Something I've never quite had a model for understanding about: Why does electricity want to flow to the earth-ground? Is it because the generator producing it somehow is involved with the earth such that touching the earth completes the circuit?
If there is no need to complete the circuit when the earth is concerned, then for what size planet-body or planetoid does this still hold true?
------------------ Doesn't, it doesn't.
First, RF uses: Earth being mostly metal and salt water is a passible RF and E field reflector, nothing more. Not all planetoids are. That's useful for transmission, so let's use it as one arm of an antenna. Most fixed broadcasters do. You get better reception from one of them with a ground, but as portable radios show, that's not necessary.
And also: Power supplies really have no need of the earth as "ground", and AC power systems only use it to replace expensive copper wire, by grounding one arm of the generator plant's alternator to the earth. This enables us to use earth as a protective shield ground from either pole of the alternator, as well as eliminating most of the danger from the grounded pole also used as "return-neutral".
Now, finally: Ground symbols on a schematic are simply a convenience to drawing and reading the schematic, as it eliminates all return paths back to the power supply, thus making the flow of signals and power OUT TO their targets more obvious, instead of having all these lines/wires looping back to the supply and confusing things. Just pretend the negative battery ground is the paper/screen background color, and the wires are insulated conductors from it.
-- That's all, there's no magic, people have simply allowed this to sound like confusing poorly explained superstition.
-Steve
does
It doesn't.
For example, connect a battery to a flashlight bulb so that the bulb lights up. Now suddenly connect one side of the circuit to a ground- rod. The brightness doesn't change one bit. Try it with a 120VAC hand-cranked generator and the results will be the same. "Grounding" one side of the circuit has no discernable effect.
On the other hand, the earth is important when dealing with single-plate capacitor effects. This is where you see "electricity wanting to flow to the Earth."
If you charge up a SINGLE capacitor plate, then whenever you touch that
plate to a large conductive object, the excess charges will share between them in proportion to the capacitance of the plate and the conductive object. For example, if you hold a metal screw with plastic tongs, then you charge up the screw to 10KV, then touch the screw to a steel mixing
bowl, the screw will lose most of its charge-imbalance. Touch the charged screw to a large object (such as the earth,) and the screw will essentially lose all of its charge imbalance.
the earth
Yes, there's a third issue here as well. One prong of the two-prong electric outlets in your home is connected to the Earth. This is done for safety; mostly for lightning safety. Since one side of the AC system is connectd to a ground-rod, then whenever you stand on wet ground with bare feet, YOUR BODY is also connected to one side of the AC system. If you touch the other side of the circuit, then 120VAC is connected across your body.
If the AC power system had no connections to ground-rods, then the entire AC circuit would be electrically "floating" (just like a flashlight is floating.) In that case, touching ground would have almost no effect. But if lightning struck the power lines many miles away, it would have a very lethal effect to anyone who was near an electrical outlet.
Also see:
WHY THREE PRONGS
(((((((((((((((((( ( ( ( ( (O) ) ) ) ) ))))))))))))))))))) William J. Beaty SCIENCE HOBBYIST website snipped-for-privacy@eskimo.com
Not quite true. AC systems normally use metal ground return lines. They only occasionally use "Single Wire Earth Return" way out in rural areas, in situations where putting an AC current into the dirt will not cause much trouble. In heavily civilized areas, railroad rails as well as sewer and water lines would intercept such earth currents, occasionally creating lethal hazards.
And as the OP guessed, one side of the AC system is connected to a ground rod for lightning safety.
(((((((((((((((((( ( ( ( ( (O) ) ) ) ) ))))))))))))))))))) William J. Beaty SCIENCE HOBBYIST website snipped-for-privacy@eskimo.com
--------------- Yes, but only because earth grounds often have poor conduction. If they were as adaquate as wires they wouldn't bother with wires.
----------------- Even where they use wire returns they use earth ground, to place the earth's potential at a well defined and controlled value.
------------------------------------------ Nonsense. Current is unimportant, only potential drops are important. Occasionally would have to be VERY near major inlets of current to ground.
