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In a normal DC circuit the circuit must be closed for current to flow. If I connect the positive end of an LED to a 1.5v battery and both their negative ends to literal dirt on ground nothing happens, the LED does not turn on as current doesn't flow through ground (or does it?)

So then how does the current flow through my body if I touch a hot wire? where is the current flowing to? What is the "circuit" that my body is shorting? Why doesn't the same happen with my LED example above?

Dan
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3 Answers3

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You have the right idea that current wants to flow in a loop.

The AC power that comes into your home uses "earth ground" as the ground potential. Literally a metal rod stuck into the earth. The ground is a decent enough conductor of electricity that any metal rod stuck deep enough into the ground has the same voltage potential as any other rod stuck into the ground (generally). Meaning, if you measure the voltage with your meter between 2 ground rods, you measure 0V.

The power generator at the power plant then develops a voltage centered around that ground potential. An AC wave, 120V in the US, arrives at your home.

When you stand on the ground, and hold the hot wire, you are completing the circuit and forming a loop from the generator to earth ground.

A battery is not referenced to earth ground. It's not referenced to anything. Measuring the voltage between 2 different battery negative terminals will give an undefined voltage, because they are not connected. Touching one end of the battery with one hand, and the ground with another, does nothing. Because the battery negative is not connected to earth ground.

If, however, you take your battery negative terminal and connect it the your earth ground rod and then touch the positive terminal, you will get the shock you are looking for.

Thomas C.
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  • ok so there is a hot wire coming from the power plant AC generator to my house. There must be a return wire/path for current to flow. You are saying the literal ground is creating this path? so electrons are returning/moving to the same generator using the dirt on ground? If ground is conductive then why won't it act like a piece of wire between my battery and LED light? – Dan Nov 10 '23 at 19:02
  • First, AC power is AC, so it will flow through the capacitance between you and ground, you don't even have to be touching the ground. Second, if the negative of your battery were connected to a ground rod, and the other end of your load connected to ground in another location you would see some current flow. – John D Nov 10 '23 at 19:06
  • There are 3 wires in your home outlet (in the US). Hot wire, earth ground, and neutral. neutral should also be earth ground but not always (beyond this discussion). The power plant develops a voltage of 120V above Earth ground potential, through transformers (also beyond this scope). Current flows from ohms law, I=V/R. So the voltage between your hot wire and literally the ground, is 120V. Put yourself between those 2 leads, and current flows. It's important to note that "earth ground" is capable of sinking an almost infinite amount of current before changing potential. Which is why it works. – Thomas C. Nov 10 '23 at 19:07
  • This is such a bizarre concept that I just learned. I always thought there are 2 wires coming into my house, a hot wire and neutral wire all the way back the power plant. so you are saying only one hot wire comes into my house and the literal ground is acting like the return wire? Is earth that conductive? What if there are multiple power planets in my vicinity both hooked up to ground, to which plant would my power return back to? – Dan Nov 10 '23 at 19:13
  • No, in some countries earth has been used as a return, but in the US and in general the neutral is the return back to the power source (usually a local transformer). The neutral is also typically connected to ground at the service entrance to the building. – John D Nov 10 '23 at 19:26
  • I don't get, if there exists a neutral wire then why is ground needed at all? why is the power plant connected to the ground if it also has a neutral wire coming all the way to my house? – Dan Nov 10 '23 at 19:32
  • The power plant doesn't have a neutral coming all the way to your house. There's a transformer (maybe on a pole near your house) that provides the neutral. The primary of the transformer goes to a substation somewhere, which will have transformers that may go to the power plant or other transformers. Grounding the neutral allows for safety features like protective earth and ground-fault interrupters. – John D Nov 10 '23 at 19:40
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where is the current flowing to? What is the "circuit" that my body is shorting?

