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Electric shocks (was: 100V appliance on 110v power supply)

Discussion in 'Electrical Engineering' started by TJ Hertz, May 31, 2005.

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  1. TJ Hertz

    TJ Hertz Guest

    When I said "I've got a Japanese Akai S1000 sampler", I actually meant "I'm
    selling a Japanese Akai S1000 sampler for someone else". As a result, I
    didn't know much about the unit and its condition.

    But I asked the owner what she used to do with regards to power supply and
    she gave me the transformer that she had been running it off for the past 7
    years or so. Surprise surprise - it's a 240V to 115V transformer, and she
    said the Akai always worked fine despite being labelled 100V. In other
    words, she'd been using an incorrect transformer for years, but it worked
    anyway.

    A buyer emailed me today from eBay asking if the screen was still bright or
    if it had gone dim with age, so I plugged in the sampler using
    aforementioned 115V transformer and promptly got an electric shock from an
    area on the sampler's case where a bit of the paint had been scratched off,
    leaving bare metal. It is worth bearing in mind that I was barefoot and
    standing on a concrete garage floor, but nevertheless, this shouldn't
    happen. I plugged it in again and the same thing happened. However, when
    turned on, the sampler worked fine - it just gave me a shock whenever I
    touched the case (a sizeable electric shock, not like static). The owner
    said that she'd never experienced this before despite using the sampler with
    the same transformer on a UK mains supply.

    One thing I noticed about the unit was that it used a 2-pin power lead with
    apparently no earth. As in, the socket on the machine only had 2 pins, so
    there was no option to use an earthed cable (even though the transformer had
    earthed sockets). What's the deal with this? Can anything be done? Obviously
    I can't sell a unit on eBay in Very Good Condition if it shocks you when you
    plug it in, even if it works fine otherwise.

    Is my only option to sell it as-seen with a buyer warning?

    Thanks,
     
  2. Generously-designed equipment can likely take a 15% over-
    voltage although it may run warmer (and may reduce its expected
    lifespan.) But lots of modern equipment is designed on the very
    edge of acceptable ranges, so I wouldn't try it as a rule.
    PLEASE DON'T do that experiment again. We would miss you
    if you electrocuted yourself! Seriously! There are SAFE ways
    of testing for this that don't put your life or limb at risk.
    Now that you know about it, and especially now that you have
    revealed it in public, you really have no ethical choice but to
    disclose it to any potential buyer. If somebody bought it and
    electrocuted themselves full disclosure would be your minimum
    defense.

    If it were me, I would be tempted to fit an IEC power connector
    on it (like the kind used for computers), with the green-wire
    ground securely bonded to the metal chassis. But mains wiring
    is not a project for amateurs. I would think that most consumer
    equipment service shops ("TV Repair", etc.) would be equipped
    for and experienced with testing and fixing this kind of problem.

    Ironically, it may be working just as it was designed back
    when it was first sold. But you can't get away without telling
    bidders about the hazard today IMHO.
     
  3. Beachcomber

    Beachcomber Guest

    If you sell this product, even with a warning, you are opening up
    yourself to servere liability issues.

    An electric shock can be painful under the best conditions and
    potentially deadly under the worst conditons.

    It's likely that the transformer that the person you bought this from
    was providing isolation and hence their claim of no shocks may have
    valid.

    Beachcomber
     
  4. operator jay

    operator jay Guest


    Someone pointed out the online manual. It has specific mention of earthing
    in certain regions. You might want to take a look. Maybe something there
    will be worth knowing.

    j
     
  5. TJ Hertz

    TJ Hertz Guest

    Yeah, I know, and that's probably what I'll do - I was just asking if there
    were any ways of fixing it. Obviously if the problem remains I will warn any
    buyer.
    I'll take it into a repair shop methinks. Thanks for the help.

    TJ
     
  6. Don Kelly

    Don Kelly Guest

    It appears that the original usage was 240V to ground and the transformer is
    an autotransformer. Using this transformer with a North American 240/120V
    system could put the case at 120V with respect to ground.
    If this unit is to be used with a transformer, it will require an isolating
    (2 winding) 240/120V transformer and then the case can be grounded.

    However, why use the transformer?

    Why not connect directly to the 120V outlet. It will be necessary to
    determine which lead is the neutral. You will need a voltmeter. If the case
    is at 120 or so Volts to ground- reverse the wires to the plug. Check
    again-you want a small or negligable voltage between case and ground. Then
    set up the plug and leads accordingly (replacing the cord and plug to a 3
    prong (hot, neutral and ground) is the best way.
    If you have any doubts, a competent electrician can sort it out for you in a
    short time- or you warn the potential buyer that it appears that the case is
    tied to the neutral and you don't know which wire is neutral and leave the
    correction to him/her. You don't want to be responsible for avoidable
    injuries.
     
