Electric arc current electromagnetic field. Electric arc and its characteristics

Arc extinguishing is facilitated by highly discharged gases or high-pressure gases used as internal insulation of devices with voltages above 1000V.

Extinguishing the arc in a vacuum. Highly discharged gas has an electrical strength tens of times greater than gas at atmospheric pressure; this is used in vacuum contactors and circuit breakers.

Extinguishing arc in high pressure gases. Air at a pressure of 2 MPa or more has high electrical strength, which makes it possible to create compact extinguishing devices in air circuit breakers. It is effective to use sulfur hexafluoride SF 6 (SF6 gas) to extinguish the arc.

AC Arc Extinguishing Conditions.

Let the contacts diverge at point a. An arc lights up between them. By the end of the half-cycle, due to a decrease in current, the resistance of the arc shaft increases and, accordingly, the voltage on the arc increases. When the current approaches zero, low power is supplied to the arc, the arc temperature decreases, thermal ionization slows down accordingly and deionization processes accelerate - the arc goes out (point 0 ). The current in the circuit is interrupted before its natural passage through zero. Voltage corresponding to current interruption – peak quenching U g.

Rice. 22. Extinguishing AC arc with active load

After extinguishing the arc, the process of restoring the electrical strength of the arc gap occurs (curve a 1 – b 1). The electrical strength of the arc gap refers to the voltage at which electrical breakdown of the arc gap occurs. The initial electrical strength (point a 1) and the rate of its increase depend on the properties of the arc extinguishing device. In the moment t 1 the voltage curve across the arc gap intersects with the curve for restoring the electrical strength of the arc gap - the arc is ignited. Arc ignition voltage - ignition peak U z. The arc voltage curve has a saddle shape.

At the point 0 1 the arc goes out again and processes similar to those described earlier occur. To the moment 0 1 due to the divergence of the contacts, the length of the arc increases, the heat removal from the arc increases. The initial electrical strength (point a 2) and the rate of its increase (curve a 2 – b 2) increase accordingly. Accordingly, the dead time pause increases 0 1 - t 2 > 0 -t 1 .

In the moment t 2 the arc ignites again. At the point 0 11 the arc goes out. The initial electrical strength (point a 3) and the rate of its increase (curve a 3 – b 3) increase again. The voltage curve does not intersect with the increasing electrical strength curve. The arc does not ignite during this half-cycle.

In an open arc at high voltage(horn gap), the determining factor is the active resistance of the highly extended arc shaft; the conditions for extinguishing an alternating current arc are approaching the conditions for extinguishing a direct current arc, and processes after the current passes through zero have little effect on extinguishing the arc.

With an inductive load, the dead time is very small (approximately 0.1 μs), that is, the arc burns almost continuously. Disabling an inductive load is more difficult than a resistive load. There is no current interruption here.

In general, the process of arc extinguishing with alternating current is easier than with direct current. A rational condition for extinguishing an alternating current arc should be considered when the extinguishing occurs during the first transition of the current through zero after opening the contacts.

Self-test questions:

· Arc discharge areas.

· Static current-voltage characteristic.

· Dynamic current-voltage characteristic.

· Conditions for stable combustion and extinguishing of DC arc.

· What phenomena are used to extinguish the arc?

· AC arc extinguishing conditions.

Switching off a circuit by a contact device is characterized by the appearance of plasma, which goes through different stages of a gas discharge in the process of converting the intercontact gap from a conductor of electric current into an insulator.

At currents above 0.5-1 A, an arc discharge stage occurs (area 1 )(Fig. 1.); when the current decreases, a stage of glow discharge occurs at the cathode (region 2 ); next stage (area 3 ) – Townsend discharge, and finally, the region 4 – the stage of isolation in which carriers of electricity - electrons and ions - are not formed due to ionization, but can only come from the environment.

Rice. 1. Current-voltage characteristics of stages electrical discharge in gases

The first section of the curve is an arc discharge (area 1) – characterized by a low voltage drop across the electrodes and a high current density. As the current increases, the voltage across the arc gap first drops sharply and then changes slightly.

Second section (region 2 ) curve, which represents the region of the glow discharge, is characterized by a high voltage drop at the cathode (250 - 300 V) and low currents. As the current increases, the voltage drop across the discharge gap will increase.

Townsend discharge (region 3 ) is characterized by extremely low current values ​​at high voltages.

Electric arc accompanied by high temperature and associated with this temperature. Therefore, an arc is not only an electrical phenomenon, but also a thermal one.

Under normal conditions, air is a good insulator. Thus, to break down an air gap of 1 cm, a voltage of at least 30 kV must be applied. In order for the air gap to become a conductor, it is necessary to create a certain concentration of charged particles in it: negative - mainly free electrons, and positive - ions. The process of separating one or more electrons from a neutral particle to form free electrons and ions is called ionization

Gas ionization can occur under the influence of light, x-rays, high temperature, under the influence of an electric field and a number of other factors. For arc processes in electrical devices, the following are of greatest importance: from the processes occurring at the electrodes - thermionic and field emission, and from the processes occurring in the arc gap - thermal ionization and push ionization.

In electrical switching devices designed to close and open a circuit with current, when disconnected, a discharge occurs in the gas either in the form of a glow discharge or in the form of an arc. A glow discharge occurs when the switched current is below 0.1 A, and the voltage at the contacts reaches 250 - 300 V. Such a discharge occurs either on the contacts of low-power relays, or as a transition phase to a discharge in the form of an electric arc.

Basic properties of an arc discharge.

1) An arc discharge occurs only at high currents; the minimum arc current for metals is approximately 0.5 A;

2) The temperature of the central part of the arc is very high and in devices can reach 6000 - 18000 K;

3) The current density at the cathode is extremely high and reaches 10 2 – 10 3 A/mm 2;

4) The voltage drop at the cathode is only 10 - 20 V and is practically independent of current.

In an arc discharge, three characteristic regions can be distinguished: near the cathode, the region of the arc column (arc shaft) and near the anode (Fig. 2.).

In each of these areas, the processes of ionization and deionization proceed differently depending on the conditions that exist there. Since the resulting current passing through these three regions is the same, processes occur in each of them that ensure the occurrence of required quantity charges.

Rice. 2. Distribution of voltage and electric field strength in a stationary DC arc

Thermionic emission. Thermionic emission is the phenomenon of the emission of electrons from a heated surface.

