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Why is a fuse connected in the live wire only?

  1. Feb 13, 2008 #1
    why is fuse connected in the live wire only? as current is same throughout the circuit( it is produced simultaneously in the whole circuit ie it does not 'originate' from any source) putting the fuse anywhere in series with the circuit should work.
     
  2. jcsd
  3. Feb 13, 2008 #2
    It's simple: safety reason. If anything, the fuse will isolate the whole circuit from the live source.
     
  4. Feb 17, 2008 #3
    what's the diff bw live wire and neutral wire then? after all they are only connecting wires in the circuit
     
  5. Feb 17, 2008 #4
    Neutral is grounded to earth, as result you won't get shocked because the potential difference between your feet and the earth is very small.

    We fuse the live wire for safety reasons like pixel said.
     
  6. Feb 18, 2008 #5

    stewartcs

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    The live wire feeds the circuit and the neutral (or return) wire completes the circuit by providing a return path to the source.

    Here is some more info on the subject.

    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/hsehld.html

    CS
     
  7. Feb 23, 2008 #6
    hey thanks
     
  8. Mar 15, 2008 #7
    Supporting Miss Photon

    I don't know whether I am right, but don't we get an ac current at home for all appliances? so the Live and the Neutral wires should keep changing , should'nt they ? How can we decide where to put the fuse or connect the earth?

    I'm sorry but plz explain. I'm still in grade 10 so I don't know much, but know more than wat my friends know

    Praveen
     
  9. Mar 16, 2008 #8

    Someone may correct me If I am wrong but:

    Which wire we chose to make neutral is more of a standard than anything. You could chose either wire to be "neutral". This is the wire that is referenced to ground. AC is alternating current, so you are right - the current flows both ways depending on the time interval and the frequency of the AC waveform. Most simple AC household appliances don't have polarity, but if you notice one plug may be bigger than the other, making it polarized. If you were to force the plug into an outlet the wrong way, the appliance would still work. Depending on the circuitry of the appliance and if it has a ground wire or not, the neutral wire just lets any components inside the appliance be referenced to ground.

    Also, the ideal fuse location would be closest to the main power supply. The fuse is protecting the appliance from high current draw, and it is also protecting any house wiring from being shorted. In a simple DC example, say you have a battery, 20 feet of a pair of wires and a load. the fuse is in series with the load, about 4 inches (of wire) before the load. the load may malfunction, and draw too much current, tripping the fuse, preventing a possible fire or more damage to the load unit. Now with the same senario lets say something sharp falls on the wiring before the fuse, shorting them together. Now there is a circuit consisting of only the battery and a very low resistance load, causing a lot of current and the wires will most likely get red hot and possibly start a fire. This is the reason the fuse is located closest to the battery, to protect the wire, or in the home case closet to where the main wiring is run into a house.



    Also just to clear up on ground wires, because I always had trouble with the concept: If an appliance in your house has a ground prong, the third middle bottom prong on a plug, this wire is usually ran to the chassis of the appliance. Say you have a toaster, either of the neutral or hot wires shorts out inside the unit, touching the chassis. Without the ground wire, the toaster chassis is now "hot", if you touch it, the current is going to want to get to ground the easiest way it can, which in an unlucky case could be through you if you are touching the toaster chassis. With the ground wire though, the ground wire is going to earth ground, literally through a big copper rod impregnated in the ground next to your house. Now if a wire shorts out to the chassis in the toaster, the current will return to ground through this wire. You may still conduct some electricity if you touch the toaster chassis, but chances are the ground wire is going to be an easier path to ground than your body, so the majority of the current will go through that route.




    hope that helped somebody.
     
  10. Mar 16, 2008 #9
    There are a couple of points anyone should be aware of. In the US, the neutral wire is white and is connected to a common bus inside the fuse/breaker box. All outlets and most fixtures have a specific wiring pattern so that the neutral is always in the correct orientation (for example, in a lamp socket the outer shell is supposed to be neutral since that is the one you are most likely touch accidentally). All modern outlets have polarized sockets and appliances that need polarization do have polarized plugs. If this polarization is defeated and insulation deteriorates, users might be shocked.

    The fuse/breaker is always inside the main entry box except in commercial buildings where there are sub-boxes. The idea is always that any wire inside a building is protected from overheating in case of an accidental short. The fuse/breaker is not intended to protect the appliance (which may have its own fuse for that purpose), but to provide fire safety. Thus, it must always be on the hot wire (usually black, sometimes red or blue) since an accidental short could occur between that hot wire and, say, a grounded water pipe.
     