-------------------------------------- There's no need for that either, a non-grounded system is not more subject to lightning. A grounded system is.
-Steve
To understand grounding principles (earthing is only one type of ground), begin with electrical nature of wire. Electricity is never same on both ends of a wire. All wire is an electronic component. Some electrical currents are so small that electricity 'appears' to be same at both ends. But one factor necessary to understand grounding - wire is an electronic component. Electricity is different at both ends.
Many different grounds exist. Motherboard logic ground connects to chassis ground, connects to wall receptacle safety ground, connects to breaker box neutral bus (the breaker box safety ground), connects to earth ground, connects to telephone NID (surge protector) ground, connects to TV antenna ground, connects to CATV shield, connects to utility's transformer neutral, connects to utility pole earth ground, connects to water pipes ... All are interconnected and all have electrical differences.
Primary reason for connecting all grounds together - human safety. We take one point in each electric circuit and call that point ground. Which point is selected as ground is based upon engineering principles. But the selected point is mostly a standard - and primarily for human safety reasons.
Some of the exampled grounds are also isolated from others by using only a single point connection. For example, motherboard logic ground is best connected to chassis ground at a single point. And motherboard logic ground must make no direct connection to earth ground. Blender safety ground cannot connect direct to motherboard logic ground even though motherboard eventually connects breaker box safety ground. Lightning rod cannot connect to chassis even though chassis eventually connects to earth ground.
Reasons why grounds are unique involve their functions. Antenna may require earth ground for lightning protection AND may use earth as part of the antenna. Two functions share a same ground. Logic ground and analog ground on a printed circuit board may be kept separate except at one point so that noise in one ground does not cause problems to circuits on other ground. Stereo components are grounded at a single point to avoid hum. But in each case, the grounds have some type of interconnection for human safety reasons. Exactly why the rock and roll band microphone should share a common ground point with the electric guitar.
Some grounds do float. Many have damaged oscilloscope probes inside computer power supplies or inside monitors because the ground is not connected to other grounds. These are called 'floating' grounds. Floating grounds are encased so that humans do not touch those grounds. A human could suffer a hundreds volt shock by touching the floating ground. When they put that high voltage warning on the case, it is quite likely that a floating ground threatens human life.
We ground for so many reasons. Grounds have unique names because they have different functions. For example, the wall receptacle ground is a safety ground and is not an earth ground. We are careful to plan how we interconnect those ground to avoid electrical phenomena such as ground loops (ie hum). Reason why grounds are slightly different: wires (conductors) are electronic devices.
Not all electricity seeks earth ground. Static electric discharge from your hand my never use earth ground. The circuit up through your body, through door knob, > Something I've never quite had a model for understanding about: Why
Now I'm really confused. I've read through all of this.
Let me unwind this a bit at a time. I was unaware that the return/neutral was actually attached to earth-ground. I thought it simply led back to the power source by wire and that the 3rd plug was the only earth-ground. Given that the neutral is now earth ground, doesn't that indicate that electricity already wants to return to the earth (in a fashion).
Note---My electrical understanding is as follows (I'm a software guy), so you can (if you like) tailor explanations accordingly. However, please be patient, since the gaps in this understanding is odd.
Car wiring A/C simple wiring (switches, etc.) Stereo line/level, and video signal issues. multi-meter testing, including resistance. Vague understanding of impedance. Digital TTL in college, including wiring up flip/flops, up/down counters, karnaugh maps, etc. Physics: hole-charges, electron propagation (some)
But in all that, I'm missing the role of earth-ground, and /why/, as I stated.
There are at least two important reasons why buildings are wired with a grounded neutral: (1) If lightning strikes the building, the wiring system will be able to conduct and dissipate most of the charge. (2) If a high-voltage line falls across a service entrance, the grounded neutral will prevent the wiring system from reaching a dangerous potential with respect to ground.
So, what is going on here is safety (first and foremost). There is no way to prevent occupants from achieving a ground reference through their feet or hands or whatever. So, the grounded wiring system tries to prevent equipment from reaching a dangerous potential with respect to ground.