When the contacts diverge, the contact resistance and current density in the last contact area increase sharply. This area is heated to the melting temperature and the formation of a contact isthmus of molten metal, which breaks with further divergence of the contacts. This is where contact metal evaporates. A so-called cathode spot (hot area) is formed on the negative electrode, which serves as the base of the arc and the source of electron radiation at the first moment of contact divergence. Thermionic emission current density depends on the temperature and electrode material. It is small and may be sufficient to create an electric arc, but it is not sufficient to burn it.

Autoelectronic emissions. This is the phenomenon of electrons being emitted from the cathode under the influence of a strong electric field.

The electrical break point can be represented as a capacitor variable capacity. The capacitance at the initial moment is equal to infinity, then decreases as the contacts diverge. Through the resistance of the circuit, this capacitor is charged, and the voltage across it increases gradually from zero to the mains voltage. At the same time, the distance between the contacts increases. The field strength between the contacts during the voltage rise passes through values ​​exceeding 100 MV/cm. Such electric field strengths are sufficient to rip electrons out of the cold cathode.

The field emission current is also very small and can only serve as the beginning of the development of an arc discharge.

Thus, the occurrence of an arc discharge at diverging contacts is explained by the presence of thermionic and field electron emissions. The predominance of one or another factor depends on the value of the switched current, the material and cleanliness of the surface of the contacts, the speed of their divergence and a number of other factors.

Ionization by push. If a free electron has sufficient speed, then when it collides with a neutral particle (an atom, and sometimes a molecule), it can knock out an electron from it. The result will be a new free electron and a positive ion. The newly acquired electron can, in turn, ionize the next particle. This ionization is called push ionization.

In order for an electron to ionize a gas particle, it must move at a certain speed. The speed of an electron depends on the potential difference across its free path. Therefore, it is usually not the speed of movement of the electron that is indicated, but the minimum value of the potential difference that must be present along the length of the free path so that the electron acquires the required speed by the end of the path. This potential difference is called ionization potential.

The ionization potential for gases is 13 – 16 V (nitrogen, oxygen, hydrogen) and up to 24.5 V (helium); for metal vapors it is approximately two times lower (7.7 V for copper vapors).

Thermal ionization. This is a process of ionization under the influence of high temperature. Maintaining the arc after it has occurred, i.e. Providing the resulting arc discharge with a sufficient number of free charges is explained by the main and practically only type of ionization - thermal ionization.

The temperature of the arc column is on average 6000 - 10000 K, but can reach higher values ​​- up to 18000 K. At this temperature, both the number of fast moving gas particles and the speed of their movement greatly increases. When rapidly moving atoms or molecules collide, most of them are destroyed, forming charged particles, i.e. gas ionization occurs. The main characteristic of thermal ionization is degree of ionization, which is the ratio of the number of ionized atoms in the arc gap to total number atoms in this gap. Simultaneously with the ionization processes in the arc, reverse processes, i.e., the reunification of charged particles and the formation of neutral particles. These processes are called deionization.

Deionization occurs mainly due to recombination And diffusion.

Recombination. The process in which differently charged particles come into mutual contact to form neutral particles is called recombination.

In an electric arc, the negative particles are mainly electrons. The direct connection of electrons with a positive ion is unlikely due to the large speed difference. Typically, recombination occurs with the help of a neutral particle that is charged by an electron. When this negatively charged particle collides with a positive ion, one or two neutral particles are formed.

Diffusion. Diffusion of charged particles is the process of removal of charged particles from the arc gap into the surrounding space, which reduces the conductivity of the arc.

Diffusion is caused by both electrical and thermal factors. The charge density in the arc column increases from the periphery to the center. Because of this, an electric field is created, causing the ions to move from the center to the periphery and leave the arc region. The temperature difference between the arc column and the surrounding space also acts in the same direction. In a stabilized and freely burning arc, diffusion plays a negligible role.

The voltage drop across a stationary arc is distributed unevenly along the arc. Pattern of voltage drop change U D and electric field strength (longitudinal voltage gradient) E D = dU/dx along the arc is shown in the figure (Fig. 2). Under the voltage gradient E D refers to the voltage drop per unit arc length. As can be seen from the figure, the course of the characteristics U D and E D in the near-electrode regions differs sharply from the course of characteristics in the rest of the arc. At the electrodes, in the near-cathode and near-anode regions, over a length interval of about 10 - 4 cm, there is a sharp drop in voltage, called cathode U to and anodic U A. The magnitude of this voltage drop depends on the electrode material and the surrounding gas. The total value of the near-anode and near-cathode voltage drops is 15 – 30 V, the voltage gradient reaches 10 5 – 10 6 V/cm.

In the rest of the arc, called the arc column, the voltage drop U D is almost directly proportional to the length of the arc. The gradient here is approximately constant along the trunk. It depends on many factors and can vary widely, reaching 100 - 200 V/cm.

Near-electrode voltage drop U E does not depend on the length of the arc; the voltage drop in the arc column is proportional to the length of the arc. Thus, the voltage drop across the arc gap

U D = U E + E D l D,

Where: E D – electric field strength in the arc column;

l D – arc length; U E = U k + U A.

In conclusion, it should be noted once again that in the arc discharge stage, thermal ionization predominates - the breakdown of atoms into electrons and positive ions due to the energy of the thermal field. With a glow discharge, impact ionization occurs at the cathode due to collisions with electrons accelerated by the electric field, and with a Townsend discharge, impact ionization predominates over the entire gas discharge gap.

Static current-voltage characteristic of electrical

DC arcs.

The most important characteristic arc is the dependence of the voltage on it on the magnitude of the current. This characteristic is called current-voltage. With increasing current i the arc temperature increases, thermal ionization increases, the number of ionized particles in the discharge increases and decreases electrical resistance arcs r d.

The arc voltage is ir d.As the current increases, the arc resistance decreases so sharply that the voltage across the arc drops, despite the fact that the current in the circuit increases. Each current value in a steady state corresponds to its own dynamic balance of the number of charged particles.

When moving from one current value to another, the thermal state of the arc does not change instantly. The arc gap has thermal inertia. If the current changes slowly over time, then the thermal inertia of the discharge has no effect. Each current value corresponds to an unambiguous value of the arc resistance or voltage on it.

The dependence of the arc voltage on the current when it changes slowly is called static current-voltage characteristic arcs.

The static characteristics of the arc depend on the distance between the electrodes (arc length), the material of the electrodes and the parameters of the environment in which the arc burns.

Static current-voltage characteristics of the arc have the form of curves shown in Fig. 3.

