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Shock from TV aerial socket

Discussion in 'Electronic Basics' started by James, Jan 15, 2004.

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  1. James

    James Guest

    I wonder if someone could explain something to me. Im an electrical
    technician in a hospital, and the other day i got a shock from the
    back of a tv, namely the aerial socket (outer side) to earth. I was
    very curious about this and done some tests. I measured 115v ac to
    earth, and 80v dc to earth. This i found on a few TV's so made a call
    to a TV engineer i know. He said its very common but OK. He said its
    to do with a floating earth and is OK because its isolated in the
    power supply. Correct me if im wrong, but surely if it's isolated,
    then a connection between live and earth would not produce this kind
    of shock.

    Could someone maybe help me out on explaining this to me.

    Thanks
     
  2. Measure the current, not the voltage.

    --
    We now return you to our normally scheduled programming.

    Take a look at this little cutie! ;-)
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    Michael A. Terrell
    Central Florida
     
  3. Baphomet

    Baphomet Guest

    "He said its very common but OK" - Tell that to the patient with an
    implanted catheter. It's definitely NOT o.k. That is what micro-shock is all
    about and why major hospitals are required to have bio-engineering
    departments. 10 micro-amps leakage current is all that is allowed from any
    electrical appliance (beds included) in any hospital, especially those
    having CCU or ICU units. That is why hospitals have floating power systems
    in operating rooms and why patient leads on electronic diagnostic equipment
    are now required to be isolated.
     
  4. Earl

    Earl Guest

    It's o.k. in the sense that all t.v's have this phenomenon, namely
    when you touch the aerial connection with a bare hand you will
    receive a small annoying shock. I am not an engineer nor am
    I talking about hospitals, but all the t.v.'s I've come across have
    this problem. btw. this is why you should never connect any audio
    video equipment into your t.v, always connect them to the vcr.
    I have no idea why this high voltage occurs, maybee someone will
    explain. I do know though that the chasis of a t.v. is live, so never
    touch it. I guess this is because there is no step down transformer
    in a t.v. since it needs all the voltage it can muster in order to
    generate the 20000V it needs for the tube.
     
  5. Guest

    | James wrote:
    |>
    |> I wonder if someone could explain something to me. Im an electrical
    |> technician in a hospital, and the other day i got a shock from the
    |> back of a tv, namely the aerial socket (outer side) to earth. I was
    |> very curious about this and done some tests. I measured 115v ac to
    |> earth, and 80v dc to earth. This i found on a few TV's so made a call
    |> to a TV engineer i know. He said its very common but OK. He said its
    |> to do with a floating earth and is OK because its isolated in the
    |> power supply. Correct me if im wrong, but surely if it's isolated,
    |> then a connection between live and earth would not produce this kind
    |> of shock.
    |>
    |> Could someone maybe help me out on explaining this to me.
    |>
    |> Thanks
    |
    | Measure the current, not the voltage.

    If there is enough current available to feel the shock, it is WAY too much
    in a hospital setting. Back when TVs used 300 ohm balanced in and the
    distribution was 75 ohm, the little transformer to convert it isolated the
    DC and low frequency AC. Now days we plug 75 ohm coax straight into the
    TV and don't think about what might be on there. Normally TVs in hospital
    are out of reach and patients use remote controls. But I would find a way
    to isolate them in that environment. Also keep in mind it could be the
    distribution amplifier putting that voltage on there.
     
  6. Guest

    | It's o.k. in the sense that all t.v's have this phenomenon, namely
    | when you touch the aerial connection with a bare hand you will
    | receive a small annoying shock. I am not an engineer nor am
    | I talking about hospitals, but all the t.v.'s I've come across have
    | this problem. btw. this is why you should never connect any audio
    | video equipment into your t.v, always connect them to the vcr.
    | I have no idea why this high voltage occurs, maybee someone will
    | explain. I do know though that the chasis of a t.v. is live, so never
    | touch it. I guess this is because there is no step down transformer
    | in a t.v. since it needs all the voltage it can muster in order to
    | generate the 20000V it needs for the tube.

