Concept of "Ground"

Discussion in 'Homework Help' started by tirantloblanc, Mar 9, 2014.

  1. tirantloblanc

    Thread Starter New Member

    Mar 9, 2014
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    I'm new to electronics and i would like to know what's the main difference between a circuit with no connection to ground, and another circuit with a ground connection.

    I read about it in the first chapter (DC) of this book. I can't understand things like this:
    [​IMG]
    [​IMG]

    I mean: In the second image, How could exist a circuit without ground? What implies that ?

    I thought ground connection was a safety measure, whose main objective was to receive the current that otherwise a person would recive by touching the circuit.

    Please note that i'm an absolute beginner in this. Thank you.

    P.D.: I have found another doubt, please answer this question if you think it's adecuate (I don't know if this is an "all engineer" forum and it's inadecuate to post this stupid doubt here)
    [​IMG]
    The last doubt is this: How could the current pass accross the tree and the person? I think that all the space between the tree and the person, including all the ground is a more resistive path than just flowing normally across the resistance.
     
    Last edited: Mar 9, 2014
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you want a definitive answer in detail, I suggest you get hold of the 'bible' of grounding, or if you have a library access, this is a book by Eustace Soares and is published by the IAEI, which is the International Association of Electrical Inspectors and is used be the NEC and others as a reference.
    I got my last copy from AbeBooks clearing house for a virtually new copy for $1.00!
    It pretty much answers all questions on Earth Grounding and bonding.
    Max.
     
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  3. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi tir,
    Your 2ndQ.

    Consider the very high current flowing away, radially from the point were the tree is in the ground when the lightning strikes the tree.

    If the current is say 10,000A and the ground resistance between your feet is 10 ohms, thats 10,000 * 10 = 100,000Volts differential between your feet... Bang!!!

    E
     
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  4. tirantloblanc

    Thread Starter New Member

    Mar 9, 2014
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    I still dont get it. When a lightning strikes a tree, it burns it down because of the high resistance of the wood.

    What I don't get is how the current can go from the voltage source to the tree, and pass through the ground or "dust", and find it's way towards the person touching the wire, and from the person, to the opposite side of the voltage source.

    I mean: The resistance of the tree plus the dust plus the person with its feet on the ground and his hands touching the wire should make that a highly resistive path, am I wrong ?
     
  5. ericgibbs

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    Jan 29, 2010
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    hi
    The ground is resistive, but if you put a very voltage across the ground, it will conduct a heavy current which will create a high voltage drop.

    I will modify one of your drawings and post back.

    The tree often explodes because the sap boils due the heat caused by the strike causing a very high current to flow in the trunk.

    The rule is if you must shelter under a tree in an electrical storm, keep you feet very close together.
     
    Last edited: Mar 9, 2014
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  6. WBahn

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    Mar 31, 2012
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    In general, the term "ground" is used to mean several different things. Here it is being used to indicate a readily accessible conducting medium, which in some of the drawings is the physical ground (i.e., dirt).

    Circuits do not have to be grounded. The circuit is a typical flashlight is not grounded. The electrical system in a car is not grounded (in the earth-ground sense). Saying that grounding an electrical system is for "safety" purposes is true up to a point and only in the proper context. If the electrical system in your house were not grounded, then you could touch any one conductor without fear of getting shocked, while because it IS grounded means that if you touch just one of the conductors you likely WILL get shocked. Doesn't sound like a move in the direction of "safety" does it? The key is to understand that by having a grounded electrical system, most (not all) faults will result in an immediate large flow of current through the fault that is large enough to blow a fuse or breaker before you have the opportunity to get tangled with it.

    In your diagrams, what you are looking for is a closed path that goes THROUGH the battery and THROUGH the human without touching the same node twice.

    Yes, the resistivity of the physical ground is much higher than that of a wire, but remember that the diameter of the wire is small (a fraction of an inch) compared to the effective diameter of the "wire" made up of the ground (tens of feet). Further, keep in mind that the amount of current needed to electrocute you is only a fraction of an ampere.
     
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  7. tirantloblanc

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    Mar 9, 2014
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    So, if someone touches a wire of a grounded electrical system (i.e. a wire of his own home), the current would find a less resistive way, and because of that, the current would grow, but before reaching it's way through the fault, the heat would blow the fuse ?

    Appart from the fact that the houses normally have a concrete floor and the electrical system it's normally grounded on the dust outside (I think that the concrete floor would be more resistive than any wire or metal surface so the chances of getting electrocuted because of making a less resistive path between you and the ground by touching a wire on your house would be small.), I can understand how the ground safety works now.
    Then, because of the effective diameter of the "wire" made by the ground, the current could find it easier to go flow from the tree touching the ground, and then back to the battery, passing through you ?

    It's hard for me to figure that because you normally can touch a wire without worrying precisely because of the resistivity of the plastic covering the wire. The idea of the ground being less resistive than the plastic covering the wire seems impossible.
     
  8. tirantloblanc

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    Mar 9, 2014
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    Thanks !

    So what the drawing means is that if both your feet are 1 meter away, with both feets on the ground you are forming a less resistive path than the own ground, so the current would electrocute you because you would have less Ω between your body that 1 meter of soil.
     
  9. ericgibbs

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    Jan 29, 2010
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    hi,
    The values I have quoted are for explanation only, the actual value will vary depending upon the ground/soil etc.

