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Shocking Experience Walking Under High-Power Lines with UmbrellaDiscovered: umbrella tines act as an antenna, turning the umbrella holder into a mini transformer. Moist weather seems to accentuate the effect. by Sterling
D. Allan
EAGLE MOUNTAIN, UT, USA -- I was out on a walk with my boy Saturday, during a
light rain. As the road passed under two sets of high-tension power lines,
I stopped to show my boy how straight the rows of towers extended miles into the
distance. Showing him how to eyeball the alignment, I began to hear a
buzzing sound, and I said, "Listen, you can hear the electricity in the
lines above." The lines sagged down over the road so they were maybe
25 feet above the road.
SAFETY
FeedbackDid it to me too
From: <Miller, Samm
>
Sent: Tuesday, August 30, 2005 8:40 AM
Subject: Shocking Experience
I just had the same experience you had. The building I work in in Rhode Island is right next to high tension power lines and I have to walk under them to get from the parking lot into work. It is very humid and rainy today and I was walking under the wires when I felt a pinch or something on my hand. My umbrella handle has a metal button to open the umbrella, I thought maybe I had pinched my finger but then I really felt a shock and heard the buzzing FROM MY UMBRELLA. I, of course, dropped the umbrella and told the guy behind me to watch out for his umbrella. It was rather scary. I was glad to see your article so I know that I'm not crazy. Samm Miller * * * * Not line loss but EMR from the power linesFrom: "frank" <suemcgate{at}gmail.com> * * * * A Little Experiment to TryFrom: "Luke" <iceweller{at}yahoo.com> There is also something else to consider: the high voltage lines ionize the air. With humidity the breakdown voltage of air (Paschen curve) is much lower (so conduction is increased). This is why you hear the sizzling of the corona around the HV line (HV elevated power lines are always uninsulated). In fact, it is called corona leakage. This is also why when you are wet, you risk much more a deadly shock than when the air is dry - you simply are less grounded when dry and the air is dry and your resistance is much higher (it's not the voltage that kills you it's the current). So as you see, there is more than 1 factor to consider: voltage for the
corona leakage, current which induces a voltage at a distance (AC) and the
humidity which affects your resistance and the air's breakdown voltage. * * * * Two Factors HereFrom: "Thomas Brand"
<tsbrand1{at}yahoo.com> First, your dad was correct. A current flow might be induced in your umbrella as if it were a transformer due to the time-varying magnetic flux from the power line. This wouldn't present a potential between you and the ground, though, so it couldn't shock you, although it might make currents flow within the umbrella's structure. This would be an H-field or inductive coupling. Second, you could pick up some of the electric field between the power line and ground with the umbrella acting as an antenna. This is the same thing you see when you stand under a high voltage power line with a fluorescent bulb in your hand. This DOES put a potential across you to ground, and you could feel it if you gathered enough lines of the electric field with your umbrella structure. I'd bet this is what you felt. The wetter your feet and the bigger the 'antenna', the more current you'll have passing through your body. This would be an E-field or capacitive coupling. The numbers you're seeing for power line losses aren't from power 'leaking out' somehow, it's for resistive (IR) losses in the lines. That's why they're at such high voltage for long runs, and the longer the run, the higher the voltage will be. The wire has a fixed resistance per foot (it does vary with temperature somewhat). The loss in the line is related to the current flowing in the line, but not to the voltage. Therefore, it makes sense to keep the current as low as possible. For a given amount of power flowing in the line, this means the voltage has to be as high as practical, because the lower the voltage is, the higher the current will have to be. In short, the higher the voltage you run, the lower the line losses will be. These losses appear as heat in the wires. Incidentally, this is why DC didn't catch on. You can't easily convert low voltage/high current DC to high voltage/low current DC. With AC, you just use a transformer. Therefore, line losses with DC were unacceptably high. I'm sorry that I'll have to disagree with the other gentleman about running your entire home from an antenna under a power line though, that just won't happen. Stealing power involves diddling with or bypassing your meter, or running a wired connection to a pole downstream of a distribution transformer. In order to inductively couple significant power, you'd have to put a coil around one of the lines to form a current transformer, not much chance of that, and using an antenna to catch the E-field, well, you've got a circuit with a 60Hz source with an equivalent of probably less than 10 pF in series with your house. I doubt you could get more than a few dozen microAmps to flow. For fun, though, you might try getting a 40W fluorescent, hold one end in your hand, and hold it up toward the power line at night. The phosphors in the light will be excited by the E-field and light up. It's not the way the bulb would normally work, but it's impressive. * * * * The Nitty Gritty of What Might Have HappenedFrom: "Thomas Brand"
<tsbrand1{at}yahoo.com> From the picture, that's a classic 230kV transmission line. The insulator height is supposed to be about 60 feet minimum, with 90 feet being more common. I guess the wires could droop to 25 feet but that seems a little low. This produces the electric field (E-field) I discussed earlier. This does not mean that it will induce a significant current, however. The E-field will deform around conductive objects. Accurately measuring an e-field requires extremely high impedance equipment. The amount of field line deformation of the E-field you get depends on the
impedance of the medium you're in. For dry air, it's nearly a non-conductor, so
you get next to nothing in terms of current flow across a somewhat conductive
object. I don't have impedance tables on hand that I trust for humid air, and
it's different still if rain is falling. Only true if no current is flowing. The effect of the E-field along your body depends mostly on the height you present, your impedance and the impedance of the medium you're immersed in. So, if you had a e-field of 10kV/foot across you while you were immersed in relatively conductive seawater, you'd be in trouble. A 10kV/foot static field in dry air would induce essentially no current at all, because the field lines would go around you. If electrostatic fields were as simplistic as stated, then you could stick a foot long piece of copper wire in the ground and net a current flow of thousands of Amperes. How could you have metallic towers carrying the REALLY high voltage lines? Obviously, the entire length of the tower is in the field, and it's steel. However, it doesn't suddenly glow blue-white and melt due to the millions of Amperes required by the E field. No, it's just not that simple. You can't say 'there's 100 feet between the
conductor and ground, the conductor has a potential of 1,000,000 Volts,
therefore the E field has a strength of 10,000 volts per foot, and any foot long
conductor in the field will therefore have a 10,000 volt voltage drop across
it'. It doesn't work that way.
