Does it literally mean that electricty goes into the earth. If so, why is that good? Doesn’t dirt have moisture, and isn’t that bad? What happens in a physics sense if you touch something and you’re grounded? And why is that better than not being grounded?
From wikipedia:
When crawling underneath my house, there’s a wire stuck directly into the ground from the house. I assume that the energy fed to this wire is spread into the earth in order so that it has no effect on anything.
Assuming you’re talking about the AC current in your house, “ground” and “neutral” should be the same thing; your neutral buss in your CB box is tied to your ground, either through some of your water pipes or a ground rod or both (not an electrician; someone will be by shortly to quote the NEC [National Electric Code]).
Back at the place that makes electricity, their generators are also wired with their neutral to ground. The Earth makes a great, stable, common reference point and safety device.
For example, your 9-volt batteries aren’t grounded. Their devices’ schematics may indicate ground, but this is really “common”; often times “common” and “ground” and “neutral” are all the same in plug-in devices. I think I like the British word “earth” better than ground, because “earth” is always an earth-ground, and ground can be whatever you want it to be.
Of course in the case of a 9-volt, this isn’t really dangerous. To “float your power” (i.e., have the reference free of ground) can be dangerous, although you’ll often see it in industrial applications where other safety precautions are present.
The Earth is big enough that any electrons a circuit might add or take away won’t appreciably effect its overall charge. Its ability to source or sink makes it a good neutral reference point.
In digital circuitry (DC), the negative terminal is typically referred to as “ground” out of convenience. This is not necessarily a true Earth ground.
Factual and good answers, those, but I’m not sure they answer your question.
Yes. The convention is that electricity flows from positive to negative, but remember that electrons are negatively charged so they flow from negative to positive. Which is to say that that when electricity flows into the earth, electrons flow out of it.
It is not good, it is not bad, it is just the way it is.
No, but I’m not sure I get the point of the question.
It might be worse. If you touch a live wire, maybe stick a knife in a live toaster, and you are not grounded, the electricity from the toaster doesn’t have much of anywhere to go. But if you are grounded, you allow a complete circuit to form and the electricity is more than happy to flow right through you to reach ground. Now it is verygood for the external surfaces of an appliance to be grounded. If there were to be a loose wire inside, and it touched the surface, the whole appliance would be live. If you are grounded and you touch it, you would be shocked. But if the surface is grounded, most of the electricity will prefer to take that route to ground rather that through you, assuming that the wire is a better conductor than you.
So, in the concatenation of entities, is it the last entity before the ground that experiences the electrical shock? Or is it every entity between the source and the ground?
One of the wires is grounded at the generator or transformer and if something goes wrong you could be the conductor between a faulty appliance and ground. Grounding metal parts means you will not be the one closing the circuit to ground if something goes wrong.
So, the entity that “closes the circuit” is the entity that receives the shock? If so, what does closing the circuit mean? How does it get opened?
All resistive items in a DC circuit will get a shock; most of them just don’t have any sense organs. 
The voltage across all the components in a complete circuit must add up to the voltage at the source. So if there is a 12V DC source and a single resistance, it will “feel” the entire twelve volts. If that is you, you might possibly have a slight tingle. 120V is another matter.
The voltage across a component is proportional to its resistance by Ohm’s Law:
V = IR, read: voltage is equal to the current times the resistance.
So for a simple example, assume a 12V source, and 100 ohms of resistance. Solving for current, we see that there is .12 amps flowing. The way you can look at this is also that current through a resistor creates what we often call a voltage drop; so in any circuit, .12 amps flowing through a 100 ohm resistor will, if you put a voltmeter across it, have 12V.
Let’s complicate the example just a bit. Suppose we have two fifty ohm resistances in it. We still have a total of 100 ohms, so we still have .12 amps flowing, but each resistor now only has .12 X 50 = 6 volts dropped across it. Note that the voltages measured across the resistances still add up to the source of twelve, though.
Finally, let’s consider another two-resistance problem. We’ll keep one hundred ohms total, and so .12 amps total, but instead of two fifty-ohm resistances we’ll have one 20 ohm and one 80 ohm resistance. In this case, using Ohm’s Law and solving for voltage we see that the larger resistance has more voltage across it. In this case, 9.6 volts of the total 12.
At any rate, any resistance in the path will have a current pass through it that is determined by the total resistance of the path by Ohm’s Law (disregarding reactive components). What tends to happen when devices malfunction is that the source itself is open. When you, a grounded individual, touches the source, you complete the circuit. Even if there are other elements in the circuit besides you that are slightly resistive, the same current flows through all elements in series. Everything gets the same shock.
