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Newb question about circuit grounding

  1. Apr 5, 2012 #1
    I'm an EE student freshman yr. I dont quite understand grounding in circuits. Can someone explain it to me?

    I have a general idea but sometimes I just get lost.

    Even in AC, when you can plug a cord in with just hot and neutral and it would work but 3 prong plugs have a ground... is that necessary?
     
  2. jcsd
  3. Apr 5, 2012 #2

    FOIWATER

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    In the context of your AC situation with the plug, and three prongs, the ground prong is essential not for the operation of the system, but for safety purposes.

    The secondary of the transformer outside your house/building has a grounded secondary, and all the circuits in your house are wired back to a ground bus in your panel that is also grounded

    Let's say you plug in something like a toaster... you have power to and from with your hot and neutral respectively.. I see that you understand that but the issue is with the ground.

    First, consider that all the electric current tends to do is travel back to the source, not directly TO ground, yes to ground but it only uses ground as a low impedance path back to the source i.e. the transformer secondary.

    So, the casing of your toaster is connected to the ground wire, and it is continent all the way back to your distribution transformer through grounding.

    Now in the event that your power wire becomes broken inside your toaster, and it for this reason or another becomes live, you or anything else that IS grounded touching the toaster will receive electric shock. With the ground, electric current can use the ground as a return back to the transformer without having to go through you.

    EDIT: Now, with the path to source only limited by ground impedance, enough current SHOULD flow to trip your circuit breaker feeding that circuit.

    PS: How are you enjoying your program & where are you studying?
     
  4. Apr 5, 2012 #3
    Grounding is a very extensive topic that is not given due importance in most of the text books and in college. It is too long to try it here.

    Just remember by heart......Ground is the other half of your signal. To every circuit, there is signal driving out to something, but in order to complete the circuit, there got to be the signal return to complete the loop. The signal return is just as important if not more important than the signal. Just like a circuit with two resistors, one right at the output of the generator driving out, the other one is in series and going back to return of the signal generator. The total current is affect just as much by the first and second resistor. Think of it one is the forward circuit, the other is the return circuit. Your signal is only as good as the combination of both resistors.

    Do you know, cook book circuit really work......if you take care of all the grounding and filtering!!!! Why people said circuits don't work when they copy from cook books or text books......mostly because they mess up at the signal return.....grounding. Why people said pcb layout is so important.........mostly grounding!!!! That's where the rubber hit's the road. You can have the biggest engine in your car, but if you have lousy tires, you are not going to get very far.

    Glad you have the presence to ask this question. From years of designing all sort of circuits, I can tell you the easy part is designing the circuit. The real work that you earn your keep is the grounding. It is that important. Just remember this through out your studies and career and you won't regret.

    This might be beyond you at this moment, get a book on EMC.....it is all about grounding, from that you learn to appreciate ground. They have special type of engineer call Signal Integrity Engineer that specialize in grounding, keeping track of the ground image current to make sure signal don't cross. When you are ready, get one of those books. Yes, it's a whole book.
     
    Last edited: Apr 5, 2012
  5. Apr 5, 2012 #4
    How does the circuit not flow to ground initially? I mean how does it detect that the path has less resistance?
     
  6. Apr 5, 2012 #5
    Could you mean, "...in the event that your neutral wire becomes broken..."?
    The problem is my toaster has only a two prong plug.

    Suppose you have a boiler to produce steam. The heater of the boiler is controlled by a pressure switch inside the boiler. When the pressure reaches a certain level it opens a switch, shutting off the heat until the pressure is reduced.

    Initially let's assume there are no grounds in the circuit and the circuit works fine. Years go by, even decades, and eventually the insulation at point A gets worn away and it makes a connection with ground. What happens? Nothing and nobody is even aware that it happened. More time goes by and the same thing happens to the insulation a point B. Now what happens - the boiler explodes.

    But suppose there was a ground at point C from the beginning. As soon as the ground develops at point A, the fuse blows and an electrician investigates to find out why the fuse blew and fixes the problem.
     

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  7. Apr 5, 2012 #6

    FOIWATER

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    No I mean the hot.


    Code rule 10-408(3) - Canadian electric code

    This rule will fill you in... you do not need to bond your toaster, because it does not have a metal frame.. or if it does, it is insulated by another frame, which is not conductive.

    That's the reason for some appliances having no ground.
     
  8. Apr 5, 2012 #7

    FOIWATER

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    Although I cannot say as to why you would not have both measures for added security..
     
  9. Apr 5, 2012 #8

    FOIWATER

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    One more thing, I think the OP, with the level of understanding they have tried to illustrate, might not understand how the ground at C and A might constitute a short, if they do not understand grounding.. however I like the example you brought forward I have kept it.

    But to the OP, if you look at the diagram that Skeptic posted, assume you do not have a ground at C, but you do have grounds at A and B.. electric current tends to return to the source.. it doesn't want to go to ground as many incorrectly assume, rather ground is the lowest impedance path back to source

    You posted: How does the circuit not flow to ground initially? I mean how does it detect that the path has less resistance?