    There are typical figures given on the web for lightning strikes.
     
  10. WBahn

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    If you touch a hot wire in your house, there will be a path through you to the ground and back to the transformer. The amount of current that will flow will depend on the total resistance of that path including your skin resistance, your shoes, the floor, and everything else. This is usually pretty high and so you will normally receive a relatively mild shock that will serve to get your attention but do little damage. This will seldom cause a fuse or breaker to blow. The fuses/breakers are there to protect the wiring, no you. By the time you have the 15A or 20A needed to blow the breaker flowing through you, your problems are probably pretty much over. Now, the situation for a GFI is different as it IS intended to protect YOU specifically and will blow when there is about 5mA of DIFFERENCE between the current flowing in the hot and the neutral.

    Concrete is usually a pretty decent electrical conductor. Concrete in the earth (a house foundation or slab) normally has a fair amount of moisture absorbed it in. For just illustration, consider a 10m x 10m concrete pad and imagine dividing it into vertical rods of concrete that are each 1cm x 1cm and that have a resistance of 10MΩ. There are 1 million of those "rods" in parallel yielding a total resistance of only 10Ω. The effective resistance back to the transformer when you are standing on concrete will be primarily determined by the contact resistance of you with the concrete.
     
  11. amilton542

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    Nov 13, 2010
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    I've read this thread with interest, and the definition of ground has boiled down to power engineering nomenclature associated with fault current.

    What is the electronics definition of ground? To measure absolute potential relative to zero?
     
  12. WBahn

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    That's because the OP's context of use is in that realm.

    There is no such thing as absolute potential or absolute zero when it comes to electric potential. The definition of electric potential is fundamentally a difference in potential energy between two point.

    In general, we get to arbitrarily pick one point and declare the potential at that point to be zero volts. But all we are really doing is saying that we will measure (refer) all voltages relative to that point. Since "all voltages" includes the voltage at that point, the voltage at that point will be zero volts (as long as we are talking about conservative electrical fields).

    In electronics, we should call that point the circuit "common" (and use something other than the normal "ground" symbol that is used in the diagrams in the OP) , but using the term "ground" is widespread and not likely to go away any time soon. But, we can accept what is meant when others use it without needlessly contributing to its continuation by using "common" instead of "ground" unless we are talking about a true earth-referred ground (or unless the context of the conversation might result in confusion if different parties are using multiple terms for the same thing without understanding that they are the same thing).
     
  13. amilton542

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    Nov 13, 2010
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    The line integral states its independent of the path how voltages accumalate relative to one point amid another; hence a potential difference.

    What I struggle to understand in electronics is how so many components can be terminated straight to ground? Even using a 'scope I'm informed the circuit element of interest must be measured as the last component to ground? I don't understand why, what difference does it make if I tie all components to a common ground and take that same measurement?
     
  14. WBahn

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    You have to read the fine print on that one. This is true only for conservative electric fields.

    I don't follow your point. Perhaps a sketch would make it clear what you are talking about. I have a feeling that you are getting at the impact of the ground connections being non-ideal and hence the measurements being influenced by the presence of ground currents due to other components. But a sketch would sure help.
     
  15. amilton542

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    Yes, ground currents were definately mentioned. Unfortunately I don't have a computer, I use an internet explorer app on my Xbox, so it makes posting diagrams virtually impossible; but I'll get back to you on this one.
     
  16. BytetoEat

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    Mar 5, 2014
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    Like WBahn explained,

    Ground is simply calling a certain voltage potential "zero"

    If you take a 9V batter and connect it to a light bulb, you can call the positive or negative terminal "ground" and it will make no difference in how the circuit behaves. It's just what you are defining as being 0Volts. Everything from that point will be in reference to that point. So if you call the positive terminal ground, then the negative terminal will be -9V from ground.. Think of everything as relative and it will make more sense. To answer the original question, The reason the man gets shocked in the first case is, the bottom of the high voltage source is referenced to ground. If the man touches the high voltage positive terminal and his feet touch the negative terminal, which is ground in this case, then he is acting as a wire shorting the high voltage source.

    In the second picture, youll notice that the high voltage negative terminal is now "floating", and the POSITIVE terminal is now ground(because the man is acting like a wire to ground. So that means the negative terminal is at -superhighvoltage!
    He is not closing the loop of the circuit in this case, so he is safe.

    In the third picture, notice that the tree is now being called ground and the positive terminal of the high voltage source is connected to this. So when the top man touches this positive rail and his feet touch ground, there is ZERO voltage difference across him (both his feet and hands are touching the positive rail of the source).

    The lower man however is touching the highly negative voltage with his hand and ground with his feet, creating a large voltage difference across his body (hence him being shocked). Remember, the current will flow from his feet to his hand in this case since ground is at a much higher potential than his hand!
     
  17. MaxHeadRoom

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  18. amilton542

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    @ Byte to eat

    You've fed me the bird on the wire as well as proving my point; the description has boiled down to power engineering and fault current.
     
  19. amilton542

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  20. PsySc0rpi0n

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    Mar 4, 2014
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    I have readed somewhere (but find hard to beieve) that a bird doesn't get electrified because the space/distance between his legs/fingers is not enough to make a potencial difference!
     
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