Rice. 3. Static current-voltage characteristics of the arc

The longer the arc length, the higher its static current-voltage characteristic lies. As the pressure of the medium in which the arc burns increases, the intensity also increases E D and the current-voltage characteristic rises similarly to Fig. 3.

Arc cooling significantly affects this characteristic. The more intense the cooling of the arc, the more power is removed from it. In this case, the power released by the arc should increase. For a given current, this is possible by increasing the arc voltage. Thus, with increasing cooling, the current-voltage characteristic is higher. This is widely used in arc extinguishing devices of apparatus.

Dynamic current-voltage characteristic of electrical

DC arcs.

If the current in a circuit changes slowly, then the current i 1 corresponds to arc resistance r D1, for higher current i 2 corresponds to lower resistance r D2, which is reflected in Fig. 4. (see static characteristics of the arc - curve A).

Rice. 4. Dynamic current-voltage characteristic of the arc.

In real installations, the current can change quite quickly. Due to the thermal inertia of the arc column, the change in arc resistance lags behind the change in current.

The dependence of arc voltage on current when it changes rapidly is called dynamic current-voltage characteristic.

With a sharp increase in current, the dynamic characteristic goes above the static one (curve IN), since with a rapid increase in current, the arc resistance drops more slowly than the current increases. When decreasing, it is lower, since in this mode the arc resistance is less than with a slow change in current (curve WITH).

The dynamic characteristic is largely determined by the rate of change of current in the arc. If a very large resistance is introduced into the circuit in a time infinitesimal compared to the thermal time constant of the arc, then during the time the current decays to zero, the arc resistance will remain constant. In this case, the dynamic characteristic will be depicted as a straight line passing from the point 2 to the origin (straight line D),T. That is, the arc behaves like a metal conductor, since the voltage across the arc is proportional to the current.

DC arc extinguishing conditions.

To extinguish a direct current electric arc, it is necessary to create conditions such that in the arc gap at all current values, deionization processes would proceed more intensely than ionization processes.

Rice. 5. Voltage balance in a circuit with an electric arc.

Consider an electrical circuit containing a resistance R, inductance L and arc gap with voltage drop U D to which voltage is applied U(Fig. 5, A). With an arc of constant length, the voltage balance equation in this circuit will be valid for any moment in time:

where is the voltage drop across the inductance when the current changes.

The stationary mode will be one in which the current in the circuit does not change, i.e. and the stress balance equation will take the form:

To extinguish an electric arc, it is necessary that the current in it decreases all the time, i.e. , A

A graphical solution to the stress balance equation is presented in Fig. 5, b. Here's a straight line 1 represents the source voltage U; inclined straight line 2 – voltage drop across the resistance R(rheostatic characteristic of the circuit), subtracted from the voltage U, i.e. U–iR; curve 3 – current-voltage characteristic of the arc gap U D.

Features of the AC electric arc.

If in order to extinguish a direct current arc it is necessary to create conditions under which the current would drop to zero, then with alternating current the current in the arc, regardless of the degree of ionization of the arc gap, passes through zero every half-cycle, i.e. Each half-cycle the arc goes out and lights up again. The task of extinguishing the arc is greatly simplified. Here it is necessary to create conditions under which the current would not recover after passing through zero.

The current-voltage characteristic of an alternating current arc for one period is shown in Fig. 6. Since, even at an industrial frequency of 50 Hz, the current in the arc changes quite quickly, the presented characteristic is dynamic. With a sinusoidal current, the arc voltage first increases in the section 1, and then, due to the increase in current, drops in the section 2 (sections 1 And 2 refer to the first half of the half-cycle). After the current passes through the maximum, the dynamic current-voltage characteristic increases along the curve 3 due to a decrease in current, and then decreases in the section 4 due to the voltage approaching zero (sections 3 And 4 refer to the second half of the same half-period).

Rice. 6. Current-voltage characteristics of an alternating current arc

With alternating current, the arc temperature is a variable value. However, the thermal inertia of the gas turns out to be quite significant, and by the time the current passes through zero, the arc temperature, although decreasing, remains quite high. Nevertheless, the decrease in temperature that occurs when the current passes through zero contributes to the deionization of the gap and facilitates the extinguishing of the alternating current electric arc.

Electric arc in a magnetic field.

An electric arc is a gaseous conductor of current. A magnetic field acts on this conductor, just like a metal one, creating a force proportional to the field induction and the current in the arc. The magnetic field, acting on the arc, increases its length and moves the elements of the arc in space. The transverse movement of the arc elements creates intense cooling, which leads to an increase in the voltage gradient across the arc column. When the arc moves in a gas environment at high speed, the arc splits into separate parallel fibers. The longer the arc, the more severe the delamination of the arc.

The arc is an extremely mobile conductor. It is known that the current-carrying part is subject to forces that tend to increase the electromagnetic energy of the circuit. Since energy is proportional to inductance, the arc, under the influence of its own field, tends to form turns and loops, since this increases the inductance of the circuit. This ability of the arc is stronger the greater its length.

An arc moving in the air overcomes aerodynamic air resistance, which depends on the diameter of the arc, the distance between the electrodes, the gas density and the speed of movement. Experience shows that in all cases, in a uniform magnetic field, the arc moves at a constant speed. Consequently, the electrodynamic force is balanced by the aerodynamic drag force.

In order to create effective cooling, the arc is drawn using a magnetic field into a narrow (the diameter of the arc is greater than the width of the slot) gap between the walls of an arc-resistant material with high thermal conductivity. Due to the increase in heat transfer to the walls of the slot, the voltage gradient in the arc column in the presence of a narrow slot is much higher than that of an arc moving freely between the electrodes. This makes it possible to reduce the length and time of extinguishing required for extinguishing.

Methods of influencing an electric arc in switching devices.

The purpose of influencing the column of the arc arising in the apparatus is to increase its active electrical resistance up to infinity, when the switching element goes into an insulating state. This is almost always achieved by intensive cooling of the arc column, reducing its temperature and heat content, resulting in a decrease in the degree of ionization and the number of electrical carriers and ionized particles and an increase in the electrical resistance of the plasma.