A closed circuit just means a completed path for the electricity to flow. (When you turn on a switch, you are closing the circuit.) Electricity always chooses the path of least resistance. The human body is a mediocre conductor, and the earth is so big that any electricity you put into it spreads out very quickly. Thus, the earth is (for our purposes) essentially an infinately big electricity sink. So if you connect a good conductor (like a wire) to the earth, then the electricity will “choose” to go along the wire instead of through you.
True to a point. I’m only qualifying your remark to avoid a dangerous connection someone could make, I’m not disagreeing with you. Electricity will choose any and all paths it can so long as there is a potential.
Current will pass through any complete circuit that is not shorted (what you suggest). More current will go through a less resistive element than a more resistive one, but whether that other element is in parallel with you or not you’ll get the same shock. 120V across a 10 megaohm resistance will create the same current no matter what else is in the circuit parallel to it.
If you can put a voltmeter across two points and measure voltage, were you to grab the same points with your hands current would certainly pass through you.
Electrical circuits are connected to the earth in order to prevent the buildup of a dangerous voltage on the lines as a result of static electricty. An electrical circuit that is isolated from the earth can develop dangerous voltages relative to the earth as a result of the wind blowing sand or dust into the wires. If anything, such as a person, standing on the earth then comes near the circuit the charge on the circuit, which can be quite large, will discharge through that thing and can be fatal for people.
The terminology can get confusing because one side of a circuit that isn’t even connected to an earth ground is often referred to as “ground.” All this means in that case is that that is the reference level to which all voltage measurements on that circuit are referred.
Hoo, boy. Just like ol’ Nott to complicate things a little more. Another use for grounding is shielding of electronic stuff. If a guitarist has a flawed ground, he might hear buzzing from the lights or taxicab conversations from his amp. If the ground strap for your car’s engine cover comes loose, your engine will mess with radios all over the neighborhood. If a restaurant owner installed grounded wire screen behind the wallpaper in his joint, nobody’s cell phone would work inside.
Disclaimers: I’m not an engineer. G.I. Joe’s weapons will not cause other action figures to die. Fire is dangerously hot. IAN (I am Nott.)
Nitpicks: DC stands for direct current, where electrons flow in one direction only. Also, ground is a zero volt reference. Whether it is attached to earth doesn’t depend on whether the circuit is an AC or DC one.
“Ground” is one of the most confusing terms when talking about eletrical stuff because the term is used in so many ways. In circuits, everything makes a big loop (that’s why they call it a circuit). Electricity goes from one side of the battery, through the wire and whatever else is attached, and completes the loop by returning to the battery. Same thing with an electrical generator, it’s just that generators make that whole alternating current thing where the current actually switches directions 60 times a second.
So how does “ground” fit into all of this?
To start with, for a lot of reasons (many of which have already been mentioned) it’s nice sometimes to have a physical connection to earth ground. So, quite often, we literally have a connection to earth ground, like the typical copper rod which is hammered straight into the earth for your home’s electrical ground.
When you talk about voltages and such, you need some sort of reference point. It makes the math a whole lot easier if you pick one big thing and call it zero, and reference everything else to it. A nice big convenient thing is the earth, so very often we’ll use the earth as our zero reference point.
Now here is where it gets tricky. Because we often call “ground” our zero reference, it has become quite common to refer to our zero reference as “ground” even when there is no physical connection to earth ground present. Take a car for example. It has a 12 volt battery. If you called the positive terminal the zero reference, then the negative terminal is 12 volts lower, or it would be at (-12). However, if you use the negative terminal as the zero reference, then the negative terminal is now 0 instead of -12, and the positive terminal is +12 instead of 0. Since it’s kinda arbitrary which one we pick, what is typically done is “ground” refers to which connection is attached to the car’s body/frame. Most cars are therefore what we call “negative ground” systems. A “positive ground” car would be one that has the positive terminal of the battery connected to the car’s body/frame.
It can get even more tricky. In manufacturing plants, the “ground” often has a lot of electrical noise on it due to the very large motors and such present. So, very often a seperate ground will be run for computers and plant controls. So not only does ground not necessarily mean a connection to earth ground, but there can also be different grounds present. Confused yet?
I’m no expert on dirt, but it’s my understanding that the water in the dirt helps to make it more conductive. Different types of dirt have different conductivities, so in some places you will get a better connection to earth ground than others. Some areas will require larger copper rods for the grounds their electrical systems as a result of this.
Since wire is expensive (especially if you need really long pieces of it), in many early electrical systems the generators only sent one wire out to the countryside, and just drove a connection into the earth of the return. Since some dirt conducts better than others, and how well it conducts can even vary with things like rainfall, these ended up not being very reliable systems, and I don’t think any systems like this are presently in use anywhere. We’ve also learned some other tricks (like 3 phase circuits) to cut down on the wiring costs.
