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Our power went down yesterday afternoon (December 12). The utility posted a message on their 1-800 number with expected repair time (a few hours later), but no explanation except that ~2000 customers were affected in our suburban area (Northeast USA).
Here's the weird bit -- LED bulbs stayed on, at reduced brightness. I got out a meter and measured 16 VAC in the house. This was enough to make useful light from "dimmable" LED bulbs (happened to be GE brand). After it got dark, we could also see that incandescent bulbs were giving off a faint reddish glow.
16VAC was also enough to keep a Netgear home router/Wi-Fi box going, it must have a switcher in the wall wart that accepts a really wide input voltage range?
Called a friend on the other side of the country who is an EE (with hardware background). He didn't have a good explanation, but suggested that in the process of bringing the grid back up there might be some big voltage swings--recommended unplugging everything we could. Went out to dinner and all was restored when we got home (no damage).
He also told a story from a rural area (near CA-Nevada border) where there was a power failure that upset the normally-balanced split phase -- instead of ~120V on both sides of neutral, the power went to 80V on one side and 160V on the other side of neutral. Equipment on the high voltage side failed due to extended over-voltage, seems that surge suppressors won't deal with this much energy.
Anyone? How does the grid fail-soft?
(Score: 2) by VLM on Tuesday December 15 2015, @04:02PM
16 VAC across maybe a 10 meg input impedance meter, sure. That's not much of a current.
Google up "induced voltage".
People think power outages are like magic spherical video game EMP bombs but reality is unless your whole metropolitan area lost power the HV lines are up. Even if your subdivision lost power the MV lines might be up. Quite likely there's power close enough to the disconnected lines to couple some in.
When you have a short the auto-reconnector thingies fire a couple times and for a second every five seconds in case it was just an intermittent, it'll reset the circuit breaker for a moment. Maybe a couple times. But if you're still drawing sparks a minute later off a downed line, either it failed open somehow and the breaker didn't trip, or if they're really small sparks its just induced current from lines near enough by that are still up.
For lack of a better place, there's high voltage towers going thru a nearby county owned hiking park and its tradition for kids to wave old fashioned fluorescent light tubes in the air under the power lines and they light right up.
You kids and your new fangled switching power supplies, the rectifiers probably have such a fast switching speed you may be seeing rectified local AM radio station pickup not induced 60 Hz current. After all a giant grid of copper wire, if not connected to power, is a hell of an AM radio antenna, and making a LED dimly glow is not that ambitiously beyond a crystal radio set's power. Of course the inductance of the transformers probably doesn't help. Then again maybe just the drop to your house, from your house to the pole to the transformer would make a decent random wire radio antenna when its not powered up. What your cheap multimeter designed for DC and 60 Hz things when fed a couple MHz AM radio signal is mysterious, could be 16 VAC on the display, who knows what an oscope would see.
So I'd give it about 50/50 odds that its simple induced power or AM radio antenna pickup making modern switching power supplies happy. You're not going to run a clothes dryer off that, but then again it doesn't take much to make a couple LEDs dimly glow.
(Score: 0) by Anonymous Coward on Tuesday December 15 2015, @04:17PM
And then, of course, there is this lovely big cell we call Planet Earth. On a good low-impedance meter you can always read a potential between any two ground rods, which usually increases with distance, and is one reason why proper bonding practices specify single-point earthing for each service drop.
You should've seen the fun when an idiot "safety director" decided to have maintenance drill and pound ground rods for every machine in a factory instead of using the building column grid. When I saw what they did I just told the plant engineer to put it back like it was and sent them my bill, along with some germain NEC reference excerpts.
(Score: 0) by Anonymous Coward on Tuesday December 15 2015, @04:22PM
Did you know... if you string a wire parallel with high tension powerlines you can get free electricity even though it's not physically connected? Don't let the utilities catch you though, they'll send you a nasty C&D and make you remove it.
(Score: 0) by Anonymous Coward on Tuesday December 15 2015, @08:25PM
I used to be our companies "safety inspector"... that is until I caught upper management changing my reports. I told them to shove it and to find someone else to do it, which was pulling me away from my main duties anyway for a lousy extra 1 hour of pay. The dipshit that took over only pencil whipped the inspection reports, without actually checking anything. About a year later a fire broke out and the first extinguisher that was used didn't work. The inspection tag on it hadn't been filled out since the last time I checked it.
(Score: 0) by Anonymous Coward on Tuesday December 15 2015, @05:54PM
i try not to give google more use than necessary. Shame on you for making them install CDNs and force them on us for your convenience. Someone should "kleenex" up the mess caused by them and "xerox" the results for us all to see. Cause will let you google it for me, but I wont search it on them myself. I'm pretty left wing, so I am going to go duck it up instead.
(Score: 0) by Anonymous Coward on Wednesday December 16 2015, @12:14AM
The most famous example of induced current was The Carrington Event. [wikipedia.org]
Telegraph equipment was welding together or exploding.
When checking whether electrical feeds are alive, an electrician doesn't tend to use a high-impedance voltmeter.
He will use something that draws a bit of current. [google.com]
That gets rid of phantom voltage readings.
I have a 10k 5W resistor I got out of my junk box and mounted on a dual-banana feedthrough connector.
I hook that to my meter when checking household voltages.
It pulls ~2W @ 120V and ~6W @ 240V.
Similar kind of idea as a Wiggy.
-- gewg_