Electrical causes of fire: description and types

Electrical causes of fires are one of the most common causes of fires - almost every fifth fire. Are electrical causes of fires always sufficiently justified?

As many years of experience and practice in studying fires have shown, in order for the investigator and investigator to put forward and finally accept a version of the cause of the fire, sometimes it is enough to detect a melted electrical conductor at the site of the fire.

Knowing that a short circuit has a sufficient thermal impulse and is capable of igniting the insulation of live parts and flammable materials located near electrical installations, some experts believe that they have correctly identified the cause of the fire. In the future, they are not interested in the remaining elements and devices for protecting the electrical network of the fire object. This conclusion about the authenticity of the cause of the fire is incorrect.

To objectively solve crimes and to reasonably determine the cause of a fire, it is necessary to conduct a complete and high-quality study of the entire electrical network of the fire site, record fragments of electrical devices found in the fire, and correctly remove the material evidence necessary for instrumental research.

When investigating fires, elements of the electrical network (protection devices, switching devices, sections of cables and wires with copper and aluminum conductors) that have characteristic traces of exposure to a short-circuit arc or temperature destruction must be removed from the fire site as material evidence.

The sequence of actions of persons involved in fire investigations has been repeatedly indicated in the specialized literature. We consider it useful to systematize and repeat them again.

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Fire in the apartment

Possible reasons and what to do

The possibility of a fire arising from electrical installations must be put forward and worked out in all cases where there was electrical equipment at the fire site. Inspection of electrical installations is quite difficult, so it is advisable to carry it out with the involvement of energy specialists. Moreover, it should be borne in mind that this inspection cannot be limited only to the premises in which the combustion occurred, because To work out versions of the possibility of a fire from electrical equipment, it is necessary to know the state of the entire electrical network, starting from the power source (transformer substation) to the most remote consumers of electricity located at the fire sites.

Versions about the causes of fires associated with the operation of electrical installations are the widest group of causes. This is primarily due to the power supply at manufacturing enterprises, in agriculture and in everyday life, the possibility of failure of electrical products, as well as the low quality of technical maintenance of electrical equipment. It should be noted that the involvement of electrical equipment in fires is often “established” without sufficient grounds. This requires a more in-depth and competent study of all those phenomena that preceded the fire and took place during its process, which are essential in establishing the true cause of the fire when working out the put forward versions of the possible cause of the fire.

It should be borne in mind that almost all ignition sources associated with the operation of electrical installations have a large reserve of thermal energy and are capable of igniting most flammable substances and materials.

Causes of electrical fires include:

electric arc;
short circuit;
overload of electrical circuits;
higher contact resistance;
sparking;
electrical network overvoltage;
transition of electric current to metal grounded structures of buildings and structures;
transfer of electric current to low-current electrical lines (radio, telephone, etc.);
thermal effects of electric heating devices;
thermal effects of electric incandescent lamps, their emergency mode and melting of bulbs;
emergency operation of fluorescent lamps.

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Children's prank with fire

Cause of fire and consequences

To improve the quality of inspection of electrical equipment during a fire, it is advisable to consider in more detail each of the reasons listed above, bearing in mind that the appearance or presence of some of them is provided for by the normal operation of electrical installations. For example, electric arcs occur during electric welding work; sparking occurs in commutator motors, magnetic starters and contactors; the presence of heated or glowing parts is present in heating devices, etc.

You need to know that overvoltage of the electrical network, high transient resistance and overload of the circuit can lead to a short circuit, the occurrence of an electric arc, and vice versa, a short circuit can lead to overload of the electrical network, to sparking, the formation of an electric arc, to the transition of electric current to metal grounded designs, etc.

That is, some emergency modes transform into others that are more dangerous in terms of the possibility of fires.

Let's take a closer look at the above ignition sources.

Electric arc

An electric arc has a very high temperature (1500-4000 °C) and can ignite almost any combustible material, in direct contact with it, as well as through radiant heat. An electric arc is formed as a result of a stable electrical discharge between two metal elements of an electrical installation that have different potentials. In an electric arc, intense ionization of the gas gap occurs, melting and burning of the metal. In addition, there is intense splashing of molten metal particles, which have a large supply of thermal energy, which, falling on flammable materials, can ignite them.