To successfully extinguish an electric arc in low-voltage switching devices, it is necessary to perform following conditions:

1) increase the length of the arc by stretching it or increasing the number of breaks per pole of the switch;

2) move the arc onto the metal plates of the arc extinguishing grid, which are like radiators that absorb thermal energy the column of the arc, and break it into a series of successively connected arcs;

3) move the arc column magnetic field into a slot chamber made of arc-resistant insulating material with high thermal conductivity, where the arc is intensively cooled in contact with the walls;

4) form an arc in a closed tube made of gas-generating material - fiber; gases released under the influence of temperature create high pressure, which helps extinguish the arc;

5) reduce the concentration of metal vapors in the arc, for which purpose use appropriate materials at the design stage of the devices;

6) extinguish the arc in a vacuum; at very low gas pressure there are not enough gas atoms to ionize them and support the conduction of current in the arc; the electrical resistance of the arc column channel becomes very high and the arc goes out;

7) open the contacts synchronously before the alternating current crosses zero, which significantly reduces the release of thermal energy in the resulting arc, i.e. promotes arc extinction;

8) use purely active resistances that shunt the arc and facilitate the conditions for its extinguishing;

9) use semiconductor elements that shunt the intercontact gap and switch the arc current to themselves, which practically eliminates the formation of an arc on the contacts.

Opening an electrical circuit at significant currents and voltages is usually accompanied by an electrical discharge between diverging contacts. When the contacts diverge, the contact resistance and current density in the last contact area increase sharply. The contacts are heated to the point of melting, and a contact isthmus of molten metal is formed, which, with further divergence of the contacts, breaks, and evaporation of the contact metal occurs. The air gap between the contacts ionizes and becomes conductive, and an electric arc appears in it under the influence of high voltage arising as a result of the laws of commutation.

An electric arc contributes to the destruction of contacts and reduces the performance of the switching device, since the current in the circuit does not drop to zero instantly. The occurrence of an arc can be prevented by increasing the resistance of the circuit in which the contacts open, increasing the distance between the contacts, or using special arc extinguishing measures.

The product of the limiting values ​​of voltage and current in a circuit at which an electric arc does not occur at a minimum distance between the contacts is called the breaking or switching power of the contacts. As the voltage in the circuit increases, the maximum switching current must be limited. The switched power also depends on the time constant of the circuit: the more
the less power the contacts can switch. In alternating current circuits, the electric arc goes out at the moment when the instantaneous current value is zero. The arc may reappear in the next half-cycle if the voltage at the contacts increases faster than the dielectric strength of the gap between the contacts is restored. However, in all cases, the arc in an alternating current circuit is less stable, and the breaking power of the contacts is several times higher than in a direct current circuit. An electric arc rarely appears on the contacts of low-power electrical devices, but sparking is often observed - breakdown of the insulating gap formed when contacts quickly open in low-current circuits. This is especially dangerous in sensitive and high-speed devices (relays), in which the distance between the contacts is very small. Sparking reduces the service life of contacts and can lead to false alarms. To reduce sparking at the contacts, special spark extinguishing devices are used.

Arc and spark extinguishing device.

The most effective way to extinguish an electric arc is to cool it by moving in the air, coming into contact with the insulating walls of special chambers that take away the heat of the arc.

In modern devices, arc-extinguishing chambers with a narrow slot and magnetic blast are widely used. The arc can be considered as a conductor carrying current; if it is placed in a magnetic field, a force will arise that will cause the arc to move. As it moves, the arc is blown with air; falling into a narrow gap between two insulating plates, it is deformed and, due to an increase in pressure in the chamber gap, goes out (Fig. 21).

Rice. 21. Design of an arc suppression chamber with a narrow slot

The slot chamber is formed by two walls 1 made of insulating material. The gap between the walls is very small. Coil 4, connected in series with main contacts 3, excites magnetic flux
which is directed by ferromagnetic tips 2 into the space between the contacts. As a result of the interaction of the arc and the magnetic field, a force appears
displacing the arc towards plates 1. This force is called the Lorentz force, which is defined as:

Where - particle charge [Coulomb],

‑charged particle speed in the field [m/s],

-force acting on a charged particle [Newtons],

- the angle between the velocity vector and the magnetic induction vector.

We can say that the speed of a particle in a conductor is equal to:
Where - length of the conductor (arc), and - time of passage of a charged particle along the arc. In turn, the current - this is the number of charged particles per second through the cross section of the conductor
. That is, you can write:

Where - current in the conductor (arc) [Amperes],

-conductor length (arc) [meters],

‑magnetic field induction [Tesla],

- force acting on the conductor (arc) [Newtons],

- the angle between the current vector and the magnetic induction vector.

The direction of the force follows the left-hand rule: magnetic lines of force rest against the palm, straightened four fingers are located in the direction of the current the bent thumb shows the direction of the electromagnetic force
. The described action of the magnetic field (induction ) is called electromechanical or force, and the resulting expression is called the law of electromagnetic forces.

This design of the arc chute is also used on alternating current, since changing the direction of the current changes the direction of the flow
and the direction of the force
remains unchanged.

To reduce sparking on low-power DC contacts, a diode is connected in parallel with the load device (Fig. 22).

Rice. 22. Turning on the diode to reduce sparking

In this case, the circuit after switching (after turning off the source) is closed through a diode, thus reducing the sparking energy.

When the switch contacts open, the current is not interrupted. According to Lenz's law, an emf E L = -Ldi/dt arises in the circuit, preventing a change in current. The latter finds a path for itself through the gas gap between the diverging contacts of the switch, which is blocked by an electric arc. To interrupt the current, the arc must be extinguished. In alternating current circuits, favorable conditions for arc extinction occur every time the current reaches zero, i.e. 2 times during each period. The diameter of the arc column, temperature and ionization of the gas decrease sharply. At some point in time, the current comes to zero and the arc discharge stops. However, the chain has not yet been broken.

After zero current in the gas gap, which is still somewhat ionized, the deionization process continues, i.e. the process of transforming it from a conductor into a dielectric, and in the electrical circuit the process of restoring the voltage at the switch contacts from a relatively small arc voltage to the mains voltage begins. These processes are interconnected. The outcome of the interaction of the arc gap with the electrical circuit depends on the relationship between the energy supplied to the gap and the energy losses in it, which depend on the arc extinguishing device of the switch.

If energy losses predominate throughout the entire transient process, the arc will not occur again and the circuit will be interrupted. Otherwise, the arc will arise again and the current will flow for another half of the period, after which the interaction process will repeat. The function of the switch is not so much to “extinguish” the arc, but rather to exclude the possibility of its re-ignition by effectively deionizing the gap by various artificial means. This uses the exceptional property of gas - quickly, within a few microseconds, to transform from a conductor into a dielectric capable of withstanding the restoring network voltage.

