Electric automatic is an incorrect everyday designation of protective devices for low voltage circuits. As a rule, it implies a single-phase network of 220 V. Those who wish can familiarize themselves with the section on Automatic Circuit Breakers.
Automatic circuit breakers
It is not correct to use the term electric circuit breaker to this type of device. Strictly in this regard, the Soviet literature since the USSR.Such products are called, for example, automatic air switches. The meaning is hidden in every word:
- Automatic - work without human intervention, with the possibility of the intervention of the latter.
- Air - the device is not sealed, not filled with medium.
- Switches - a device for interrupting the circuit.
Any automaton inside contains two types of releases: thermal and electromagnetic. The range of operation currents is set at the factory, but the main thing is that the devices react to the amount of charge flowing per unit of time and additionally trigger on:
- minimum current threshold;
- reverse current occurrence;
- minimum or maximum voltage threshold;
- rate of increase or decrease of current.
. Devices that operate on a number of criteria are called universal, replacing a group of devices. The critical time is the response time, which consists of several components when the current rises to a specific limit according to the law of the exponent in a matter of seconds. This becomes the first term of the formula, and the second is the time of the release mechanism, called the proper one. Finally, the arc does not go out immediately, becoming the third term in the formula for the total response time. Safety automats with a response time of less than 0.01 s are called high-speed ones.
According to the concept of adequacy, the automatic protection allows you to configure at least two parameters. For example, the response time and the magnitude of the current. The necessary operations are performed at the factory, and the case, as a rule, is not collapsible, does not provide for changes in the parameters during operation. There are classes of automaton protection groups of values. For example, it is known that the threshold current of operation B is lower than that of C, and is measured in proportion to the nominal. It is useless here to give a complete classification, specific figures depend on the manufacturer and vary widely. In the catalogs, as a rule, the current time is given, where the thresholds and response delays are indicated.
Contacts of the
protection machines After a response, that is, an abnormal situation, the device is able to return to normal operation. Therefore, a parameter is entered - the maximum tripping current. This gigantic number, unattainable in everyday reality, showing a certain limit, below which the circuit breaker will remain intact, will not burn and will not melt. Design features are noted:
- . For rated currents below 200 A, the current-carrying part consists of single contacts. Even the owner of a small estate can be put here: the power consumption of the devices will be 44 kW.For the administrative building it is already not enough, if we recall that electric boilers for 100 kW and more were created.
- Rolling contacts are T-shaped or T-shaped with a rounded tip. At the moment of closing and opening the arc arises and goes out on the sidewall. And the working area used in normal mode is not affected by destructive processes. As a result, the service life of the product is much higher.
- The use of arcing contacts is a typical solution for burning the surface of the working surface from burning. The chain is broken in two stages. First, the main contacts come out of interaction with each other. When the process is complete, discharge formation begins. But the arc burns between the protective contacts, there is formed carbon. The work surface does not suffer.
- End contacts asymmetrical. One looks like a pipe, the second looks like a mushroom, the cap of which is spring-loaded and rocking for better contact of both surfaces. As a result, wear and misalignment are leveled. In addition, the contact area increases.
Resistance is almost independent of contact area, but working conditions vary greatly. For example, massive surfaces allow more current to flow. Copper and alloys are most readily used as conductive materials. To reduce the level of oxidation processes tin plating and protective silver pads are used. Periodically applied aluminum and steel. They are protected from corrosion, respectively, by a layer of zinc and cadmium. But the most durable recognized tungsten contacts, allowing heating to high temperatures.
protection circuit. Those where high accuracy is needed, but there are no significant currents, use silver, platinum and nickel. AgO film quickly covers the metal with remarkable electrical conductivity. Platinum does not oxidize at all.
At the time of contact opening and closing, vibrations occur due to strong changes in the electromagnetic field. Numerous current surges cause non-uniform arcing. Vibrations cause the contacts to collide and destroy the working surface, being one of the limiting factors of the service life of the automatic machine protection.
