Grounding 101

Discussion in 'General Electronics Chat' started by LearningElectrician, Aug 24, 2008.

  1. LearningElectrician

    Thread Starter New Member

    Aug 24, 2008
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    Okay, this is the beginning of my journey to understanding grounding.



    Consider the diagram I posted below (I hope it actually shows up when I submit this thread), but just in case it doesn't, I'll give a description). Consider a circuit containing a source voltage, a load, and a ground connecting the conducting wire to earth; in this circuit there is no ground wire connecting a chassis to earth. In other words, say a circuit contains an outside source voltage (a power plant) connected to a metal household toaster, and a ground rod is connected from the conducting wire outside to earth, but the toaster was built with no ground wire connecting it's metal chassis to earth. The diagram i'm describing is of the picture posted right below...

    [​IMG]


    ....In this picture, there is a man being shocked because the hot side of the circuit is shorted to the metal chassis of the toaster. Supposedly, he is creating a complete circuit between negative and positive (neutral and hot) sections of the circuit.

    From the drawing above, it seems as though the short circuit on the hot side is touching the chassis while still remaining in contact with it's original circuit (as though it is fraying or chaffing against the chassis). I'm assuming this would give the electrons 2 seperate paths to complete a circuit. One through the originally designed circuit (through the toaster resistance) and the other path going into the earth and traveling through the man directly to the hot side of the circuit (completely bypassing the toaster's resistance). I read that electricity takes the path of least resistance. Armed with this knowlegde of electricity taking the path of least resistance, my question is simply, why does the man in the picture have to get shocked? I would assume that the electricity would rather just go through the toaster (bypassing the new option of going through the earth) since the toaster's resitance is most likely much less than the earth's and person's resistance. So the question is, why doesn't the electricity just continue it's originally designed path through the toaster since it poses less resistance than the electricity going through the new "shorted" path. Given the choice, doesn't theory say that the electricity should choose not to go through the highly resistive earth and the person if it can go through the less resistive toaster?




    LearningElectrician
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Because while electricity takes the path of least resistance, it doesn't do so exclusively. What you are looking at is a parallel circuit, visualize the same diagram as two resistors, each connected to the hot lead and neutral. Both form a separate circuit.
     
  3. mik3

    Senior Member

    Feb 4, 2008
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    The idea that electricity goes for mthe path with the least resistance is used by the kids at high school when they start to learn electronics. According to your drawing, you have two loads, the toaster and the man. They are connected in parallel to the same voltage source and each load draws the current it needs according to its resistance or more accurate its impedance. If you know the basics about current division in parallel circuits you will understand why the man gets shocked.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    If the current always only followed the path of least resistance, then insulating conductors would hardly be necessary. But, if there is a voltage difference between two points, any conductor, however poor, will have current in it if connected between those points.
     
  5. rp_1

    New Member

    Aug 22, 2008
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    To add to the answers of Bill, Mik and beenthere, please see the attachment.

    The human body acts as another resistor between the hot wire and ground. Current will flow in a wire if there is a potential difference between the two points.

    Kirchoff's current law states that at any junction, incoming current is equal to the sum of outgoing currents.

    Hope it clears your doubt.

    RP
     
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  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Yes it's true the mains has enough capacity to drive current through both the man and the toaster, or 100 men and 100 toasters all at once and still have capacity to drive current through something else if you connect that as well.

    This has already been said.

    What has not been said is the relatively small amount of current required to kill a man, comapred with the amount required to toast a slice of bread.

    Bread toasting takes 10amps plus, human killing takes 50 milliamps or less, a ratio of 50 : 10, 000
     
  7. LearningElectrician

    Thread Starter New Member

    Aug 24, 2008
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    Okay so this diagram ......

    [​IMG]



    ....is equal to this diagram....

    [​IMG]

    Minus R3 (the third branch) of course...

    Would that mean that if I touch the broken hot wire toaster while it is toasting a slice of bread and I get shocked by creating an unintended parallel circuit through earth, in theory, I will not affect the current flowing into the toaster (therefore, not affecting the operation of the toaster's bread toasting ability); I will only be adding total current before outside of the branch paths. This this correct?

    Also, would that mean if I were to touch the broken hot wire toaster while toasting a slice of bread and at the same time another person touched the toaster and got shocked, the added current running through his body would not affect the amount of current running through my body, since he would be just another parallel path added to the circuit. This would also mean that the current running through both our bodies would not have any affect on the current running through the toaster for the same reasons already mentioned.

    Is this all correct so far?
     
  8. studiot

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    Yes and Yes
     
  9. studiot

    AAC Fanatic!

    Nov 9, 2007
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    It is flawed to say
    .
    Whilst this is certainly true, it is incomplete. Worse it is could be taken to mean that electricity does not take any other path, which is not true.

    It is not much better to say that most of the electricity takes the path of least resistance, as this does not tell you how much is most.

    A good working statement is

    Electricity takes all paths available to it, in inverse proportion to the resistance of that path.

    this is what other meant by parallel paths etc.
     
  10. PRS

    Well-Known Member

    Aug 24, 2008
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    There was an argument about this between Westinghouse, favoring AC and General Electric (Edison) in the beginning of the creation of electrical systems for urban use. The big thing in favor of AC was the use of transformers to reduce voltage without energy loss.

