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AC plug wires are reversed and Above ground potential

Discussion in 'General Electronics Discussion' started by LouAlder, May 17, 2015.

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  1. LouAlder

    LouAlder

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    May 16, 2015
    How can a technician know if a piece of equipment before plugging it into the wall that the hot and neutral wires have been reversed internal inside the equipment. Some test fixtures or test equipment has either their hot and neutral wires reversed, no safety chassis ground or the ground is above zero volts. I have to take a fluke volts meter and measure the DC and AC voltage from ground of one test fixture or test equipment and measure the voltage difference against the ground or chassis ground of another test equipment. I measure the AC and DC voltage difference from ground to ground and chassis to chassis. The oscilloscope will short out when probing test point on a circuit board or on a test fixture test points. Why it shorts out is because it can be either of these problems.

    The circuit under test Hot and Neutral wires are reversed
    The circuit under test has no Safety ground
    The circuit under tests grounds are above ground


    I know if I use an isolating transformer this will solve all these problems, but how can a technician measure before plugging it into the wall if the circuit under test, test fixture or piece of equipment has any of these issues.
     
  2. poor mystic

    poor mystic

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    Apr 8, 2011
    I don't believe it's possible to test for all potential faults by any method. If the equipment you're working on is as bad as that, use the isolating transformer - it's a lifesaver.
     
  3. LouAlder

    LouAlder

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    May 16, 2015
    Do you measure the AC or DC voltage difference from ground to ground and chassis to chassis?

    If there is a voltage difference or potential difference you have to use an isolation transformer or the oscilloscope will short out?
     
  4. signalman72

    signalman72

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    Jan 26, 2014
    A ground potential above zero volts sounds symptomatic of a floating ground to me. Are there schematics for the equipment and can you open up them up and and inspect them?
     
  5. LouAlder

    LouAlder

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    May 16, 2015
    I can't open them up. But when I measure the chassis ground to the oscilloscope chassis ground its above ground and also the test fixtures ground is above ground to the circuit boards ground. I measure the AC and DC voltage difference from each equipment ground and chassis earths.
     
  6. shrtrnd

    shrtrnd

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    Jan 15, 2010
    I don't know what the heck is going on here, but I'll submit my two-cents worth.
    1) When electricians wire power outlets, they drop the 480VAC three-phase from the outside power line, into three 120VAC single-phase lines. (If you're in the U.S.)
    It's called 'balancing the load', of turning one 480V 3-Phase line, distributing it into 3 each 120VAC lines for use.
    If you run power from one input line to equipment, and then tie that equipment into somthing that's drawing power from one of the other original 3-phase input lines, you're 'cross-phasing'.
    That means your 'neutral' at your euipment, could be seeing it's neutral tied to 240VAC, the 'hot' wire from one of those other original 3-phase input power lines.
    Is that what's happening here? Or are we looking at something else going on?
    2) My other thought here, is that there are plenty of circut voltage testers on the market to check power outlet voltages. Are you just using electronc test and measurement equipment here,
    or do you have access to basic electrician's test equipment?
     
    Last edited: May 18, 2015
  7. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    Not sure what is going on either.
    If we are talking about a single phase 120v plug (cord cap) there is technically no neutral. There is a hot, grounded, and when it's 3 prong plug; an equipment ground that hooks to the grounding pin.

    You should have a polarized plug with the grounded pin (neutral) physically wider than the hot.

    You should use an ohmmeter on the cord cap (plug) to verify infinite resistance between hot (non polarized skinny pin) and the chassis.
    On old equipment, they would bond a chassis to the grounded (neutral) conductor, but this is not safe, primarily for anything metallic because the grounded conductor carries current.
    The chassis should be bonded to the grounding pin (equipment ground) conductor only.
     
  8. poor mystic

    poor mystic

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    Apr 8, 2011
    I think it might be time to ask an electrician for help.
     
  9. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    I am an electrician
     
  10. poor mystic

    poor mystic

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    Apr 8, 2011
    Sorry, I mean an electrician right there, to clear what might be a potentially lethal fault.
    The OP is in another country, isn't he?
     
  11. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    I hear you, it's better to understand something in front of you, rather than going on basic descriptions on a forum.

    If we knew more about this "equipment" perhaps we could be of help.
    In the meantime, cut off the plug so no one plugs in this dangerous thing.
     
  12. davenn

    davenn Moderator

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    Sep 5, 2009
    guys, this thread was supposed to be deleted when I banned the user the other day


    now Im closing it to further comments

    Dave
     
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