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What is the wisdom and purpose of a Ground Node in a circuit?

  1. Jul 20, 2012 #1
    Grounds are always present. Even in a dual supply circuit with +15v/-15v power supply we still need a ground. The question is WHY?

    This question is raised in my mind especially because, battery powered circuits do not have a ground. You may say that we need all voltages with a reference point but what does it happen to be the earth itself? Why can we get away without having one in battery powered circuits?
     
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  3. Jul 20, 2012 #2

    jedishrfu

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  4. Jul 20, 2012 #3
  5. Jul 20, 2012 #4

    cepheid

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    Remember that only differences in electric potential (also known as voltages) are significant, not the absolute values of the potentials. If you claim that a point in a circuit is at a potential of 15 V, the automatic question is, "relative to what?" So, one of the purposes of a ground conductor in a circuit is to serve as the point with respect to which all other voltages in the circuit are measured. In other words, it's the reference point -- the point that you define to be at an electric potential of 0 V.

    So, if you have a power supply with a bipolar output (+/- 15 V), both of those voltages have to be defined relative to a fixed reference point in order to be meaningful. This is the ground reference of the power supply. If you want to connect 15 V across some load (e.g. a lamp), you'd have to supply two wires to it, one to connect to either end. One of wires should be at a potential that is 15 V higher than the potential of the other wire. So, we attach the wires to the +15 V output and the ground of the power supply respectively. (So, in this example, the ground connection is also the return path for current, which is commonplace as well). The +15 V output is at 15 V relative to that ground reference specifically. It's not necessarily at 15 V relative to your shoe or the desk, or the Earth. I hope this helps.
     
  6. Jul 21, 2012 #5

    jim hardy

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    "ground" is a term widely mis-used, and consequently widely misunderstood.

    Establish in your mind the concept of "Circuit Common", for example the steel chassis of an automobile or the aluminum skin of an airplane, to which are connected to the low side of those vehicle's respective batteries and alternators.. Those are "Circuit Common" for the vehicles.
    Next realize that niether of those "Circuit Common"'s is connected to Mother Earth's ground, and though many people call them "ground" that nomenclature causes beginners to make incorrect assumptions about the concept of "Circuit Common" and "Ground".

    So to your question: the purpose of having a "Circuit Common" is to provide a convenient collection point for current that needs to get back to its source per Kirchoff's current law.
    Sometimes that "Circuit Common" is connected to earth by a green wire and sometimes it is not. When it's not it shouldn't be called "Ground" , but "Common" .
    In many appliances it IS connected to earth for safety because your feet are also connected to earth and they want the appliance's skin at local earth potential.
    That way there's no potential difference between it and you, as Cepheid was saying...

    When working in the lab you encounter a device with a terminal marked by the Ground symbol, always check with an ohmmeter whether that terminal is connected to the third prong on power cord.
    If so it's ground
    if not it's circuit common.
    A circuit common thet's not ground may or may not tolerate being connected to ground - i have seen sparks and smoke result from lack of awareness of the difference.


    Used to be standard in electrical drawing circles that "Circuit Common" symbol was either a triangle or the three horizontal lines short medium and long one above the other;
    while "Chassis Ground" looked like a garden rake.

    old jim
     
    Last edited: Jul 21, 2012
  7. Jul 21, 2012 #6
    So Ground/Common just provides a means for reference point and a point to which all current is returned after passing through the components so it can get back to the source.

    Your explanations are quite good but one thing stays unanswered always everywhere.
    The ground as we call it is the mother Earth itself. Its soil, dry or wet, Or perhaps even sand, may be even consisting of very hard Solid Rocks at some places. None of these things "conduct" electricity right?

    This is main reason why Ground and Earth make so little sense to me. Soil, Sand, Hard Solid Rocks and the like are all insulators. So how does connecting them to a circuit work? How can electrons flow out or into the earth? This is actually my main confusion.
     
  8. Jul 21, 2012 #7

    cepheid

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    In AC mains electricity, the Earth doesn't actually complete the circuit in the sense of acting as a return path for current.1 Normally, a wire does that (the neutral wire). However, the neutral wire is grounded (or "earthed" as the British would say) at a single point (for each house, I think) by means of stake driven into the ground. So the idea is that the neutral wire is maintained at a constant potential equal to the potential of the Earth. Earth ground is useful as a fixed reference point, because the idea is that it is so large, that it acts as an ideal conductor in the sense that it is able to absorb an arbitrarily large amount of electric charge without its electric potential changing significantly. So if you define it to be at 0 V, you can be reasonably sure that this zero-point doesn't fluctuate around too much.

