Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

One-phase, three-phase?

  1. Dec 4, 2005 #1
    I have a question about electricity. In Europe, an electrical power plug has two "sticks" or whatever you call them. I understand here that power comes in through one of them - goes through the apparatus - and out the other stick. This makes sense. This is a so called "one-phase" system. What does "one-phase" actually mean? Three-phase systems have three "sticks" I know, so how does the power flow here? Help!
     
  2. jcsd
  3. Dec 4, 2005 #2

    FredGarvin

    User Avatar
    Science Advisor

  4. Dec 4, 2005 #3
    I thought the third prong was a ground, but im not sure.
     
  5. Dec 4, 2005 #4

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    The third prong in typical house outlet is a ground. Its purpose is to protect against electrical shock. Suppose you are using an old electric drill whose wiring has just about had it. Now suppose the wiring inside the drill fails while you are using it such that a live wire comes in contact with the case. You, who are holding the case, now form a new pathway through which the electricity can flow. To protect against that, the case is connected to the ground wire. If the ground wire is truly connected to ground, the path through the ground offers less resistance to the current flow than do you. The new path through the ground wire is a short circuit. This throws the circuit breaker and protects you against a nasty electrical shock. The ground wire nominally performs no useful function. Its utility arises from intentionally creating a short circuit under off-nominal conditions.

    The third prong in a high-powered industrial application is a completely different beast. All three lines are "live", phase-separated by 120 degrees. A three-phase socket look nothing like the single-phase + ground socket found in your house.
     
  6. Dec 4, 2005 #5
    DH said that

    "If the ground wire is truly connected to ground, the path through the ground offers less resistance to the current flow than do you".

    Now, wouldn't the electricity need to flow through you in order to find out if this is actually the case...? I mean, it can't know which way offers the least resistance before it has even tried it...or? I know this is a silly question, but I truly can't understand why...
     
  7. Dec 5, 2005 #6

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Short answer: Electricity does flow through both the person and the ground wire. However, a whole lot more flows through the ground wire than through the human. Net result: the circuit blows before the person suffers too much harm.

    Long answer, classical view:
    The path through the person and the path through the ground wire are parallel circuits. The current flow through the two paths is inversely proportional to the resistance along the paths. The wire offers a very low resistance compared to the path to circuit ground through the person also includes a high-resistance path through the ground on which the person is standing. Net result: the circuit blows before the person suffers too much harm.

    Long answer, quantum view:
    Each electron virtually "tries" all possible paths simultaneously. The act of sensing the existence of the electron by some measuring device binds that particular electron to the path on which it was sensed. Human cells are electron sensing devices along the path through the human; the circuit breaker is such a device along the ground path. The probability of the human cell electron sensors versus the circuit breaker electron sensor seeing a particular electron is inversely proportional to the resistance along the path on which the sensor lies. The circuit breaker senses a whole lot more electrons than does the human. Net result: the circuit blows before the person suffers too much harm.
     
  8. Dec 5, 2005 #7

    Danger

    User Avatar
    Gold Member

    Fascinating explanations, DH. Thanks.
     
  9. Dec 5, 2005 #8

    turbo

    User Avatar
    Gold Member

    It might be helpful to note that in the US, the fat ground conductor at the plug is connected by your house wiring all the way back to the ground connection at your circuit breaker box. The neutral connector at the plug (the wider of the two flat prongs) is also connected by your house wiring all the way back to the ground connection at the circuit breaker box. That's right - neutral and ground wiring are both grounded.

    This is pretty important, because it is possible for a dangerous situation to occur when neutral is not correctly referenced to ground. A few years ago, I started to see some lights dimming and others brightening when some heavy electrical loads came on (refrigerator, freezer, etc). Initially I thought it was a problem with the power company's transformer, but it turns out that the clamp in my electrical entrance that connected the neutral leg to the local ground had loosened and when loads on one hot leg came on, they shifted the neutral's potential relative to ground and put more voltage on the opposite hot leg. Just a few minutes with a screwdriver and the problem was fixed - after a couple of weeks of head-scratching, getting visits from the power company, etc.

    There are appliances, etc, that have polarized plugs on them. In other words, there are just the two flat prongs and no ground prong. There is only one way that plug will fit in the socket, since the neutral prong is too wide to fit the "hot" socket. This way, products can be designed so the on/off switch interrupts the hot wire so that no other part of the circuit beyond the switch can be energized. If you ever have to replace such a polarized plug, you must observe that polarity.

    Safety tip:
    If you have never checked the outlets in your house, do not assume that they are all wired correctly. Just go to the hardware store and for a couple of bucks you can buy an outlet tester. This is especially important if your house or apartment has been "modified" by a do-it-yourselfer in the past.
     
  10. Dec 5, 2005 #9

    Danger

    User Avatar
    Gold Member

    Excellent safety tip, Turbo. I'll just add a quickie memory aid for anyone thinking of rewiring a light fixture, wall outlet, etc.. Think of oil--Black Gold. All such items have one lug (or pair) that is gold in colour, and one that is silver. The black wire goes to the gold connector. If a ground connection is available, it's green.
     
  11. Dec 5, 2005 #10

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The way I remember it is:

    Black to Brass; White to bright; Green to ground.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: One-phase, three-phase?
  1. Phase trajectory (Replies: 2)

  2. Phase AC (Replies: 1)

  3. Phase diagram (Replies: 2)

  4. Phase changes (Replies: 1)

Loading...