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As legend goes it is maybe possible to steal electricity from power lines in a wireless way (also see this and this related questions). The legend continues that once someone does that he gets caught by the electric company.

Suppose I actually do it - dig enough wire under a power line, plant some fast growing weeds so that the fact of digging is concealed so that there's no obvious disruption and this way steal say one kilowatt of power.

How would the electric company detect that wireless theft?

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I recollect reading in a newspaper in england, maybe 20 years ago, that someone had been prosecuted for stealing electricity from a radar. This was detected because the air defense radar had a black spot in its propagation field and apparently he hooked up a load of wires in his roof. I an certain I read this, but it seems physically impossible. Is this false memory syndrome or could it have happened? Could it happen with a microwave link if they pass over head? Thanks – dave Nov 26 '12 at 15:37
@dave: This is actually a separate question - no powerlines involved. IMO you should just ask it as a separate question - use this link electronics.stackexchange.com/questions/ask – sharptooth Nov 27 '12 at 7:11
up vote 10 down vote accepted

They can't. They monitor the line and notice that a very small fraction of the produced power doesn't arrive at the transformer at the end of the line. Conclusion: over those several kilometers there's a leakage. There's no way to know where. BTW, the amount of power they lose this way is probably far less than cable and transformer losses.

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Why can't they detect the location? An impedance mismatch on a transmission line reflects waves back to the source, and the waves travel at a measurable velocity, so you can detect the location of discrepancies along a line. en.wikipedia.org/wiki/Time-domain_reflectometer – endolith May 24 '11 at 15:41
@endolith - The amount of tapped power versus the transmitted power is probably too low to be detectable. Also I guess the resolution for the exact location is a function of the wavelength, which is 6000 km for a 50 Hz wave. – stevenvh May 25 '11 at 8:59
TDR uses pulses with lots of high frequency energy, not 60 Hz waveforms. Not sure if this would allow them to detect a load that's tuned to 60 Hz, though. – endolith May 25 '11 at 10:35
@endolith - Apart from the problem of having to couple this signal onto a 380 kV line, how far do you think it will travel? I don't know what frequency you would use. (My only experience with TDR is OTDR for detecting faults in optical fiber and there you use the same signal as the carrier, like 1.5 um) – stevenvh May 25 '11 at 14:26
@stevenh: I don't know, but it seems plausible to me. One document even says "Time-domain reflectometry (TDR) is a cable-testing technique that was originally developed to detect faults along power transmission lines" – endolith May 25 '11 at 14:50

There are several problems here:

  1. Power-theft in this manner is not feasible: Even if you had a couple kilometers of wire paralleling the power lines, you would only get a few watts of energy.

  2. Furthermore, the inverse-square law comes into play. Mythbusters tested this, and cot a few milliwatts with a great big wire coil right up against the wires. This would be rather noticeable, however. The available energy at the base of the tower is much less.

Note: It's possible to harvest a bit more power from the fields of a power line if you have a circuit that wraps completely around the power line (therefore basically acting like a 1-turn transformer). However, you are stuck with capacitive coupling if you cannot wrap entirely around the wire. (This is how parasitic power monitors work, like the one AngryEE mentions. Also, most parasitic monitors use microwatts of power anyways, so they don't need much power to work.)

Basically, the only real way the power company can find you did something like what is described in the question is to physically observe it. Realistically, if you did what you describe, the company would pretty much come after you for trespass and (maybe) vandalization, not theft.

Interestingly enough, on some power lines, there is no way to tap the line at all without a physical connection - On some long distance transmission power lines, they use high-voltage DC to reduce losses. On those power lines, there is no way to capacitively or inductively couple power out of the wires.

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On your point about trespass/vandalisation, power lines normally go across farmland with the power companies paying rent to the landowner for the privilege of putting a pylon on their land. If that landowner wanted to lay a cable alongside the transmission lines on his own land then what would stop him? The only thing a power company could do to stop this is to move their transmission lines! – BG100 May 25 '11 at 11:14
can you explain this a little more? On some long distance transmission power lines, they use high-voltage DC to reduce losses? I'm an EE grad but I barely passed my power classes (3 phase power is confusding ;_;) But I don't understand in what situation using DC power over long distances reduces losses. – Falmarri May 25 '11 at 19:17
@Falmarri - en.wikipedia.org/wiki/High-voltage_direct_current , Basically, if you think about it, a long high-voltage line is a giant capacitor, with the wire forming one terminal, and th earth the other. Therefore, every AC Cycle, this large capacitor has to be charged and discharged. While it is a capacitive load, and therefore purely reactive (which means the capacitor itself dissipates no power), it does mean that a lot of current has to flow to charge the capacitor, which results in ohmic loss, and the need for the supply generator to be able to charge it every cycle [continued] – Connor Wolf May 25 '11 at 22:58
[continued]... in addition to the load current. However, with High-voltage DC power lines, since the voltage does not change, there is no reactive loading of the power lines, so the only current the generator needs to drive is the load current. – Connor Wolf May 25 '11 at 22:59
depends on scale: extremetech.com/extreme/… – Fizz Sep 30 '15 at 12:18

Whatever you try, you cant harvest a considerable amount of power from these lines. These lines have shields that reduces electromagnetic loses, even though they may not be implemented specifically to stop wireless theft.

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What shields? They're bare aluminum wires. – endolith May 24 '11 at 15:43
Agreed - I worked for a company that developed a prototype power line monitoring robot that derived its power from the field around the line. It's possible. – AngryEE May 25 '11 at 1:30
I was talking about underground electric wires. – 0xakhil May 25 '11 at 15:12

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