Faraday cage and linemen

Discussion in 'General Electronics Chat' started by Cerkit, Feb 10, 2011.

  1. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
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    I have read a bit on how the faraday cage suit can protect linemen from current flowing through them when working on HV lines but it keeps talking about electric field, how is there an electric field if anything I see it as a magnetic field created by the current in the wire...??
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Current creates a magnetic field, voltage creates an electric field. High tension lines have 110,000 volts, sometimes more.
     
  3. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    Neither field can penetrate a perfect conductor ( and no conductor is perfect ).

    There is a great movie on utube of a lineman working in South Africa, well worth finding.
     
  4. magnet18

    Senior Member

    Dec 22, 2010
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    Not practically, but lets not forget superconductors :)
     
  5. timrobbins

    Active Member

    Aug 29, 2009
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    Cerkit, are you ok now about how the E-field can be quite high between an energised line and an earthed structure - and how the faraday cage can provide 'protection'.

    The typical example is someone holding up a long thin flourescent tube in a high e-field (with moist air).
     
  6. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
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    Can you please explain how an electric field arises from the high voltage line? If I manage to visualise that things will become clear I think..
     
  7. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
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    If I am correct the E field from such a wire if dran moves radially out from the wire, is that righ?? If that is correct why is it in fact like that?
     
  8. magnet18

    Senior Member

    Dec 22, 2010
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    I found it useful to think of voltage using the older terminology.
    Voltage is essentially "electrical pressure"; you have a strong positive charge, or positive pressure, and negative charge, or less pressure ("suction"), and ground is neutral, it can supply or take as many electrons as you need.

    The difference between pressure in a pipe and electrical pressure is that electrical pressure isn't physical, its electric, hence the term electric field.
    When theres a strong positive charge on a wire, its "pulling" at the electrons in the ground, or any object with a more negative charge. A metal cage will provide a path of less resistance should the electrons begin to jump between, say, a HV wire, a lineman, and earth, so they flow around him.

    The field lines radiate outward because the electric pressure creates a field through the air.

    Hopefully this helps :)
     
  9. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The magnetic field might not be very well screened by a practical Faraday cage made of ordinary conductors, but that would not normally be of much consequence. Human beings can usually survive quite strong magnetic fields at power frequencies. This might not be so if the operator was fitted with a conductive implant like a cardiac pacemaker, for which reason users of such devices should avoid this type of occupation.
     
  10. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
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    Ok. So does this mean that there is generally always a small connection to ground through the air from High voltage lines??
    When a lineman is getting close to the line with a chopper and he creates the arc with the rod is that static discharge? what is going on at that point?
     
  11. magnet18

    Senior Member

    Dec 22, 2010
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    Not exactly, it's like two magnets held apart, they might tug on each other a bit, but they wont have enough energy to jump together until they get close enough.

    The arc isn't exactly static, but its pretty much the same. The air between the rod and the line you're talking about ionizes when the distance to voltage ratio is low enough, and at this point it becomes a good conductor and electrons will leap across to try to equalize the voltage.
     
  12. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
    275
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    This has been very helpful thanks a lot!
     
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