When switching electrical devices or overvoltages in the circuit between live parts, an electric arc may appear. It can be used for useful technological purposes and at the same time be harmful to equipment. Currently, engineers have developed a number of methods to combat and use the electric arc for useful purposes. In this article, we will look at how it arises, its implications, and its scope.
Content:
- Arc formation, its structure and properties
- Why does an electric arc occur?
- Harm and fight against it
- Useful application
Arc formation, its structure and properties
Let's imagine that we are conducting an experiment in a laboratory. We have two conductors, for example metal nails. We place them with a tip to each other at a short distance and connect the leads of an adjustable voltage source to the nails. If we gradually increase the voltage of the power source, then at a certain value of it we will see sparks, after which a stable glow similar to lightning is formed.
Thus, you can observe the process of its formation. The glow that forms between the electrodes is plasma. In fact, this is an electric arc or the flow of an electric current through a gas medium between the electrodes. In the figure below you can see its structure and current-voltage characteristic:
And here are the approximate values of the temperatures:
Why does an electric arc occur?
Everything is very simple, we considered in the article about electric fieldand also in the article about distribution of charges in the conductorthat if any conducting body (a steel nail, for example) is introduced into an electric field, charges will begin to accumulate on its surface. Moreover, the smaller the bending radius of the surface, the more they accumulate. In simple terms, charges accumulate on the tip of the nail.
Air is gas between our electrodes. Under the influence of an electric field, it is ionized. As a result of all this, conditions arise for the formation of an electric arc.
The voltage at which the arc occurs depends on the specific environment and its state: pressure, temperature and other factors.
Interesting: according to one version, this phenomenon is so called because of its shape. The fact is that in the process of burning the discharge, air or other gas surrounding it heats up and rises, as a result of which a distortion of the rectilinear shape occurs and we see an arc or arch.
To ignite the arc, it is necessary either to overcome the breakdown voltage of the medium between the electrodes, or to break the electrical circuit. If there is a large inductance in the circuit, then, according to the laws of commutation, the current in it cannot be interrupted instantly, it will continue to flow. In this regard, the voltage between the disconnected contacts will increase, and the arc will burn until the voltage disappears and the energy accumulated in the magnetic field of the inductor is dissipated.
Consider the ignition and combustion conditions:
There must be air or other gas between the electrodes. To overcome the breakdown voltage of the medium, a high voltage of tens of thousands of volts is required - this depends on the distance between the electrodes and other factors. To maintain the burning of the arc, 50-60 Volts and a current of 10 or more Amperes are enough. The specific values depend on the environment, the shape of the electrodes and the distance between them.
Harm and fight against it
We have considered the causes of the occurrence of an electric arc, now let's figure out what harm it does and how to extinguish it. An electric arc damages the switchgear. Have you noticed that if you turn on a powerful electrical appliance to the network and after a while unplug the plug from the outlet, a small flash occurs. This arc is created between the pins of the plug and the socket as a result of an interruption in the electrical circuit.
Important! During the burning of an electric arc, a lot of heat is released, the temperature of its combustion reaches values of more than 3000 degrees Celsius. In high-voltage circuits, the arc length reaches a meter or more. There is a danger of harm to human health and the condition of the equipment.
The same thing happens in light switches, other switching equipment, including:
- automatic switches;
- magnetic starters;
- contactors and more.
In devices that are used in 0.4 kV networks, including the usual 220 V, special protection means are used - arc extinguishing chambers. They are needed to reduce the harm done to contacts.
In general, the arc-extinguishing chamber is a set of conductive partitions of a special configuration and shape, fastened by walls of a dielectric material.
When the contacts are opened, the formed plasma bends towards the arc extinguishing chamber, where it is separated into small sections. As a result, it is cooled and extinguished.
In high-voltage networks, oil, vacuum, gas switches are used. In the oil circuit breaker, the damping takes place by switching the contacts in the oil bath. When an electric arc burns in oil, it decomposes into hydrogen and gases. A gas bubble is formed around the contacts, which tends to escape from the chamber at a high speed and the arc cools down, since hydrogen has good thermal conductivity.
In vacuum circuit breakers, gases are not ionized and there are no conditions for arcing. There are also high pressure gas filled switches. When an electric arc is formed, the temperature in them does not increase, the pressure rises, and because of this, the ionization of gases decreases or deionization occurs. A promising area is considered SF6 circuit breakers.
Zero AC switching is also possible.
Useful application
The considered phenomenon has found a number of useful applications, for example:
- Lighting. For example, arc-discharge lamps (DRL, xenon and other types). If you add salts of certain metals to the electrodes, the color of the electric arc will change.
- Arc welding. When the electrode touches the metal surface, a high current flows, which heats the metal. When you break off the electrode, the current cannot be interrupted, the heated surfaces emit the electrodes and an arc occurs. When the metal surfaces to be welded are melted and the electrode itself melts, the two parts can be joined or cut. There are various types of welding, for example, using electrodes or gas - carbon dioxide or argon. It is used universally and has made a huge contribution to residential and industrial construction.
- Arc melting. The electric arc depends on the electrical parameters of the power sources, thus it is possible to regulate its combustion. Due to the high temperature, it is possible to melt a large number of metals.
Finally, we recommend watching a useful video on the topic of the article:
Now you know what an electric arc is, what are the causes of this phenomenon and possible applications. We hope the information provided was clear and useful for you!
Related materials:
- Causes of a short circuit
- Welding cable
- Welding wires in a junction box
(0)I do not like
(0)