There are two types of electrical systems, grounded and un-grounded. The un-grounded ones are usually refered to as “isolated” systems. In a grounded system, one conductor is always connected to earth. So, if you touch that one conductor you are safe. But if you touch the other conductor, since you are usually standing on the earth, you make contact between both conductors and complete the circuit, and you get zapped. In an isolated system, you can touch either conductor and ground, and not get hurt at all. In that respect, isolated systems are much safer. So, why don’t we use isoalted systems for our power distribution? The answer is simply mother nature. If you try to run a very large isolated system, mother nature likes to randomly insert ground connections all through your system, which ruins your isolation. This is worse than a grounded system, because at least in an intentionally grounded system you can detect the faults a lot easier, and you always know which conductor is grounded and therefore “safe” to touch. So, rather than use a randomly grounded system, we instead use a system where we know where the grounds are.
Isolated systems are used in some places though. If you are ever in a hospital, look for the red outlets. Those outlets are fed from isolation transformers. Any equipment used in a “wet” location (like an operating room where blood may be present on the floor) is required to use isolated power. Hospitals have to check their entire systems quite often to insure that they are staying isolated. Somewhere near all the red outlets you’ll also usually see an ammeter with a test button. If you push the test button it creates a ground fault. If more than 5 mA of current flows, then the isolation isn’t good enough and out come the electricians to fix it.
The part about the water:
The fact is that water in the soil does affect the nature of “grounding” a circuit. Mostly, not much, but in extreme examples, sometimes enough to matter. When I was in El Paso, TX, studying electronics I found a specific example where that mattered.
El Paso is very dry, and its soil is very sandy. Sand is mostly quartz, a very good insulator. Water dissolves some elements in soil, and can increase its conductivity. Dry sandy soil makes a poor circuit material. In El Paso, we had many different types of highly sensitive electronic measuring devices, and all of them needed to be grounded for safety and for accuracy as well. The army’s answer was to drive aluminum cored iron rods into the ground every few hundred feet all over the base, and string bare copper wire between them, and into every building.
So, ground was determined by the potential of a specific grid of wires. That grid was actually connected to the Earth by multiple paths of highly conductive wire.
In most environments, and for most purposes, just connecting the common terminal of the main circuit breaker to the input cold water pipe from the street is enough.
Tris
In the UK, and I’d guess in most industrial advanced nations this would probably not be a good idea.
We have our own set of wiring codes in the UK (just as they have in the US).
In those UK wiring codes, earthing cannot be considered to have been adequately achieved by bonding to water or gas pipes (it is possible that good earths can be obtained this way, but it is not considered to be reliable enough)
The reason for this is that utility companies have been making great efforts to replace old metallic pipework with uPVC and glass reinforced plastic pipes.This reduces leaks massively.
Often the pipework in the street will be replaced but not the metallic stretch from there into the household.
Householders might see a metal pipe entering the house and make the wrong and unsafe assumption, so now all installed systems must either use the incoming neutral from the electricity supplier(this neutral is actually a combined earth and neutral cable) or they must use earthing rods.
The real reason that the gas and water utilities are bonded to earth is not to use them as the earth point, far from it, the real reason is to ensure that these are tied to the incoming earth referance from the electrical utility supplier.
This is an extremely common misunderstanding from those who do not actually work in the electrical distribution industry, it is so common that very many others who work elswhere in the electrical/electronic industry also get this wrong
Earthing rods are more usually used in rural areas such as farms etc.
The other plce you will see earthing rods installed is around tall structures where there is risk of lightening strikes - but these earth systems are completely separate from the electrical supply system.
This is so incredibly over my head. Although I appreciate the work and effort put into the responses, I’m convinced that I will just never get this. I don’t know what my mental obstacle is.
Libertarian,
Rather than pound the rubble some more, let me recommend this site:
http://amasci.com/ele-edu.html
The owner/author is a frequent SDMB poster and has a very practical explanitory style. The first 3 articles on that page are an excellent start on really understanding electricity.
If you scroll about 1/2 way down that page there’s a ref to an article titled “Why 3 prongs?”, which specifically discusses grounding in the commonsense household power context.
Very nice post.
Another place where ungrounded systems are used are U.S. Navy ships. U.S. submarines use 3-phase ungrounded systems. The idea there is to increase reliability of the system. If one phase gets a short to ground, the system will continue to work. If another phase gets a short to ground, circuit breakers will trip and knock out the system.
The disadvantage of the ungrounded system is that there is no indication of a short to ground until the second short occurs. This makes it necessary to frequently check for grounds. In practice, it is done hourly.