Arc Extinguishing Methods
Various sources state that a special camera is used to extinguish the arc in a circuit breaker. It cuts a stream of ionized air, disrupting the burning process. And the method is not the only one. In industry, a number of other methods are used, and used for automatic protection. It is a mistake to assume that the arc is characteristic of high voltage circuits. Sources claim that ionization is already observed at 15–30 V, if at least 100 mA of current flows through the circuit. A characteristic feature of the process is the participation in it of the charges of both signs formed from air molecules. However, positive ions are much less mobile. The lion's share of the current falls on electron transfer. Positive ions approach the cathode and help the emission of negative carriers from its surface. Falling on the electrode, give him their energy. The cathode is heated to high temperatures( up to 5000 ° C).This enhances the outcome of electrons. It turns out that positive ions immediately contribute to two similar processes:
- Autoelectronic emission under the influence of a positive field external to the cathode.
- Thermionic emission.
Fast moving electrons become current carriers, performing impact ionization of air molecules. The reverse process is much weaker and is called recombination. In the process of burning the arc, the area of negative resistance is especially dangerous: when the current increases with decreasing external voltage. In addition to the current-voltage characteristics, the parameters of the consumer circuit affect the quenching. High inductive resistance provokes the occurrence of reverse EMF.
The main method of arc extinction is the increase in its length, which naturally reduces the field strength in the spark gap. In low-voltage circuits there is no difficulty, but at industrial consumers the critical distance between the electrodes is sometimes so great that conventional methods are fundamentally inappropriate for solving the set task. Arc suppressors are used here. The above technique is complemented by the following methods:
- Reducing the temperature of the gap due to the forced blowing of air or gas. In fact - the failure of the flame. It is possible to come across structures where the plasma is blown out by a jet of compressed air. A simple design, considering that cans with various gases are sold everywhere today.
- Divide the arc into a series of short. Implied when mentioning Russian written sources. In this case, a series of arcs is formed, the voltages of which are added. And the amount exceeds the original value. As a result, the burning condition is not always satisfied, because the potential difference applied to the circuit breaker is not enough to support the process.
Industrial circuit breakers are often filled with oil. Then the burning of the arc is prevented by the cooling effect of the medium, blocked by the hydrogen released under these conditions. Without oxygen explosion does not occur. The technique is applied exclusively in AC circuits.
In practice, techniques are usually combined to enhance the effect of arc extinction.
Contacts with protective horns
The flameout occurs faster with a sharp increase in the arc length. With seeming impossibility, if the horizontal contacts are in contact with the ends and two horns scatter from the border with the letter V, the situation looks like this:
- The arc burning when the contacts open, heats the surrounding air.
- The flow rises, enticing ionized gas to the horns.
- Divergent surfaces as the height increases further from each other.
- The moment comes when the conditions of the arc are violated.
The contact surface is largely saved( it is easy to change the horns), in addition, the device of the automatic circuit breaker is as simple as possible. The role of the arc and the interaction of the arc with the magnetic field of the contactor, which seeks to push it out, becomes an additional accelerating factor. To enhance the effect in the design include a special arcing coil, the magnetic field of which is much stronger than that of a conventional conductor. As a result, flameout is faster.
Arc suppression chambers
This arc extinction method is often used in home security machines. The bottom line: the flame is pushed into a metal construction cut by the labyrinths, where the plasma gives up the temperature and as a result goes out. As in the previous case, the field of the special coil becomes the driving force. The labyrinth sometimes takes bizarre forms, because the bending of the arc becomes an additional extinguishing factor.
Sometimes the construction is supplemented with a deionic lattice. This is a set of steel plates insulated from each other by a dielectric. The principle of operation is based on the fact that the total arc voltage consists of two components:
- The potential difference of the plasma column.
- Electrode voltage drop.
When the arc is divided into a series of consecutive, the first parameter is added along all the pillars and remains unchanged. And the second one is increased by as many times as the parts were divided. As a result, the voltage applied to the circuit breaker is no longer enough to maintain the arc burning. A feature of the deionic lattice is its efficiency for direct and alternating current. In the latter case, the number of plates can be significantly reduced.
should be spoken correctly. The concept of an electric automaton today is not defined by state standards. There are no similar terms. Instead, it is necessary to use the well-established word combination of a circuit breaker, residual current device or circuit breaker. All this is similar, but not identical. In this case, there is no danger of any misunderstandings and misunderstandings: is it necessary to respond to a differential current, for example.