When an electric arc occurs. Arc formation and properties. Voltage drop across an electric arc

January 17, 2012 at 10:00

When the electrical circuit is opened, an electrical discharge occurs in the form of an electric arc. For an electric arc to occur, it is sufficient that the voltage at the contacts be above 10 V with a current in the circuit of the order of 0.1 A or more. At significant voltages and currents, the temperature inside the arc can reach 10...15 thousand °C, as a result of which contacts and current-carrying parts melt.

At voltages of 110 kV and higher, the arc length can reach several meters. Therefore, an electric arc, especially in powerful power circuits, at voltages above 1 kV is a great danger, although serious consequences can also occur in installations at voltages below 1 kV. As a result, the electric arc must be limited as much as possible and quickly extinguished in circuits with voltages both above and below 1 kV.

Causes of electric arcs

The process of formation of an electric arc can be simplified as follows. When the contacts diverge, the contact pressure and, accordingly, the contact surface initially decrease, the transition resistance (current density and temperature) increases - local (in certain areas of the contact area) overheating begins, which further contribute to thermionic emission when, under the influence of high temperature, the speed of electron movement increases and they break out from the surface of the electrode.

At the moment the contacts diverge, that is, the circuit breaks, the voltage is quickly restored at the contact gap. Since the distance between the contacts is small, a high-intensity electric field arises, under the influence of which electrons are ejected from the surface of the electrode. They accelerate into electric field and when they hit a neutral atom, they give it their kinetic energy. If this energy is enough to remove at least one electron from the shell of a neutral atom, then the process of ionization occurs.

The resulting free electrons and ions make up the plasma of the arc barrel, that is, the ionized channel in which the arc burns and continuous movement of particles is ensured. In this case, negatively charged particles, primarily electrons, move in one direction (towards the anode), and atoms and gas molecules lacking one or more electrons - positively charged particles - in the opposite direction (towards the cathode). The conductivity of plasma is close to the conductivity of metals.

A large current passes through the arc shaft and a high temperature is created. This temperature of the arc barrel leads to thermal ionization - the process of formation of ions due to the collision of molecules and atoms with high kinetic energy at high speeds of their movement (molecules and atoms of the medium where the arc burns disintegrate into electrons and positively charged ions). Intense thermal ionization maintains high plasma conductivity. Therefore, the voltage drop along the arc length is small.

In an electric arc, two processes continuously occur: in addition to ionization, also deionization of atoms and molecules. The latter occurs mainly through diffusion, that is, the transfer of charged particles into environment, and the recombination of electrons and positively charged ions, which recombine into neutral particles, releasing the energy expended in their decay. In this case, heat is dissipated into the environment.

Thus, it is possible to distinguish three stages of the process under consideration: arc ignition, when, due to impact ionization and electron emission from the cathode, an arc discharge begins and the ionization intensity is higher than deionization; stable arc combustion, supported by thermal ionization in the arc barrel, when the intensity of ionization and deionization is the same, arc extinction when the intensity of deionization is higher than ionization.

Methods for extinguishing arcs in electrical switching devices

In order to disconnect the elements of the electrical circuit and thus prevent damage to the switching device, it is necessary not only to open its contacts, but also to extinguish the arc that appears between them. The processes of arc extinguishing, as well as combustion, are different for alternating and direct current. This is determined by the fact that in the first case, the current in the arc passes through zero every half-cycle. At these moments, the release of energy in the arc stops and the arc spontaneously goes out each time, and then lights up again.

In practice, the current in the arc becomes close to zero somewhat earlier than the transition through zero, since as the current decreases, the energy supplied to the arc decreases, and the temperature of the arc decreases accordingly and thermal ionization stops. In this case, the deionization process occurs intensively in the arc gap. If in at the moment open and quickly separate the contacts, then subsequent electrical breakdown may not occur and the circuit will be disconnected without arcing. However, in practice this is extremely difficult to do, and therefore special measures are taken to accelerate the extinguishing of the arc, ensuring cooling of the arc space and reducing the number of charged particles.

As a result of deionization, the electrical strength of the gap gradually increases and at the same time the recovery voltage across it increases. The ratio of these quantities determines whether the arc will light up for the next half of the period or not. If the electrical strength of the gap increases faster and there is more restoring voltage, the arc will no longer ignite, otherwise a stable arc will be ensured. The first condition determines the task of extinguishing the arc.

In switching devices they use various ways arc extinction.

Arc lengthening

When the contacts diverge during the process of disconnecting the electrical circuit, the resulting arc stretches. At the same time, the cooling conditions for the arc improve, since its surface increases and more voltage is required for combustion.

Dividing a long arc into a number of short arcs

If the arc formed when the contacts open is divided into K short arcs, for example by drawing it into a metal grid, then it will go out. The arc is usually drawn into a metal grid by electrical action. magnetic field induced in the grid plates by eddy currents. This arc extinguishing method is widely used in switching devices for voltages below 1 kV, in particular in automatic air circuit breakers.

