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If I'm not mistaken, on DC there needs to be a circuit for current to flow. If I just touch the positive side of a battery while standing on ground, and connect its negative side to the ground battery won't drain as earth is non conductive, correct?

When touching an AC hot wire the current would flow through me into the ground, but there is no circuit created, earth is non-conductive, the electricity isn't going back to its origin to create a closed loop (or is it?)

How is the current flowing in this case?

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    \$\begingroup\$ Earth is conductive - at least a little bit, depending on its moisture content. \$\endgroup\$
    – brhans
    Feb 2, 2022 at 14:49
  • \$\begingroup\$ Current only passes through something if there is a difference in voltage potential between two objects and they are connected with some conductive material. It's really as simple as that. \$\endgroup\$
    – Lundin
    Feb 2, 2022 at 15:06
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    \$\begingroup\$ This video of high voltage maintenance has been shared here before youtu.be/9YmFHAFYwmY high voltage electricity lines are inspected while powered. While being 100 feet in the air adequately prevents a short circuit to the earth, capacitative coupling of the helicopter requires an equalization wand to bring the helicopter (and the person) to line voltage. The inspectors themselves are wearing fireproof suits built in a fashion to function as faraday cage for any other stray fields around the high voltage conductors. Worth a watch. \$\endgroup\$
    – crasic
    Feb 2, 2022 at 15:42

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Whether it be AC or DC, for current to flow through your body you need two things:

  • voltage to ‘push’ it (potential difference)
  • a complete path for it to flow

First, DC.

A battery just sitting there without a connection is insulated. Touch one of its terminals and that terminal becomes the same voltage as you. Once that happens, no current flows*.

*If you and/or the battery have some static charge, some current will flow until the charge is equalized. That is, your body has capacitance to ground, even if you aren’t grounded yourself. We'll come back to this.

Now touch the second battery terminal at the same time. A path is formed and current flows. That’s why you get a bit of a shock when you touch a 9V battery to your tongue, but only if you touch both terminals.

Now, let’s make things interesting. Consider a grounded, high-voltage battery pile: more than 100V, say, like you might encounter with a home solar setup. If you touch that, and are grounded, a complete circuit is formed through you and back to the battery, and you will get a shock, possibly a lethal one.

(There is some debate as to which is more 'lethal': AC or DC. More about that here: https://www.electronicsforu.com/resources/ac-dc-current-body-dangerous Nevertheless, non-touch-safe DC deserves your caution and respect, same as AC.)

If you are insulated, your body will assume the same potential as the battery: your body capacitance will be charged to the same voltage as the battery, after which no current will flow.

Now let's talk about AC.

An AC power line has a reference to earth ground, in multiple places: at the utility head end, from time to time on the distribution towers and poles, and near where it enters your home. If you touch a power line, and you are also grounded, again, current flows from the line, through you, then to ground (which, by definition, has almost no resistance.)

Again, if you are insulated from ground, perhaps by your shoes or, say, by dangling in mid-air from a helicopter, (mostly) no current flows. We'll talk about that weasel-word 'mostly' below.

More here: Professor said no current flows to ground

Your insulated body nevertheless still has capacitance to adjacent ground, which provides an AC path. That’s why you can still get a shock from a live wire by touching just one terminal, even if you're not touching ground or another live wire (and, also why that helicopter in the video above has a huge arc to the powerline while they're depositing the high-wire guy on it.) The AC will alternately charge and discharge your body capacitance, and if the voltage is high enough, the resulting current can be fatal. In comparison, DC will only charge you up just once, to the DC potential.

Body capacitance is also why you get a shock from static electricity: your body capacitance stores or releases charge if you come into contact with something else at a different relative-to-ground potential. This is why ground straps are needed when working on sensitive electronics: to bleed off static charge, equalizing your body static voltage to the gear you're working on.

What about ground?

Again, by definition, earth ground has practically no resistance by itself. The Earth is such a large object with a lot of conductive matter in it, especially iron. Thus, it has a very, very low internal resistance. Of course, earth resistance varies locally depending on the earthen material, but in aggregate it's low enough to not matter.

In fact, Earth ground is so low-resistance that early telegraph and even some power systems relied solely on ground as a return, using only one wire to send a signal.

With grounding systems, any local resistance to Earth is largely due to the grounding connection itself. Example: a grounding rod installed at a residence must be 25 ohms or less to be code-compliant; usually they're a few ohms. Earth grounds used by larger systems like utilities will be much, much lower than that. So this is why we assume that 'ground' has no resistance.

