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  1. #61
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    Default Re: Can someone explain the difference between phases and poles??

    Quote Originally Posted by Sabio View Post
    Kentvw:

    I used the vom and forced a current between phase and ground by hooking up a simple light bulb and took voltage drop and current measurements. Then using ohmīs law calculated a ground impedance of 90 ohms. Seems high... possibly because this is not the prefered method of doing the test? Unfortunately I donīt have a ground impedance tester.

    Appreciate the info.
    How about a pic of your test set-up and some calculations ?

  2. #62
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    Default Re: Can someone explain the difference between phases and poles??

    Let's see if I can oversimplify this discussion here.

    Imagine a teeter-totter (or see-saw). Tommy is on one end and Norm is on the other. Tommy and Norm represent the two poles and the post in the center represents the grounded neutral. A standard 240V single-phase system in the typical North American household is like Tommy and Norm see-sawing away 60 times per second (ugh). Both Tommy and Norm are in phase, but opposite polarity. When one goes up, the other goes down. (For 120V, imagine that the teeter-totter is broken and just Norm is going up and down.)

    Now, imagine this playground has three teeter-totters. Scott & Kevin are on the middle one; Richard and Roger are on the one on the other end. They're all going at it 60 times per second, BUT they aren't quite in sync; while Tommy is going up, Scott is at the top and Richard is going down. What you've got here is a "three phase" system; on each individual teeter-totter, the boys are in phase but opposite polarity; the three teeter-totters are said to be "120 degrees out of phase" (120 * 3 = 360 degrees).

    It's not unlike a V-6 engine.

    MATHEMATICALLY if you were to graph the poles of an idividual "phase" relative to ground, you would see two sine waves that appear to be "180 degrees out of phase". This is not correct, because the poles are of the same system. You can only say that something is out of phase when you are comparing two systems. Phase shifting implies a time difference. In the case of three phase power, there is a time shift of 1/180th of a second between each "phase".


    Hope that helps!

  3. #63
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    Sep 2008
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    Default Re: Can someone explain the difference between phases and poles??

    Quote Originally Posted by djohns View Post
    How about a pic of your test set-up and some calculations ?
    Here you go...

    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	Ground Impedance.jpg 
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  4. #64
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    Sep 2008
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    Default Re: Can someone explain the difference between phases and poles??

    Oops...not sure what happened there... Picture is too small.

    Letīs try again.
    Attached Files Attached Files

  5. #65
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    Default Re: Can someone explain the difference between phases and poles??

    Sabio, are you on a split phase system like the U.S. (120 vac and 240 vac houshold) or on a 220 vac system only. What where you readings for V1, V2, and I1?
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  6. #66
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    Sep 2008
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    Default Re: Can someone explain the difference between phases and poles??

    Quote Originally Posted by JLMCDANIEL View Post
    Sabio, are you on a split phase system like the U.S. (120 vac and 240 vac houshold) or on a 220 vac system only. What where you readings for V1, V2, and I1?
    Jack
    Yes, I am on a split phase sytem. I have both 120V and 240V.

    Measurements were:

    V1 = 128.7 Volts
    V2 = 123.4 Volts
    I1 = 0.058 Amps

    If you use the formula in my previous link, you get Ground Impedance about 91 ohms. All this without a ground impedance tester.

    I used a 40 Watt regular lightbulb and a digital VOM to take the measurements. What I canīt figure out is why the impedance is so high? I have read that it should generally be less than 25 ohms. Obviously the lower the better.

  7. #67
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    Default Re: Can someone explain the difference between phases and poles??

    I1 = 0.058 Amps
    It looks as though the decimal point is in the wrong spot .... I would think it should be 0.58 amps.

    Based on the difference between V1 & V2 which is 5.3 vac ..... 5.3 / 0.58 = 9.1 ohms
    "" an ounce of perception -- a pound of obscure "
    - Rush

  8. #68
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    Default Re: Can someone explain the difference between phases and poles??

    The 128.7 vac is close to the no load voltage of the system
    The 123.4 vac is the voltage drop across the measured points
    The 5.3 vac is the voltage drop across the rest of the wiring
    The 9.1 ohms is the total impedance of the phase wiring and the ground wiring. The find the impedance of the ground wire alone you would have to know the impedance of the phase wiring from the source and subtract that from the 9.1 ohms. Because this a ac the resistance of the wiring is only a contributing factor to the impedance of the circuit so it makes it very difficult to estimate accurately with only the length or ga. of the wiring.
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  9. #69
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    Aug 2007
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    Denver, CO
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    Default Re: Can someone explain the difference between phases and poles??

    If this is like a US system then the grounding conductors are bonded (connected) to the neutral conductor at the main service to the residence.

    You could disconnect all of the grounding electrodes, re run the test and get the same results. What is being measured is the impedance or resistance of the wiring inside of the structure and not the “ground impedance.”

    Interesting discussion none the less.

  10. #70
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    Default Re: Can someone explain the difference between phases and poles??

    When you consider the run of conductors from the transformer to the service panel ...... through the breaker .... through the wire run in the home ..... through the fixture .....through the load .... to the return .... the net result of 9 ohms is pretty darn good.
    "" an ounce of perception -- a pound of obscure "
    - Rush

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