    Check out the intro to this article:

    http://en.wikipedia.org/wiki/Ground_(electricity [Broken])

    Another thing to bear in mind is that it's not a binary situation where a material either conducts electrically, or it doesn't at all. There is a whole spectrum of conductivity, with some materials conducting better than others. The materials that you mentioned (silicates etc.) may not conduct as well as metals, but if you have large enough voltage, and a strong connection to Earth by means of a large conductor, then charges will flow to ground when needed, making an Earth ground connection effective in its use as a safety path to divert dangerous levels of current away from the user when a short happens. An extreme example of this is a lightning rod. With a such a strong conductor giving the built up electric charges on clouds a direct path to Earth, you can rest assured that the discharge will travel along this path directly to ground, where it will be absorbed safely. It won't pass through anything else on the way. (EDIT: But the voltage is SO high in this situation, that even air, which is a terrible electrical conductor, becomes ionized, giving the lightning a strong conductive path to ground even in the absence of a lightning rod. The lighting rod is just there to make sure that it doesn't pass through you on the way. Either way, it's finding some path to ground).

    -------------------------------------
    1although apparently it does in some telegraph systems and some low cost rural power distribution networks. Neat. So the answer to your question must be that the Earth conducts *enough* in these situations. I guess if there were enough moisture, then it might conduct pretty well. See here: http://en.wikipedia.org/wiki/Single-wire_earth_return
     
    Last edited by a moderator: May 6, 2017
  9. Jul 21, 2012 #8
  10. Jul 21, 2012 #9

    nsaspook

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    The systems are slowly being harmonized with IEC standards.
    http://www.electrical-installation.schneider-electric.com/ei-guide/pdf_files/EIG-E-LV-distribution.pdf [Broken]
     
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  11. Jul 21, 2012 #10
    Your responses have answered much of my confusions. I then wish to know: Is it possible to have just 1 wire coming from the power plant to our homes. We connect that to our appliance and the other to the ground (earth). In the power plant it has the live wire going out and come to our home, while the other path is connected to ground in the power plant as well. This must work. Why don't we do it this way?

    To: Studiot, yes I have read those posts as well. They have answered some of my confusion especially the one with the electric rod that conducts lighting strike. Its just this image of closed loop conducting path from school days that is really becoming an obstacle. Ground wasn't explained well at that time.

    I have understood now that "Ground" and "Earth" do not have to be the same thing. They may be if ground (common path for all electrical current on return) is to the mother earth but not otherwise. Otherwise it is a case of floating ground e.g in flying machines and battery powered devices.
     
    Last edited: Jul 21, 2012
  12. Jul 21, 2012 #11

    cepheid

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    Oh ok. Sorry. I thought I had heard that said before.
     
  13. Jul 21, 2012 #12
    Hello schippo, glad you are getting the idea.

    Yes indeed this system works and is still used in some remote areas, particularly in Canada and Australia. However it is not in general use for a variety of safety reasons.

    @cepheid.

    Hope I didn't give the wrong impression yes we call grounding earthing.
    But we just don't do this to the neutral supply conductor. Mostly the supply company provides a separate earth conductor, connected to the cable armour, in addition to the supply phase and neutral conductors. If not (unusual) the consumer has to provide his own via ground stakes.
    US practice is to supply the consumer via pase and neutral conductors connected to the primary of a CT transformer at the consumer's end. Thus the consumer would see a floating supply from the transformer secondary if one side was not earthed at his end.
     
    Last edited: Jul 21, 2012
  14. Jul 21, 2012 #13

    cepheid

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    Ah, but I gather from the end of your post #8 that the neutral end of the transformer secondary is indeed earthed (in the US) as I thought?

    So, in the UK, how does it work? Where does the transformer figure into things? Does the separate safety ground conductor (that connects to the cable shield) go directly to the third earth prong that supplies appliances etc, while the live and neutral conductors go to a transformer first?
     
  15. Jul 21, 2012 #14
    There is no transformer is UK practice.

    The company supply will be a three phase and neutral cable buried in the road.

    One phase and neutral will be tailed off (connected) through the company isolation switch and meter to the fused consumer distribution box.

    It is easy to provide the earth however as most supplied are underground so the external cable armour is well earthed. All the consumers protective conductors (including building cross bonding of services) are connected to the company provided earth point at the dist box
     
    Last edited: Jul 21, 2012
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