    A person is not a resistor. He or she is a resistor in parallel with a capacitor. And the AC current flows through people due to this. The higher the frequency of the current, the more of the electicity goes through the body. That's one reason a low frequency was used, 60 cycles per second. A higher frequency would have sent the whole thing through you! LOL! ;)
     
  11. theamber

    Active Member

    Jun 13, 2008
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    Kirchoff's laws sumarize the whole thing. It is very good you are questioning every detail and understanding you will become a great electrician.
     
  12. JoeJester

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    Apr 26, 2005
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    And your reference for that is?

    Are you saying that people can't be affected by DC?
     
  13. LearningElectrician

    Thread Starter New Member

    Aug 24, 2008
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    Okay, so by looking at the diagram below, if I take a simple series circuit with one source voltage and one load, then add a ground on one of the conductors, I automatically create a hot wire and a neutral wire. If I touch the neutral wire nothing happens, if I touch the hot wire while standing on the ground, I create a parallel circuit where I'm completing the circuit - and I get shocked. This makes perfect sense by now. But...

    [​IMG]


    But what happens someone decides to connect both sides of the voltage source to earth? Does he still have a "hot" and "neutral" or "Both HOT" or "Both Neutral" situation? Such as this diagram below.....


    [​IMG]


    Further illustrating the above diagram into 2 scenarios below where a person touches the bottom conductor and then a person touches the top conductor. I'd assume that no matter which wire a person touches, a path completing the circuit will be created. Therefore, i'd assume that by adding another ground on the opposite sides creates a "Both HOT" situation - in other words, no matter which side of the voltage source is touched, a parallel circuit is created through earth where the person is the load and gets shocked; therefore making it a bad idea to add connections to earth on both sides of a voltage source. Illustrated below is the path I'd assume the current takes in both situations...

    [​IMG]
    [​IMG]


    Am I correct so far?
     
  14. studiot

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  15. Wendy

    Moderator

    Mar 24, 2008
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    True that. The other thing is, do you have a fire extinguisher to put the fire that used to be your battery? You've just created a dead short across it. The ground symbol in your schematic is a common electrical point, they are connected electrically.

    There are actually several sorts of grounds, loosely related concept wise. One is the electronic version, where a common point (such as a machines chassis) is used as ground. This in turn goes to earth ground. The earth is pretty conductive, with some exceptions. This thread discussed this very issue.

    My ESD article discusses yet another version of ground.

    Generally, an electrician is more interested in chassis ground, followed by earth ground. The reason is if the chassis of something (say a drill motor) develops an internal short between chassis and hot the breaker on the line will blow (kinda like that battery fire) instead of making the chassis a hot conductor.

    Earth doesn't have to be extremely conductive to allow a fatal current, a little electricity will kill you as dead as a lot. Might not be as spectacular, but I suspect the victim won't care.
     
    Last edited: Aug 26, 2008
  16. thingmaker3

    Retired Moderator

    May 16, 2005
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    In the most recent example, replace the ground symbols with a conducting path. Then look at the battery current.
     
  17. LearningElectrician

    Thread Starter New Member

    Aug 24, 2008
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    I believe I see. Two connections to earth is the same as a parallel circuit. All it does is make things more dangerous and create more total current - a very counterproductive scenario.

    How about this scenario. From the diagram from the E-book, it seems that in a circuit with no intended connection to earth, if two people (who are both connected to earth) contact the wires on opposite sides, they complete a circuit and both get shocked. Illustrated below...then..

    [​IMG]


    On the other hand, if a connection to earth of any sort is established on that circuit, only one person (the person on the opposite side of the earth connection) gets shocked while the other is safe. Illustrated below.... but then I get confused because ....
    [​IMG]


    ...If current doesn't just follow the path of least resistance and a connection to earth sets up a parallel circuit, then wouldn't there be a connection between the two men (standing on earth) even in the event of an earth connection (in this case caused by the tree)?


    [​IMG]
     
  18. PRS

    Well-Known Member

    Aug 24, 2008
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    Joe, I take it you are asking me about my saying people are to be modelled as a resistor in parallel with a capacitor? Before I answer that try switching to the threaded mode. Click on Display Modes then on Threaded Mode. That gives you a map so that you can see who it is you're talking to.

    As for the model, no I don't have a reference, it is, I think, just good sense. That you questioned it took me by surprise and I was about to look it up when it hit me, I could do a test myself. Have you ever touched your tongue to a 9v battery and felt the tingle? I touched my tougn to the leads from my function generator with the frequency at its lowest setting and turned up the voltage until I felt a tingle on my tongue. I got a good tingle at 3 volts and 1 hertz. Then, monitoring the output voltage with a meter in order to make sure its amplitude remained constant, I increased the frequency. I could not even get to 50 hertz before I had to quit. The tingle became too painful.

    Well, there you have it. The reactance of a capacitor declines as the frequency increases, hence more current. The places on my tongue where I put the leads were the capacitor's 'plates,' and the body fluid in my tongue was the dialectric, in this case an electrolite. Hence our bodies are big electrolytic capacitors! LOL. ;)
     
  19. PRS

    Well-Known Member

    Aug 24, 2008
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    You need to use threaded mode in order to see who exactly you are talking to. Click on Display Modes, then on Threaded Mode. This gives you a chart that clears the situation up immediately. Try it, look at it; you'll immediately see what I mean.
     
  20. PRS

    Well-Known Member

    Aug 24, 2008
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    That's right. The term came about because the power company connected one lead to the ground and sent the other over a wire while the user hooked one post of his service entrance to the ground and used the wire as a hot. The term stuck even though its usage changed. There is no earth ground in a transistor radio for instance. The radio has a common node that all other voltages are referenced to and it came to be called ground. A more exact term is common.
     
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