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  1. #131
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    Quote Originally Posted by SG-1 View Post
    My research into this indicates that water vapor has a higher Voltage Break Down than dry air. Ions are recombined into the water droplets before they can form into an electric circuit.

    The presence of water on the contaminated surface of insulators & other materials does make a flashover more likely in the presence of corona discharge.
    dry air is more of an insulator than humid air
    humid air is more of a conductor than dry air

    https://www.physicsclassroom.com/cla...and-Insulators


    Pure H2O is a good insulator, and if you displace air molecules with pure H2O the mixture moves towards better insulator. Outside of special lab tests and environment (aka "real world"), the higher %RH the easier it is for charge to migrate, hence a better conductor.

  2. #132
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    190112-0923 EST

    Distilled water exposed to air has moderately low conductivity, but nothing close to ultra pure water.

    Several sources indicate that distilled water exposed to air has a resistivity of about 1 megohm-cm. Measurements I have made with grocery store distilled water might imply somewhat lower. Rain or fresh snow water is quite a bit lower than store distilled water, but not nearly as low as tap water. Note there is a 100 to 1 ratio difference between megohm-cm and megohm-meter.

    Several references:

    https://www.google.com/search?q=ohm-...hrome&ie=UTF-8

    https://www.researchgate.net/post/Wh...ater_DI_water2

    https://www.labmanager.com/white-pap...r#.XDoAXyMrLAw

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  3. #133
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    On the other hand:

    E. Miscellaneous correction factors. Changes in the air medium that forms the insulation influences the strength of an air gap. A brief discussion of each factor follows.


    1. Dielectric strength of air. The dielectric strength of air in a uniform electric field at standard atmospheric conditions is approximately 3 kilovolts per millimeter.


    The pressure, temperature, and humidity of the air, the shape, dimensions, and separation of the electrodes, and the characteristics of the applied voltage (wave shape) affect the disruptive gradient.


    2. Atmospheric effect. The empirically determined electrical strength of a given gap is normally applicable at standard atmospheric conditions (20 °C, 101.3 kilopascals, 11 grams/cubic centimeter humidity). An increase in the density (humidity) of the air inhibits sparkover for a given air gap. The combination of temperature and air pressure that results in the lowest gap sparkover voltage is high temperature and low pressure. This combination of conditions is not likely to occur. Low air pressure, generally associated with high humidity, causes increased electrical strength. An average air pressure generally correlates with low humidity. Hot and dry working conditions normally result in reduced electrical strength. The equations for minimum approach distances in Table R-3 assume standard atmospheric conditions.


    3. Altitude. The reduced air pressure at high altitudes causes a reduction in the electrical strength of an air gap. An employer must increase the minimum approach distance by about 3 percent per 300 meters (1,000 feet) of increased altitude for altitudes above 900 meters (3,000 feet). Table R-5 specifies the altitude correction factor that the employer must use in calculating minimum approach distances.



    Source: https://www.osha.gov/laws-regs/regul...0/1910.269AppB
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  4. #134
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    190112-1341 EST

    Circuit breakers, such as those made by Mechanical Products, for aircraft use have to be tested under low pressure conditions.

    In the early 1960s I did some consulting work at MP. One project was to solve a production problem on an order for the breakers for the prototype B-1 bomber. Yield was initially only about 5% because of the very tight specifications. Finding the problem I was able to improve the yield to about 95%.

    At the time I did work at MP it was family owned by Mr. Knapp, and his son in law Lyle Trolz was president. I liked brownies, and when we went to lunch Mr. Knapp always made sure I had brownies for dessert. In the early 1960s MP was sold to Howard Aiken, and that is how I met him. Aiken was the driving force behind the Harvard Mark I. Approximately 1935 Aiken proposed the idea of a digital computer, but his idea received no support at that time. James B. Conant was president of Harvard at that time. Thru a Harvard VP Aiken was put in contact with IBM and that got the Mark I started.

    https://en.wikipedia.org/wiki/Harvard_Mark_I
    https://en.wikipedia.org/wiki/James_B._Conant

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  5. #135
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    Quote Originally Posted by LarryFine View Post