If you are talking about mains then it's simple: The current flows through your body to the earth. Because the generator's voltage is referenced to the earth:

schematic

simulate this circuit – Schematic created using CircuitLab

NOTE: If this voltage source was isolated from the earth you wouldn't get shocked. You may have heard of "isolation transformers". They help to reduce the shock risk.

What is the "circuit" that my body is shorting?

Your body doesn't short anything. But your body (from touchpoint e.g. finger, to your feet assuming you are standing), along with some EXTRA see above resistance between your feet and the earth, becomes a load for the earth-referenced power source (As can be seen from the diagram above, you don't have to touch Neutral at the same time because Neutral is already connected to the earth at the generator or at the building, depending on the country and some other related standards/requirements). Therefore a current set by this total resistance flows through your body.

For the feel thresholds and effects see the popular diagram below in IEC 60479-1, and here:

enter image description here

Rohat Kılıç
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  • Thanks you for proving the diagram, great help. so an important question arises, why is the generator/neutral connected to earth? Isn't the natural wire a return path to the generator as well as a common reference? what's earth doing here? – Dan Nov 10 '23 at 19:55
  • @Dan Isn't the natural wire a return path to the generator no. The generator generates AC i.e. changes direction 50 or 60 times a second so there's no return path. as well as a common reference That's the point of the earth! It's the largest mass and the largest conductor we can use, and is also common for all humans! What would a better reference be? And is also can be used for general electrical safety as a path to fault currents (e.g. protection against lightning as those huge currents flow ground). – Rohat Kılıç Nov 10 '23 at 20:37
  • changes direction 50 or 60 times a second so there's no return path, by return path I mean a closed loop. There must be closed loop for electrons to move around (AC or DC) so you are saying ground here is acting like that path (a second wire) for electrons to move between my house and the power generator? I thought earth is a poor conductor, apparently not? – Dan Nov 10 '23 at 20:41
  • @Dan when we say "return path" we mean the path the current follows to go back to its source. For AC, N on its own is not the return path because, since the current changes direction, N and L alternately become the return path. you are saying ground here is acting like that path for electrons to move between my house and the power generator? No. The ground is a reference point and is not supposed to carry any current but fault currents. RCDs work this way, for example. I thought earth is a poor conductor, apparently not? Earth is a good conductor but not as good as copper or silver. – Rohat Kılıç Nov 10 '23 at 20:46
  • I still don't get it. Looking at your diagram, N and L can create a closed circuit from my outlet all the way to the generator. What is ground doing here? I thought common reference is the second wire which closes the loop (i.e N or L depending the way current is moving), I don't get what you mean by saying earth is the common reference. What happens if I cut ground from the image above? – Dan Nov 10 '23 at 21:50
  • I.e in a simple LED circuit common reference is the negative side of the battery connected to my LED, nothing is connected to the actual ground. – Dan Nov 10 '23 at 21:52
  • Is the ground just a safety measure here? So both my equipment and the AC generator are grounded for safety measures and nothing more? – Dan Nov 10 '23 at 22:42
  • @Dan well I can't teach you the basics tbh. The nodes having the same symbols or names can be joined together, this is a basic schematic rule. We use symbols sometimes for better readability. Now look at my schematic again. You can simply draw a line from the bottom end of the EXTRA resistor to the source's EARTH connection. When you are standing, this is pretty much what happens. Now you have a closed circuit. Is it understandable now? N and L can create a closed circuit from my outlet only when your appliances are connected to it. Without anything connected, it's still open circuit. – Rohat Kılıç Nov 10 '23 at 23:15
  • @Dan as for selecting the earth as the reference, you can't simply compare the whole mains network to an LED circuit because 1) an LED circuit with battery is a local, isolated system, 2) the mains network goes hundreds of kilometres and is exposed to environmental effects such as lightning. If you don't select a proper reference, everyone in the country will have a different voltage at the outlets. – Rohat Kılıç Nov 10 '23 at 23:17
  • @Dan as for the safety measure, as I said before, we can't allow the earth to carry current because it is a reference point i.e. it sets the "zero volt reference". And also it's not as good conductor as a copper so it'd be lossy. But we can use it for fault currents. For example, we have current flowing through N, and you are being shocked. Normally we expect the current flowing through L and N to be equal but when you are being shocked they won't be because some of the current flows through your body. RCD senses this, breaks the circuit and saves your life. Another example is ... – Rohat Kılıç Nov 10 '23 at 23:20
  • ... your washing machine. It has to be a metal body as a design requirement. And the body is grounded. In case of a wiring failure inside (e.g. L wire snapped inside and touched the body) you can get shocked if you touch the body. But if the body is grounded, L wire touches the ground and completes the circuit (closest way) therefore a very high current flows through L (or ground) so the breaker trips. Grounding also helps EMI and noise problems e.g. your computer doesn't get affected by anything else around it. – Rohat Kılıç Nov 10 '23 at 23:23
  • Your statements are contradicting to me. You say earth is a common reference but no current goes through it. I guess I don't know what a common reference is than. – Dan Nov 10 '23 at 23:56
  • To me a common reference is the negative side of battery connected to an led, the return connection, but that definitely has current going through it. – Dan Nov 10 '23 at 23:57
  • @Dan my statements are not contradicting because you don't know the basics therefore you don't understand. All of my explanations are for people who have at least a basic understanding/background of electricity. You sure need to learn the basics. The reference point is a "point" that we define voltages with respect to. For your battery, if you take its negative as "reference" then the positive lead will show 1.5V w.r.t. that point. That's it! If you run a wire to that point from your LED, then the return current flows through the reference. But, for mains, we don't want this to happen. – Rohat Kılıç Nov 11 '23 at 00:16
  • This answer explains the "ground" connection is just a safety measure and nothing more. N and L are all we need to complete a path: https://electronics.stackexchange.com/a/232753/305610 – Dan Nov 11 '23 at 02:41
  • What I want to know is why is the local transformer grounded to begin with? What would happen if it's not grounded and we only rely on N and L to complete a path? – Dan Nov 11 '23 at 02:51
  • I'll ask ask separate question for this. – Dan Nov 11 '23 at 03:38
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This answer may shed some light on all this: Professor said no current flows to ground

