Electrical resistance of air 3

[enginesrus]

I know it will change due do moisture, as well as pressure. What is the standard and number?

 

[waross]

Air makes a good insulator;
It's plentiful and easily obtainable.
It's cheap.
It's light.
It's easy to cut and drill.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[zeusfaber]

This is one of those cases where Ohm's thingummy is more of a guideline than a law. The relationship between the applied voltage and the current through a column of air is sufficiently nonlinear that resistance stops being a particularly useful concept.

The one figure you will find is the dielectric strength, which comes in at about 3MV/m for dry air at atmospheric pressure (noting that atmospheric pressure is more variable than you expect and that dry air is a rare beast in itself).

A.

 

[7anoter4]

Usually the shunt conductance depends also on pollution and it could be between 1 to 2 /10^9 S/km
[for 130-230 kV transmission line].

 

[7anoter4]

Sorry. I think you are interested in ELECTRICAL PROPERTIES OF ATMOSPHERIC MOIST AIR. See-for instance ELECTRICAL PROPERTIES OF ATMOSPHERIC MOIST AIR /1988 Research.

http://dtic.mil/dtic/tr/fulltext/u2/a199599.pdf

 

[enginesrus]

I realize air is a good insulator, but I also know that electricity does also travel through air, and it doesn't need to be very humid for that to happen. And no I'm not talking about an arc or plasma. Thank you for the information above.

 

[cuky2000]

The resistance of the air is fairly high. The air dielectric characteristics change with temperature, moisture and pressure among other factors. The electrical resistivity (ρ) is estimated between 1.30×10¹⁴ to 3.30×10¹⁴(Ω·m)at20 °C
See the enclosed graph for additional information.

 

[Compositepro]

Electron flow through air occurs due to ion mobility. Air must be ionized before there is any flow. Ionization of air is often initiated by a comic rays or other radioactivity, and/or a strong electric field, which is measured in volts/meter. If the electric field across a gap is enough to ionize air you will get a plasma arc where the current is limited only by the circuit resistance and source voltage.
If the electric field is non-uniform, as it usually is at the tip of a sharp electrode, then the electric field strength may be enough to ionize the air near the tip of the electrode but not to ionize it further away. In this case you get a corona discharge where the ions that are generated at the electrode will move toward the counter electrode or ground due to the electric field. So electron flow is through ion flow. You may have heard of air purifiers called an ion breeze. There actually is a breeze generated by this type of ion flow.
Resistance can be calculated using Ohm's law from the measured current and voltage but this resistance is not a material property.

 

[enginesrus]

When a fellow climbs a high tension galvanized steel tower and is looking right at the wires just feet away and can feel
the affects of the electrical current leaking off with no arcing at all and no plasmas. I suppose all conductors in a sense ionize.

 

[davidbeach]

Don’t need anywhere that much exertion. In a 500kV switchyard all you have to do is raise your arm above shoulder level and you can feel the electric field.

 

[cuky2000]

To illustrate a practical application example of air insulation for high voltage, below is a picture and a video link of linemen working on energized air insulated 500 kV transmission line.

https://www.youtube.com/watch?v=9tzga6qAaBA

 

[edison123]

That's some scary s..t. Thanks cuky.

Muthu

http://www.edison.co.in

 

[enginesrus]

The simple fact that you feel the current leaking means there is conduction though very high resistance. Key word is resistance.

 

[ScottyUK]

Davidbeach,

We got an unofficial tour of a 275/400kV open terminal substation a few years ago. I walked around with my hands in my pockets.

The big GIS substations didn't bother me as much, although the thought that my hands were separated by 1/4" of aluminium and four inches of SF₆ gas from touching a quarter-million volt busbar was a little unnerving at first.

 

[Mbrooke]

@Edison: Id rather be up on a 500kv line several times over than a 480 volt service in a large building. Your chances of survival are actually higher if something goes wrong or bang.

 

[bacon4life]

Close high voltage lines, it is capacitance rather than resistance that makes your hair stand on end.

 

[enginesrus]

If you where on one plate or the other how can you feel the charge? You won't feel it unless electron flow comes from some place? If your on a capacitor plate you are part of that charge on it, and you can't feel or know there is a charge till you complete a circuit.

 

[SwinnyGG]

Anything that bears a charge and is placed in a field will 'feel' effects- there does not need to be current flow.

 

[bacon4life]

Charge doesn't have to be moving for us to feel in the impact of charge buildup. Picture what happens when you rub a balloon in your hair to build up static electricity. After you stop rubbing, you will still feel the charge making your hair stand on end.

Typically we think about only conduction current. However, most of what you feel near HV equipment is due to [link tps://en.wikipedia.org/wiki/Displacement_current]displacement_current[/url].