Arc cooling in narrow slots

Extinguishing the arc in a small volume is easier. Therefore, in switching devices, arc-extinguishing chambers with longitudinal slots are widely used (the axis of such a slot coincides in the direction with the axis of the arc shaft). Such a gap usually forms in chambers made of insulating arc-resistant materials. Due to the contact of the arc with cold surfaces, its intense cooling, diffusion of charged particles into the environment and, accordingly, rapid deionization occur.

In addition to slots with plane-parallel walls, slots with ribs, protrusions, and extensions (pockets) are also used. All this leads to deformation of the arc barrel and helps to increase the area of ​​contact with the cold walls of the chamber.

The drawing of an arc into narrow slots usually occurs under the influence of a magnetic field interacting with the arc, which can be considered as a conductor with current.

The external magnetic field to move the arc is most often provided by a coil connected in series with the contacts between which the arc occurs. Arc extinction in narrow slots is used in devices for all voltages.

High pressure arc extinguishing

At a constant temperature, the degree of ionization of the gas decreases with increasing pressure, while the thermal conductivity of the gas increases. All other things being equal, this leads to increased cooling of the arc. Extinguishing the arc using high pressure, created by the arc itself in tightly closed chambers, is widely used in fuses and a number of other devices.

Arc extinction in oil

If the switch contacts are placed in oil, the arc that occurs when they open leads to intense evaporation of the oil. As a result, a gas bubble (sheath) is formed around the arc, consisting mainly of hydrogen (70...80%), as well as oil vapor. The released gases penetrate directly into the arc shaft area at high speed, cause mixing of cold and hot gas in the bubble, provide intense cooling and, accordingly, deionization of the arc gap. In addition, the deionizing ability of gases increases the pressure inside the bubble created during the rapid decomposition of oil.

The intensity of the arc extinguishing process in oil is higher, the closer the arc comes into contact with the oil and the faster the oil moves relative to the arc. Taking this into account, the arc rupture is limited by a closed insulating device - an arc extinguishing chamber. In these chambers, closer contact of the oil with the arc is created, and with the help of insulating plates and exhaust holes, working channels are formed through which the oil and gases move, providing intense blowing of the arc.

An electric arc is a powerful, long-lasting electrical discharge in a highly ionized mixture of gases and vapors between energized electrodes. Characterized by high gas temperatures and high current in the discharge zone.

The electrodes are connected to sources of alternating current (welding transformer) or direct current (welding generator or rectifier) ​​with direct and reverse polarity.

When welding with direct current, the electrode connected to the positive pole is called the anode, and to the negative pole is called the cathode. The space between the electrodes is called the arc gap region or arc gap (Figure 3.4). The arc gap is usually divided into 3 characteristic areas:

1. anode region adjacent to the anode;
2. cathode region;
3. arc pillar.

Any arc ignition begins with a short circuit, i.e. from the connection of the electrode with the product. In this case, U d = 0, and current I max = I short circuit. At the point of closure, a cathode spot appears, which is an indispensable (necessary) condition for the existence arc discharge. When the electrode is removed, the resulting liquid metal is stretched, overheated and the temperature reaches boiling point - an arc is excited (ignited).

The arc can be ignited without contact of the electrodes due to ionization, i.e. breakdown of the dielectric air (gas) gap due to increased voltage by oscillators (argon arc welding).

The arc gap is a dielectric medium that must be ionized.

For the existence of an arc discharge, U d = 16÷60 V is sufficient. Passage electric current through the air (arc) gap is possible only if there are electrons (elementary negative particles) and ions in it: positive (+) ions - all molecules and atoms of elements (me metals form more easily); negative (–) ions – more easily form F, Cr, N 2, O 2 and other elements with an affinity for electrons e.

Figure 3.4 – Arc burning diagram

The cathode region of the arc is a source of electrons that ionize the gases in the arc gap. Electrons released from the cathode are accelerated electric field and move away from the cathode. At the same time, under the influence of this field, + ions are directed to the cathode:

U d = U k + U c + U a;

The anode region has a significantly larger volume U a< U к.

Arc column - the main part of the arc gap is a mixture of electrons, + and – ions and neutral atoms (molecules). The arc column is neutral:

∑charge.neg. = ∑charges of positive particles.

The energy to maintain a stationary arc comes from the IP power supply.

Different temperatures, sizes of the anodic and cathodic zones and different amounts of heat released determine the existence of direct and reverse polarity when welding with direct current:

Q a > Q k; Ua< U к.

• upon request large quantity heat to heat the edges of large thicknesses of metal, direct polarity is used (for example, when surfacing);
• for thin-walled metals being welded that do not allow overheating, reverse polarity (+ on the electrode).

In the article you will learn what an electric arc is, a flash, how it appears, the history of its origin, as well as its danger, what happens during an electric arc and how to protect yourself.

Electrical safety is of paramount importance to maintaining any efficient and productive facility, and one of the greatest threats to worker safety is electric arc and arc flash. We recommend this article to you.

