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Difference between neutral and ground

  1. Feb 3, 2012 #1
    What is the difference between neutral and ground??
    one more question what is the practical example of current source
     
  2. jcsd
  3. Feb 3, 2012 #2

    tiny-tim

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    welcome to pf!

    hi sirsajid! welcome to pf! :smile:
    neutral is part of the circuit … touch it and you die :redface:

    ground (or earth) is not part of the circuit (unless something goes wrong)
    not sure what you mean by "current source" :confused:
     
  4. Feb 3, 2012 #3

    cmb

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  5. Feb 3, 2012 #4
    A voltage source in series with a resistor.
     
  6. Feb 3, 2012 #5
    Re: welcome to pf!



    I mean example of voltage source is battery. Can u give me the example of current source?
     
  7. Feb 3, 2012 #6
    Re: welcome to pf!

    Really? I thought neutral is tied to ground at the tansformer output place. Isn't only dangerous to touch if there is a short circuit somewhere so that neutral is no longer at ground potential?
     
  8. Feb 3, 2012 #7

    Averagesupernova

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    Tiny-tim is certainly not telling the whole story. It is generally not a good idea to touch anything connected with mains electricity until the power is off. However, touching the neutral will not kill you unless there are some wiring problems. It is done all the time.
    -
    Here is an example of where you can get into trouble with a neutral: Suppose you have a ceiling light you wish to replace and in doing so you don't bother switching the power off since the wall switch is off. Again, not a good idea, but it is done all the time. You assume since the wall switch is off there should be no problems. BUT, power is routed in to the ceiling light box and continues farther down the circuit to the next light/outlet. The light itself you are working on does not have power on it since the wall switch is off, but when the neutrals are disconnected in order to replace the light you have interupted the devices down the circuit. Now the neutral coming back will have a voltage on it.
     
  9. Feb 3, 2012 #8

    psparky

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    In other words....there is no such thing as a current source. There are only voltage potentials with potential loads. V=IR. The V and the R determine the current. Current sources are just tools used in school to help you learn.

    So you can say that a voltage source in series with a resistor is a current source....but really the Voltage and the load (resistance) are determining the current. Good question though....I remember thinking the same thing in college.

    What is difference between nuetral and ground. Another good question. In simplest terms...take your breaker panel at home. Assuming you are USA....the ground and nuetral are actually tied together in the breaker panel! The ground and the nuetral obviously go to the loads.....but it's probably the other end you are interested in...well, the ground simply goes to the ground. In other words, it gets buried in the Earth. The non load side of the neutral goes to the center tap of the transformer on the pole......delivering 120 Volts instead of line to line voltage of 240V. If you are holding a 120V hot wire (dry condition...insulated boots...don't ever grab a hot wire, just hypothetically saying).....touching either nuetral or ground will complete the circuit.

    On the load side, the grounds are used to keep everything at zero potential....and equally if not more important, the ground is there to trip the breaker in the case that the hot wire comes loose and sits on the washing machine for example. Without the ground the hot wire will make the washing machine "hot". Mom walks up to it touches it and gets electricuted. If the washing machine has a ground...the second the hot wire touches the chasis.....it immediately trips the breaker. Mom goes to reset breaker and it doesn't reset. Mom calls repair man....he installs hot wire back in place.....resets breaker....all is well....mom is happy.
     
  10. Feb 3, 2012 #9
    I would not go this far. When you talk about a source, usually you mean it is supplying something to a load. A voltage source is equally valid as a current source, they are a duality (for example, thevenin and norton equivalence). The difference between the two I think is more of a convention issue, as it is which is being regulated/held constant or which your load is dependent on.

    You can design a source that's voltage changes as the external load changes to keep a constant current, and then I'd call it a current source. Ion chambers are also usually considered current sources, since the ion pairs are generated by radiation and source a current.
     
    Last edited: Feb 3, 2012
  11. Feb 3, 2012 #10

    psparky

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    I agree 100%
     
  12. Feb 3, 2012 #11

    Thanx for giving me good response. I am new to this forum. I never joined any forum before. I ll be a regular user of this forum now onwards.
    i have seen on net written van de graaff generator is a current source.
     
  13. Feb 3, 2012 #12

    jim hardy

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    the bottom end of a lightning bolt would be a good example of a current source.