To understand the design and operation of switches, it is necessary to become familiar with the physical processes in the arc gap during the shutdown process. This article discusses arc extinguishing methods in air and oil circuit breakers.

Physical processes in the arc gap of a switch at high pressure

An electric arc, or more precisely an arc discharge, is called an independent discharge in a gas, i.e. a discharge occurring without an external ionizer, characterized by a high current density and a relatively small voltage drop at the cathode. Below we consider the high pressure arc, i.e. arc discharge at atmospheric and higher pressure.

The following arc discharge areas are distinguished:

• area of ​​cathode voltage drop;
• area near the anode;
• arc pillar.

The region of the cathode voltage drop is a thin layer of gas at the cathode surface. The voltage drop in this layer is 20-50 V, and the electric field strength reaches 10 5 10 6 V/cm. The energy supplied from the network to this area is used to release electrons from the cathode surface.

The mechanism for releasing electrons can be twofold:

• thermionic emission with refractory and refractory electrodes (tungsten, coal), the temperature of which can reach 6000 K and higher
• field emission, i.e. the ejection of electrons from the cathode by the action of a strong electric field when the cathode is “cold”.

The current density at the cathode reaches 3000-10000 A/cm 5 . The current is concentrated in a small brightly lit area called the cathode spot. The released electrons move through the arc column to the anode.

At the anode, positive ions become accelerated towards the cathode. Electrons go to the anode and form thin layer negative charge. The voltage drop at the anode is 10-20 V.

Processes in the arc column are of greatest interest when studying switches, since various types of influence on the arc column are used to extinguish the arc. The latter is plasma, i.e. an ionized gas with a very high temperature and equal amounts of electrons and positive ions per unit volume.

The high temperature in the arc column is created and maintained by electrons and ions that participate in the thermal chaotic movement of neutral molecules and atoms, but also have directional movement in electric field along the arc axis, determined by the sign of the particle charge. This movement is prevented by neutral gas. Frequent collisions of electrons and ions with neutral particles occur. Since the mean free path of electrons at high pressure is small, the energy loss during elastic collisions with molecules and atoms per collision is small and insufficient to ionize particles. However, the number of collisions that electrons undergo is very large. As a result, the energy of the electrons is transferred to the neutral gas in the form of heat.

The average energy of the “electron gas” cannot appreciably exceed average energy neutral gas, since the additional energy acquired by electrons and ions in their directed movement along the axis of the meadow column is small compared to the thermal energy of the gas. Consequently, ions, electrons, as well as neutral atoms and molecules are in thermal equilibrium. In this case, the specific ionization of the arc column is completely determined by temperature, and when one of these quantities changes, the other inevitably changes.

Because at high gas pressure atoms and molecules overwhelmingly outnumber electrons and have almost the same high temperature, most excited and ionized atoms and molecules are produced by collisions between neutral particles rather than by collisions with electrons. Thus, electrons do not ionize directly through collisions with neutral particles (as happens in a vacuum), but indirectly, increasing the temperature of the gas in the arc column. This ionization mechanism is called thermal ionization. The source of energy required for thermal ionization is an electric field.

There are energy losses in the arc column, which in a steady state are balanced by the energy received from the network. The bulk of the energy is carried away from the arc column by excited and ionized atoms and molecules. Due to the difference in the concentrations of charged particles in the arc column and the surrounding space, as well as the temperature difference, the ions diffuse to the surface of the arc column, where they are neutralized. These losses must be compensated by the formation of new ions and electrons associated with the expenditure of energy. In steady state, the voltage gradient in the arc column is always such that the ionization that occurs compensates for the loss of electrons through recombination. The voltage gradient depends on the properties of the gas, the state in which it is located (quiet, turbulent), as well as pressure and current. As gas pressure increases, the voltage gradient increases due to a decrease in the free path of electrons. As the current increases, the voltage gradient decreases, which is explained by an increase in the cross-sectional area and temperature of the arc column. The arc column tends to take on a cross-section such that, under the conditions under consideration, energy losses are minimal.

The dependence of the voltage gradient E = dU/dl in the arc column on the current with a very slow change in the latter is a static characteristic of the arc (Fig. 1, a), depending on the pressure and properties of the gas.

Fig.1. Current-voltage characteristics of the arc:
a - static characteristic;
b - dynamic characteristics

In a steady state, each point of the characteristic corresponds to a certain cross section and temperature of the arc column. When the current changes, the arc column must change its cross-section and temperature in relation to the new conditions. These processes take time, and therefore the new steady state does not occur immediately, but with some delay. This phenomenon is called hysteresis.

Let's assume that the current suddenly changes from the value I 1 (point 1) to the value I 2 (point 2). At the first moment, the arc will retain its cross-section and temperature, and the gradient will decrease (point 2"). The supplied power will be less than that required to conduct current I 2. Therefore, the cross-section and temperature will begin to decrease, and the gradient will increase until a new steady state occurs at point 2 on the static characteristic. With a sudden increase in current from value I 1 to value I 3, the voltage gradient will increase (point 3"). The power supplied to the arc will be greater than that required to conduct current I 3 . Therefore, the cross section and temperature of the column will begin to increase, and the voltage gradient will decrease until a new steady state occurs at point 3 on the static characteristic.

When the current changes smoothly at a certain speed, the voltage gradient does not have time to follow the current change in accordance with the static characteristic. When the current increases, the voltage gradient exceeds the values ​​determined by the static characteristic, and when the current decreases, the voltage gradient is less than these values. Curves E=f(I) when the current changes at a certain speed represent the dynamic characteristics of the arc (solid lines in Fig. 1, b).

The position of these characteristics in relation to the static characteristic (see dotted curve) depends on the rate of change of current. The slower the current change occurs, the closer the dynamic characteristic is to the static one. Under a given arcing condition there can be only one static characteristic. Number dynamic characteristics not limited.

When analyzing electrical circuits, it is customary to operate with the concept of resistance. Therefore, they also talk about arc resistance, meaning by this the ratio of voltage at the electrodes to current. Arc resistance is not constant. It depends on the current and many other factors. As the current increases, the arc resistance decreases.

Fig.2. Arc voltage at alternating current:
a is the arc voltage as a function of current;
6 - arc voltage as a function of time

The current-voltage characteristic of an alternating current arc is shown in Fig. 2, a. During the quarter period, when the current increases, the voltage curve lies above the static characteristic. The next quarter of the period, when the current decreases, the voltage curve lies below the static characteristic.

