For a long time there have been stories floating around about people supposedly stealing electricity by building a coil underneath power lines. I am assuming the idea is that the coil would act as the secondary of an air transformer, and the overhead power line would be the primary winding.
But I am skeptical it has really happened. I have yet to see a legitimate news story about it. And I am skeptical an appreciable amount of power can be extracted using a coil on the ground.
Even if you could achieve any level of coupling between a coil on the ground, and the overhead lines, any real power transfer would be eliminated since the overhead line carries balanced AC currents. For single phase (end of line residential) distribution, the two phases on the wires are 180 degrees apart, cancelling each other. For three phase power, again, the net coupled sum of phase power to the coil would yield zero.
We have had a recent case where the chain on a children’s swing under a power cable had enough current induced to give a mild shock. It’s significant that the swing was on a wooden frame, so not earthed until someone touched the steel chain.
Aren’t the lines on high-voltage systems widely separated to prevent arcing? That wide separation could allow for useful magnetic coupling at ground level. The article bob++ would seem to confirm this.
Wouldn’t it be possible to calculate how much power you can get with reasonable parameters?
I can calculate/estimate/wild-guesstimate that, at 10 meters distance from a conductor carrying 1kA DC, the magnetic field strength is smaller than the earth’s magnetic field.
Far from a proper calculation but seems to me you need a very large coil (tons of expensive copper) to get any usable power out of such a setup.
E.g. assume balanced three phase AC conductors, 10m above ground, spaced 1m horizontally from each other, carrying 1kA AC RMS 50Hz. A 1 meter diameter coil on the ground somewhat off to the side. How many turns of coil needed to light up a 100W light bulb?
That’s somewhat above my pay grade to calculate, but perhaps not hard for an expert?
Even though they use the example of a farmer stealing electricity, they include this:
As it turns out, Mythbusters actually did test this, though it was an online extra and wasn’t a part of their regular show. They found out that it was technically possible, though not terribly practical due to rather high initial investment costs.
Note that the voltage you get out of your coil varies with the amount of current going through the line, and the amount of current on a typical transmission line can vary a lot, highest on hot, summer days and significantly less current when everyone isn’t running their air conditioners 24/7.
As for actual cases of stealing electricity, all of the real world cases I am aware of involved someone actually clamping connections on to existing wiring. They weren’t using induction.
Poking around on google, I was able to find several stories like these:
In all of the stories I looked at, no cite was given. My best guess is that even when you have articles like this that can be cited, ultimately they are just urban legends of sorts. I am particularly skeptical of the radar story.
The link in the OP is to a Snopes message board where they discuss this, and it contains several stories people doing just that. Some sound like a FOAF stories, and others sound (to me, anyway) purely made-up. Years (decades) ago, when I was in Engineering school, is when I first heard of this. The version I remember was slightly different, however.
In the version I heard, it was a guy, in England, who put a “coil antenna” in his attic and ran his house off of it. It was explained that England was using very powerful transmitters (more powerful than usually found in the US) to broadcast to the entire island and this fellow’s house was close to the transmitting antenna. In addition, the only electricity he used was for his lights and one radio (they also explained how he had built his own power supply for the radio, but these were Electrical Engineering students), so his power usage was low. He was found out, not by a deterioration of the radio signal, but simply the fact he stopped paying for electricity.
I think the physics side of the question has been answered. What about the legal aspects?
First case: attaching conductors to transmission lines. Obviously trespassing and theft. Note that it’s still trespassing if you hook onto the power line before your meter.
Second case: building an induction loop on your own property. Not trespassing, but still obviously stealing power, since they are not intending to give you power and your taking of power causes a loss to them.
Third case: building an antenna to collect power from a transmitter. This is legally fuzzy. Obviously it’s the intended use case for some transmissions (radio, TV, phone). But other transmissions, like radar I’m not sure. Their radar is intentionally broadcasting power–it’s already on their books as a power loss. Once their radiation enters your property, I’m not sure they have any claim to it. From their point of view, an antenna absorbing their power for lighting is no different than a wall of absorbent material using the power to heat a room. Surely there’s no duty to allow radar radiation to pass through your home.
