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Electrical Plugs

  1. Aug 7, 2007 #1
    Why is it that some electrical appliances (especially high wattage ones) use a 3-pin plug instead of a 2-pin plug? I know that the 3rd pin is the ground pin but I don't know why do we need to ground some appliances and not others?

    Also, why is it that for a 2-pin plug, we sometimes have one pin slightly wider than the other? I am guessing this has something to do with the polarity of the voltage, right? If so then which pin corresponds to which polarity?
     
    Last edited: Aug 7, 2007
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  3. Aug 7, 2007 #2

    berkeman

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    It mainly has to do with the construction of the electrical device, and what safety standard the device is built to. The most straightforward construction uses a 3-prong plug, and all user-accessible metal on the outside of the package is grounded to the Earth ground connection of the 3rd prong. This is done so that if hot comes loose inside the metal package, it will contact Earth-grounded metal and blow the main breaker, without energizing the metal enclosure and posing a shock hazard to the user. Using a grounded metal enclosure also has the benefit of making it easier to pass radiated EMI testing (the tests to make sure that you aren't making so much RF noise that you interfere with broadcast radio and TV transmissions).

    Devices with 2-prong plugs have stricter input insulation requirements for the hot and neutral connections. There is a class of devices called "double-insulated", where there is an extra layer of insulation and separation between the hot/neutral wires and all other circuitry in the device. So even though corded power drills have exposed metal parts that the operator can touch, the hot/neutral power input wiring has extra insulation and separation from everything user accessible, and so the ground connection is not needed.

    The polarized 2-prong plugs are mainly used with light fixtures. The wider prong is the neutral, and the narrower one is for hot. This matches the polarization of electrical outlets, and is meant to keep neutral the outer connection for screw-in light bulbs, since that outer screw part is easy to touch by accident when screwing in a light bulb. The polarized plug is meant to keep the hot part of the light bulb socket just the bottom button part, which you have to be a lot dumber to reach down and try to touch.
     
  4. Aug 7, 2007 #3
    What exactly do you mean by "if hot comes loose inside the metal package"?
     
  5. Aug 7, 2007 #4

    berkeman

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    In traditional electronic device/enclosure construction, the hot/neutral/ground wires come in through the enclosure wall through a strain relief washer, and the wires are separated, stripped and have metal grommets attached. The ground wire must be bolted to the metal enclosure, and the hot/neutral wires can either be bolted to the input circuit (transformer or PCB or whatever), or they can be friction-fit. If, through vibration, mechanical shock, or whatever reason, there is a single fault of the hot wire coming disconnected or breaking, having the solid metal Earth ground connection to the chassis ensures that the AC mains breaker will pop before the metal chassis becomes energized. The safety regulations are mainly about preventing shock/fire hazards due to a single fault, like a single wire breaking.
     
  6. Aug 7, 2007 #5

    mgb_phys

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    In electrical engineering 'hot' means the wire with voltage on it, it doesn't have anything to do with heat. In simple terms it's the one you don't want to touch!

    One problem with 3 wire earthed enclosures is that if for some reason the earth wire is broken the device still functions but is no longer safe - and you have no way of knowing. If an earth wire breaks and somehow comes into contact with the live (hot) wire then the metal case can become live! This is why in industry you have to perform periodic earth tests on equipement.
    2 pin double shielded equipment doesn't have this problem and because it can use plastic cases is generally cheaper to build.
    Which equipment requires grounding usually depends on safety rules rather than any particular engineering requirement.
     
    Last edited: Aug 7, 2007
  7. Aug 7, 2007 #6
    I see. So if the hot wire means that it has a voltage, then what is the difference between neutral and ground wires?
     
  8. Aug 7, 2007 #7

    mgb_phys

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    Very little, which most people don't realise. In simple terms:
    You have a voltage on the hot/live wire which flows into your equipment and does some work, the electricity flows out on the neutral wire.
    The neutral wire has to be at a lower voltage than the live for current to flow, often the neutral wire is connected to earth at the supply to the building.
    So you could simply connect the case to the neutral. Except suppose the neutral breaks, then the case is still connected to the live via the equipment but has an infinite resistance to earth and so will end up at the same voltage as the live!
    Similairly if the resistance of the neutral wire is high because of poor connections to ground or poor wire and there is enough current flowing, the neutral at the equipment can be at a higher voltage than ground.
    The ground wire is generally only a safety device since it doesn't carry any current except in an emergency it is always at 0 volts.
     
    Last edited: Aug 7, 2007
  9. Aug 7, 2007 #8
    I see. Thank you guys.
     
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