The arc ignites at points 1 and 3 and goes out at points 2 and 4. Figure 2,b shows the characteristics of the arc as a function of time. Intervals 2-3 and 4-1 correspond to an unstable state in which there is intense interaction of the arc with the circuit constants R, L and C. These short time intervals, which last a few microseconds, are used to intensively deionize the gap between the switch contacts to prevent new arc ignition. Depending on the conditions, the interaction process can end in two ways: either the arc goes out and the circuit is interrupted, or the arc appears again and the interaction process repeats after half a period with more favorable conditions.

Arc extinction in air circuit breakers

In air circuit breakers, the arc is extinguished in a high-pressure air stream. The switch extinguishing device (Fig. 3a) is a chamber in which two nozzles are placed, which simultaneously serve as contacts. The exhaust sides of the nozzles are connected to the low pressure area. When the contacts are separated, due to the pressure difference, an air flow occurs, directed into the nozzles symmetrically in both directions.

Fig.3. Arcing device of air circuit breaker with two-way blowing:
a - diagram;
b - pressure distribution along the axis

Figure 3b shows the pressure distribution along the axis. In the middle of the gap between the nozzles there is a flow stagnation point, the pressure at which is indicated by p o .

On both sides of this point the pressure decreases and reaches approximately half p o at the nozzle necks. Behind the throats, the pressure continues to drop to exhaust pressure.

The arc extinguishing process proceeds as follows. An arc appears between the opening contacts, which, under the action of the air flow, is quickly transferred along the axis. In this case, the arc support spots move inside the nozzles along the flow, as shown in Fig. 3. The arc in the space between the nozzles has a cylindrical shape.

Fig.4. Temperature distribution in the transverse direction in the area between the nozzles:
a - arc;
c - thermal boundary layer

The temperature distribution in the transverse direction is shown in Fig. 4. In the arc zone a it is approximately 20,000 K and drops sharply towards the thermal boundary layer b formed near the arc. Here the temperature varies from 2000 K to cold air temperature. As the current approaches zero, the diameter of the cylindrical part of the arc rapidly decreases. When the current is zero, it is less than 1 mm. However, the temperature in this part of the arc is still very high (15000 K).

The most important factor contributing to arc extinction is turbulence in the boundary layer between the arc and the relatively cold air surrounding it. Due to the high temperature of the arc, the gas density in the column is approximately 20 times less than in the environment. Therefore, the gas velocity inside the arc column is significantly higher than the velocity in the neighboring layers (the velocity is inversely proportional to the square root of the density). Due to the diffusion of particles from the region with high speed to the region with low speed and back in the boundary layer, significant shear forces arise, vortices are formed and the entire volume acquires high turbulence. A relatively cold non-ionized gas is introduced into the arc column, as a result of which the column loses its homogeneity. It splits into thousands of the finest conductive threads, continuously changing their shape and position (Fig. 5).

Fig.5. The influence of turbulence on the arc column near zero current (scheme)

They have a high temperature and high specific ionization and are surrounded by cold, weakly ionized gas. It is known that the rate of diffusion from a cylindrical volume is inversely proportional to the square of the diameter. The thinner the ionized filaments, the faster the exchange of particles with the surrounding colder and less ionized environment occurs. Turbulence increases diffusion many times over. It manifests itself especially sharply in the throats of the nozzles, where the plasma speed is maximum - 6000 m/s. After zero current, for a short period of time, measured in microseconds, the conductive channel disintegrates and a further decrease in temperature is determined by the thermal boundary layer, the cooling of which occurs much more slowly.

Fig.6. Equivalent diagram explaining the influence of arc resistance and capacitance

Fig.7. Interaction of the arc with the electrical circuit

The arc resistance and capacitance connected parallel to the arc gap have a significant influence on the shutdown process (Fig. 6). If we neglect the arc resistance, the current i 0 =I m sinɷt approaches zero almost linearly (Fig. 7). However, the arc resistance is not zero. Therefore, the current i B in the arc gap of the switch decreases:

(1)

where t 0 is the moment of contact opening.

As can be seen from the figure, the arc voltage changes in accordance with the current-voltage characteristic. The rate of current decrease decreases significantly during the last 5...10 μs before it reaches zero. This time is small, but it is several times greater than the arc time constant and therefore significantly affects the state of the arc at zero current (point 1). The arc goes out easily. The arc resistance also modifies the PVN curve. The voltage recovery process begins at point 1; the voltage reaches its maximum at point 2, when i L =i C =0.

Stage of possible thermal breakdown

If the temperature of the gas in the gap does not decrease to a certain critical value determined by the gas property and pressure, the gap will retain its conductivity after zero current (point 1) and under the influence of the PVN a residual conductivity current will arise (Fig. 8).

Fig.8. Delayed arc extinction
caused by the appearance of residual conduction current

Under favorable conditions, it is small and quickly fades (point 2). However, if the cooling process is not intense enough, the residual conduction current increases; The plasma is reheated, the ionization process resumes, and the arc occurs again. This phenomenon is called thermal breakdown, since electrical breakdown is impossible, since the gap is ionized and has not yet acquired electrical strength.

Whether such a breakdown occurs or not depends on the outcome of two interrelated processes occurring in the gap, one of which is determined by the time integral of the supplied power (the product of current and voltage across the gap), and the second by the time integral of losses caused by thermal conductivity and convection. This means that the interaction process will continue until the current disappears or the arc occurs again. The phenomenon of thermal breakdown is typical for the first 20 μs after zero current under conditions when the speed of voltage recovery is high, for example, with unremoved short circuits.

Stage of possible electrical breakdown

If thermal breakdown does not occur, the intercontact gap continues to be exposed to the influence of the PVN. The arc channel has an even higher temperature and lower density. A few hundred microseconds after zero current, when the PVN reaches its maximum value, the stage of possible electrical breakdown begins. It is based not on energy balance, but on the process of electron formation in an electric field. If the increase in electron concentration exceeds a certain critical value, a spark will form, which will turn into an arc discharge.

Arc extinction in oil circuit breakers

In oil switches, the contacts open in oil, but due to the high temperature of the arc formed between the contacts, the oil decomposes and the arc discharge occurs in a gaseous environment. Approximately half of this gas (by volume) is oil vapor. The rest consists of hydrogen (70%) and hydrocarbons of various compositions. These gases are flammable, but combustion in oil is impossible due to the lack of oxygen. The amount of oil decomposed by the arc is small, but the volume of gases produced is large. One gram of oil gives approximately 1500 cm 3 of gas, reduced to room temperature and atmospheric pressure.

