Phase question- or- why have half my outlets failed?

Discussion in 'General Electronics Chat' started by joshg1973, Oct 30, 2012.

  1. joshg1973

    Thread Starter New Member

    Oct 30, 2012
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    We had a 3 hour power cut last night and afterward I discovered half of my apartment's outlets were dead. I flipped through the circuit breakers to no avail. I called maintenance and the woman who answered the phone said the utility said one of the phases didn't come on and they were working on it.

    So- why don't I have a uniform problem throughout the apartment? The refrigerator, stove, modem, laptop charger, and glass tube TV work perfectly. The florescent lights fixtures are dead but the compact florescent bulbs work.

    I'm going to throw a few more details in-
    A 4V LED light with in line transformer strobes when plugged into a "dead" outlet, works fine on a good one.
    And I live in a 100 unit building built in 1974 (Concord NH) and have no idea if the power supply to the building is single or three phase.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    Electricity to homes in North America is 240VAC center-tapped. The center-tap is the common wire and there are two 120v phases. Some outlets are on one 120V and other outlets are on the other 120V. Your problem is that you lost one of the 120V phases.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    By "phase" the women was referring to the two 220V output wires of a standard single-phase mains transformer, not whether it was single-phase or three phase. The center of the transformer winding is grounded to give two 110V outputs to ground. These two outputs are distributed in the building to help balance the load for the transformer, with about half the outlets connected to one of the transformer outputs and the rest to the other output. Thus if one of the "phases" fails the outlets connected to that will be dead, as you observed.
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
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    120 volt power for homes in the United States is supplied from a center tapped transformer. The center tap is tied to ground and also acts as the neutral return line. The two ends of the transformer each supply 120 volts AC when referenced to neutral and at the same time supply 240 volts AC for things like close driers, cooking ranges, central air conditioners, etc. In an attempt to balance loads between the windings, some 120 volt outlets are connected to one end and other 120 volt outlets are connected to the other end. In your situation, only one end of the transformer is connected to the supply feeding your apartment complex.

    As for the LED light, any 240 volt load still connected and turned on can cause a backfeed to the dead line.

    Obviously, I type too slow!
     
  5. joshg1973

    Thread Starter New Member

    Oct 30, 2012
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    Thanks all for clearing things up. This also answers why is one of the bank drive ups is not working, why the emergency generator is still running, and why adjacent buildings are dim but still operating. When 8000 customers lost power yesterday I assumed a feed (for lack of a better term) was blown down in the "hurricane". Is what caused the blackout what knocked out one phase? How does that work? At the age of 39 I've been through many power cuts for many reasons, but the lights were always on or off.
     
  6. mbohuntr

    Active Member

    Apr 6, 2009
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    In industrial areas, you will see three lines on the poles. (three phase) As you leave industrial areas and move into residential areas, most power is single phase. The phases are split up into different geographical areas. You will commonly see one or two phases on poles in your neighborhood.

    The higher voltage on the lines is broken down by the pole pigs to house power. You might have different feeds coming in from different phases, or the same phase to sub panels. (What Bill said...)

    You can lose a breaker in your house from an overload, but the rest of the house stays on. See the relationship?
     
  7. joshg1973

    Thread Starter New Member

    Oct 30, 2012
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    I understand the single and triple phases and overloaded circuits, and now I know there are two 120v AC phases coming from the transformer.

    Really I'm asking a different question without starting a new thread. What would cause just one of those phases to fail, with reference to the preceding widespread blackout? I can't see everybody in our area overusing the same half, especially considering most of the neighborhood was vacant offices at the time.

    And as an aside I tested the kitchen range by putting a pot with a half inch of water on to boil and an hour later… just warm water.
     
  8. BillB3857

    Senior Member

    Feb 28, 2009
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    When you turned the electric range on, the power from the live side of the feed went through the range and then through other loads connected to the dead side of the feed, then to neutral. Some folks may have seen some of their lights come on dim when you turned your range on.
     
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  9. MrChips

    Moderator

    Oct 2, 2009
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    In an apartment complex you likely have 3-phase wiring and your apartment gets two of the three. As the woman says, one of your phases is out. What BillB3857 says makes sense. When you turn on your electric range, which requires both phases, you are feeding the live side through the dead side through any lights that are switched on. Those lights should come on dim getting power via the range.
     
  10. joshg1973

    Thread Starter New Member

    Oct 30, 2012
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    Tried it with an incandescent bulb- dim but lit. Wow. Still have to make coffee in the living room.
     
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