Electrical fires cause catastrophic damage, and in industrial settings they are often caused by electrical arcs of one type or another. While some types of electrical arcs are hard to miss, "an arc flash is loud and accompanied by a large, bright explosion," some electrical arcs, such as arc flash, are more subtle but can be just as destructive. Arc faults are a common cause of electrical fires in residential and commercial buildings.

Simply put, an electric arc is an electrical current that is discharged, either intentionally or unintentionally, across the gap between two electrodes through gas, steam or air and produces a relatively low voltage across the conductors. The heat and light produced by this arc are usually intense and can be used for special applications such as arc welding or lighting. Unintentional arcs can have devastating consequences such as fires, electrical hazards and property damage.

Electric arc

Electric arc origin story

In 1801, the British chemist and inventor Sir Humphry Davy demonstrated the electric arc to his comrades at the Royal Society of London and proposed a name - electric arc. These electrical arcs look like jagged lightning strikes. This demonstration was followed by further research into the electric arc, shown by Russian scientist Vasily Petrov in 1802. Further advances in early electric arc research led to industry-leading inventions such as arc welding.

Compared to a spark, which is only instantaneous, an arc is a continuous electrical current that generates so much heat from the charge-carrying ions or electrons that it can vaporize or melt anything within the range of the arc. The arc can be maintained at electrical circuits direct or alternating current, and it must include some resistance so that the increased current does not go unchecked and completely destroy the actual source of the circuit with its heat and energy consumption.

Practical Application

At correct use electric arcs can have useful purposes. In fact, each of us performs a number of daily tasks thanks to the limited use of electrical arcs.

Electric arcs are used in:

• camera flashes
• spotlights for stage lighting
• fluorescent lighting
• arc welding
• arc furnaces (for the production of steel and substances such as calcium carbide)
• plasma cutters (in which compressed air is combined with a powerful arc and converted into plasma, which has the ability to instantly cut steel).

Electric arc hazard

Electrical arcs can also be extremely dangerous if not used intentionally. Situations where an electrical arc is created in an uncontrolled environment, such as in an arc flash, can result in injury, death, fire, equipment damage, and property loss.

To protect workers from electrical arcs, companies should use the following arc flash products to reduce the likelihood of electrical arcs and reduce damage if they occur.

Arc-Protective Gloves- These gloves are designed to protect your hands from electrical shock and minimize injury in the event of an electrical incident.

Arc flash definition

The definition of arc flash is an unwanted electrical discharge that travels through the air between conductors or from a conductor to ground. An arc flash is part of an arc discharge, which is an example of an electrical explosion caused by a low-impedance connection that passes through air to ground.

When an arc flash occurs, it creates very bright light and intense heat. Additionally, it can create an arc that can cause traumatic force that could seriously injure someone in the area or damage anything nearby.

What happens during an arc flash?

An arc flash begins when electricity leaves its intended path and begins to travel through the air towards a grounded area. Once this happens, it ionizes the air, which further reduces the overall resistance along the arc path. This helps attract additional electrical energy.

The arc will move in such a way as to find the closest distance to the ground. The exact distance that an arc flash can travel is called arc flash boundary. This is determined by potential energy and many other factors such as air temperature and humidity.

When working to improve arc flash safety, the installation will often mark the arc flash boundary using floor tape. Anyone working in this area will be required to wear personal protective equipment (PPE).

Arc Flash Potential Temperature

One of the biggest dangers associated with an arc flash is the extremely high temperature it can create. Depending on the situation, they can reach high temperatures at 35000 degrees Fahrenheit or 19426.667 degrees Celsius. This is one of the highest temperatures in the world, approximately 4 times higher than the surface of the Sun.

Even if the actual electricity does not touch the person, the person's body will suffer enormous damage if he is near it. In addition to direct burns, these temperatures can ignite something in the area.

What does an arc flash look like?

The following video shows how fast and explosive arc flash can be. This video shows a controlled arc flash with a "test dummy":

How long does an arc flash last?

An arc flash can last anywhere from a fraction of a second to several seconds, depending on a number of factors. Most arc flashes do not last very long because the source of electricity is quickly cut off by circuit breakers or other safety equipment.

The most advanced systems now use devices known as arc eliminators, which detect and extinguish the arc in just a few milliseconds.

However, if the system does not have some type of protection, the arc flash will continue until the flow of electricity physically stops. This can happen when a worker physically cuts power to an area or when damage caused by an arc flash becomes severe enough to somehow stop the flow of electricity.

Look at real example arc flash that continues for an extended period of time in the following video. Luckily, the people in the video were wearing their personal protective equipment and were uninjured. Powerful explosion, loud noise, bright lights and extreme temperatures are all extremely dangerous.

Arc Flash Damage Potential

Due to the high temperatures, intense explosions and other effects of an arc flash, arc flashes can cause a lot of damage very quickly. Understanding the different types of damage that can occur can help businesses plan their safety responsibilities.

Potential property damage

• Warm- Heat from an arc flash can easily melt metal, which can damage expensive machinery and other equipment.
• Fire- The heat from these flashes can quickly lead to a fire that can spread through the facility if left unchecked.
• Explosions- The arc flash that can result from an arc flash can break windows, split wood in the area, bend metal, and more. Anything stored within the arc's blast radius can be damaged or destroyed in a matter of seconds.

