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Lightning Protection

Discussion in 'Misc Electronics' started by tube2ic, Jul 28, 2003.

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  1. tube2ic

    tube2ic Guest

    Hi Halfgaar,
    I think your original question was "Is there a way to reasonably
    reliably protect electronic equipment from indirect lightning hits".
    The short answer is yes.

    Now for the long answer.
    Indirect lightning hits induced large currents in metalic objects.
    This includes but is not limited to telephone and AC mains. I will get
    into more detail a little later.

    Electronic equipment are protected from "Indirect" lightning strikes
    by using fast acting gas discharge tubes, Sidactors, Metal Oxide
    Varistors, etc. on the power line and on any interface (such as
    telephone interface on modems) that could be a source of lethal
    surges. The function of these devices are to "fire" at a particular
    voltage (In your case the device inside your surge supressor is rated
    at 750V) and short the lethal surge either to ground in a 3 wired AC
    mains circuit or across the circuit it is protecting.
    Tremendous currents flow in such cases via the device for a very short
    time (typical for lightning) hence the fairly large value of surge
    current on your power strip (1250 Amps).

    A common misconception is that lightning only hits ground. This is not
    so. Lightning will hit any object that is "Relatively" lower or higher
    in potential to the source (the storm cloud in this case). So
    lightning can strike an AC mains cable too. The AC grid can easily
    absorb a lightning strike as it designed to withstand and safely
    discharge such tremendous energy surges. But there is bound to be some
    surge on the wires before this happens.
    So lightning could hit your AC mains cables miles from where you live
    and the currents would travel down the wire into your home and zap all
    your applicances that are plugged in. This is a case of indirect
    lightning strike.

    Now most power supplies on electronic appliances are designed to
    withstand voltages much higher than their normal operating voltages
    for short durations before they burn out. Hence plugging in an
    appliance to a surge supressor will limit the high voltage spike to a
    short interval and save your appliance. So the best advice I can give
    you is to check the specification on your appliance's power supply for
    it's max voltage and get a spike suppressor will limit the voltage
    spike to that.

    The light and reset switch on your power strip are for the overload
    protection. This is a reverse protection as you are protecting the AC
    mains wiring in your home from a shorted appliance or one that draws a
    larger current than the outlet is rated when it is switched on.
    Typically devices that have large capacitors or inductors such as
    motors or high wattage amplifiers with speakers draw a much larger
    current than normal when switched on. The overload protector saves you
    the trouble of going down to the basement or wherever to reset the
    circuit breaker when this happens.

    I am not even going into the modem protection as it is not relevant to
    your original question.

    As for how to test that it works? Well that is already done by the
    manufacturer. One of my duties at my job (I work for a telecom
    equipment manufacturer) is to zap equipment with an artificial
    lightning machine to see that it not only withstands an indirect
    lightning strike safely but also continues to work during that time.
    There are many standards for equipment surge protection which
    manufacturers have to meet. (Example K21 for modems)just ensure that
    when you buy new gear they meet the specs in your country.

    As for direct strikes, your only protection is the dude upstairs. Make
    sure he is happy with you:)

    Also all you dudes who are fans of Ben Franklin style lightning
    arrestors, get it done by a professional. Remember that such an
    arrestor will not protect equipment from the indirect surges induced
    when lightning strikes the arrestor. The equipment has to be designed
    to have protection or should be connected to the mains or phone outlet
    via surge arrestors..

    Hope this helps.

    Vinay a.k.a tube2ic
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