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First aid for electric shock

A stable electric arc can most often occur during a short circuit in gas pipes or armored cables and much less often in electrical wires. In this case, as the current-carrying core of an electrical conductor, armor, pipe, or other protective shell melts and burns, the arc can move along their surfaces towards the power source, leaving point or distributed penetration along the length. During an electric arc, short-term storage currents flow through the circuits. Therefore, when an electric arc is formed in emergency mode, secondary (side) phenomena characteristic of a short circuit occur in the electrical circuit. In this case, ignition sources often appear not only at the site of arc formation, but also in other places in the electrical circuit, but towards the power source. In cases not provided for by the normal operation of electrical installations, the occurrence of an electric arc most often occurs during a short circuit.

One of the well-known examples of the use of an electric arc in production is electric welding, in which significant currents flow through the conductors and a large amount of thermal energy is released.

The process of electric arc welding is usually accompanied by the occurrence of:

parts, structures or their individual sections heated to a high temperature or even hot to be welded;
scattering of relatively large particles of molten metal over considerable distances;
heating of contact elements and electrical conductors in places of loose connections;
sparking in places of poor quality connection or connection of electrical wires to the welding machine, welded parts and structures.

Short circuit

Among the causes of electrical fires, a short circuit is the most common, although it can often be a consequence of some other emergency situation in the electrical circuit.

A short circuit occurs when electrical wires with damaged insulation are connected, wires come into contact with metal grounded structures of buildings and structures, foreign metal objects come into contact with bare wires, or breakdown of charred or broken insulation of wires and other electrical installation products. As a result of a short circuit, due to a sharp increase in current in the electrical circuit, the temperature of the conductors increases significantly, which leads to ignition of the insulation of electrical wires and cables and is most often accompanied by melting of the metal of the conductors.

Overload of electrical circuits

Overload is a phenomenon in which current loads appear in the electrical network, windings of electrical machines, instruments and apparatus that exceed the long-term permissible ones.

The most common causes of overloading electrical circuits are:

incomplete or non-metallic short circuit through some contact resistance;
overvoltage in the electrical network;
operation of a three-phase motor on two phases due to a break in the third or tripping of one of the fuses;
jamming, overloading of a mechanism driven by an electric motor (for example, a conveyor line motor);
incorrect choice of electric motor for a given working mechanism (underestimated power in relation to the required);
jamming of the electric motor shaft due to insufficient lubrication, or destruction of bearings and shaft misalignment;
inclusion into the electrical network of powerful electricity consumers not provided for in the calculations.

High contact resistance

High transition resistance is the resistance of a section of an electrical circuit at the junction of individual elements (the junction of wires, connecting them to electrical receivers, contact elements, etc.) in which, if they are not executed correctly, the resistance is higher compared to the resistance of the electrical circuit before these areas and after them

Most often, large transient resistances occur in the following cases:

in places where wires are connected to each other, when instead of soldering, welding, crimping or bolt clamps, twisted wires with aluminum and copper conductors are used;
in places where wires are connected to switches, electric motors and other devices without special clamps and tips;
in switches, magnetic starters, switches, plug connectors (sockets, plugs) on contact elements with a decrease in the effort applied to turn on, failure to turn on, burning, etc.;
at contact points. made using threaded connections in electrical equipment that is subject to vibration during operation, and especially in cases where there are no devices against self-unscrewing;
at the junctions of wires made by soldering, but with the use of acids in surface preparation, which almost always remain at the soldering site and subsequently cause increased oxidation of the junctions or nearby sections of the wires.

The formation of ignition sources when large transient resistances occur, as a rule, is possible in the bridges of the appearance of transient resistances described above. The direct source of ignition in this case can be:

elements of electrical installations heated to a high temperature by heat generated by electric current in a place of high transition resistance;
electrical sparks or particles of molten and hot metal that occur at the site of a “poor” electrical contact.

High contact resistance may cause a short circuit.

How and with what to extinguish a burning wiring?

To extinguish fires on elements or sections of the electrical network that are energized, it is strictly prohibited to use water. Since if it comes into contact with a person, there is a risk of electric shock. Therefore, to extinguish fires in electrical installations defined by the classification in paragraph 1 of GOST 27331-87, only fire extinguishers are used that use aerosol, carbon dioxide, or powder as extinguishing material.

Figure 4: extinguishing electrical installations

But they also make it possible to fight fires only in devices up to 1 kV. And all electrical installations over 1 kV must first be de-energized.

Sparking

Sparking in electrical installations is a very common phenomenon and occurs both during normal operation of individual consumers of electrical energy and in emergency mode. Sparking is formed during contact and arc electric welding, turning switches, magnetic starters, contactors, switches on and off, on rings and commutators of electric motors when brushes do not fit tightly to them, and in places of poor-quality connection of wires to consumers of electrical energy, when individual sections of wires come into contact with each other or with grounded structures, etc. During sparking, ignition sources are formed that have energy and temperature sufficient to ignite many flammable substances and materials.