Potential personal injury from arc flash

• Burns- Second and third degree burns can occur in a split second when someone is near an arc flash.
• Electric shock- if an arc flash passes through a person, he will receive a shock, like in the electric chair. Depending on the strength of the current, this blow can be fatal.
• Hearing damage- Arc flashes can produce very loud noises that can cause permanent hearing damage to those in the area.
• Damage to vision— Arc flashes can be very bright, which can cause temporary or even long-term eye damage.
• Arc Explosion Damage“The arc explosion can create a force that amounts to thousands of pounds per meter. This can knock a person down several meters. It can also cause broken bones, collapsed lungs, concussions, and more.

Wearing personal protective equipment can provide a significant degree of protection, but cannot eliminate all risks. Employees who are present when an arc flash occurs are always at risk, regardless of what PPE they are wearing.

Potential causes of arc flash

Arc flashes can occur for a variety of reasons. In most cases, the root cause will be a damaged piece of equipment, such as a wire. It could also be the result of someone working on equipment that allows electricity to escape from the path it is normally attached to.

Even when there is a potential path outside the wiring, electricity will follow the path least resistance. This is why an arc flash does not necessarily happen as soon as something is damaged or an alternative path becomes available. Instead, the electricity will continue to follow its intended path until another option with less resistance becomes available.

Here are some things that can create a path of less resistance and therefore cause an arc flash:

• Dust- In dusty areas, electricity may begin to pass through wiring or other equipment through the dust.
• Dropped Tools- for example, if a tool is dropped on a wire, it can damage it and allow electricity to flow into the tool. From there, he must find another path to continue his movement.
• Accidental touch- If a person touches the damaged area, electricity can spread through his body.
• Condensation- When condensation forms, electricity can escape from the wiring through the water, and then an arc will occur.
• Material failure- If the wire is damaged to the point where there is a problem with the passage of electricity, the path may be more stable than going beyond the wire.
• Corrosion— Corrosion can create a path outside the wire, followed by an arc flash.
• Incorrect installation— If equipment is not installed correctly, it can make it difficult or impossible for electricity to follow its intended path, which can cause an arc flash.

Preventing Electric Arc Flashes

The first step in arc flash safety is minimizing the risk of an occurrence. This can be done by performing an electrical risk assessment, which can help determine where the biggest hazards are located on site. IEEE 1584 is good option for most objects and will help identify common problems.

Regular inspections of all high voltage equipment and all wiring is another important step. If there are any signs of corrosion, damaged wires or other problems, they should be repaired as soon as possible. This will help keep electrical currents safely inside machines and wires.

Some specific areas that should be checked include any electrical distribution boards, control panels, control panels, socket housings and motor control centers.

Proper Labeling

Any location in a facility where high electrical currents may exist should be properly marked with arc warning labels. They can be purchased pre-made or printed on any industrial label printer as needed. Article 110.16 of the National Electrical Code clearly states that this type of equipment must be labeled to alert people to the hazards.

De-energizing equipment when performing maintenance

Whenever the machine requires any work, it must be completely de-energized. Powering down your car is more than just turning it off. All machines must be turned off and physically disconnected from any power source. After disconnecting, you should also check the voltage to ensure that latent energy has not accumulated.

Ideally, there should be a lockout policy that will physically lock the power supply so that it cannot be accidentally plugged back in while someone is working on the machine.

Fuses

If possible, circuit breakers must be installed on all machines. These circuit breakers will quickly detect a sudden power surge and immediately stop the flow. Even with circuit breakers, an arc flash may occur, but it will only last part of the time because the electrical current is cut off.

However, even a very short arc flash can be fatal, so circuit breakers should not be considered a sufficient arc flash safety program.

Safety Standards

All facilities must comply with the various arc flash safety standards that have been established by public and private agencies. Determining which standards must be met can help ensure a facility is compliant with local codes and regulations while also ensuring the safety of the facility.

The following are the most common electrical arc flash safety standards:

• OSHA - OSHA has several standards, including 29 CFR parts 1910 and 1926. These standards cover requirements for the generation, transmission and distribution of electrical power.
• National Fire Protection Association (NFPA) - NFPA Standard 70-2014, National Electrical Code (NEC) pertains to safe electrical installation and practice. NFPA 70E, Standard for Electrical Safety in the Workplace, details various warning label requirements, including warning labels regarding arc flashes and arc explosions. It also offers guidance on implementing best practices in the workplace to help keep employees working with high-voltage equipment safe.
• Canadian Standards Association Z462 - This is very similar to NFPA 70E standards, but applies to Canadian companies.
• Underwriters Laboratories of Canada - This set of standards is intended for any situation where electrical energy is generated, transmitted or distributed and covers safety requirements. Similar to OSHA standards, but for Canada.
• IEEE 1584 is a set of guidelines for accurately calculating arc flash hazards.