> The "buzzing sound" you were hearing was the I'll propose two sources for the buzzing sound, take them as you will. If you get the same sound in the dry as well as when it's raining, then it's an electromagnetic induction phenomenon. If not, what you were hearing was corona, which is what I'd bet on. If it was corona, I'd expect the sound to vary depending on how you held the umbrella and where you were under the lines. However, a corona discharge would allow the umbrella to conduct more current than a simple capacitive coupling with the 60Hz line. In this case, the air is ionized near the sharp edges of the metallic parts of the umbrella, and carries a charge away with it. Trying to dig into this in depth to solve for absolute values of current you were likely to be seeing looked solvable at first, if only in approximation, but it's a bag of snakes. First off, the umbrella has a lot of sharp edges on it. Not just the point on top, and the little balls on the ends of the ribs, but the ribs are made of sharp-edged bent metal with pretty small radii. Next, the E-field from the power line is really hard to solve for without a real good measurement of the geometry. The total field you'll see at different places under the line varies with your relation to the three phase lines. Worse, the corona breakdown values of air are all over the place for pressure, temperature, humidity and whether it's raining. The calculation for dealing with rain involves wind speed, the droplet sizes, the rain density, the rain acidity and so on. Nasty. Anyway, at a conceptual level, what happens is this. There is a breakdown value for air that varies all over the place, but for dry air between two infinite flat plates it's about 3 million Volts per meter. For flat plates, as you exceed that voltage you will get an arc. However, for pointy things like your umbrella parts, something else happens. The breakdown voltage around a non-flat object is lower. The sharper and pointier the object is, the lower the breakdown voltage becomes in the area around the object. This is due to the E-field concentrating at the points and edges, which is also why lightning usually strikes the highest point in an area, but that's another subject. When you put your umbrella with sharp pointy edges and surfaces in the e-field under the power line, interesting things happen. First, the air around the sharp edges and points begins to ionize, and becomes orders of magnitude more conductive than normal for some radius around the point. This ball of conductive air has a larger smoother radius, and to the situation as a whole, it looks like your umbrella also became larger and less pointy/edgy. As it gets smoother looking in terms of the field density, it reaches a balance where the umbrella/corona system is smooth enough that there isn't enough field density at the edge of the corona to get any bigger, and the breakdown doesn't go any farther, like, say, to a full out arc. The current to sustain this corona passes through YOU. It's not a lot but it's enough to feel, and the lower the breakdown voltage of the air around you, the more current it takes to make the corona ball large enough to smooth out the apparent radius. Rain drops in air make the breakdown voltage lower, so does humidity, salt particulates and so on. So even on humid days, you felt it, in rain it was much worse, when bone dry you didn't feel anything. This is due to the breakdown value of the air changing with different atmospheric conditions. Hotter temperatures lower it as well, as does lower barometric pressure. Bad enough corona can be heard as a hissing noise. A DC corona is a steady low level hissing, but an AC corona's hiss is modulated by the field. With three phases 60 degrees apart over head producing the field, you'll get a modulated hissing noise which will sound like a freaky buzz. I believe you'd get a straight 60Hz buzzing off to one side, but maybe you'd get 180 and 120Hz in the mix when you were underneath all three phases. And of course, at some point with a sharp enough object and a low enough breakdown voltage, if you get close enough you can get an arc. I haven't been able to find a reference that says when that happens in rain with 230kV. At the average value for air, you'd have to be within about 10 centimeters to get an arc to a large smooth object. Even with the Macky effect going all out in the rain, and something pointy in your hand, you'd still have to be within a foot or three. So you weren't in any imminent danger of an arc. However, the sharp edges and points on the umbrella could easily reduce that breakdown voltage by a factor of 20-30, in the region around the umbrella. Thus, the corona. As you bring something sharp, pointy and conductive into a strong E-field, you first will get corona, then a brush discharge, then an arc. I think you made it up into the starting point of corona. A really good reason to stay away from power lines in the rain is lightning. The lines get hit all the time, because they're conductive and the highest thing in the vicinity. If it picks a tower (or the line) in your vicinity, you can easily get a flashover that CAN kill you. You can also be killed if the lightning burns a phase wire off and it lands anywhere on the ground near you. In that case, it will set up a voltage gradient in a much more conductive medium than air, and you can easily be killed by the ground currents. * * * * Had a Similar Experience on a BikeFrom: Simoens, Joe A. <Joe.Simoens {at}
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D. Allan June 14, 2005 |
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