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  • \$\begingroup\$ right but i'm still wondering if I ground the battery and myself, and touch the + side of the battery would current still flow, but very very slowly? \$\endgroup\$
    – Dan
    Feb 2, 2022 at 15:16
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    \$\begingroup\$ Yes, if you are also grounded (bare feet on a concrete floor for example.) if you are wearing shoes with rubber soles that insulate, no. \$\endgroup\$ Feb 2, 2022 at 15:18
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    \$\begingroup\$ Relatively speaking, yes. With AC there is also charging/discharging of your body capacitance even if you are insulated, which is how you feel a ‘tingle’. But worst case is grabbing both hot leads, which can be fatal even with household voltage. The body tends react by contracting muscles which if you’re lucky will pull you off that line. \$\endgroup\$ Feb 2, 2022 at 15:25
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    \$\begingroup\$ You start out by saying, "For current to flow you need...a complete path," But then the rest of your answer contradicts that by acknowledging that that the complete path is only needed for a continuous, DC current to flow. A transient spike of current, or a continuous AC current does not need a continuous DC path when capacitance is significant. \$\endgroup\$ Feb 2, 2022 at 19:12
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    \$\begingroup\$ No, I don't contradict myself. I introduce ideas in stages. You'll see that I lead into discussing capacitance for AC paths as well as for static electricity (equalization of charge). \$\endgroup\$ Feb 2, 2022 at 19:27
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A closed circuit is needed for current to flow, as always.

If your body is indeed absolutely isolated from earth/neutral, you can touch a live wire without getting shocked. Just look at a bird sitting on a power line.

But in reality, your surroundings are usually not perfect insulators. The resistance/impedance to earth usually is not infinite, causing a closed circuit that may shock you. You will most likely not be shocked as badly if you just stand on a wood floor touching a live wire, as if you touch an earth wire at the same time. But you often still get chocked.

Also worth noting, with AC it is not only resistance that applies, but impedance. A capacitor can conduct AC, and you may be capacitively coupled to earth even if the resistance is infinite.

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  • \$\begingroup\$ So in the DC example, the battery would actually drain, but very slowly? \$\endgroup\$
    – Dan
    Feb 2, 2022 at 14:55
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    \$\begingroup\$ @Dan also voltage must be considered. If you are not perfectly isolated from ground, a battery makes little current pass - you don't notice or you can't measure it. But a live mains wire has maybe 20 times that voltage, so 20 times the current. And sometimes good insulators break, like in a thunder. \$\endgroup\$ Feb 2, 2022 at 15:10
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If I just touch the positive side of a battery while standing on ground, and connect its negative side to the ground battery won't drain as earth is non conductive, correct?

No. Almost everything you typically stand is somewhat conductive, so the battery will drain, but only very slowly. The current is function of the voltage (which is low for a battery) and the resistance (which is high for a residential floor, but low for, say, a water faucet or a metal radiator).

When touching an AC hot wire the current would flow through me into the ground, but there is no circuit created, earth is non-conductive, the electricity isn't going back to its origin to create a closed loop (or is it?)

In this case there is not a lot of difference between AC and DC. Both need a "closed" circuit for current to flow which is determined by the impedance of your ground connection. AC is a bit more complicated because the impedance is also determined by your capacitance and/or inductance to ground but in practice that doesn't make a whole lot of difference.

For a residential AC outlet, the circuit can be closed though ground because residential AC always uses earth ground as "0V". If you touch one wire of an AC function generator and leave the other pole of the generator unconnected, no current will flow.

The main reason why you get shocked from an outlet and not from a battery has little to do with AC vs DC: it's simply the outlet voltage is a lot higher. If you touch 120V DC you will get a good whack as well (so DON'T try this).

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As an addition to the other answers, consider why an isolation transformer is often used for safety, when you have equipment under test / development.

Without a transformer the live wire is relative to ground. If you are at the same "ground" potential, then touching the live wire makes you part of the return path.

enter image description here

If you "isolate" L and N from E (which you are standing on), you "remove" the resistive return path and further reduce the capacitive coupling. Its still not 100% safe. And of course you still get shocked when you touch both L and N on the isolated side.

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The body is mostly water and other chemicals.

Water molecules have an electric dipole moment which tends to align with an external electric field. Alternating electric fields cause dynamic motion of the water molecules which we associate with the flow of displacement current.

There are also ions (charge carriers) dissolved in the tissues of the body. An ionic solution can flow direct current, when the imposed field does not change polarity, and this would accumulate net charge in some region, which can be harmful when charge accumulates in regions of the body. An ionic solution can also efficiently conduct alternating current.

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A very popular (especially in the past) application of this phenomenon is the neon-lamp line tester. Its operation is surprising and incomprehensible to many people, because at first glance it does the impossible - it measures the voltage with just one probe!

For the participants in this discussion, the answer is clear - a very weak (but enough to light the neon lamp) current flows mainly through the capacitive coupling and leakage of the human body to the ground. So the human body acts as the other probe. See also How does a "neon tester" work?

I remember that in the past I had noticed an interesting detail - the tester did not work well (the neon lamp glowed very dimly) when I tried to measure the voltage while standing on a ladder. Guessing about its operation principle then I used a simple trick - I touched my other hand to the wall... and the lamp glowed brightly.

But later I realized that this is dangerous because the built-in resistor is quite unreliable (they usually make it from some pressed graphite mass). So don't do that!

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