  7. TJ Hertz

    TJ Hertz Guest

    So basically, you're saying find out which way the thing should be connected
    in order to let the case be neutral, replace the mains end of the cable with
    a 3-pin plug so that it can't be connected the wrong way, and glue/fix the
    sampler end of the cable into the sampler so it can't be reversed (or label
    which way it should go)? And this would fix the electric shock problem, but
    still leave the unit unearthed?

    I'll let an electrician deal with it because I have neither a
    volt/multimeter nor any spare 3-pin plugs (you don't see many in the UK),
    but I'd like to know what's going on anyway.

    Thanks
     
  8. Steve Urbach

    Steve Urbach Guest

    On the shaving mirror in hotels <G>. I just would not plug good
    electronics into it :O


    , _
    , | \ MKA: Steve Urbach
    , | )erek No JUNK in my email please
    , ____|_/ragonsclaw
    , / / / Running United Devices "Cure For Cancer" Project 24/7 Have you helped? http://www.grid.org
     
  9. Should be no problem, except they are limited to 25W with
    a slow acting thermal cutout. The output is isolated via
    an isolating transformer (floating).
     
  10. w_tom

    w_tom Guest

    Ioslation transformer must still have the secondary (the 120
    volt side) connected back to the building safety ground. If
    not, then the appliance will 'float' - can become electrically
    hot. If the transformer is properly designed, then either it
    has such a connection OR it has a dedicated grounding wire
    terminal to make that necessary connection.
     
  11. Phil Allison

    Phil Allison Guest

    "w_tom"

    ** Making it no longer an "isolation " transformer.


    ** The appliance's AC supply wiring will float - but nothing else since
    it is all insulated from that wiring.


    ** A step-down / isolation transformer ought to have a ground terminal on
    the outlet that connects to the METALWORK of the appliance via the third pin
    of the plug.




    ........... Phil
     
  12. Roy Q.T.

    Roy Q.T. Guest

    From: (Phil Allison)
    "w_tom"
      Ioslation transformer must still have the secondary (the 120 volt
    side) connected back to the building safety ground.
    ** Making it no longer an "isolation " transformer.
    If not, then the appliance will 'float' - can become electrically
    hot.
    ** The appliance's AC supply wiring will float - but nothing else since
    it is all insulated from that wiring.
    If the transformer is properly designed, then either it has such a
    connection OR it has a dedicated grounding wire terminal to make that
    necessary connection.
    ** A step-down / isolation transformer ought to have a ground terminal
    on the outlet that connects to the METALWORK of the appliance via the
    third pin of the plug.
    ..........   Phil

    All those observations seem all for naught to me, since he as a Hot
    Chassis Item.
    It behooves whom ever get's to keep it to isolate the works 100% from
    that chassis with plastic or insulated offsets to avoid any
    objectionable current flowing through it as he's experienced., Or keep
    it as is and Fancy Oneself in those days.

    Point: No Manner of Transformer will correct the condition & a seperate
    ground conductor will only disguise the fault.

    He should have plugged it in his safe insulated floored section, but
    then again it was all for the better :)

    ®oy
     
  13. Scott Dorsey

    Scott Dorsey Guest

    No, that's the point of isolation. Because the appliance floats, it can
    become "hot" but the leakage current has no reference to the building
    ground so if you touch the hot case, no current will flow through you.

    Not that a safety ground isn't _still_ a good idea, mind you.
    --scott
     
  14. Yes, that's why in all around the world the neutral of LV distribution
    transformers is earthed (so I learned in my studies) so that you get
    schocked and know that electricity is dangerous (no joke) AND because in a
    neighborhood e.g. someone would be touching a live phase and someone else
    another, so that both would be exposed to the line to line voltage which is
    380 V in Europe (220 V in USA).However this happens only in LV, in MV, HV
    and EHV there's no need for a reference to ground, because the windings of
    the substation transformer are in delta, ungrounded.This doesn't make the
    voltage not dangerous, I was told some horror stories back in Kozani.

    --
    Tzortzakakis Dimitrios
    major in electrical engineering, freelance electrician
    FH von Iraklion-Kreta, freiberuflicher Elektriker
    dimtzort AT otenet DOT gr
     
  15. They are often grounded even so, sometimes through a resistor
    and current measuring equipment to monitor leakage in the line.
    Also to prevent a transformer leak letting the lower voltage
    windings float up to the voltage of the HV windings relative to
    ground, which would exceed the insulation rating of the line and
    downstream gear.
     