Material from Wikipedia - the free encyclopedia

Electric arc (voltaic arc, arc discharge) - physical phenomenon, one of the types of electrical discharge in gas.

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 section of it there are the same number of charged particles of opposite signs. The voltage drop in the arc column is proportional to its length.

Welding arcs are classified according to:

• Electrode materials - with consumable and non-consumable electrode;
• Degrees of column compression - free and compressed arc;
• According to the current used - DC arc and AC arc;
• According to the polarity of direct electric current - direct polarity ("-" on the electrode, "+" - on the product) and reverse polarity;
• When using alternating current - single-phase and three-phase arcs.

Arc self-regulation

When external compensation occurs - changes in network voltage, wire feed speed, etc., a disturbance occurs in the established equilibrium between the feed speed and the melting rate. As the length of the arc in the circuit increases, the welding current and the melting speed of the electrode wire decrease, and the feed speed, while remaining constant, becomes greater than the melting speed, which leads to the restoration of the arc length. As the arc length decreases, the wire melting speed becomes greater than the feed speed, this leads to the restoration of the normal arc length.

The efficiency of the arc self-regulation process is significantly influenced by the shape of the current-voltage characteristic of the power source. The high speed of arc length oscillations is processed automatically with rigid current-voltage characteristics of the circuit.

Fighting an electric arc

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 circuit breakers, 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.

The mechanism of arc occurrence in this case is as follows:

• 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).

To fulfill the above requirements, the following arc control methods are used:

• arc cooling by a flow of cooling medium - liquid (oil switch); 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 barrel (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 a certain value, a vacuum circuit breaker leads to effective extinguishing of the arc (due to the absence of carriers for arc formation).
• use of more arc-resistant contact material;
• use of contact material with a higher ionization potential;
• 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 extinguishing 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 across 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).

See also

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Literature

• Electric arc- article from.
• Spark discharge- article from the Great Soviet Encyclopedia.
• Raiser Yu. P. Physics gas discharge. - 2nd ed. - M.: Nauka, 1992. - 536 p. - ISBN 5-02014615-3.
• Rodshtein L. A. Electrical devices, L 1981
• Clerici, Matteo; Hu, Yi; Lassonde, Philippe; Milian, Carles; Couairon, Arnaud; Christodoulides, Demetrios N.; Chen, Zhigang; Razzari, Luca; Vidal, François (2015-06-01). "Laser-assisted guiding of electric discharges around objects". Science Advances 1(5):e1400111. Bibcode:2015SciA....1E0111C. doi:10.1126/sciadv.1400111. ISSN 2375-2548.

Links

Notes

Terminology Electric arc Pressure welding Resistance welding Other types of welding Equipment and gear Occupational diseases Professional organizations

