Grounding, basic but tough topic

Discussion in 'General Electronics Chat' started by unsaint32, Feb 23, 2005.

  1. unsaint32

    Thread Starter Member

    Jan 13, 2004
    14
    0
    There are many questions I have about grounding. But I am willig to solve them one at a time. I am intentionally avoiding the term "ground" since it has such many usages.

    Question 1) Among many reasons, connecting an electrical system to earth provides a reference point for input voltage, therefore achieving steady voltage to loads. But, since we connect all neutral conductors together at the main service panel, why can't the neutral conductor be used as the reference point, instead of connecting the circuit to earth? If I have a neutral conductor bonded at the service panel and no part of my system (not even the service transfromers) is connected to the earth, wouldn't I still get stable voltages to the loads in the circuit?

    Question 2) Along the same question ... I also read that connecting circuit to earth stabilizes the "voltage to earth" or "voltage to ground" under normal operation. But, why do I care about the voltage to earth as long as I have voltage to loads?



    I'll appreciate your answer.
     
  2. Firestorm

    Senior Member

    Jan 24, 2005
    353
    0
    this is a question whos answer is basically "because"...u need a ground also for safety...ill dig around in some books and try to come up with a professional answer...hope i can help

    -fire
     
  3. n9xv

    Senior Member

    Jan 18, 2005
    329
    1
    Regarding question #1;

    For what you are saying to work it would mean that the entire power transmission system would have to be an isolated circuit. That is, nothing being tied to earth as ground. The reason that idea is not employed is because it would require hundereds of miles of a"ground loop" cabeling in addition to the hundereds of miles of cabeling carrying the voltage & current. The inherent losses of the system would be so great that power transmission would not be feasable. The grounding (or neutral) connection would electrically be one big resistor! There is actually tens of thousands of kilowatt losses in the system as it is. (I wonder who pays for that :D ). One scenario where that would apply would be a home generator system in which the complete scheme was indipendant of the power grid.

    Regarding question #2;

    The implication of "stabilizing" voltages to earth means that the power station generates X amount of voltage "with respect to ground/earth". You have the luxury of getting that same voltage where you live by simply connecting one wire to the "hot" side carrying voltage and the other side to "earth/ground". What better common tie point than the earth? its everwhere!
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    As far as any circuitry is concerned, the earth ground is of no concern. Circuit ground will work just as effectively even if that potential is thousands of volts different from earth.

    The aspect of the earth ground that is important is the one of electrical safety. Take any electrical hardware that is isolated, and imagine the effect on anyone coming into contact with a potential several hundred volts different from the earth. All the makings of a crispy critter.

    I can recall my mother getting knocked across the kitchen in a new house in Florida because the stove was ungrounded and she was barefoot on a terrazo (concrete) floor. Life is much nicer if everything you touch has the same potential.

    In support of my stability contention, I saw a Univac computer 32 years ago that nobody could get sensible voltage readings from. It worked fine, but internal voltages were all wrong and different every time anyone checked. We finally tore it down and found that a big copper stud on the transformer has literally vaporized. The ceramic ring around it had lots of verdigris, but about 4 oz. of copper had just vanished. The stud was the attachment point for the earth ground strap. That's still a mystery...
     
  5. unsaint32

    Thread Starter Member

    Jan 13, 2004
    14
    0
    Thanks for the answer n9xv

    ==================================================
    You said: For what you are saying to work it would mean that the entire power transmission system would have to be an isolated circuit. That is, nothing being tied to earth as ground. The reason that idea is not employed is because it would require hundereds of miles of a"ground loop" cabeling in addition to the hundereds of miles of cabeling carrying the voltage & current. The inherent losses of the system would be so great that power transmission would not be feasable. The grounding (or neutral) connection would electrically be one big resistor! There is actually tens of thousands of kilowatt losses in the system as it is.
    ==================================================

    Here is my question:
    Why would I need a separate cabling if the entire power transmission system wasn’t grounded? Doesn’t current flow through the neutral conductor back to the power plant during normal operation?

    I guess I don’t know exactly when current flows through the ground? Let me make a multiple answer type question to make it easier for you to answer.

    When does current flow through the ground?
    A) at ground-fault situations
    B) with lightning surges
    C) during normal operation (all the time). Current never flows through the neutral conductor once current finds ground path.
    D) All of the above

    *For B, I know that lightning sends high voltage to ground. I was wondering why lighting doesn’t trip the circuit breakers. Does the lightning voltage simply dissipate in the ground or does it take low impedance path to a supplying system?
     
  6. n9xv

    Senior Member

    Jan 18, 2005
    329
    1
    Well, lets take one step at a time.

    In your residential breaker box you have three wires coming in from the pole. There is 2 insulated wires and 1 bare/uninsulated wire. The 2 insulated wires carry 120-volts each. These 2 120-volt lines are 180-degrees out of phase with each other. The other bare/insulated wire is ground. The ground wire is physically grounded to earth at various points throughout the system. There may be a ground connection at the pole to your house or it may be at a point 2 or 3 poles down the line.