Arc extinguishing in oil switches occurs most effectively when using extinguishing chambers, which limit the arc zone, contribute to an increase in pressure in this zone and the formation of a gas blast through the arc column. Figure 9 shows a diagram of the simplest extinguishing chamber.

Fig.9. Diagram of the simplest extinguishing chamber of an oil switch

During the shutdown process, contact rod 1 moves downwards. An arc occurs between pins 1 and 2. Intense gas formation occurs and the pressure in the chamber quickly increases. The relatively cold gas formed on the surface of the oil mixes with the arc plasma. The boundary layer enters a turbulent state, promoting deionization. However, the arc cannot go out until the distance between the contacts reaches a certain minimum value determined by the recovery voltage. This minimum gap occurs when the moving contact is still in the chamber. When the rod leaves the chamber, the gases are thrown out with force. A gas blast occurs, directed along the axis, helping to extinguish the arc.

After the arc is extinguished, the contact rod continues its movement to ensure the required insulation distance in the off position.

The arc voltage of an oil circuit breaker is at least 3 times that of an air circuit breaker. The electrical strength of the gap is restored faster (at a rate of about 2 kV/µs). Therefore, with the same short-circuit current, the arc extinguishing device of the oil circuit breaker can be designed for twice the voltage and twice the characteristic impedance than the air blast device.

Characteristic properties of air and oil circuit breakers

In air circuit breakers, the blowing in the arc gap is created from an external energy source and does not depend on the switched current. After zero current, the recovery voltage is applied to a short gap filled with hot ionized gas. The rate of restoration of the electrical strength of the gap is determined by cooling the gas and removing it from the gap by flow fresh air. This takes time and therefore the process of restoring the electrical strength of the gap is delayed.

Fig. 10. Characteristics of recoverable electrical strength
arc gap of air circuit breaker

Figure 10 shows typical curves of the recovering electric strength of the arc gap of an air circuit breaker. They are S-shaped. In this case, the main stage of the process of restoring the electrical strength of the gap occurs at a speed not exceeding 1-2 kV/μs, and begins 10-15 μs after the zero current value. As the switched current increases, the delay increases, and the rate of recovery of the electrical strength decreases. The lower dotted curve corresponds to the case of unsatisfactory operation of the switch, since the process of restoring the electrical strength of the gap is too slow. The rated breaking current of an air circuit breaker is limited by the recoverable dielectric strength of the gap.

In oil switches, the energy of the arc itself is used to create a gas blast. The pressure in the extinguishing chamber and the blowing force are, to a first approximation, proportional to the current being switched off. The larger the latter, the more effective the deionization of the gap and the faster its electrical strength is restored. However, as the current increases, the mechanical stresses in parts of the damping chamber increase. Therefore, the rated breaking current is limited by the mechanical strength of the extinguishing chamber.

The characteristic properties of air and oil circuit breakers appear when the asymmetrical short-circuit current is switched off. As is known, high-speed switches, in the presence of appropriate relay protection, open their contacts when the aperiodic component of the switched current has not yet had time to die out. Therefore, these switches must be capable of switching off both symmetrical and asymmetrical current, i.e. current, not displaced or displaced relative to the time axis, depending on conditions. Current asymmetry β (the relative content of the aperiodic component in the short-circuit current) is defined as the ratio of the aperiodic component to the amplitude of the periodic component of the short-circuit current at the moment τ of opening the switch contacts

(2)

The asymmetry of the disconnected current depends on the circuit time constant T a =Х/(ɷR), as well as on τ - the opening time of the switch contacts, taking into account the response time of the relay protection. The longer the time constant and the faster the switch contacts open, the greater the asymmetry of the switched current. Generators, transformers and reactors have the greatest time constant. Therefore, the greatest asymmetry should be expected for short circuits near generators and station busbars. Calculations show that the asymmetry of the current switched off by high-speed switches installed in the main switchgear of powerful stations can reach 80%. Less high-speed switches under the same conditions can encounter an asymmetry of the order of 40-50%. Switches installed in distribution networks are encountered with asymmetry not exceeding 20%.

If there is an aperiodic component in the switched current:

• the effective current value increases;
• the time intervals between the moments when the current reaches zero become unequal: they are alternately more or less than a half-cycle;
• the rate of change of current di/dt decreases as it approaches zero;
• the returning voltage at the switch pole is reduced.

An increase in the effective current value and a change in the time intervals between zero current values ​​can, under unfavorable conditions, lead to a significant increase in the released energy compared to the energy released in the absence of the aperiodic current component. The energy released in the arc determines the ionization of the gas in the gap, and in oil switches it also determines the amount of gases formed and the pressure in the chamber, therefore, the mechanical stress in the switch elements, the degree of contact melting, etc.

Reducing the rate of change of current as it approaches zero reduces the ionization of the gap by the time the arc goes out, which facilitates the shutdown process.

Reducing the return voltage also makes the shutdown process easier.

Fig. 11. Returning voltage when switched off current is unbalanced

As can be seen from Fig. 11, the periodic component of the short-circuit current i p is shifted relative to the network voltage by an angle φ close to π/2. If the closing phase is α=φ, then the aperiodic component of the current is absent, the moment the current reaches zero and the arc goes out is close to the moment of maximum voltage. The returning voltage is determined by the ordinate ab. When a circuit is closed at any other time, an aperiodic component appears in the current being switched off and the moment the current reaches zero is shifted. In the case under consideration, at α = 27°, the returning voltage after a large half-wave of the current is determined by the ordinate a"b", and after the small half-wave - by the ordinate a"b" (when constructing the curves, the periodic and aperiodic components of the current are assumed to be conditionally undamped).

From the above analysis it follows that in the presence of an aperiodic component in the switched-off current, a number of new factors appear that influence the switch-off process, some of which make this process more difficult, while others make it easier.

The final effect of the aperiodic component depends on the properties of the switch.

Oil circuit breakers, the breaking capacity of which is limited by the mechanical strength of the extinguishing chamber, have a significant reserve in the recovering electrical strength of the arc gap when switching off a large current. An increase in the effective value of the switched-off current, due to the presence of an aperiodic component, increases the severity of the shutdown, since the energy released in the arc increases, and the facilitating factors introduced by the aperiodic component of the short-circuit current (a decrease in the speed of the current approaching zero and a decrease in the returning voltage) are not used by oil circuit breakers. Such switches are said to be current sensitive because the energy released in the arc is determined mainly by the current.