An excerpt characterizing the electric arc

– On fera du chemin cette fois ci. Oh! quand il s"en mele lui meme ca chauffe... Nom de Dieu... Le voila!.. Vive l"Empereur! Les voila donc les Steppes de l"Asie! Vilain pays tout de meme. Au revoir, Beauche; je te reserve le plus beau palais de Moscow. Au revoir! Bonne chance... L"as tu vu, l"Empereur? Vive l" Empereur!.. preur! Si on me fait gouverneur aux Indes, Gerard, je te fais ministre du Cachemire, c"est arrete. Vive l"Empereur! Vive! vive! vive! Les gredins de Cosaques, comme ils filent. Vive l"Empereur! Le voila! Le vois tu? Je l"ai vu deux fois comme jete vois. Le petit caporal... Je l"ai vu donner la croix a l"un des vieux... Vive l"Empereur!.. [Now let's go! Oh! as soon as he takes charge, things will boil. By God... Here he is... Hurray, Emperor! So here they are, the Asian steppes... However, good-bye, Bose. I will leave you the best palace in Moscow. Goodbye. Have you seen the emperor? If I am made governor of India, I will make you the minister of Kashmir... Hurray! Here he is! I saw him twice like you. Little corporal... I saw how he hung a cross on one of the old men... Hurray, emperor!] - said the voices of old and young people, of the most diverse characters and positions in society. All the faces of these people had one common expression of joy at the beginning of the long-awaited campaign and delight and devotion to the man in a gray frock coat standing on the mountain.
On June 13, Napoleon was given a small purebred Arabian horse, and he sat down and galloped to one of the bridges over the Neman, constantly deafened by enthusiastic cries, which he obviously endured only because it was impossible to forbid them to express their love for him with these cries; but these screams, accompanying him everywhere, weighed on him and distracted him from the military worries that had gripped him since the time he joined the army. He drove across one of the bridges swinging on boats to the other side, turned sharply to the left and galloped towards Kovno, preceded by enthusiastic Guards horse rangers who were transfixed with happiness, clearing the way for the troops galloping ahead of him. Arriving at the wide Viliya River, he stopped next to a Polish Uhlan regiment stationed on the bank.
- Vivat! – the Poles also shouted enthusiastically, disrupting the front and pushing each other in order to see him. Napoleon examined the river, got off his horse and sat down on a log lying on the bank. At a wordless sign, a pipe was handed to him, he placed it on the back of a happy page who ran up and began to look at the other side. Then he went deep into examining a sheet of map laid out between the logs. Without raising his head, he said something, and two of his adjutants galloped towards the Polish lancers.
- What? What did he say? - was heard in the ranks of the Polish lancers when one adjutant galloped up to them.
It was ordered to find a ford and cross to the other side. Polish Lancer Colonel, handsome old man, flushed and confused in words from excitement, asked the adjutant if he would be allowed to swim across the river with his lancers without looking for a ford. He, with obvious fear of refusal, like a boy who asks permission to mount a horse, asked to be allowed to swim across the river in the eyes of the emperor. The adjutant said that the emperor would probably not be dissatisfied with this excessive zeal.
As soon as the adjutant said this, the old mustachioed officer happy face and with shining eyes, raising his saber, shouted: “Vivat! - and, having ordered the lancers to follow him, he gave spurs to his horse and galloped up to the river. He angrily pushed the horse that had hesitated beneath him and fell into the water, heading deeper into the rapids of the current. Hundreds of lancers galloped after him. It was cold and terrible in the middle and at the rapids of the current. The lancers clung to each other, fell off their horses, some horses drowned, people drowned too, the rest tried to swim, some on the saddle, some holding on to the mane. They tried to swim forward to the other side and, despite the fact that there was a crossing half a mile away, they were proud that they were swimming and drowning in this river under the gaze of a man sitting on a log and not even looking at what they were doing. When the returning adjutant, having chosen a convenient moment, allowed himself to draw the emperor’s attention to the devotion of the Poles to his person, a small man in a gray frock coat stood up and, calling Berthier to him, began to walk with him back and forth along the shore, giving him orders and occasionally looking displeasedly at the drowning lancers who entertained his attention.
It was not new for him to believe that his presence at all ends of the world, from Africa to the steppes of Muscovy, equally amazes and plunges people into the madness of self-forgetfulness. He ordered a horse to be brought to him and rode to his camp.
About forty lancers drowned in the river, despite the boats sent to help. Most washed back to this shore. The colonel and several people swam across the river and with difficulty climbed out to the other bank. But as soon as they got out with their wet dress flopping around them and dripping in streams, they shouted: “Vivat!”, looking enthusiastically at the place where Napoleon stood, but where he was no longer there, and at that moment they considered themselves happy.
In the evening, Napoleon, between two orders - one about delivering the prepared counterfeit Russian banknotes for import into Russia as soon as possible, and the other about shooting the Saxon, in whose intercepted letter information about orders for the French army was found - made a third order - about the inclusion of the Polish colonel, who unnecessarily threw himself into the river, into the cohort of honor (Legion d'honneur), of which Napoleon was the head.
Qnos vult perdere – dementat. [Whoever he wants to destroy, he will deprive him of his mind (lat.)]

Meanwhile, the Russian emperor had already lived in Vilna for more than a month, making reviews and maneuvers. Nothing was ready for the war that everyone expected and for which the emperor came from St. Petersburg to prepare. There was no general plan of action. Hesitation about which plan, out of all those that were proposed, should be adopted, only intensified even more after the emperor's month-long stay in the main apartment. The three armies each had a separate commander-in-chief, but there was no common commander over all the armies, and the emperor did not assume this title.
How lived longer The emperor in Vilna prepared less and less for war, tired of waiting for it. All the aspirations of the people surrounding the sovereign seemed to be aimed only at making the sovereign, while having a pleasant time, forget about the upcoming war.
After many balls and holidays among the Polish magnates, among the courtiers and the sovereign himself, in June one of the Polish general adjutants of the sovereign came up with the idea of ​​giving a dinner and ball to the sovereign on behalf of his general adjutants. This idea was joyfully accepted by everyone. The Emperor agreed. The general's adjutants collected money by subscription. The person who could be most pleasing to the sovereign was invited to be the hostess of the ball. Count Bennigsen, a landowner of the Vilna province, offered his country house for this holiday, and on June 13 a dinner, a ball, boating and fireworks were scheduled in Zakret, country house Count Bennigsen.
On the very day on which Napoleon gave the order to cross the Neman and his advanced troops, pushing back the Cossacks, crossed the Russian border, Alexander spent the evening at Bennigsen’s dacha - at a ball given by the general’s adjutants.
It was a cheerful, brilliant holiday; experts in the business said that rarely so many beauties gathered in one place. Countess Bezukhova, along with other Russian ladies who came for the sovereign from St. Petersburg to Vilna, was at this ball, darkening the sophisticated Polish ladies with her heavy, so-called Russian beauty. She was noticed, and the sovereign honored her with a dance.
Boris Drubetskoy, en garcon (a bachelor), as he said, having left his wife in Moscow, was also at this ball and, although not an adjutant general, was a participant for a large sum in the subscription for the ball. Boris was now a rich man, far gone in honor, no longer seeking patronage, but standing on an even footing with the highest of his peers.
At twelve o'clock at night they were still dancing. Helen, who did not have a worthy gentleman, herself offered the mazurka to Boris. They sat in the third pair. Boris, coolly looking at Helen's shiny bare shoulders protruding from her dark gauze and gold dress, talked about old acquaintances and at the same time, unnoticed by himself and others, never for a second stopped watching the sovereign, who was in the same room. The Emperor did not dance; he stood in the doorway and stopped first one or the other with those gentle words that he alone knew how to speak.
At the beginning of the mazurka, Boris saw that Adjutant General Balashev, one of the closest persons to the sovereign, approached him and stood un-courtly close to the sovereign, who was speaking with a Polish lady. After talking with the lady, the sovereign looked questioningly and, apparently realizing that Balashev acted this way only because there were important reasons, nodded slightly to the lady and turned to Balashev. As soon as Balashev began to speak, surprise was expressed on the sovereign’s face. He took Balashev by the arm and walked with him through the hall, unconsciously clearing three fathoms of wide road on both sides of those who were shunning in front of him. Boris noticed Arakcheev's excited face while the sovereign walked with Balashev. Arakcheev, looking from under his brows at the sovereign and snoring his red nose, moved out of the crowd, as if expecting that the sovereign would turn to him. (Boris realized that Arakcheev was jealous of Balashev and was dissatisfied that some obviously important news was not conveyed to the sovereign through him.)
But the sovereign and Balashev walked, without noticing Arakcheev, through the exit door into the illuminated garden. Arakcheev, holding his sword and looking around angrily, walked about twenty paces behind them.