    Modern TVs tend to use solid state power supplies. But I do remember
    lots of older TVs from the 1950's and 1960's that had big hefty power
    transformers. The primary side had the power switch and the safety
    cord connector (the one that was pulled off when the back was removed).
    Several voltages came out, including filament for the CRT, filaments
    for the various tubes, around 60 volts or so to power most circuits,
    and 400 or so to feed the flyback circuits that generated the higher
    voltage to drive the CRT. At least that's what I saw in schematics of
    the ones a friend of mine took apart. Color sets probably had different
    voltages, but transistors were starting to be common around then, and
    the transformers eventually disappeared, too. Oldest set I saw was a
    Philco unit made around 1949. Only 15 inch CRT in a box about 24 inch
    by 24 inch by 30 inch deep, plus 8 more for the CRT yoke. The power
    transformer on this beast was at least 8x8x8 inch cube. Took about 3
    minutes to warm up. We had it until around 1971. And oh yeah, I did
    get shocked on the antenna connection.
     
  7. Baphomet

    Baphomet Guest

     
  8. Ian Stirling

    Ian Stirling Guest

    Why would such be messing with the antenna connector?
     
  9. Baphomet

    Baphomet Guest

    Actually, they wouldn't have to directly. Assume that a tech was touching
    the antenna terminal and also the bed frame. Further assume that the bed
    wasn't grounded because of a bad outlet or broken ground lead. Further
    assume that the patient catheter was contacting the bed frame. ZAP! One
    always has to assume absolutely worst case conditions in these types of
    critical or sensitive environments, if for no reason other than liability
    which is not insubstantial.
     
  10. First of all, TV sets are not built for "hospital settings". The are
    common household appliances, and the standard is for leakage current,
    not open circuit voltage.

    Second, when you connect the coax, the voltage is pulled into the low
    millivolt range.

    Third, any current you would get by touching the coax and another
    ground should fall well below your safety requirement, which you didn't
    state.

    Fourth, I have never seen a TV the patient can reach in any hospital.
    They are hung from the wall or ceiling and use a remote control, either
    with a standard remote, or through a membrane keyboard on the hospital
    bed. I have seen too many hospital rooms over the years watching as
    family and friends lay there in pain.

    Last of all, tell us which hospital you work for. I don't want to be
    in one without proper written standards for their electrical or
    electronic systems.

    --
    We now return you to our normally scheduled programming.

    Take a look at this little cutie! ;-)
    http://home.earthlink.net/~mike.terrell/photos.html

    Michael A. Terrell
    Central Florida
     
  11. James

    James Guest

    Ok, firstly, thanks for all the reply's.

    Anyway..

    Today i set up a test to measure the current from the aerial socket to
    earth. I used a 1k resistor, paralled up with a .15uF cap for the
    meausurement test circuit (This i took from my notes many years ago
    when i used to repair medical equipment). 1mV / uA of current. I
    measured 200 uA which is below the threshold of perception, and also
    below the maximum value for a piece of type B medical equipment which
    is about 500uA from memory. It does seem strange though that i got a
    shock from this. You are correct in assuming that patients should not
    be able to touch a TV, but in the instance of our wards, the TV aerial
    goes behind the patients beds, because that is where the socket is.
    These Tv's should not even be here these days because all beds in the
    NHS should well be on their way to having their own TV's which ours
    do. Removing the bay TV's though is very political as you can imagine.

    Thanks all for the replys, im sure this discussion will go on with
    much interest. Other peoples opinions in this situation surely makes
    you think, especially what you may have forgotten, and even
    overlooked.