    Lets stop here for a moment and see what we measure with our DMM/voltmeter.

    >>> if you actually take these measurements, BE CAREFULL!!! There is plenty of power there to KILL YOU instantly!!! Use your own judgement. <<<

    First check continuity to ground. Black lead to bare/unisulated wire and red lead to earth should read full continuity (no resistance). There should be a ground wire running from the "neutral" or grounding block in the breaker pannel to a ground rod outside near the service entry point. If you dont have a ground rod then the ground wire should be going to the cold water pipe where the water main enters the residence.

    Now, from ground/neutral to one of the insulated wires you will read 120-volts. From ground/neutral to the other insulated wire you will read 120-volts. Now take the black lead off the uninsulated/ground wire and touch it to one of the insulated wires and then touch the red lead to the other insulated wire and you will read 240-volts. (each 120-volt line being 180-degrees out of phase with each other).

    In a residential wireing scheme you have three wires. A black wire ("hot" side carrying 120-volts), a white wire (generally called system ground), and a bare/uninsulated wire (earth ground).

    For all the 120-volt items in your house the Black wire carries 120-volts and the white and bare wires are connected together at each outlet, switch, etc. The so called "neutral" or system ground is simply a common tie point for all the interior wireing. This White "neutral" wire is connected to the grounding block at the breaker box. Neutral and ground are the same thing. They are physically connected together at the breaker box and then ran directly to earth ground.

    For all the 240-volt items, A black wire carries 120-volts, and another black wire (sometimes red) carries 120-volts. Between these two wire you have 240-volts. and the bare wire is ground.

    These voltages can only be stable with a ridged multiple connection point grounding system. Current is always flowing to ground. That is, ground is the refference that gives the voltage its potential difference from ground. There has to be a path for current to return to or make its path complete. If you connect an outlets "hot" wire to one side of a 120-volt light bulb and the other side of the bulb left unconnected, the bulb will not light because no current is flowing. Now, connect the previously unconnected side of the bulb to earth and - behold - there shall be light. Which is to demonstrate that current must have a return path for the circuit to be complete. This return path is ultimately earth.

    I'll wait to see if that cleared up any questions (or created more :) )

    As far as lightning is concerned, it is the most unpredictable form of raw power to say the least. Like any flow of current it seeks the path of least resistance. But lightning at 100s of 1000s of amps to mega-amps and more, its not like any other current flow! I've seen guys use a little #14 scrap piece of wire and connect it to a TV antenna mast and then to ground. There thinking was that lightning would simply follow this nice little path to ground and not cause any other damage. Well, I guess you get what I'm saying. Thats about as logical as sticking your tounge on the mains coming in to the breaker box to see if it's "live". "It" might be live, but now the person on the other end of the tounge aint! :unsure: .

    n9xv
     
  7. unsaint32

    Thread Starter Member

    Jan 13, 2004
    14
    0
    thanks for the very detailed explanation, n9xv.

    I however tried the experiment you suggested with slightly different circumstances I beleve. I knew ground is a good conductor. However, when I put two leads of my ohmmeter onto the cement floor in my utiliy room in my house, I didn't even get continuity. I got good continuity when I apply the two leads on a same piece of metal.
    Either way, I know that earth is not as good of a conductor as a copper wire. Therefore, I believe during normal operation current travels through the return wire, not through earth. And when ground fault occurs, the fault current travels also through the wire (or at least that shall be the intention) because the impedance of the earth path is not low enough to trip circuit breakers.
    If my assumption stated above is correct, I believe the connecting circuits to earth is good for only two reasons. 1. To stablize voltage to loads. 2. To redirect and make high lightning surges dissipate into the earth.
    So, if my assumptions are correct, I wouldn't need a separate grounding cable to avoid voltage drop across the return wire to the power plant, as you stated before, because current is already traveling through existing wires back to the powerplant.

    I have a feeling that either some of my assumptions are wrong, or there is something else I do not know that you do. I'll wait for your reply. Thanks.
    You can reply directly to me to my email address, if you like. volleyballkim@hotmail.com (I don't know if that's allowed)

    Sung
     
  8. n9xv

    Senior Member

    Jan 18, 2005
    329
    1
    I think your assumtions are basically in check. Just realize that earth is the ultimate reference for the potential difference. Ultimately, when one uses the term "ground", a physical connection is to the actual planet earth is implied. This could be locally or at some distant point. This old ball of dirt where living on seem to have unlimited uses :) . Feel free to respond via email. Sometimes I have trouble logging on.

    Kevin,

    n9xv@msn.com

    The n9xv is my amatuer radio callsign. (somthing I can easily remember).

    You can call me by Kevin or n9xv - doesnt matter.
     
  9. Intrigued

    New Member

    Mar 14, 2005
    1
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  10. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    Just remember that the hot wire is an "ungrounded" conductor; the neutral is a "groundED" conductor; and the ground wire is a "groundING" conductor. the neutral and the ground are bonded in the panel board in your house for safety. The wires will work without the ground.
     
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