    In response to this, current tends to return to the source through all circuit paths, but with a limited amount of voltage, current cannot take a path through ground to return to the source if there is not point for the current to RE ENTER the circuit.. now if you look at what Skeptic posted imagine grounds at A and B. Current has a way to leave the circuit (at A) and re enter (at B). The infinite impedance of the open switch presents too large an impedance for the current to flow, but there is much less resistance to ground, so this is the path it takes.. even with the boiler pressure maintaining that switch. Therefore the pressure builds until it explodes... On the other hand, If you have C point grounded initially... the circuit operates fine, because no current will go to this point initially (why would it, since current tends to flow to source, not to ground?) But once A becomes grounded, current can leave through A, return on C, and return to source... this nullifies the impedance of the boiler heater, and the only current limiting factors are that of the circuit wire, and that of ground which are low. Now rather than waiting for B to ground and cause disaster... the circuit will blow the fuse.

    Nice post Skeptic, sir

    As yungman said though, this is just one application, where grounding is added for safety..

    Also Skeptic, what did you mean by Could I have meant neutral? would it happen in both instances? if the neutral became grounded, nothing would happen in the circuit.. since the neutral is grounded at the transformer..? right?
     
  10. Apr 5, 2012 #9
    You have not specified whether you are referring appliance, motor type that you have separate wire for ground or what. I am concentrate on electronic circuits on pcb and system. If you have discrete wires, the signal follow the wire. I take that when you ask how the circuit flow to the ground initially, I take that you have a ground plane....and with that:

    Ground is part of the circuit. As I said, don't think ground, think signal return. It is very important to have this mind set.

    You are getting into a very complicated subject. To say in very simple terms, for AC signal particular higher frequency and for STEADY STATE, we call the return path "image current" where the path is chosen to minimize the loop area of the signal loop. This is electromangetics. I cannot answer your question without going very deep. This is the main part of predicting emission or EM radiation from circuit. For very initial condition at t=0, I don't even know and we actually had a debate as how the current even take on the path of minimum loop area or " path of least inductance".

    One important thing, in the straight sense, unless the signal is a NON VARYING, signal does not travel in form of current and voltage, it is the EM wave that is propagate down the wire or trace. The current and voltage are ONLY the consequence of the boundary condition of the EM wave. Books that talk about circuits only looking at the result of the EM theory as current and voltage are much easier to measure. Remember, electrons move very slow in good conductor, if you can label one electron and injects into one end of a wire few feet long and put a voltage across the two end of the wire. After you inject the electron, you can go get a cup of coffee, come back and maybe the electron will come out from the other end!!!! It is not the current that travel at all, if it is the current and voltage that travel, circuits will be so slow that they would be completely useless.

    That is the reason it takes a lot of theory to predict how the image current take on the path. It is not necessary to take on the path of least resistance. I can meander the trace on top of the ground plane and make the image current go on fancy route as will.

    Again, I am not helping your question at all. The only answer I gave is in bold letters above. I am not going to answer you a question by a simplified way to miss lead you something like a path of least resistance. If you refer to motor and appliances that have separate wire for signal and return, of cause it follow the wire even though it is EM wave if it is not DC. I rather see you not have the answer and knowing that the answer is out there. Be encourage that at least you are the very few that actually have the presence to even ask. Most of the students just take ground as a triangular symbol that some how magically complete the circuit.

    The first pass answer is in the EMC book and you have to take the time to read it. Then when you study EM theory later, you will really understand this. Just remember you are doing good.
     
    Last edited: Apr 5, 2012
  11. Apr 5, 2012 #10

    jim hardy

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    to OP - here's a somewhat unconventional writeup on the subject. I enjoyed it. I hope you do too.

    http://amasci.com/amateur/whygnd.html
     
  12. Apr 5, 2012 #11
    Again, assuming you referring to circuit board with ground plane.

    I have been think about this more and try to come up with anything acceptable, this just my thinking, anyone has idea feel free to join in:

    In circuit boards, signal trace on top of ground plane is simply either stripline or microstrip. They have what is called characteristic impedance and the impedance has some proportional to the distance between the two conductor. So if the two conductors are farther apart, the impedance goes up. The lowest impedance will be if the current return path is right underneath the signal trace. So if you start a current, and injects the return current into the ground plane at a point. Then it "choose the path of lowest impedance" which is the path of closest to the signal trace.....which is right under the trace. So the return " image current" follow the signal trace back to the source.

    You see, it is not the path of least resistance, resistance don't matter, I can meander the signal trace any which way I want and the ground current will follow. That's the reason it's called the "image current" that image the signal path.

    This is the only thing I come up with if you can accept the notion of EM propagation and impedance of the stripline and microstrip line without asking why!!!

    Please comment on this as this is only what I pull out from my behind.
     
    Last edited: Apr 5, 2012
  13. Apr 5, 2012 #12
    I may have misunderstood you Foiwater.
     
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