Air circuit breakers, the breaking capacity of which is limited by the electrical strength of the gap, use facilitating factors introduced by the aperiodic component of the current (reducing the rate of decrease in current and returning voltage). An increase in the effective value of the switched-off current caused by the aperiodic component does not increase the severity of the switch-off, since the introduced heavier and lighter factors are compensated. It is customary to say about such switches that they are voltage sensitive.

When choosing a circuit breaker based on its breaking capacity, the asymmetry of the switched short-circuit current should be taken into account. However, the normalized (nominal) asymmetry values ​​β nom are set the same for both oil and air circuit breakers.

Electric arc- the phenomenon of electric discharge in a gas (gas environment). Electricity, flowing through an ionized channel in gas (air).

When the voltage between two electrodes increases to the level of electrical breakdown in the air, an electric arc occurs between them. The electrical breakdown voltage depends on the distance between the electrodes, the surrounding gas pressure, ambient temperature, humidity and other factors that potentially affect the onset of the process. The ionization potential of the first electron of metal atoms is approximately 4.5 - 5 V, and the arcing voltage is twice as much (9 - 10 V). It is necessary to expend energy to release an electron from the metal atom of one electrode and to ionize the atom of the second electrode. The process leads to the formation of plasma between the electrodes and the burning of an arc (for comparison: the minimum voltage for the formation of a spark discharge is slightly higher than the electron output potential - up to 6 V).

To initiate breakdown at the existing voltage, the electrodes are brought closer to each other. During a breakdown, a spark discharge usually occurs between the electrodes, pulse-closing the electrical circuit.

Electrons in spark discharges ionize molecules in the air gap between the electrodes. With sufficient power of the voltage source in the air gap, a sufficient amount of plasma is formed for a significant drop in the breakdown voltage or resistance of the air gap. Wherein spark discharges turn into an arc discharge - a plasma cord between the electrodes, which is a plasma tunnel. The resulting arc is, in fact, a conductor and closes the electrical circuit between the electrodes. As a result, the average current increases even more, heating the arc to 4700-49700 C. In this case, it is considered that the ignition of the arc is completed. After ignition, stable arc combustion is ensured by thermionic emission from the cathode, heated by current and ion bombardment.

The interaction of electrodes with arc plasma leads to their heating, partial melting, evaporation, oxidation and other types of corrosion.
After ignition, the arc can remain stable when the electrical contacts are separated to a certain distance.

When operating high-voltage electrical installations, in which the appearance of an electric arc is inevitable, it is combated using electromagnetic coils combined with arc extinguishing chambers. Among other methods, the use of vacuum, air, SF6 and oil circuit breakers is known, as well as methods of diverting current to a temporary load that independently breaks the electrical circuit.

Electric arc structure

The electric arc consists of cathode and anode regions, arc column, and transition regions. The thickness of the anode region is 0.001 mm, the cathode region is about 0.0001 mm.

The temperature in the anodic region when welding with a consumable electrode is about 2500 ... 4000 ° C, the temperature in the arc column is from 7,000 to 18,000 ° C, in the cathode region - 9,000 - 12,000 ° C.

The arc column is electrically neutral. In any of its sections there are the same number of charged particles of opposite signs. The voltage drop in the arc column is proportional to its length.

The influence of an electric arc on electrical equipment

In a number of devices, the phenomenon of an electric arc is harmful. These are primarily contact switching devices used in power supply and electric drives: high-voltage switches, circuit breakers, contactors, sectional insulators on the contact network of electrified railways and urban electric transport. When the loads are disconnected by the above devices, an arc occurs between the opening contacts.

Arc occurrence mechanism

• Reducing contact pressure - the number of contact points decreases, the resistance in the contact unit increases;
• The beginning of contact divergence - the formation of “bridges” from the molten metal of the contacts (at the last contact points);
• Rupture and evaporation of “bridges” from molten metal;
• Formation of an electric arc in metal vapor (which contributes to greater ionization of the contact gap and difficulty in extinguishing the arc);
• Stable arc burning with fast burnout of contacts.

To minimize damage to the contacts, it is necessary to extinguish the arc in a minimum time, making every effort to prevent the arc from remaining in one place (as the arc moves, the heat released in it will be evenly distributed over the contact body).

Methods for dealing with electric arcs

• arc cooling by coolant flow (oil switch);
• cooling the arc with a flow of cooling gas - (air circuit breaker, autogas circuit breaker, oil circuit breaker, SF6 gas circuit breaker), and the flow of the cooling medium can pass both along the arc shaft (longitudinal quenching) and across (transverse quenching); sometimes longitudinal-transverse damping is used;
• use of the arc-extinguishing ability of vacuum - it is known that when the pressure of the gases surrounding the switched contacts is reduced to certain value, leads to effective extinguishing of the arc (due to the absence of carriers for arc formation) vacuum circuit breaker.
• use of more arc-resistant contact material;
• use of contact material with a higher ionization potential;
• the use of arc extinguishing grids ( circuit breaker, electromagnetic switch).
• The principle of using arc extinguishing on gratings is based on the use of the effect of near-cathode drop in the arc (most of the voltage drop in the arc is the voltage drop at the cathode; the arc extinguishing grating is actually a series of serial contacts for the arc that gets there).
• use of arc suppression chambers - entering a chamber made of an arc-resistant material, such as mica plastic, with narrow, sometimes zigzag channels, the arc stretches, contracts and is intensively cooled from contact with the walls of the chamber.
• the use of “magnetic blast” - since the arc is highly ionized, it can be considered as a first approximation as a flexible conductor with current; By creating a magnetic field with special electromagnets (connected in series with the arc), it is possible to create arc movement to uniformly distribute heat over the contact, and to drive it into the arc-extinguishing chamber or grid. Some switch designs create a radial magnetic field that imparts torque to the arc.
• bypassing of contacts at the moment of opening by a power semiconductor switch with a thyristor or triac connected in parallel with the contacts; after opening the contacts, the semiconductor switch is turned off at the moment the voltage passes through zero (hybrid contactor, thyricon).

Notes

• Electric arc - article from the Great Soviet Encyclopedia.
• Spark discharge - article from the Great Soviet Encyclopedia.
• Raiser Yu. P. Physics of gas discharge. - 2nd ed. - M.: Nauka, 1992. - 536 p. - ISBN 5-02014615-3.
• Rodshtein L. A. Electrical devices, L 1981