    James
     
  12. Earl

    Earl Guest

    o.k.

    now to sum up:

    the shock which is received is only when toucing the aerial connection
    if there is no wire there, or if there is a wire, by touching the other
    bare end of the wire.

    we had many responses, no one has explained why this occurs.

    all the stories about the hospitals is not relevant and are a figment of
    imagination of the OP and some of the commentators.

    this prooves that you can most of the time write on usenet almost anything
    you like, and get away with it. People comment not because they
    necessarily know the answers, but because they want to talk.

    have a wonderful sunny day.
     


  13. It was explained, but you missed it. It is leakage current if you are
    touching two items. If you only touch one item, then it is a simple
    static discharge. USe Google and do some research like "Tutorial leakage
    current" or "Measuring leakage current".



    How is the discussion of the leakage current between the antenna
    connector and the coax not relevant?



    You must be speaking about what you post.



    Its supposed to rain.

    --
    We now return you to our normally scheduled programming.

    Take a look at this little cutie! ;-)
    http://home.earthlink.net/~mike.terrell/photos.html

    Michael A. Terrell
    Central Florida
     
  14. Baphomet

    Baphomet Guest

    Should I forget, please remind me never to hire you for electronics safety
    related work in a hospital setting. Clearly, if you don't think the
    responses about the hazard that this type of situation poses in critical
    environments is relevant, you are inhabiting a planet other than terra
    firma.
    Just making sure you don't get away with your nonsensical uninformed
    blather.
    and you ;-)
     
  15. Then what were all those Zeniths I saw back in the 1980s with 3-wire
    cords? I saw them only in hospitals and nursing homes, while hotels
    had almost the same model with 2-wire cords.
     
  16. They were also used in classrooms and air traffic control towers, but
    they are long extinct. They were built before the leakage standards were
    tightened, and even then, they were not to be used in a room where a
    patient was connected to monitoring equipment. They were a standard
    consumer TV chassis, with a three wire cord added, and a metal cabinet
    for rough service. I repaired a lot of them at Ft. Rucker Alabama in
    1972 & 1973. I also repaired a lot of early '60s version for the
    Middletown, Ohio school system that only had a two wire cord, but were
    metal cased, "Institutional" TV set. They were strapped to heavy metal
    carts so they could be moved from one classroom to another without
    worrying that they would be damaged by rolling them into a wall or a
    door frame.

    --
    We now return you to our normally scheduled programming.

    Take a look at this little cutie! ;-)
    http://home.earthlink.net/~mike.terrell/photos.html

    Michael A. Terrell
    Central Florida
     
  17. There is in my opinion only three sources for the problem.

    One of your TV's has a problem and you'll need to disconnect them one at a time to find it.

    or

    Something is shorting to the TV's input cable from within the wall or service boxes.

    or

    They're a problem with the amplifier that is driving the system.
     
  18. Or, you don't know what you are talking about. If there is a
    "defective piece of equipment" you won't find it with this method. You
    have to measure the leakage current for each item, and do it to the
    manufacturers specifications. All TV sets have a low leakage current
    that will appear as a high voltage when read with a high impedance volt
    meter. What you are concerned with is the CURRENT that can flow. Volts
    might hurt, but Current flow is what kills.

    --
    We now return you to our normally scheduled programming.

    Take a look at this little cutie! ;-)
    http://home.earthlink.net/~mike.terrell/photos.html

    Michael A. Terrell
    Central Florida
     
  19. It is astonishing what voltage some entertainment electronics has on
    touchable parts. I was once setting up a Philips satelite receiver when
    I got a mild shock from touching the case. It turned ot that there were
    about 80 V DC towards ground. The voltage had a high source impedance,
    simply connecting the audio output to a stereo system made that voltage
    go away.

    I think the main problem is that audio/video equipment usually does not
    have a saftety ground connection (2-pole plug instead of 3), to avoid
    ground loops.

    Nevertheless, I was kind of surprised that equipment with such high
    voltages on the case would pass safety standards.
     
  20. They do pass the safety standards, which limit the leakage current.
    You can't have a current flow without a voltage to push it.

    --
    We now return you to our normally scheduled programming.


    Michael A. Terrell
    Central Florida
     
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