R. Steve Walz coughed up:
...[rip]...
Ok, so I see two scenarios, and the world seems to have picked the 2nd:
---The way it is?---> 2. A generator extending only the hot, and using the earth as the return, sort of one big round horribly conductive wire.
Correct so far?
...[rip]...
Earth as an electrical conductor was true for early telegraph. But earth was not sufficient which is why telephones could not use earth. Earth is a conductor just like wire. Earth is also an electronic component of the circuit.
Earth is not used by AC electric generators to transmit electricity for same reason. In the case of an electric system, where you find a transformer, instead think of that transformer as a battery (an AC battery). Then forget about everything from that transformer back to generator. You are now ready to discuss earthing from the perspective of that circuit. That circuit and the circuits inside the house are many different electrical loops.
To understand grounding, one must first take the perspective of each circuit - each loop. Then step back, look for a common point between each of the many circuits, and declare that common point ground.
You are having trouble with grounding because you are trying to think in terms of only one circuit - one electric loop. Furthermore all conductors are electronic components. Everywhere in that loop, the electricity can be different even though the entire loop may share a common current.
From that transformer into the house are numerous circuits - loops - again because wire is an electronic device. We wire the many loops so that one point in each loop is common to other loops. We then declared that one point a 'ground'. Then we take the many different 'grounds' and connect them to a common point. Eventually all different 'common point grounds' come together at one 'big time common point' - the earth ground. We have constructed circuits that protect human life.
The point of earth ground is not to conduct electricity. It conducts electricity because we have weaknesses, leakages, or unexpected transients. The most important of these adversities are the currents that can kill humans. Earth ground is first and foremost to create a safe human environment. The primary reason why we arbitrarily select a point in each electric circuit and call it ground: human safety. There are secondary reasons. But first we must define the primary reason - human safety - so that AC electric does not kill humans.
Earth is not a good conductor - depending on the parameters we are discussing. But when it comes to keeping electricity out of a human body, then earth becomes a superb conductor.
Sidebar - what happens to a car when we go all plastic body? Now we have a major grounding (not earthing) problem - involving things not related to human safety. IOW there are other reasons for declaring a point in each circuit as ground. But for Grounding 101, the only reason to first worry about is human safety - so that utility AC electric cannot even harm even when you are at greatest risk - in the shower or steel bathtub.
"Thomas G. Marshall" wrote:
---------------------- Has nothing to do with any "all reaches of the universe".
Potential only exists BETWEEN the two poles of a generator/alternator. If you drag around the hot, you have to drag around the neutral, or have a damned good ground stake.
-Steve
return/neutral
Yep, that's the key concept.
Given
electricity
Nope. While it's true that the neutral is connected to a ground rod, that doesn't affect the currents in the circuit. Think: if the neutral WASN"T connected to a ground rod, the charges in the "hot" wire would still want to flow to the neutral (since hot and neutral are essentially the two terminals of an AC generator.)
Under normal conditions, the value of current in the wire which leads to the ground rod is zero. Under normal conditions, the ground rod does nothing.
That really only applies to unbalanced charges on single capacitor plates, i.e. "static electricity" physics. Where power circuits are concerned, there will be no currents leading to ground UNLESS you connect some OTHER part of the circuit to a different ground rod, then use the dirt between the two ground rods as a conductor.
As I said before, if you have a flashlight circuit, and you connect one side of that circuit to a ground rod, it has no effect. All the current remains in the closed circuit. The current in the wire leading to the ground-rod is zero.
IUnder normal conditions, the ground connection HAS no role.
How about under abnormal conditions? For example, during a lightning strike to the power lines? Or during a wind storm, when a 14KV power line falls onto the 120V lines leading to your home? Ah, that's totally different. In that case, having part of the AC circuit connected to a ground rod will keep your little body from being turned into charcoal.
((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer snipped-for-privacy@chem.washington.edu UW Chem Dept, Bagley Hall RM74 snipped-for-privacy@eskimo.com Box 351700, Seattle, WA 98195-1700 ph206-543-6195 http//staff.washington.edu/wbeaty/
2nd:
using the
No. There is a 3rd scenario, where a generator extends to lengths of cable all over the place... and then we connect one of those cables to a ground rod. Since the ground rod is not part of any complete circuit, the ground rod normally does nothing at all.
