitors in the power supply; as > a result, the voltage waveform is rounded off and the current is rich in > harmonics of the mains frequency, 50 or 60 Hz. > > Nearly all power distribution uses 3-phase wiring, and "triplen" harmonics > (those whose number is divisible by 3) add in the neutral, no matter how well > balanced the load between phases. This turned into a major problem roughly 50 > years ago, when a very high fraction of current in these systems was > delivered to electronic loads, everything from fluorescent lights to > computers to anything with an AC to DC power supply. The most dangerous side > effect is that triplen currents in the neutral of 3-phase systems can be > almost twice the current in the phases, which can cause destructive failures > in older systems, where the neutral used conductors and hardware rated for > half the current in the phases! The movie "The Flaming Inferno" is based on > the true story of a massive fire in a high rise that was started by that > neutral current, spread through the building in vertical "riser" spaces, and > that was fueled by the insulation on the cables themselves! Sometime in the > last century, a major TV station almost went off the air because excessive heating of their power distribution was close to catching on fire! I attended an SBE (Society of Broadcast Engineers) meeting led by the consultant who worked with the station engineers on the problem. > > MAJOR changes were made to standards for mains wiring after this event, > including the rating of transformers and neutral hardware to handle the peak > currents and the harmonic content, conduit requirements for the wiring, and > requirements for insulation on the wiring that does not contribute to flame > spread and that does not produce toxic fumes. > > BTW -- the triplen harmonics also appear on equipment ground, and are the > reason why we mostly hear power line noise in the audio as "buzz" (triplen > harmonics of 50/60 Hz) rather than "hum" (pure 50/60 Hz). > > There's a not-too-technical discussion of all of this written for hams in > http://k9yc.com/GroundingAndAudio.pdf > and a more detailed one written for sound and video professionals in > http://k9yc.com/SurgeXPowerGround.pdf > > 73, Jim K9YC > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [email protected] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