    It's going to develop whatever voltage is necessary to dump its charge into earth.
    ..................

    read those old threads on neutral vs earth. It is a matter of definition and is widely mis-understood.
     
  14. Feb 3, 2012 #13
    This is commonly used as a (constant) current source:
     

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  15. Feb 4, 2012 #14
    In American electrical wiring there is a black covered wire that is hot in relationship to the white and green wires. the reason there are two of these is that the white one is to provide the other side of the electrical circuit. current flows back and forth through these two in that it is alternating current so depending what the potential of one of those is determines which way it flows. The green wire is the 'safety ground', it is connected to any exposed metal of anything connected to power to carry any current that is inadvertently on that metal due to what is called a fault caused by defect.
    both the white and green wires are connected to a grounding stake at the main disconnect.
    Michael
    Sr Consultant
    EMC
     
  16. Feb 4, 2012 #15

    jim hardy

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    The white neutral wire is one half of the normal ppower circuit, you could think of the black "hot" as supply line and white "neutral" as return line.

    The green "ground" wire does not carry any current unless there is an insulation failure in the appliance. The idea is for 'fault' current (from failed insulation) to return to the supply transformer through the green "ground wire" instead of through your hands, torso, feet, and ground. . If there is any fault current that Green wire should be the path of least resistance, keeping tiny hands safe..
     
  17. Feb 4, 2012 #16
    As Rogerio showed, a current source is generally the output of a transistor or tube or other nonlinear active circuit element. Batteries and generators are considered voltage sources. It is generally a secondary, dependent source.

    The neutral wire (generally color coded white I believe) is the 'return' leg for the 110 volt circuit. The idea is that the black 'hot' wire supplies current to the load and the white 'neutral' wire gives the current a return path to the breaker panel to complete the circuit. Of course that's rubbish since with AC current the direction of the current is constantly changing 60 times every second. So every 1/30th of a second the white wire is the (positive) 'hot' one. The real physical difference between the hot and the neutral wire is that, in a 110 volt circuit, the neutral wire connects to the ground bus in the breaker panel and the black hot wire connects to the hot bus. I believe the ground bus itself connects to two points. An actual earth connection, like a copper or aluminum stake actually pounded into the ground and a center tap connection from the distribution transformer at the street. So the ground bus basically serves two purposes. A connection that allows for 110 volts from a 240 volt secondary winding and a safety connection directly to earth to reduce shock hazards.

    Both the white neutral wire and the green ground wire are connected to the ground bus in the breaker panel. The difference is at the other end. The neutral wire connects to one side of the load and carries the current under normal operation of the device. The green wire just connects to whatever part of the device the manufacturer wants to keep at ground potential. If the metal case ends up with a voltage the ground wire will give the electrons a path straight to ground along a copper wire with a much lower resistance than a human body. Since the human body, even wet, has a much higher resistance than a copper wire most of the current will flow through the wire instead of the human.

    Note that on a 240 volt load the white wire actually also connects to the hot bus. Well there are actually two hot buses in a panel representing two legs of a center tapped transformer. In that case I believe only the ground wire connects to the ground bus. So for 240 volt circuits there really is no neutral wire. Just two hot wires and an earth/ground connection.
     
  18. Feb 4, 2012 #17

    cmb

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    Very confusing/confused here. (Notwithstanding the 1/30th sec typo) Why you associate +ve with 'hot' I am unclear. What counts is the work that can be done per Coulomb of charge (viz. volts - and for mains that is with respect to the neutral). It matters not if it is + or -.

    In polyphase AC circuits, the neutral is the line back to the star point of the polyphases. This is usually earthed at the distribution, so, nominally, the neutral line should be at ground potential. It is therefore never 'hot' (whatever that means), unless you also regard the whole of the Planet Earth becoming 'hot'.

    The reason the neutral is not always at the potential of where it is earthed (i.e. why it is not always at ground potential) is if there is a fault current from the live lines running in to ground. If this happens, then effectively the circuit will then push the neutral line around. In this case, the neutral might become 'hot', but only in a fault condition. Usually, the neutral of a house ring circuit does, indeed, float around a few volts off earth potential, just because of inductive effects, length of mains distribution trunks, &c., together with the fact that if each phase of the distribution is having different demands on it then there will likely be some deviation from ground for the neutral line.

