The size of the plane is not at all critical, since the size of the electric field is not critical. Thus, the electric field resulting from a sphere in very close proximity to a plane approaches uniformity.
We used a 2-inch diameter steel sphere at distances from 0. The scale illustration indicates the worst-case appearance of the two conductors. The second variable is the detection of the breakdown of air. Fortunately, modern hi-pot testers have electronic trip mechanisms, which are uniform in tripping when an arc occurs. Any trip current is acceptable provided an arc truly occurs just before the trip. This is easy to confirm visually. The third variable is the measurement of voltage at the time of trip.
Here, a digital meter can be very helpful if the voltage is increased very slowly when approaching the breakdown. After performing the experiment and experiencing the repeatability, it seems appropriate to hypothesize why the differences between our data and the lEC data.
When the lEC data is plotted point-by-point, the data agrees below 5 kV or so, and diverges seriously at 6. The hi-pot tester available to me was limited to 6 kV rms and 6 kV dc, so we could not collect data as the ratio increased.
One hypothesis could he that nonlinearity can occur as the ratio increases. At the lower voltages, the IEC data is not as precisely linear as our measurements.
This could be explained by non-uniform observations of the breakdown or by poor control or measurement of the voltage. With the experience of performing the measurement, we found that these are critical to the uniformity and repeatability of the measurement. We chose steel as the material for the electrodes. Massive, thick steel. Whenever an arc occurs, the power dissipated in the arc can melt the metal at either end of the arc. But, with a good thermal conductor and lots of thermal mass, this is minimized.
In attempting to do the point-to-plane test, we burned up a hardened steel needle when the hi-pot failed to trip and let the arc continue for an undue amount of time.
The other extreme is the perfectly non-uniform field and, consequently, non-uniformly distributed equipotential lines. Such a field is that resulting when the diameter of the sphere approaches zero. A practical point is an extremely small sphere compared to the distance between the sphere and the plane. Since the lines of force must emanate at right angles to the surface of the small sphere, they are bent in the region of the small sphere and are therefore longer than the single line of force at the end of the sphere.
Because the equipotential lines must be normal to the lines of force and equally spaced along the lines of force, the equipotential lines are severely bent near the small sphere. This bending of the equipotential lines increases the total force on any charged particle in the region of the point as compared to a homogeneous field. Figure 3. We also repeated the same test with a point-to-plane system.
Immediately, we see significant differences. The first is a current indication well below breakdown. The second is lack of repeatability. The third is the slope of the line is about one fourth that of the sphere-to-plane. First, the highly bent equipotential lines lead to partial discharge at voltages very much less than the breakdown voltage. What happens is that the air actually breaks down in the region very near the point, but not across the entire gap.
This is the same phenomenon as St. Second, because the point has a very small thermal mass, the breakdown arc actually melts the steel at the point, and therefore changes the shape of the point. This statement may pose many questions if you are unfamiliar with insulation or the properties of air.
In general, gases are better insulators than liquids, which are better insulators than solids. Density is a large factor which affects a substances insulation capability. In its solid state, as ice, its molecules are packed much closer together in relation to its gaseous state as water vapor. Air is a good insulator because it is a gaseous substance, therefore its spread-out molecular configure resists heat transfer to some degree. Think of the last time you had a hot cup of coffee or tea.
When you pick the cup off the table to take a sip, you can feel the warmth of the drink get hotter as your face gets close. The reason why you do not feel the same warmth of that coffee or tea from the table is because the air is acting as an insulator. An example of air as an insulator is a double-pane window. Double-pane windows are two sheets of glass with a stagnant space of air or gas between the two panes. This small space of air between the two layers of glass reduces the ability of heat transfer via convection.
Convection is the transfer of heat by the movement of fluid matter, fluids being only liquids and gases.
Because this region of air has very minimal movement, it makes it very hard for heat to transfer through this medium.
Other examples of air as an insulator include insulated coffee cups. If you are drinking out of a paper cup, your hand will feel the warmth of the coffee, maybe even so much that you need a cup sleeve to comfortably hold it. But if your drink is in an insulated cup, you may barely feel any heat while holding it. Not only is it cooler to the touch, but it also keeps the coffee hot for a longer period of time.
Two types of insulated cups are thermos style cups and polystyrene cups, often referred to as styrofoam. Keeping warm involves stopping the transfer of heat from one object to another. This can be done by insulating the object.
Keeping the inside of buildings warm and people warm can be done by covering them or any other object in layers of an insulator. Trapping air in layers is a very effective way of insulating an object. Cats fluff up their fur and birds do the same to their feathers when it is cold.
This traps air inside the fur or feathers and reduces the amount of heat energy these animals lose. People wrap themselves in layers of clothes for the same reason. The thicker the layers and the more layers of clothing someone is wearing, the better the insulation. Polystyrene and plastic foam are both used as insulators as they have small air bubbles trapped inside them.
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