Electrical arcing can be extremely destructive to equipment and, more importantly, dangerous to people. An alarming number of accidents caused by it occur each year, often resulting in serious burns or death. Fortunately, significant progress has been made in the electrical industry in terms of creating means and methods of protection against arc exposure.

Causes and places of occurrence

Electrical arcing is one of the deadliest and least understood electrical hazards and is prevalent in most industries. It is widely accepted that the higher the electrical system voltage, the greater the risk to people working on or near live wires and equipment.

The thermal energy from an arc flash, however, may actually be greater and occur more frequently at higher temperatures. low voltage with the same devastating consequences.

An electric arc usually occurs when there is accidental contact between a live conductor, such as a trolleybus or tram line contact wire with another conductor, or a grounded surface.

When this happens, the resulting short circuit current melts the wires, ionizes the air and creates a fiery channel of conducting plasma with a characteristic arc-shaped shape (hence the name), and the temperature of the electric arc at its core can reach over 20,000 ° C.

What is an electric arc?

In fact, this is the common name for an arc discharge, well known in physics and electrical engineering - a type of independent electric discharge in a gas. What are physical properties electric arc? It burns in a wide range of gas pressure, at constant or alternating (up to 1000 Hz) voltage between the electrodes in the range from several volts (welding arc) to tens of kilovolts. The maximum arc current density is observed at the cathode (10 2 -10 8 A/cm 2), where it is contracted into a cathode spot, very bright and small in size. It moves randomly and continuously over the entire area of ​​the electrode. Its temperature is such that the cathode material boils in it. Therefore, there are ideal conditions for thermionic emission of electrons into the cathode space. A small layer is formed above it, charged positively and providing acceleration of emitted electrons to speeds at which they impact ionize atoms and molecules of the medium in the interelectrode gap.

The same spot, but somewhat larger and less mobile, forms on the anode. The temperature in it is close to the cathode spot.

If the arc current is of the order of several tens of amperes, then plasma jets or torches flow out of both electrodes at high speed normal to their surfaces (see photo below).

At high currents (100-300 A), additional plasma jets appear, and the arc becomes similar to a bundle of plasma filaments (see photo below).

How does an arc manifest itself in electrical equipment?

As mentioned above, the catalyst for its occurrence is strong heat generation in the cathode spot. The temperature of the electric arc, as already mentioned, can reach 20,000 ° C, about four times higher than on the surface of the sun. This heat can quickly melt or even vaporize the copper of the conductors, which has a melting point of about 1084 ° C, much lower than in an arc. Therefore, copper vapors and splashes of molten metal often form in it. When copper changes from solid to vapor, it expands to several tens of thousands of times its original volume. This is equivalent to a one cubic centimeter piece of copper changing to a size of 0.1 cubic meters in a fraction of a second. This will create high-intensity pressure and sound waves propagating around at high speed (which can be over 1100 km per hour).

Exposure to electric arc

If it occurs, serious injuries, and even death, can occur not only to persons working on electrical equipment, but also to people nearby. Arc injuries can include external skin burns, internal burns from inhaling hot gases and vaporized metal, hearing damage, vision damage such as blindness from ultraviolet flash light, and many other devastating injuries.

A particularly powerful arc may also cause it to explode, creating a pressure of more than 100 kilopascals (kPa) and releasing shrapnel-like debris at speeds of up to 300 meters per second.

