We just had an exciting moment when an extension cord became unplugged while the compressor was cranking. Being the guilty party I plugged the extension cords together and it sounded like an arc welder for 15 seconds or so. The plastic cord ends melted and burst into an instant 2′ flame that was more like gasoline burning than a lazy wood or paper fire. None of us, including our electrican, had ever seen something so dramatic from a simple unplugged cord. Does anyone know if it had something to do with the capacitor-run motor of the compressor or if this was just a construction gremlin?
Cheers, Don
Replies
IdahoDon,
My guess is that the unloader valve didn't finish bleeding off the back pressure on the compressor before you plugged it in again. the motor had to do a standing start against maybe up to a 100 pounds of pressure. This really puts a load on the motor. In addition the extension cord may have been long, and or, light gauge wire for the job and in any case the voltage really dropped as the motor was trying to pull more amps than could flow through the cord. With the voltage drop this prevented the motor to reach cutout speed for the start winding which get the temporary kick from the capacitor. This same situation occcures when power is interupted on a air conditioning system and will blow the circuit breaker and sometimes damage the compressor, and or, motor.
Remember to take a couple minutes before restarting equipment under heavy start loads.
Regards,
Virginbuild
Virgin's right, the unloader didn't. Motor was trying to start under load and drawing megga amps.
Joe H
IMHO your both on target. Classic 'hard start' situation. The motor has a high start current under normal conditions. Then starting working against head pressure the motor likely drew close to the LRA, Locked Rotor Amps, the maximum the motor will draw.This wasn't made any easier because the compressor was at the end of an extension cord so the voltage was likely a bit low.Which further increased the current draw. When you reconnected the cord the cord cap, possibly a bit dirty and worn, had perhaps 100 amps coming through it for a short time. This immediately overheated the cord and caps which was pyrolized, broke down to its fairly flammable petroleum constituents, by the heat and caught fire. It also might have damaged the cords. I would retire them for anything but sacrificial uses. This sort of thing doesn't happen too often but, as you now know, it does happen. Mostly electricity plays the way we think it will but occasionally it will jump up and do something surprising like this. Sometimes it costs far more than a couple of cords and, maybe, a compressor motor.
Yep, starting under load. On an extension cord (or two). Possibly in combo with a bad contact in the extension cord end.Toss the extension.
"toss the extension"
Send them to me, new ends and I'll have another good cord <G>. If the breaker didn't trip the wires themselves are OK, the high impedance arc is what generated the heat and did the damage.
Likely your right. The damage might be localized to the cord cap. Depends on how hot the inside of the cord got along its length. Likely a function, mostly, of the duration of the load.On the other hand I have seen a cord where after a very high fault load, at seemingly random locations, the insulation melted. The conductors were free to wander in this softened mess. In a few locations the conductor, normally centered, was just under the outer surface of the insulation and in a few spots the conductor was exposed. Only the paper filler and wrapping maintaining electrical separation between conductors. The cord had been damages from the inside. The jacket in most of these locations showed no major damage. Just normal wear.When in doubt I tend to decommission the wound up cord with my cable cutters. Three or four quick cuts and the roll is reduced to a pile of two-foot pieces. If you don't do it this way it will circulate around the job site for months. Hard to throw anything away on large jobs. I have seen cords so sorry that they shot sparks and often had to be wiggles to make work. But you know construction folks. As long as they could make it work they kept it around. Cord might be shocking the piss out of them, this was before GFIs were required, and be a technicolor wonder with patches, this was before OSHA pushed stricter cord rules, but they won't give up on it.Which comes to how I found the sections with the hidden damage. The crew was mostly waiting for materials. Filling the slow days with make-work and low priority jobs. Times were tough so we were glad the boss was going the extra mile to keep us in paychecks. He had me strip out the remains of the 100' cord for the copper. Wish he had told me before I had cut it into short lengths. On the up side I got to closely examine the cord, a couple of feet at a time.Hard to say if the cord that took such a load in this particular is damaged internally. My fist instinct would be to toss it out and replace it. Cost of a cord is slight in the context of other costs. On the other hand if If it was a high quality unit and relatively new and blemish free it might be worth saving. If it hasn't shorted yet I doubt a meter would tell you much. I used to immerse cords in water but and check with a meter but this is mostly a check for nicks and pinholes.You could hook it to a megger and run it at say 500v to see if anything gives. Might be useful. Lacking the equipment and having a desire to save the cord if it is still useful I think I would give it a detailed examination. You can tell a lot about a cord by looking at it carefully.Stretch it out and remove any twist. This is a good time to clean the cord. Unplugged of course. Then go over it inch by inch looking for faults. Use the hand to feel any defects or bulges in the jacket. Also flex it and feel how it lays. If the insulation has been heated enough to fuse or deform it might cause the jacket to bulge or pucker or make the cord slightly stiffer in that location. Worth a shot and cheap. IMHO it pays to give all the cords this treatment every few months. Given your experience you likely do a light version of this every time you wind up a cord. Makes me wonder when I see contractors drag out ratty, filthy and abused cords. Saw a roofer, on a wet metal roof no less, using a common two-conductor brown lamp cord, patched with duct tape, to run his screw gun. I guess he had his dues paid up with the patron saint of construction workers. Not sure if fools have a patron.
St. Jude, patron saint od lost causes?
Quite a selection of possible patron saints might apply.Saint Thomas - Patron saint of construction workers.Saint Vitus - Protection from lightning. Also did a mean jig, as so many do when they grab the wrong wire.Saint Dymphna - Insane asylums, protector of the insane.
Thanks for the info. I hadn't considered how much of a load there is on a compressor when restarting before the unloader has time to operate--that makes perfect sense.
The cords are fairly new 12 gauge 25' yellow jackets plugged in about 2' from the circuit panel on a 20 amp breaker. Unfortunately the connection that arced burned 1/4" off one prong and probably as much from the female end so my pretty current-indicating cord ends are toast. I'll keep using the cords with replacement ends since the wires and insulation under the jacket don't appear to be damaged. Along happier lines this was a good excuse to buy another 50' 12g cord :-)
If it were #10 I'd say there was no hazard at all. With #12 you're not quite home free, but probably so if there is no sign of overheated insulation after cutting back the burned end 6-8 inchs.With #14 the cord would be toast.
Bought a silicone insulated set of battery cables at a garage sale once for a buck, looked good. Turns out somebody had shorted a battery (NO breaker) with the + side for who knows how long, all the copper was oxidized (completely open) into particles to withing 6 inches of each end - still had 1/2 of a good jumper set <G>.
BTW, almost all of my extension cords are somebody's discards, usually put the nylon Hubble plugs on the ends.
I had a cord the other day that the male end ground pin broke off when in use. I picked up a good quality replacement end and checked everything out. Surprised that I still had an open ground. Found the ground conductor broken about 3 inches from female end. No apparent damage to sheath. It pays to check these things; fat, dumb, and happy can lead to oh sh1t.
>> the compressor was at the end of an extension cordBetter at the end of the cord than in the middle ;-)
The best policy is less wire, more hose. I keep the compressor plugged in right at the panel. I also use light sash cord to tie off power cords so they don't get kicked out.
-- J.S.