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is third transformer hole an earth?

Discussion in 'Electronic Repair' started by john downie2, Nov 8, 2008.

  1. Arfa Daily

    Arfa Daily Guest

    You see, there you go getting your apples and oranges mixed up again. A
    static charge from a carpet is not lethal to you because it is an extremely
    high resistance source. That is the nature of a static charge. No matter
    whether it is 1kV, 10kV or 40kV, the source resistance will ensure that only
    uA will flow for decimals of a uS. Enough to give you a nasty little crack,
    yes, but not enough to come within several orders of magnitude of being
    lethal. The same applies to the taser. It is designed to not be able to
    supply enough current (theoretically) to be lethal. However, its aim is to
    incapacitate by disrupting nerve activity and causing extreme pain. To do
    this, the charge is applied to the receiving body in multiple short-duration
    pulses. In some cases, this has proven to be fatal, I believe, due to the
    length of time that the pulses are delivered for. On the other hand, a high
    voltage power line, or even a fairly low voltage household supply, *is*
    potentially lethal not because it can deliver huge amounts of amps, but
    because the voltage that is present, is sufficient to drive *enough* (mili)
    amps from that low resistance source through the conduction path within the
    body.
    Yes, arc welding can occur at 12v and, in fact, at lower voltages than this,
    although there are many different methods these days of generating and
    controlling the welding arc, though none that operate by charging caps to
    the sorts of voltages that you are talking about, as far as I am aware. In
    general, arc welding is a high current rather than high voltage system. Spot
    welders sometimes work by charging very large capacitors to low voltages.
    The reason that the low voltage arc welder is not capable of killing you,
    any more than a car battery can, is because although both are extremely low
    internal resistance sources, and hence capable of supplying large amounts of
    current to a low resistance load, the human body is a much higher resistance
    load, so the 12v is not capable of pushing enough miliamps through that
    path, to cause electrocution - or even electric shock.

    Well, if you understood the principles of isolation transformers, which as
    well as being used on building sites for obvious reasons, are also to be
    found in every professional electronics workshop, to render safe, the need
    to work on live equipment employing such potentially lethal nasties, as
    switch mode power supplies, then you would know that it isn't too good to be
    true - it simply *is* true. Sketch it down on a piece of paper then have a
    good think about it.

    Arfa
     
  2. Arfa Daily

    Arfa Daily Guest

    This is unmitigated nonsense. You clearly have little understanding of how
    the terms that you are bandying about, relate to one another, and more
    importantly, to current flow within the human body. I don't mean to be
    offensive, but there's a good old phrase which goes something like -

    "When the hole that you're digging gets too deep to climb out of, stop ... "

    'Nuff said, I think.

    Arfa
     
  3. Whiskers

    Whiskers Guest

    Precisely; it isn't the voltage that's lethal, it's the current. If the
    current isn't available, the voltage is pretty harmless. High-voltage
    mains power lines carry a /lot/ of current, whereas high-voltage 'Tazers'
    don't.
    OK, I'm not familiar with arc welding, and I'm surprised that 12V is
    enough to strike an arc. But your comment that it is the high current
    that matters for creating a weld, rather supports my contention that
    current is more dangerous than voltage.

    Electron beam welding (which you probaby won't find happening in a garden
    shed or on a building site) requires many thousand volts, but only a small
    current (less than 1A).

    The only welding I've ever done myself, involved a coke-burning forge and
    a heavy hammer.
    The only 'isolation transformers' I've come across, are earthed - and
    either have an output restricted to 50V or less (that threshold at which
    the human body's resistance drops substantially) or provide a good earth
    connection for the appliance. An isolation transformer /does/ disconnect
    the connection that exists between the power station and the ground and is
    carried by the 'live' or 'hot' cable, but that is /not/ the same as the
    safety 'earth' connection which is meant to provide a relatively safe
    return path in case exposed metal parts of the appliance accidentally
    become connected to the power supply.

    The 'earth' connection provided by an 'isolation transformer' to the
    appliances running off it, may be arranged to effectively create a
    'short-circuit' on the output windings of the transformer if any current
    flows through it, and that should trigger the safety cut-off (probably a
    'residual current device'). Or the 'earth' connection will pass straight
    through to the same 'earth' as the mains supply provides.

    A double-insulated appliance has no exposed metal parts that can become
    connected to the power supply, and that's why they are permitted to be
    used with no 'earth'.
     
  4. Arfa Daily

    Arfa Daily Guest


    Neutral and ground are bonded together at the substation

    Hmmmm. Your understanding of the principles of ground isolation by use of a
    transformer seem very flawed I'm afraid, as also is your knowledge of
    electricity distribution schemes, and earth / neutral commonality.

    Arfa
     
  5. ian field

    ian field Guest

    In that case you shouldn't have any problem grabbing the high voltage
    terminal of a 2kV microwave oven transformer - its only rated for 500mA.

    On the bright side, we wouldn't have to put up with you talking complete and
    utter bollox.
     
  6. ian field

    ian field Guest

    PHucker usually hangs out on alt.binaries.chatter , I can't recall seeing
    whiskers there. PHucker sometimes trolls on the sci.electronics groups -
    usually asking stupid questions then criticizing the answers he's given.
     
  7. ian field

    ian field Guest

    He claims to have a degree in physics, yet according to him he found physics
    too boring so he settled for a tech support job in a computer dept - funny
    how he visits this group yet never seems to contribute in any meaningful
    way.
     
  8. He (Dave P.) is right. Once you're about the amount of current
    sufficient to be lethal (which is measured in milliamps, not amperes),
    the ampacity of a circuit you're touching--whether 15, 20 or 200
    amps--matters not.

    No reason to be overly impressed by a 60-amp circuit. No more deadly
    than a 1-amp circuit.
     
  9. VanguardLH

    VanguardLH Guest

    David Nebenzahl replies to 13-month old thread:
    Google Groups copy of old thread: http://preview.tinyurl.com/ylf7uo8

    You thought any participant in that *OVER 1-YEAR OLD* thread was still
    waiting for a response?
     
  10. You were reading an *OVER 1-YEAR-OLD* thread? and commenting on it? (And
    look--I *did* get a response!)

    In my defense, I neglected to look at the year; thought it was about a
    month old.
     
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