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Question about reversed-polarity in an AC outlet

Discussion in 'Electronic Basics' started by wylbur37, Oct 9, 2004.

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  1. wylbur37

    wylbur37 Guest

    Someone I know had recently purchased a piece of computer equipment (a
    flatbed scanner, I think) and discovered that it didn't work when he
    plugged it into his AC power strip. To make a long story short, he
    discovered that the power strip had a "reversed-polarity" (which I
    assume meant that the left-hand prong of the power strip plug was
    connected to the right-hand slot in the power strip's outlets instead
    of the left-hand slot, and vice versa).

    Back in the old days of DC, polarity obviously made a difference.
    But how would reversed-polarity make a diference in Alternating
    Current?

    Furthermore, if he used a power strip, I assume he had other computer
    components plugged into the same power strip (such as a printer and
    monitor) which apparently still worked despite the reversed-polarity.

    So how come reversed-polarity affects some AC components and not
    others?





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  2. There is no obvious way for this to happen. Normally, modern equipment doesn't
    care about the polarity of the Hot and Neutral, even in terms of safety. (By
    regulation, it must NOT affect safety.) I would guess that if the problem was
    actually due to the reverse polarity, then it may have been the resulted
    from excessive hum/noise on the USB or whatever interface was used.
    As noted, really shouldn't matter but each piece of equipment will be
    more or less susceptible to reverse polarity.

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  3. Richard

    Richard Guest

    Most of the time, a bad or improper ground is the culprit. We were having
    numerous problems with the computers at our office. Somethings would work
    while others did not. Weird printer problems and reboots. We found a
    ground wire in the wall was spliced by twisting two bare copper ground wires
    together. They were loose. We retwisted and installed a wire nut on there
    and the problems have gone away.
     
  4. CJT

    CJT Guest

    The bare wires are safety ground, and should not be active under
    normal circumstances. I would be concerned. RF bypass capacitors
    with excessive leakage might be the culprit.
     
  5. R.Lewis

    R.Lewis Guest

    Best guess is that re-wiring the plug merely made good a poor contact.
    Household mains powered equipment does not have a 'polarity'.
     
  6. This may be some kind of safety interlock.

    For a 3-wire plug, a circuit can easily be created to detect whether the
    neutral line has a large potential with respect to the ground line, and
    to disable the appliance. That may be whats going on here.

    The other possibility is that the appliance is using the ground wire as
    the current return, either through internal miswiring, or by design. In
    that case, if the hot and neutral wire is switched, there won't be a
    potential difference to run the appliance. Neutral is usually only a few
    volts away from ground.
     
  7. NSM

    NSM Guest

    | Someone I know had recently purchased a piece of computer equipment (a
    | flatbed scanner, I think) and discovered that it didn't work when he
    | plugged it into his AC power strip. To make a long story short, he
    | discovered that the power strip had a "reversed-polarity" (which I
    | assume meant that the left-hand prong of the power strip plug was
    | connected to the right-hand slot in the power strip's outlets instead
    | of the left-hand slot, and vice versa).
    |
    | Back in the old days of DC, polarity obviously made a difference.
    | But how would reversed-polarity make a diference in Alternating
    | Current?
    |
    | Furthermore, if he used a power strip, I assume he had other computer
    | components plugged into the same power strip (such as a printer and
    | monitor) which apparently still worked despite the reversed-polarity.
    |
    | So how come reversed-polarity affects some AC components and not
    | others?

    If it does you have SERIOUS problems, and I would recommend finding a smart
    electrician to take a look.

    N
     
  8. Rich Grise

    Rich Grise Guest

    ....
    Oh, it most certainly does! It's just not "plus" and "minus," it's "hot"
    and "neutral." Reversing them is a safety issue.

    Good Luck!
    Rich
     
  9. Same in the UK but for slightly different reasons. Our Neutral connection is now
    bolted up to the incoming mains Earth, so it doesn't float above earth as it
    used to years ago.

    The USA 120V is parts of a centre-tapped 240V if my memory serves me correctly?

    Peter

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  10. CWatters

    CWatters Guest

    I think that might depend on which country you are in but I'm not 100% sure.
     
  11. Its 110V and 220V.

    http://www.school-for-champions.com/science/acwiring.htm

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  12. James Sweet

    James Sweet Guest


    I had a very weird one like this once, a cheap CMOS color video camera which
    was powered by a standard off the shelf magnetic transformer type 9v DC
    output wall wart. It would work fine plugged in normally, but plug the wall
    wart in upside down (reverse polarity) and the camera would power up but not
    produce an image, never did figure that one out.
     
  13. James Sweet

    James Sweet Guest

    The bypass capacitors in the power supply usually connect to ground, so no
    ground, no bypass.
     
  14. CJT

    CJT Guest

    No ground -> chassis isn't necessarily at ground potential.

    If another piece of equipment is closer to ground, and is connected via
    a cable carrying signals referenced to ground, you can induce noise.

    Ground loops can be lots of fun to diagnose.
     
  15. CWatters

    CWatters Guest

    These things don't always make good contact in the socket - Perhaps the
    weight of the adaptor on the pins one way up caused the problem?
     
  16. Richard

    Richard Guest

    Uhh this is definately not normal. I recently saw something like
    this. The guy who wired up the circuit didn't have a neutral
    available at the end of a 3way switch so he simply tied the light
    there to the ground wire. Tracing the ground wire back I discovered a
    place where it had just been touching the inside of an electrical box
    run with conduit. It had completely arced out and lost it's
    connection. If the conduit had ever become discontinuous all of the
    conduit (which was exposed work ) would have become hot.
     
  17. Rich Grise

    Rich Grise Guest

    No, it can actually make a difference. It's kind of surprising that
    a wall wart has that problem - it usually shows up in old equipment
    that didn't isolate neutral from ground properly, so if the hot/
    neutral are reversed, you get all kinds of power line noise in the
    equipment and people get shocked.

    You can get an outlet tester for about five bucks that will check
    all three wires and ground - I'd recommend that. Check the electrical
    dept. at your local home store.

    Good Luck!
    Rich
     
  18. James Sweet

    James Sweet Guest

    The camera would power up and produce the video timing, but no picture, it
    was strange.
     
  19. Comteck

    Comteck Guest

    First of all, I'm not sure what you mean by "the good old days". DC is
    very widely used today.

    Secondly, AC does have a hot and a neutral. You can buy AC testers at
    Canadian Tire. It's looks sorta like a pen with a single plastic blade on
    the end. If you stick the blade into the hot side of a receptacle, the
    light will turn on, but if you put it on the neutral side, it will not.

    This simple test proves that, even though there is no plus and minus with
    AC, there is still a polarity.
     
  20. NSM

    NSM Guest

    Not widely distributed in homes.
    No, it proves that one side is grounded and the other is not.
     
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