Power lines are referenced to earth ground, and so are you, assuming you're not wearing insulated shoes or are suspended in mid-air. The ground behaves like a huge, low-resistance wire between you and the power generator.

If you're unlucky enough to touch the power line, you complete the path from generator, through the line, through you, then through ground back to the power generator. Because the voltage is so high, even with a resistive body and body-to-ground path enough current can flow to be painful, if not fatal.

Now, why didn't your LED light when you connected it to the battery through the dirt? It takes some effort to make a low-resistance connection to ground. Just sticking a wire into the ground isn't going to do it: resistance between the wire and dirt is way too high.

House electrical feeds are grounded with copper spike 8 feet long (and usually more than one), which gives a resistance of a few to tens of ohms. You'd need something like this to make your LED battery work 'through the dirt'.

hacktastical
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  • Thank you, but what i'm not understanding from all these answers (even googling didn't help) is that why is a power generator grounded at all? If we have two wires coming out of the generator all the way to my house (a hot line and a neutral line) then aren't they enough to complete a path? I hook up a light bulb to it, current alternates and my bulb turns on. What does ground have to do with any of this? what's this "earth is the reference point" everyone keeps talking about. From my understanding of DC, a reference is just the wire which connects everything to the negative side of a battery. – Dan Nov 11 '23 at 02:12
  • This answer explains the "ground" connection is just a safety measure and nothing more. N and L are all we need to complete a path: https://electronics.stackexchange.com/a/232753/305610 – Dan Nov 11 '23 at 02:41
  • What I want to know is why is the local transformer grounded to begin with? What would happen if it's not grounded and we only rely on N and L to complete a path? – Dan Nov 11 '23 at 02:51
  • I'll ask a separate question. – Dan Nov 11 '23 at 03:38