    2. Atmospheric effect. The empirically determined electrical strength of a given gap is normally applicable at standard atmospheric conditions (20 °C, 101.3 kilopascals, 11 grams/cubic centimeter humidity). An increase in the density (humidity) of the air inhibits sparkover for a given air gap. The combination of temperature and air pressure that results in the lowest gap sparkover voltage is high temperature and low pressure. This combination of conditions is not likely to occur. Low air pressure, generally associated with high humidity, causes increased electrical strength. An average air pressure generally correlates with low humidity. Hot and dry working conditions normally result in reduced electrical strength. The equations for minimum approach distances in Table R-3 assume standard atmospheric conditions.

    Where does the term "electrical strength" come from? We describe items in terms of resistance (R), or conductance (1/R).

    Dry add is a better conductor?? Seems to be contradictory.

    Static electricity is caused by an imbalance of electrons on a surface. Atoms normally contain an equal number of protons (positively charged particles) and electrons (negatively charged packets of energy). When they don't, the result can be shocking. Why? Because when two materials in motion make contact, electrons jump from one to the other to fix the imbalance!
    So what do temperature and humidity have to do with static electricity? Moisture makes the air more conductive, so it can absorb and more evenly distribute excess charges. On humid (wet) days, objects don’t hold static charges quite as well. Also, temperature changes can generate a temporary voltage. This is known as the pyroelectric effect.

    Author: Judee Shipman
    Ref: https://www.education.com/science-fa...-charges-last/

    Perhaps when looking at leakage from HV lines, the flash voltage # is not as important as the conductivity of the air. That is, if I believe you get higher flash V # when the air is less %RH.

  6. #136
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    Quote Originally Posted by FionaZuppa View Post
    Where does the term "electrical strength" come from? We describe items in terms of resistance (R), or conductance (1/R).

    Dry add is a better conductor?? Seems to be contradictory.



    Ref: https://www.education.com/science-fa...-charges-last/

    Perhaps when looking at leakage from HV lines, the flash voltage # is not as important as the conductivity of the air. That is, if I believe you get higher flash V # when the air is less %RH.
    Electric strength is measured in volts per meter and is purely a measure of breakdown voltage characteristics. It can be measured from one side of a solid insulator to the other or from point to point in dry air. It can also be measured from point to point along the surface of an interface between two insulators, such as ceramic and air.
    Electric strength can be measured between two (approximately) infinite parallel plates to avoid the complication of electric field gradients. A point or small radius of curvature can make breakdown easier because the electric field is stronger close to the point

    Sent from my XT1585 using Tapatalk

  7. #137
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    Quote Originally Posted by LarryFine View Post
    [B]An increase in the density (humidity) of the air ... Low air pressure, generally associated with high humidity,
    Something doesn't seem to fit. Above is self contradictory?

    Higher humidity air is less dense, just ask any aircraft pilot. To understand that ... just recognize that water vapor is H2O and air is about 78% nitrogen, 21% oxygen each of which is a diatomic molecule.

    So, going back to my high school chemistry, 1 mole (22.4 liter at STP) of air weighs 0.78*2*14 + 0.21*2*16 grams, or about 60 grams. 1 mole (again, 22.4 liter at STP) of water vapor is 2*1+16 grams, or about 18 grams.

    gar, you're the expert on these things ... isn't this true?

  8. #138
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    I would ask whether at temperatures well beow its boiling point water vapor behaves as an ideal gas. But your argument seems logical at first examination.

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  9. #139
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  10. #140
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    It is not the humidity or water vapor in the air that prevents static electricity build up, but a thin film of water on the surface of the materials that lowers the resistivity.
    http://amasci.com/emotor/humid.html

    Here is a more technical paper, see page 39 for a plot of humidity vs breakdown voltage of air.
    http://ethesis.nitrkl.ac.in/2875/1/F...04.07.2011.pdf

    This does not mean that powerlines are better off during periods of high humidity or rain, because they produce corona & it's presence during high humidity increases the likelihood of a flashover.
    https://www.pupman.com/listarchives/.../msg00428.html
    Advise is a dangerous gift, even from the wise to the wise.

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