    In a RCD protection system, the neutral is attached via a relay to ground, so if neutral ends up floating away from ground for some reason by more than a few volts (to accommodate the usual effects, as above), the relay is activated and the circuit shuts down. It is set up like this because no current should be flowing from the live wire(s) straight to ground (thus pushing the neutral line away from ground reference), all live circuits should all route back to the star point of the polyphase distribution and NOT directly to ground [except in the fault condition].

    I do not claim this is the setup for all international mains systems, I am sure somewhere in the world there are various permutations on such arrangements, but this should serve to describe basic principle.
     
    Last edited: Feb 4, 2012
  19. Feb 4, 2012 #18

    psparky

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    Yes, I agree that the black hot is the hot. The neutral is never the hot...even for 1/30th of a second. Again, if you are holding the black wire (120V) in your hand with insulated boots in a dry condition.....and you other hand is not completing the circuit....current doesn't flow. However, if you touch the neutral OR the ground with your other hand, current flows and you feel the shock.

    Conversely, if you are holding the neutral wire and touch ground with your other hand, you will feel nothing. They are tied into the same bus bar in your panel. The potential voltage between them is zero!!!
     
  20. Feb 4, 2012 #19
    Yes. cmb is correct. I was mistaken. Neutral = ground = center tap = safe. It is the same as the ground of a DC circuit. It can't hurt you unless you also touch a conductor with an electric potential above or below ground. Neutral is called neutral because it is always at 0 volts. Hot is called hot because it can hurt you (even burn you) if you touch it. By that definition both -170 volts and +170 volts are 'hot'.

    Incidentally the 1/30th was not a typo but a dumb mistake. The current actually flows in one direction for half of a cycle which is half as much time (not twice as much time) or only 1/120th of a second. So the voltage waveform is between 0 and +170v for 1/120th of a second and between 0 and -170v for 1/120th of a second (assuming 120v rms), adding up to 1/60th of a second in total.

    So to answer the OPs question the only difference between a ground wire and a neutral wire is what part of the load it connects to. The green 'ground' wire typically connects to the case of the device, and the white 'neutral' wire, which is also a ground wire, connects to the functional circuit or load. I guess another difference might be that the ground wire can connect to any earth connection: a water pipe, a stake in the ground, but the neutral wire must (I think) connect to the ground bus in the breaker panel and then back to the center tap of the secondary winding.
     
    Last edited: Feb 4, 2012
  21. Feb 7, 2012 #20
    This seams a little off topic for a physics forum but a few words of caution are in order so that the answers already given here do not cause someone an injury or worse. I have wired structures from Argentina to Alaska and from Malaysia to Uganda so I am aware that the explanations I am offering here are only applicable to North American practice and some of the other places were a Multi Grounded Neutral system is used for utility power distribution.

    Many panel boards are not service equipment / customer service units. That is important because Service equipment is the only place were the "Grounded Current Carrying Conductor;" that most of us call the "Neutral;" is properly earthed or connected to ground. That is done by a conductor that is, in North American Practice, called the main bonding jumper. In panel boards that are not service equipment the Neutral is kept aloof from the "Equipment Grounding Conductor" or ground wire at all times.

    The actual grounding / Earthing of the Service Equipment Enclosure is accomplished by connecting it to conductive materials that are buried in the earth or are effectively functioning as if they are the earth, albeit on a much smaller scale. The Conductive materials that are buried in the earth are called grounding electrodes. All of the grounding electrodes are deliberately bonded together to form the Grounding Electrode System. The conductor that connects the Service Equipment Enclosure to the Grounding Electrode System is called, strangely enough, the Grounding Electrode Conductor. I guess by that point in the code writing process the code officials were too tired to invent yet another obscure term but I digress. The purpose of Grounding or Earthing the Neutral is to "limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation."

    The reason for all the specificity is to bring this to an important point. In any system that has Grounded Current Carrying Conductors the voltage drop caused by the normal flow of current in the grounded conductor will raise it's potential above that of the earth / ground. Further if that "Neutral" becomes damaged or overloaded that voltage above ground can be at a level that is sufficient to cause injurious or destructive current to flow.

    For all of these reasons Neutral conductors are not safe to come into contact with until all circuits of which they are a part have been deenergized, locked out and tagged out.

    --
    Tom
     
    Last edited: Feb 7, 2012
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