Individuals who have suffered electrical arc injuries may require extensive medical treatment and rehabilitation, and the cost of their injuries can be extreme—physically, emotionally, and financially. Although businesses are required by law to carry out risk assessments for all work activities, the risk of arc hazards is often overlooked because most people do not know how to assess and effectively manage the hazard. Protection against the effects of an electric arc involves the use of a whole range of means, including the use when working with energized electrical equipment, special electrical protective equipment, special clothing, as well as the equipment itself, especially high-low voltage switching electrical devices designed using arc extinguishing means.

Arc in electrical apparatus

In this class of electrical devices (circuit breakers, contactors, magnetic starters), the fight against this phenomenon is of particular importance. When the contacts of a switch not equipped special devices to prevent an arc, open, then it must ignite between them.

At the moment when the contacts begin to separate, the area of ​​​​the latter decreases rapidly, which leads to an increase in current density and, consequently, to an increase in temperature. The heat generated in the gap between the contacts (the usual medium is oil or air) is sufficient to ionize the air or evaporate and ionize the oil. The ionized air or steam acts as a conductor for the arc current between the contacts. The potential difference between them is very small, but it is enough to maintain the arc. Consequently, the current in the circuit remains continuous until the arc is eliminated. Not only does it delay the interruption process, but it also generates a huge amount of heat that can damage the breaker itself. Thus, main problem in a switch (primarily high-voltage) - this is extinguishing the electric arc in as soon as possible so that the heat generated in it cannot reach a dangerous value.

Factors for maintaining an arc between switch contacts

These include:

2. Ionized particles between them.

Accepting this, we note additionally:

• When there is a small gap between the contacts, even a small potential difference is enough to maintain the arc. One way to extinguish it is to separate the contacts at such a distance that the potential difference becomes insufficient to maintain the arc. However, this method is not practical in high voltage applications where separation over many meters may be required.
• Ionized particles between the contacts tend to support the arc. If its path is deionized, then the quenching process will be facilitated. This can be achieved by cooling the arc or removing ionized particles from the space between the contacts.
• There are two ways by which arc protection is provided in circuit breakers:

High resistance method;

Zero current method.

Extinguishing the arc by increasing its resistance

In this method, the resistance along the arc path increases over time so that the current decreases to a value insufficient to support it. Consequently, it is interrupted and the electric arc goes out. The main disadvantage of this method is that the extinction time is quite long, and enormous energy has time to dissipate in the arc.

Arc resistance can be increased by:

• Arc elongation - the resistance of the arc is directly proportional to its length. The arc length can be increased by changing the gap between the contacts.
• Cooling the arc, or more precisely the medium between the contacts. Effective fan cooling must be directed along the arc.
• By placing the contacts in a difficult-to-ionize gas environment (gas switches) or in a vacuum chamber (vacuum switches).
• Decrease cross section arc by passing it through a narrow hole, or by reducing the contact area.
• By dividing the arc - its resistance can be increased by dividing it into a number of small arcs connected in series. Each of them experiences the action of elongation and cooling. The arc can be divided by introducing some conductive plates between the contacts.

Arc extinction using zero current method

This method is used only on AC circuits. It keeps the arc resistance low until the current drops to zero, where it extinguishes naturally. Its re-ignition is prevented despite the increased voltage at the contacts. All modern high-alternating current circuit breakers use this arc extinguishing method.

In an alternating current system, the latter drops to zero after each half cycle. At each such reset, the arc goes out for a short time. In this case, the medium between the contacts contains ions and electrons, so its dielectric strength is low and can be easily destroyed by increasing voltage across the contacts.

If this happens, the electric arc will burn for the next half cycle of the current. If immediately after it is reset to zero, the dielectric strength of the medium between the contacts increases faster than the voltage across them, then the arc will not ignite and the current will be interrupted. A rapid increase in the dielectric strength of the medium near zero current can be achieved by:

• recombination of ionized particles in the space between contacts into neutral molecules;
• by removing ionized particles away and replacing them with neutral particles.

Thus, real problem in interrupting an alternating current arc is the rapid deionization of the medium between the contacts as soon as the current becomes zero.

Methods for deionization of the medium between contacts

1. Gap lengthening: The dielectric strength of the medium is proportional to the length of the gap between the contacts. Thus, by quickly opening the contacts, a higher dielectric strength of the medium can be achieved.

2. High blood pressure. If it increases in the immediate vicinity of the arc, the density of particles making up the arc discharge channel also increases. Increased particle density leads to high level their deionization and, consequently, the dielectric strength of the medium between the contacts increases.

3. Cooling. The natural recombination of ionized particles occurs faster as they cool. Thus, the dielectric strength of the medium between the contacts can be increased by cooling the arc.

4. Explosion effect. If the ionized particles between the contacts are swept away and replaced by non-ionized ones, the dielectric strength of the medium can be increased. This can be achieved using a gas explosion directed into the discharge zone, or by injecting oil into the intercontact space.

These switches use sulfur hexafluoride (SF6) gas as the arc extinguishing medium. It has a strong tendency to absorb free electrons. The switch contacts open with a high pressure flow of SF6) between them (see picture below).

The gas captures free electrons in the arc and forms an excess of low-mobility negative ions. The number of electrons in the arc quickly decreases and it goes out.