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Evening Reading

by Steve Gibson, Aug 19, 2004 6:38pm PDT
Related Topics – Wack News

I dont understand how ranch sauce can taste good on so many things. I swear I didnt like that stuff much at all when I was younger but now it seems that if it doesnt have ranch sauce on it then its just not worth sticking in my mouth. (Hi mom!) Yeah I'm not sure what's going on either...

- P2P wins one in court - The beer inhaler has arrived - The masturbating judge. :( suSTAINED! - Your final Google IPO story - Flying robots are here
Lastly, TV viewing record broken!


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  • #2063
    Refresh Thread
    By: monohydrate x
    Reply
    Let’s take a look at a worst-case
    scenario in which a technician
    is performing measurements
    on a live three-phase motor
    control circuit, using a meter
    without the necessary safety
    precautions.
    Here’s what could happen:
    1. A lightning strike causes a
    transient on the power line,
    which in turn strikes an arc
    between the input terminals
    inside the meter. The circuits
    and components to prevent
    this event have just failed or
    were missing. Perhaps it was
    not a CAT III rated meter. The
    result is a direct short between
    the two measurement
    terminals through the meter
    and the test leads.
    2. A high-fault current–possibly
    several thousands of amps–
    flows in the short circuit just
    created. This happens in
    thousandths of a second.
    When the arc forms inside
    the meter, a very high-pressure
    shock wave can cause
    a loud bang!–very much like
    a gunshot or the backfire
    from a car. At the same instant,
    the tech sees bright
    blue arc flashes at the test
    lead tips–the fault currents
    superheat the probe tips,
    which start to burn away,
    drawing an arc from the
    contact point to the probe.
    3. The natural reaction is to pull
    back, in order to break contact
    with the hot circuit. But
    as the tech’s hands are
    pulled back, an arc is drawn
    from the motor terminal to
    each probe. If these two arcs
    join to form a single arc,
    there is now another direct
    phase-to-phase short, this
    time directly between the
    motor terminals.
    4. This arc can have a temperature
    approaching 6,000 °C
    (10,000 °F), which is higher
    than the temperature of an
    oxy-acetylene cutting torch!
    As the arc grows, fed by
    available short circuit current,
    it superheats the surrounding
    air. Both a shock blast and a
    plasma fireball are created.
    If the technician is lucky, the
    shock blast blows him away
    and removes him from the
    proximity of the arc; though
    injured, his life is saved. In
    the worst case, the victim is
    subjected to fatal burn injuries
    from the fierce heat of
    the arc or plasma blast.
    In addition to using a multimeter
    rated for the appropriate
    Overvoltage Installation Category,
    anyone working on live
    power circuits should be protected
    with flame resistant
    clothing, should wear safety
    glasses or, better yet, a safety
    face shield, and should use
    insulated gloves.
    Transients–the hidden danger
    Dec 10, 2004 6:07pm PST
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