However, if lightning strikes either of the cables, it can find a direct path to ground. Without that ground rod, you'd see long sparks leaping out of the entire power system. If you happened to be holding a blow-dryer while standing on a wet bathroom floor, and then lightning strikes the power lines outside your home, the path of the lightning could be out of the blow-dryer wiring, jump a small gap to your hand, travel through your body (say, 500 amperes which permanently destroys your nervous system), then flows out through your wet feet and into the nearby metal bathtub and sewer lines.
Trust me. If your locality has lightning storms, or if sometimes a broken HV power line falls across the 120V cables leading to your house, you DO want one side of your electric outlets connected to a ground stake!
((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer snipped-for-privacy@chem.washington.edu UW Chem Dept, Bagley Hall RM74 snipped-for-privacy@eskimo.com Box 351700, Seattle, WA 98195-1700 ph206-543-6195 http//staff.washington.edu/wbeaty/
Figure of speech meaning the same thing you say about "dragging around the neutral".
Ok. So from all electrical power plants are extended a minium of two wires? hot and neutral. And those two need to go everywhere in pairs. Right?
So it's not likely nor possible to have a power plant simply send 1 line everywhere, and have everything return to the earth?
If so, that makes much more sense to me, since my electrical background, as odd as it is, always speaks of completing the circuit without the earth (wires, computer chassis, car chassis, etc.)
No. Wires from power plant do not connect to your building. There is no neutral wire leaving the power plant. Your building is galvanically isolated from the power plant. This was explained in previous posts. Earth ground (one of many different grounds) does not become the conductor as also explained in a previous post:
To understand the circuit, think of that utility pole transformer as a battery. Pretend there is no other connection. Pretend that utility pole 'battery' provides all the power to you building. Now you are ready to understand how your AC electricity works, and how earthing and other different grounds works.
You are c> ...
To complicate the matter further: in Swedish, the word for "earth" and the word for "ground" is the same: jord.
w_tom coughed up:
But this itself is vague. It says /transmit/. I was referring to simply using the ground as a return.
And besides, R. Steve Walz just got done telling me:
R. Steve Walz: If you drag around the hot, you have to drag around the neutral, or have a damned good ground stake.
This implies what then? That either:
The generators I've fooled around with always have two leads, just like a battery does. Where does that neutral lead of the power plant generator go. To the earth?
Yep, that part was explained already. I'm trying to phathom the generator part now.
That's fairly easy to understand. What I don't understand is how that "battery" can be fed by only a hot wire from the generator. How is that circuit completd?
Yes, yes, yes, I understand all that. I've done car wiring, pc wiring, and even wired up counters using all TTL.
But given that you need to *always* complete a circuit, how is the circuit completed from, say, a diesel generator at the plant outward to that transformer on the pole.
A generator has two leads. One for power and one for.....the return?
...[rip]...
------------ Yup.
-------------------- If the earth was a perfect conductor, sure, but since...
----------------------- Yes, the whole "ground" mystique is baloney. It gets confusing because of RF broadcast and the use of the earth as one arm of a reflective antenna. Combine that with the safety uses to determine relative voltage to a universal level for power distribution and it makes it completely so.
-Steve
---------------------- Sure there is, sortof, it consists of the three-phases WRT each other, and this finally goes through transformers that DO connect an arm of the supply to a more local ground stake.
-Steve
------------------ Sort of, instead the alternator at the power plant has three wires coming out, three phases of AC that are sine waves 120 degrees apart.
It goes into a three-phase transformer to crank it up for transmission and then back down locally for lower voltage, and it passes through a delta-wye transformer to separate the phases and then center-tapped transformers are used to generate 240VAC for local home supply. It is this local center-tap that gets connected to the ground stake to reference the local voltage to the ground for safety.
Is that better, everyone?
-Steve
Have something to add? Share your thoughts — no account required.
Ask the community — no account required