  16. w_tom

    w_tom Guest

    If a transformer is properly grounded as required by codes,
    then if the appliance has an internal short, that short will
    trip a circuit breaker. Grounding transformer secondary is
    necessary for breaker to trip. Leakage across to a floating
    transformer secondary may put enough voltage on chassis to
    cause shocks - with or without an internal appliance
    problems. But transformer secondary must be safety grounded -
    as codes require. When grounded, further useful information
    (ie a tripping circuit breaker) would then tell us more,
    immediately eliminate any human safety threat, and cause
    humans on the scene to fix any existing problems. Even if the
    appliance does have an internal fault, that fault would not be
    fixed because a transformer secondary is not grounded.

    BTW, I have assumed the appliance has a three prong
    receptacle. I have mostly ignored a two prong receptacle
    because the problem could be more complex. First most, does
    the transformer have secondary properly safety grounded?
     
  17. Don Kelly

    Don Kelly Guest

    ---------------
    In LV, MV , HV and RHV. the transformer windings can be and often are in Y.-
    .. The most common connection is Delta Wye with the HV side in Wye with
    direct or impedance grounding. There are good technical and economic reasons
    for that. The next most common connection is probably Y-Y -delta for
    supplying EHV lines . Note that even at 4160V distribution the supply
    transformer is Y on the 4160V side.
    Ground faults are the most common faults on transmission lines. This means
    that the unfaulted phases will see overvoltages- not good. In addition,
    protective relaying problems are increased and arcing faults which are not
    detected and often undetectable can occur because of capacitive coupling.
    The grounding eliminates these problems as well as problems due to the line
    floating up and down due to atmospheric effects (thus stressing the
    insulation). The use of a Y system makes life a lot easier.

    --
    Don Kelly

    remove the urine to answer

     
  18. Don Kelly

    Don Kelly Guest

    Sorry, given that it was e-bay, I didn't cjheck the origin. The solution
    that I suggested was for North American usage.
    The key in the UK case is to pitch the original transformer and get an
    isolation transformer (2 winding 120/240V)

    --
    Don Kelly

    remove the urine to answer

    Have you helped? http://www.grid.org
     
  19. Roy Q.T.

    Roy Q.T. Guest

    Fire Mouth: It So Happens It doesn't have a 3Prong plug, If You Had been
    Attending the thread conscientiously you'd not assume erroneously ., and
    he said the trany from japany is well grounded and supplies for
    agrounded 3P plug, it is simply a hot chassis assembly and I stated he
    can isolate it 100% from the circuits and supply with "stand offs" and
    other plastic isolating materials if that is desirable., Shock Proofing
    It !

    The Fact that he got a harrowing Shock from it while testing it in his
    cement floor garage is noteworthy enough to take measures to make the
    Old For Sale Unit desirable of it's own volition irregardless of Supply.

    Agreed: a Grounding conductor is always desirable and safest. Roy

    From: (w_tom)
        If a transformer is properly grounded as required by codes,
    then if the appliance has an internal short, that short will trip a
    circuit breaker.[ }:-oooh Not NEC ] Grounding transformer secondary is
    necessary for breaker to trip. Leakage across to a floating transformer
    secondary may put enough voltage on chassis to cause shocks - with or
    without an internal appliance problems. But transformer secondary must
    be safety grounded - as codes require. When grounded, further useful
    information (ie a tripping circuit breaker) would then tell us more,
    immediately eliminate any human safety threat, and cause humans on the
    scene to fix any existing problems. Even if the appliance does have an
    internal fault, that fault would not be fixed because a transformer
    secondary is not grounded.
        BTW, I have assumed the appliance has a three prong
    receptacle. I have mostly ignored a two prong receptacle because the
    problem could be more complex. First most, does the transformer have
    secondary properly safety grounded?
    :
    All those observations seem all for naught to me, since he as a Hot
    Chassis Item.
    It behooves whom ever get's to keep it to isolate the works 100% from
    that chassis with plastic or insulated offsets to avoid any
    objectionable current flowing through it as he's experienced., Or keep
    it as is and Fancy Oneself in those days.
    Point: No Manner of Transformer will correct the condition & a
    separate ground conductor will only disguise the fault.
    He should have plugged it in his safe insulated floored section, but
    then again it was all for the better :)
    ®oy
     
  20. Rich Grise

    Rich Grise Guest

    This is correct. Standing barefoot on a concrete garage floor while
    dicking around with anything plugged into mains should never happen.

    Good Luck - you'll need it.
    Rich
     
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