Correct my misunderstanding(s):
Electricity takes the easiest path it can to ground. So take a large, uninsulated wire or cable with like 100,000 volts running at 20 amps through it. Why can’t I just grab it? It would seem that the most efficient path for the juice is to just keep on going down the line, and not through me.
If you’re grounded, you would be an easier path to the ground.
No, this is wrong. Generally electricity takes every path available. How much current will go through each path depends on how easy or hard each path is. The path “hardness” is called resistance and measured in Ohms. Google “resistances in parallel” to get an idea of the concept.
Now when you touch a cable your body is a very bad path to the ground so just a tiny portion of the current will pass through you. Most of it will continue its merry way along the cable. But when we are talking high voltage, that tiny portion will be more than enough to kill you.
Also, unless there is a constant current source providing these 20 amps at 100K Volts (which is not the case for high voltage lines), do not expect to have just a portion of these 20 amps pass through your body, there will be a lot more than that because you are creating a short circuit.
The most efficient path is for the current to both keep going down the line and go through you. Even if you have much more resistance than the other path, you’ll get a fraction of current through you.
Also, If the wire voltage stays at 100,000 volts after you grab it in your example, the wire current won’t change, but you’ll get a large amount of current also flowing through you. That is, there will be more than 20 Amps flowing after you grab the wire.
just like when you let water flow downhill, it will split and take every path it can.
Ohms law. Current(amps) equals voltage divided by resistance.
The human body has an approximate resistance of 300 - 1000 ohms. So let’s say 1000 ohms for this example. Then 100,000 volts / 1,000 ohms = 100 amps.
This will cause a great surge on the generator producing the circuit and will fry your ass in an instant.
Even with a constant current source, your body will pass most of the current, since a 20 amp load at 100 kV has 5,000 ohms of resistance; assuming a resistance of 300 ohms (internal body resistance, which also drops with higher currents, so it could be even less; Darwin Awards has a story about a sailor who electrocuted himself on 9 volts, implying an internal resistance of around 100 ohms), about 18.9 amps of current will flow through your body. Even with dry skin, which might have up to 500 k ohms of resistance, it will burn through the skin (0.2 amps/20 kW) and negate its resistance (never mind that 0.2 amps is still deadly, and potentially more so than higher currents due to ventricular fibrillation; higher currents usually just paralyze the heart, so a brief shock may not be fatal). Note also that with a constant current source, the power dissipated in the body will be a lot less than 2 MW (100 kV x 20 amps), although 106 kW (5.66 kV x 18.9 amps) is far more than enough to literally fry your body.
A related misconception is that any source of high voltage can kill, but just as static electricity doesn’t kill, the same goes for a low-current HV supply; e.g. the HV supply to a CRT (as implied in this video; the capacitance of the CRT doesn’t store enough energy either).
Doesn’t this depend where you grab the wire? If it goes into earth and you grab it just before it goes into earth, the potential between your hands and your feet isn’t going to be very great. If it’s passing overhead and you grab it though…
If you are grabbing a wire that is going to earth, that is going to be a different wire, just as household power is supplied over two wires, one at earth potential (neutral) and one at mains potential (hot). Touching the neutral won’t give you a shock unless something is wrong (break in the circuit).
Just checked. I am about 240K ohms fingertip to fingertip. Also, the way humans are put together, we actually provide impedance, not resistance to electricity. Both are measured in ohms though.
Water is often used as an analogy for electricity. Though imperfect it works here pretty well. Imagine a hose with water running downhill that splits into two parts, one fat and one skinny. Some water will run into the skinny part, no matter how skinny, in proportion to its cross-sectional area compared to the fat part. The comparison between you and a wire is not as simple–you are a lot fatter but not quite as good a conductor. Kind of like putting a sponge inside a fat hose instead of using a skinny hose. But it will still flow. You’ll fry.
The overhead line does go to earth eventually. The things that affect resistance (assuming material is held constant) are cross-sectional area and length. The longer the distance until the wire finds ground (and it will have all kinds of intervening loads and voltage step-downs when you are talking about power lines but IANAEE) the higher the resistance and the more current that would be shunted down your salt-watery body.
Electricity does not want to go to ground.
We (engineers, insurance companies, NFPA) have made earth attractive to elecricity with grounding. Grounding does not make it safe, it makes electricity dangerous to humans, protects buildings and equipment.
Delta systems, ungrounded, are safe to touch.
Just noting that measuring your body resistance with a multimeter does NOT give an accurate representation of your body resistance at say, 120 vac, or otherwise nobody would ever die from touching household power, even 240 volts (which would let only 1 mA through at 240 k resistance, which is to say, the threshold of a tingling sensation and only deadly if it went through a heart catheter or something).
Distance doesn’t really have anything to do with current flow or anything; it sounds like you are talking about a wire so long that the resistance of the wire itself limits current (which is true in some causes; for example, a DC relay coil, or a resistive heating element). Real power lines need to have a low enough resistance so that the voltage at the end user stays within a specified range, since all that regulates the voltage is the output of the generating plant.