Of course, even in the radar case, you better be sure you’re not reradiating any power on your antenna, or the FCC will be very upset with you.
Actually the two phases in your home are 120-degrees apart. When you want ‘220-volts’ for your electric stove or clothes dryer you run the two phases through the load instead of one phase and neutral like you do for 110v everywhere else. The RMS value of the two sine waves 120-degrees out of phase is more like 208 but everybody calls it 220 anyway.
The big generators at the power plant have three coils in a delta configuration so they are putting out three sine waves 120-degrees apart. When you see those high-voltage towers marching to the horizon, that’s why they are carrying three wires. This configuration is kept all through the system until in your neighborhood the poles are carrying wires with a nominal 440v in them. That last transformer, the one you can probably see from your back yard, does the final stepdown for delivery to you and your neighbors. It also has the three primary coils in the delta configuration and the secondaries in a Y configuration. You’ll get, say phases A, B and the center tap, a neighbor B, C and the tap and another neighbor C, A and the tap.
Would the radar operators even care? Assuming there’s no reradiating that interferes with the radar, that is. Radars get a lot of return signals from things on the ground near their antenna. It’s called ground clutter and they have circuits in the radar to block it out. So why would they care if someone is using some of that otherwise wasted radar signal to generate a bit of power?
There are two types of electrical service that are common in the U.S.
By far the most common is a single “split phase” service. This is essentially a single transformer with a center tap. The center tap is neutral, and from either line to neutral is 120 (110 to some), and from line to line is 240 (or 220). Since they come off of a single transformer coil, the two lines are 180 degrees apart.
In some places you get two lines from a 3 phase system, but that is far less common. Parts of New York City are still connected this way, and it is in use in a few other places (mostly densely packed housing). In this system, the two lines are 120 degrees apart. Line to neutral is 120, and line to line is 208.
While split phase is much more common (to the point where 110/208 is almost unheard of), split phase still ends up being part of a three-phase system. It’s just that they take the three phases and split them off into individual phases. One phase might go off in one direction and the other phases in different directions, or sometimes you’ll even see things like rows of homes were one street is on the first phase, the next street over is on the second phase, etc. As long as all three phases are pretty close to balanced as far as the load goes, the power company is happy.
While 120/208 is rarely used for residential house current, it is very common in apartment buildings and commercial buildings which are fed directly by 3 phase power.
No, 208v and 220-240v are two very distinct services. 120/240v split-phase is typical residential service. 120/208v 3-phase is found in larger apartment buildings and commercial applications. In practice, a lot of 208v appliances can run on 240v or vice-versa, but heating appliances like dryers, ovens, stoves, etc. simply run at reduced capacity on 208v.
Inside a commercial building with 3-phase service, the lighting and convenience receptacles are fed from different phases in the breaker panel to try to keep them balanced. A couple years ago at work we had a “single-phasing” event where a car crash down the road took out one of the three distribution wires but not the other two. So most of the lights in the building went out, but not all of them. Some were partially lit (fluorescents with an eerie red glow). Some people’s computers went out, but in some cases their monitors stayed on or vice versa (the power system for our cubicles had two circuits). Some emergency lights came on but some didn’t. Once we figured out what was going on it was a scramble to shut down the air conditioners. I think they were 3-phase, but apparently they didn’t have any sort of cutoff in case of an event like this. My understanding is that a 3-phase motor will keep running if a phase drops out (though it can’t start if stopped), but it causes the current in the other two phases to jump, overheats the motor, and also throws it off balance causing excessive vibration. If not shut down quickly the motor windings can melt.
When I took electronics in high school, I read some plans for a small setup to steal bit of power from power lines. It promised a few milliamps at a couple of volts when underneath residential lines. It also said it would do much better next to a transfer station.
The late Senator Barry Goldwater was a staunch proponent of this view: “Any radiation falling on my property is mine to use – sunlight, radio signals, satellite transmissions, etc.”. And when he was a power in the US Senate, he threatened to introduce legislation to make this legal doctrine clear.
That’s one of the reasons that satellite dish makers, cable TV providers, etc. all now provide encrypted transmissions, where you need a box to decrypt & use the signal.