  11. Mar 16, 2008 #10


    What if the neutral wire were to touch ground? Would nothing happen because the neutral wire is referenced with ground? In a different world, what if the hot wire were referenced with ground, would the same thing happen if it were shorted to ground but referenced to ground?
     
  12. Mar 16, 2008 #11
    -As in pokemon's question, I am wondering if I am wrong about the wiring not electrically mattering which side is referenced to ground?
     
  13. Mar 16, 2008 #12
    In a properly wired system, there may be a very small voltage on the neutral, perhaps 0.5 V, but nothing bad will happen if the neutral touches ground. Remember that the neutral is connected to earth at the breaker box, so the most voltage you can ever see with respect to ground is the IR drop.

    The hot wire cannot be connected to ground.
     
  14. Mar 16, 2008 #13
    You are wrong. The entire US power system is based on a common ground.
     
  15. Mar 17, 2008 #14

    stewartcs

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    Technically speaking, yes.

    It is a matter of naming convention (in the US per the NEC) that we call one power conductor "neutral" and the other "hot". The "neutral" conductor is neutral with respect to ground. Hence the name.

    In the US, the NEC specifies and requires this. From a design point, you would put the circuit breaker at a location in the circuit that de-energizes it at the source to protect the user and/or the equipment from damage.

    As a practical example, suppose you needed to change a blown fuse in a piece of equipment. It would be safer to have a disconnect switch located at the source (up-stream from the fuse) so that you could change the fuse without any voltage on it. Same thought process with circuit breakers, interrupt the current at the source thereby isolating the entire circuit.

    CS
     
  16. Mar 17, 2008 #15

    stewartcs

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    In a US residential household circuit, if the neutral wire touches the ground, nothing will happen since the neutral is grounded at the service entrance.

    If the neutral power conductor had the ground removed, and one was added to another power conductor, then the new power conductor with the ground attached would become the neutral.

    Therefore, if the "new" neutral or "old hot" where to touch the ground nothing would happen since there would be essentially no potential difference between them.

    Just remember that in order to have an electrical shock you need two things to happen: First - you need a complete path for current to flow, Second - you need a potential difference. If you have a potential difference without a conducting (i.e. return path) no shock can occur.

    Of course the magnitude of the shock depends on some additional factors.

    CS
     
  17. Mar 17, 2008 #16

    stewartcs

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    Technically speaking, it does not matter which power conductor is chosen to be the neutral as long as the load connected to it is designed and wired to handle it.

    In the US it does matter due to the NEC and the way in which households are uniformly wired. If you were to start moving the transformer's center tap ground to another power conductor then you would have a larger potential being supplied to the connected loads. This would most likely result it the connected load being damaged and/or fire hazards.

    CS
     
  18. Mar 17, 2008 #17


    That paragraph sums up a lot of electrical theory pretty well. You answered all of my questions, i was about to reword them. Basically that's all i wanted to know is if the neutral was just our standard.


    So now I have another question, if NO wire were referenced to ground, you should theretically be able to touch either wire one at a time, and not be shocked: correct?


    If the answer to that is yes, then what if you had a 120vAC line coming in that has a ground reference on neutral and put it through a 1:1 ratio transformer. If you touched either terminal of the secondary on the transformer, would there be a potential difference between you and ground, or would it just be a "floating" voltage? Since the two currents are magnetically coupled, does the ground on the primary winding have any effect on the secondary winding?




    This seems all seems pretty easy to understand so far, I guess I just never took the time to fully think about all of this.



    Thanks stewartcs
     
  19. Mar 17, 2008 #18

    russ_watters

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    As stated, the neutral wire is connected to the ground. The hot wire really is the hot wire and the neutral wire really is the neutral wire.

    One important thing to note, this is not the configuration the power comes from the power company in. All power from the power company is three phase - there are three hot wires and no neutral. The neutral in your house is derived from a transformer.
     
    Last edited: Mar 17, 2008
  20. Mar 17, 2008 #19

    stewartcs

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    Yes.

    Isolation transformers, as their name implies, do not transfer grounds. They electrically isolate or decouple the circuits.

    Therefore, the ground on the primary has no effect on the secondary.

    CS
     
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