As an example to show how the resistance of an overhead power line can be considered to be negligible for most purposes, using size 0000 AWG wire (copper; real power lines usually use aluminum but can be much thicker), a 100 mile long run (200 miles both ways) would have only 52 ohms of resistance, dropping 1,035 volts at 20 amps (5,000 ohm load resistance), for a end-load voltage of 98,965 volts for a generator voltage of 100 kV; stepped down to 120 volts (833.333:1), that would drop the load voltage to 118.76 volts. Going back to my original post (#7) where I calculated how much current would flow through your body, even with a constant current source, you can basically ignore the difference caused by voltage drop over 100 miles of wire.
Even if the generator can maintain 100 kV at its output when the ends of the transmission line are shorted together, which would deliver nearly 2,000 amps, the total round-trip resistance is only around 1/10 of your body resistance, so current will still be nearly as high (333 amps at generator, 284 amps at end of line, assuming 300 ohms body resistance, variation will be less if you use 1,000 ohms instead).
Very true.
Electrocution is a complex phenomenon, because it’s very difficult – if not impossible – to create an accurate electrical model of the human body. This is for the following reasons:
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There is a large amount of variability from person-to-person.
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The impedance of the human body depends on the connection points. Electrodes making contact with broken skin, for example, will result in greater current than electrodes making contact with dry skin, all else being equal.
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Unlike a wire, current through the human body is not due to mobile electrons. It is due to mobile ions. Ionic current in a fluid is a very complex phenomenon. The impedance is a function of everything… voltage, time, frequency, amplitude, etc. Even today, we do not fully understand how to characterize the conductivity of contaminated water. And the human body is much more complex than a glass of salt water.
None-the-less, we sometimes make an attempt to create a simple electrical model of the human body. Not because it’s accurate (it’s not), but because we need a standard procedure & methodology for evaluating test and measurement equipment. One example is the HMB used to evaluate ESD instrumentation. The human body is modeled as a 1500 ohm resistor in parallel with a 100 pF capacitor.
One thing researchers have discovered is that the magnitude of the current through the human body can be used to approximate risk to life. Current above 10 mA is considered pretty dangerous. The current through the body is a function of the voltage across the body and the impedance of the body. The voltage is pretty simple… if it’s AC wiring, then it’s simply the peak (not RMS) voltage. The impedance through the body? Who knows. As explained above, there is no reliable way to determine a person’s impedance.
Your home has an AC power system that is referenced to earth ground. This has both advantages and disadvantages.
The primary advantage is that it keeps the 120 VAC in your house from “floating up” to the primary voltage of the transformer that is hanging on the pole next to your house. The primary voltage of the transformer is around 8000 VAC, and if the electric company did not tie one of the secondary terminals to earth ground, it is possible the secondary (3-wire split phase 120/240 VAC) could float up to a high voltage, perhaps even up to the primary voltage of 8000 VAC. (Such an event would affect the common mode voltage, but not the differential mode voltage. In other words, you would still have 120 VAC between each hot leg and neutral, but all of the legs – including neutral – could be as high as 8000 V relative to earth ground.)
So to keep this from happening we connect one of the secondary terminals (the center tap, or “neutral”) to earth ground. Of course, this leads to an obvious disadvantage… each hot leg is always 120 VAC relative to earth ground. So if you were to somehow make contact with a hot leg, and another body part makes contact with earth ground, you will get zapped. You are grounded more often than you think. Touch the kitchen faucet or sink? You’re grounded. Standing barefoot in the grass? You’re grounded.
One solution is to install a whole-house isolation transformer. The primary would be referenced to earth ground, but the 120/240 VAC secondary would be floating. It would eliminate the most common scenario for electrocution – between a hot leg and earth ground – but would not eliminate the more rare scenario of making contact between both hot legs, or a hot leg and neutral.
Impedance is a purely AC phenomenon.
It takes into account the capacitive or inductive properties of a load (reactance), as well as it’s resistance.
So, humans have a resistance, as well as a reactance, and for 60Hz, the resistance is by far the bigger contributor.
Sorry for the nitpick, but these two sentences are contradictory. The first is incorrect, while the second is correct.
Impedance (Z) is a DC + AC phenomenon (Z = R + jX). If X = 0, then the impedance is strictly resistive. If R = 0, then the impedance is strictly reactive. In all real cases, both are in play to a greater or lesser extent.
In high school we attended an assembly at which the man conducting the experiment had a girl named Lori sit in a chair on the stage. He prepared an electric generator to send current out so it would travel over the surface of her body; in the process it made her hair stand out like Patti LaBelle’s. She didn’t feel a thing.
Also, I was working on something on my car once, in our garage, which had a concrete floor. I was holding a small, ordinary AC table lamp with a metal base, but gave up using it because I would feel a slight shock every time I touched any metal part of the lamp.
One way to avoid this is to have a high resistance from the secondary to earth ground, which would prevent charge from accumulating on the otherwise floating secondary side; something similar is done in electronics that use a two-wire cord; a resistor of several megohms is connected between the secondary and neutral (usually also a capacitor for diverting high frequency noise, sometimes the resistor is omitted but it should always be included to avoid high voltages from building up, such as from static). On the other hand, if a short ever occurred between the primary and secondary you’d have 8,000 volts on the secondary side, which would be a very dangerous situation (8 kV would also arc over inside of grounded equipment, even outlets and switches, causing damage and fire).