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240V mains?

Discussion in 'Electronic Basics' started by Jon Slaughter, Jun 10, 2007.

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  1. So is the mains really 240V CT and our 120V comes from one one of those or
    is it different? i.e. does 240V come from the transformer outside the
    residence and the then for 120V only one side being used and the neutro is
    from the CT and is grounded somewhere while the ground on a receptacle is
    grounded again but at a different point(so there could exist a potential
    between ground and neutral?)?

  2. Guest

    Yep - but in rare installations (we had one in the last building) we
    had 120V CT so your meter reads 60 VAC from either wire to ground.
    Why the 'goofy' power? We have a thousand pieces of gear with power
    filters. The folks who built those filters use a balanced toroidal
    core and capacitors from each power line (hot and neutral) to chassis
    ground. This causes significant ground loop currents that get into the
    audio and video. The proper solution is to replace all the filters but
    a more practical one is to use 120CT though each circuit requires a
    double breaker and twice the wire.

  3. Eeyore

    Eeyore Guest

    One presumes you're talking of the USA.

    I don't know of any other country distributing power the way the USA does.

  4. In the U.S., 240 VCT, with the center tap grounded is the
    normal residential service.
    The 240 volt center tapped is the secondary of the pole
    transformer (or on or underground distribution transformer).
    The center tap is usually (in my experience) grounded both
    at the transformer, with the under the pole and overhead
    ground line, and also at the fuse panel (down stream of the
    meter) with a ground electrode.

    That ground electrode is also connected to the safety ground
    system in the building. The fuse or breaker panel is the
    only place that the neutral point of the system (the center
    tap) and the safety ground system are to be connected
    directly together.

    The only potential difference there should be between the
    safety ground at the receptacle and the neutral at the
    receptacle is the load caused voltage drop across the
    neutral branch (from power panel to receptacle) conductor.
    However, during a direct fault between hot and neutral
    (until the over current protection opens the hot side) the
    neutral conductor drop may approach half of the normal hot
    voltage. I say, 'approach', because some of the total lone
    voltage will be lost in the distribution transformer impedance.
  5. Doug Miller

    Doug Miller Guest

    As far as I know, it's exactly the same in Canada.
  6. John Larkin

    John Larkin Guest

    In the US, residences usually get 120-0-120 (center-tapped 240) coming
    in off the street. Small outlets have 3 pins 120/neutral/ground, where
    neutral is nominally at ground potential. Big outlets, like for
    clothes driers and air conditioners, get 120/120/ground, which lets
    them run off 240 volts, the line-to-line voltage.

    Our building is wired 3-phase, 240 line-to-line, with the center tap
    of one side of the triangle being neutral. So we have 120 outlets, 240
    single-phase outlets, and 240 3-phase outlets.

    Ground and neutral are nominally equal but are sometimes a few volts

  7. Doug Miller

    Doug Miller Guest

    More typical now is a four-wire circuit containing both hots, neutral, and
    ground. This is required by current code for devices that have both 240V and
    120V loads; an example of this would be a clothes dryer, in which the heating
    elements are 240V but the control circuitry (and usually the motor as well) is
  8. Eeyore

    Eeyore Guest

    I can see how that might be but certainly wasn't sure.

    Given that the insulation requirements are barely any more problematic, I reckon the European (and
    most other countries) method of using 220-240V for all domestic use makes a lot of sense.

    For example, over here, the majority of major appliances (other than very small cookers) can be run
    off a wall socket.

    Nor do we ever have 'neutral problems' that cause the voltage to vary.

  9. Doug Miller

    Doug Miller Guest

    U.S., Canada, Mexico, Saudi Arabia, all of Central America, much of South
    America, most of the Caribbean, and a few other locales, according to this:

  10. Martin

    Martin Guest

    I was under the impression that if you DO manage to get shocked, 240
    is considerably more hazardous than 120.
    Agreed, although most appliances which need special power really
    should be on their own circuit anyway.
    Nor do we in properly functioning wiring ...

    The 'neutral problems' usually involve a high resistance in the
    neutral wire.
    If you have a high resistance in one of your wires, other items on the
    circuit will still see a drop in voltage ... in our case, it's just
    that it could be a drop or _increase_ in voltage in some cases.
  11. Doug Miller

    Doug Miller Guest

    Or a disconnected neutral altogether -- which is definitely a Bad Thing, in
    that it completes a 240V circuit through any 120V loads which happen to be in
    use on opposite legs of the service. That's hard on parts, and risks a fire.
    And that increase is a significant hazard. If you're lucky, it will simply
    damage appliances by burning out transformers, blowing diodes, etc. and
    opening the circuit. If you're not lucky, it may overheat something and start
    a fire.
  12. Don Bowey

    Don Bowey Guest

    The electrical code provides that 240V CAN be taken into a duplex box. The
    hot link between the two outlets is removed, and each one is connected to a
    different side of the 240V. Each outlet shares the neutral and protective
    ground. This is common in modern kitchens, and is useful in other places,
    such as an entertainment center.
  13. DJ Delorie

    DJ Delorie Guest

    Don't know if it's code or not, but you should still use a 240v duplex
    circuit breaker for such outlets, so that you can't turn off one
    outlet (say, for servicing it) without realizing that the other one is
    still hot.
  14. Don Bowey

    Don Bowey Guest

    You do use a duplex breaker. In the breaker box it just looks like a 240V
    circuit. Kitchen circuits MUST have 20A breakers.
  15. Doug Miller

    Doug Miller Guest

    Not quite. The outlet for the refrigerator is permitted to be on a 15A
    circuit. [Article 210.52(B)(1), Exception 2]

    And there is no requirement for the kitchen lighting to be on a 20A circuit.
  16. Don Bowey

    Don Bowey Guest

    I had in mind the duplex outlets provided for portable kitchen appliances,
    but I'll check with my attorney next time.
  17. Doug Miller

    Doug Miller Guest

    I had in mind the duplex outlets provided for portable kitchen appliances,
    but I'll check with my attorney next time.

    You're quite right that the small appliance circuits are required to be 20A.
    And the Code also specifies that the 20A requirement applies to *all* outlets
    in the kitchen, even the ones that are there to meet the "no more than 6 feet"
    interval required on walls. But there is that one, specific exception for
    "refrigeration equipment".
  18. Doug Miller

    Doug Miller Guest

    Cite, please?
  19. Jamie

    Jamie Guest

    Actually, the way it's worded, you're suppose to kill the distribution
    panel. not just a breaker.
  20. Jamie

    Jamie Guest

    If you were any electrician or close, you would know that lock and tag
    out is a must when working on electrical. The exception to the rule is
    if you have to do live testing that is required to analyze a problem.
    In which case, you're suppose to be warning arc flash clothing.

    Now, depending on the energy level you're disconnecting, you're
    suppose to be wearing the arc flash clothes as a minimum to the
    point of wearing an arc flash hood along with a full safety jumper,
    gloves etc...

    You should never assume your breaker to be functioning correctly and
    you should never assume that you know which is the correct breaker..
    Normally, there is only 1 distribution center that kills it all! and
    even then, one should first check to ensure there isn't any leakage in
    which case, your main switch should be replaced.

    Below 50 Volts does not fallow these strict rules.

    This is for the USA, I don't know about other places.
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