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Last week my compressor kicked on at a job and I saw a large blue arc in a circular pattern around the extension cord and compressor plug. I knew not to grab the cords and quickly turned the compressor off. Both the extension cord and plug were melted together. I was using a heavy duty (10 gauge) extension cord about 15 ft. long. It didn’t appear that any breakers blew although we stopped using both the compressor and the extension and also stopped using that outdoor outlet. I’m replacing the cord on the compressor and tossed the extension. Any explanations as to what happenned?
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Tell us more. What kind of job site is it? What's between that outdoor outlet and the utility? Is it an industrial site where somebody elsewhere could have screwed up working with high voltage? Did the arc last for a while, or was it gone in a fraction of a second? The main possibilities I see are human error or static discharge, kinda like your own private lightning hit. Have you asked your utility company about it? Hang on to the physical evidence to show to them.
-- J.S.
*One possibility is that the extension cord receptacle and/or compressor plug had become eroded away from improper use. It takes only a few manual connections or disconnections for the large amperage arc to erode the metal contacts and coat them with high resistance oxide. Once it gets bad enough, it gets worse on its own. The few remaining points of contact are high resistance and heat up, accelerating the process.Ideally all devices are turned on and off with a switch - not by plugging a cord in and out of a receptacle. This is especially important in high amp equipment (like heaters) and even more so in induction motors (like compressors) which pull MORE than their name-plate amperage for a few seconds as they start.You were right to be using a short, heavy gauge extension cord. I try to stay on top on it and yet still other workers are yanking my compressor cord in and out of receptacles without turning it off first.I'm pretty sure this is the cause of your problem since the plug/receptacle junction got so hot. If that junction had been in good shape, then huge amperage or the wrong voltage wouldn't have effected the junction any more than the rest of the cord.Quick fix for a few more hours of work: Emery cloth the plugs back to clean and shiny. Try to clean up the !!UNPLUGGED!! cord's contacts with a small file. But replace both ends ASAP. And educate others about how to turn equipment off and on (i.e. with the switch). -David
*Was this just an arc at the connectors, or was it a corona surrounding the entire cord? If it was just at the connectors, then David has it right, and I read your post wrong.-- J.S.
*I think yer on the right track John, probably was corona effect at area of least insulation(gap between connectors).
*John, I don't know what a corona is but it sounds like thats what it did. It was a blue halo about the size of a coffee can and surrounded the entire connection at the junction of the compressor plug and extension cord. It lasted until I shut the compressor off.... maybe 2 seconds. This was an older residential home and we were using an outside electrical outlet. Knowing the compressor uses a large load, nothing else was plugged into that oulet or on the entire circuit. Upon checking out the panel, we did'nt find any fuses blown but my partner wonders if someone placed a penny behind the 20 amp fuse. Specs on the compressor are: 3 HP, 15 amps, 60 hz., 115 volts, 3550 rpm. 20 gal. tank. Looking at the plug from the compressor with the ground prong at 6 oclock the prong at 9 oclock is melted halfway down. All prongs appear clean and without any corrosion.
*John: The coronal effects I'm familar with are astronomical in nature. In wiring, could a 60-Hertz arc, going back and forth between plug and receptacle, cause such a coronal effect centered around the plug connections? Almost all the resistence would be right there so most of 120 voltage drop would be there as well. A 0.004" gap would do it and that gap could increase in size as the air between the contacts got ionized. On an almost-related topic, we had some spectacular auroras up here last week. Horizon to horizon and all red, which is very rare. -David
*It sure is interesting reading all about the physics of the corona etc... But what about the facts? "older residential home"?? were the poles reversed,was it even grounded? Some previous improper repair/replacment etc..... The one thing I have learned and continue to learn is never assume. Assumption is the MOTHER of all f**k ups. How many times have we all had to "wiggle a plug to get a tool to work. Cheap outlets espically have weak tabs that grab the blades of the plug. For all we know the recp was back wired and crossed. With todays penny pinching/not caring builders I can't begin to count how many days were shot to s**t because of bad/broken f**ked up temp poles that are so far from the house I'm working on that I have invested in 300'8/4 that I plug or sometimes direct wire to do my job. I know all the sparkies will probably toss me under the bus for this but.......A guys gotta eat. Sorry for the ranting but sometimes.............. GEO
*George: How does an air compressor's motor know or care that if the "poles" (hot and neutral?) are reversed? Sorry if the pursuit of electrical trivia seems inappropriately off-topic to you, but one of things I like about this site is the opportunity to learn new things. -David
*The blue halo the size of a coffee can sure sounds like a ball lightning/St. Elmo's Fire kind of thing. But those are extremely rare. The fact that it stopped immediately when the compressor was turned off, and that half of one of the prongs on the plug was blown away points more to it being a sustained arcing of the AC power rather than anything exotic. Light from the arc lighting up the smoke from burning plastic could account for the halo description. The remaining plastic might well have contained the ionized air and helped to sustain the arc. The compressor would have acted as a resistance in series with the arc, and helped to stabilize it, like the grid on an old fashioned carbon arc lamp. Did it make the typical AC buzzing noise? If so, we can probably conclude that it was just simple sustained arcing of the power rather than anything exotic.-- J.S.
*Rich,On startup, a compressor motor will draw 7-10 times its running load. So figure 130 amps+/-.Probably, the cords were dirty at the connection. What kind of shape were the plugs in? Most cord caps (tool cord and extention cord) get beat to crap real fast on the job. So if there's a little slop at the plug, you get an arc, and it spreads along the cords for a ways because all the dust/dirt is somewhat conductive.I agree with your partner, there may be a penny behind that fuse. If an arc was sustained for two seconds, any fuse (even a 30 amp fuse) ought to have opened. Mind you, seeing an arc like that can throw off your sense of time...So, you did everything you could to prevent a problem, and made a good save when the juice got loose. You ought to advise the homeowner about the possible problem...the fun will be doing that without freaking them out that they could have had a dandy electrical fire if you hadn't seen it and been heads up.And BTW, did you have a GFI on the circuit? It may or may not have opened...if it was a hot-neutral short, the GFI wouldn't have done anything.
*Electrical corona is a low pressure phenomenon that precedes arcing, as graphically shown in the attached file - corona does not have enough energy to totally ionize the gas or air, hence does not arc, and is better described as a glow. Corona is always a precursor to arcing in any gas when contactors don't touch. (arcs from breaking a contact and vacuum arcs are another story). What you likely saw was copper plasma produced by arcing after a short or plug contact opening (from oxidation or bad contact) as previous posts have described. At 50kV & many kA, I've seen vaporized 4/0 produce 3M long 1M dia bright blue fireballs. What you describe is possible with #10 and vaporized copper plug terminal.
*Dave ,No harm, No foul. I'm here to listen and learn also but it seemed at the time all posts were discussing the result, not addresing the possible causes.As far as hot/neut a motor won't recognise but when strange things happen it usually pays to look at everything because it usually the little ones that make you @$(*&$%#. We all learn every day and I don't feel it was "inappropriate" Cold cyber beers to all! GEO
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Last week my compressor kicked on at a job and I saw a large blue arc in a circular pattern around the extension cord and compressor plug. I knew not to grab the cords and quickly turned the compressor off. Both the extension cord and plug were melted together. I was using a heavy duty (10 gauge) extension cord about 15 ft. long. It didn't appear that any breakers blew although we stopped using both the compressor and the extension and also stopped using that outdoor outlet. I'm replacing the cord on the compressor and tossed the extension. Any explanations as to what happenned?