Sparking in non-flammable and non-explosive environments, as well as in the absence of flammable materials and structures in the immediate vicinity, does not pose a great danger.

What to do in case of a wiring fire?

If the apartment smells of burnt wiring, you notice smoke or an open fire in places where wires run or connect, or on electrical appliances that are turned on, you must take the following measures:

Turn off the electricity, preferably at the input circuit breaker, in order to ensure a simultaneous break in both phase and zero. Without this, there is a high probability of electric shock in case of fire.
If possible, protect the source of the fire or localize the fire. Under no circumstances should you touch hot elements to prevent burns.
If there is a fire, begin to extinguish it.

If for some reason the operating conditions of the electrical wiring do not allow it to be de-energized, then the wiring must be extinguished using special means that do not conduct electric current.

Overvoltage in an electrical circuit

Due to the fact that power supply sources have limited power, connecting or disconnecting electrical consumers from them leads to a change in voltage in the electrical network. To compensate for the decrease in voltage, when a large number of consumers are turned on simultaneously, the voltage of the power supply is increased. Therefore, when most consumers are turned off, the voltage in the electrical network becomes higher than the nominal one (127, 220, 380 V). The magnitude of overvoltage can vary and the differences are most often observed in rural areas.

The cause of overvoltages in the electrical network can also be the failure of the speed controller at local power plants, when, figuratively speaking, the generator engine goes into overdrive. Overvoltage can occur: during short circuits; when “high” voltage enters low-voltage networks; during lightning discharges; electromagnetic induction, etc.

The fire hazard of overvoltage, depending on the specific conditions, may include the following:

increasing the likelihood of a short circuit;
increasing the current load in certain sections of the electrical circuit and the possibility of overload;
increasing heat generation in electric heating devices;
increasing the likelihood of emergency conditions in incandescent lamps;
increasing the likelihood of failure of individual elements of household electrical consumers (TVs, radios, power supplies, etc.), as well as industrial electrical equipment.

Main reasons

All causes of electrical fires are based on two factors - poor electrical contact or its occurrence where it should not exist, that is, a short circuit. The first causes excessive heating at the site of poor contact. The second leads to an instant increase in current, the magnitude of which is proportional to the network resistance. Due to the exclusion of household appliances or other load from the circuit during a short circuit, the resistance becomes negligible, and the current increases to tens and hundreds of amperes.

In practice, the causes of wiring fires are:

Aging of wire insulation is especially typical for old wiring, where, due to natural processes, the dielectric material has already lost its basic functions (cracked, crumbled), which determines the presence of channels for the flow of current. When the insulation is completely destroyed, channels appear that accumulate dirt, which is a conductor, and cause a short circuit with subsequent fire in the wiring.
Contact failure can be characterized by both oxidation of the contacting surfaces of conductors and their weakening. In most cases, both factors occur due to excessive heat. With thermal expansion, the metal is deformed each time, which reduces the contact area. Also, heated metal interacts much better with surrounding substances and undergoes ionization.
Connecting an aluminum cable together with a copper cable - in the case of such installation of electrical wiring, diffusion of charged particles occurs from aluminum to copper wires. Because of this, aluminum deteriorates over time, which significantly worsens contact. This leads to heating and poses a further risk of fire.

Figure 1: Copper and Aluminum Connection
Laying wiring over flammable elements - typical for plastic, wood or paper materials with which the wiring has direct contact. When it is heated, smoldering and subsequent fire of such a structure are possible. This factor is especially dangerous in the case of hidden wiring, when the initial stages of a fire remain hidden inside the wall.
The cross-section of the wires does not match the connected network load - excessive load leads to overheating of the wires. In this case, the wire breaks due to thermal destruction. In places where the insulation is weakened or in the presence of flammable elements, such overheating will lead to burnout of both the conductor itself and the surrounding structures.
Malfunction of electrical appliances causes short-circuit currents to flow through the wires. Since some devices provide reinforced insulation, which is much newer and better than that on wires, the wiring burns out much faster and can lead to a fire in the wiring and a further fire.
Connecting non-standard devices - the lack of any testing before putting them into operation, the use of ineffective insulation or installation on a flammable base makes them potentially dangerous. Because of this, it is strictly prohibited to connect devices of dubious origin in order to avoid their fire or fire of the wiring.
Failure or non-compliance of protection devices - to prevent any consequences from short circuits, a machine is installed at the entrance to the apartment. If the circuit breaker has a much higher permissible limit than the cable can withstand, then gradual overheating of the wiring occurs with destruction of the insulation and thinning of the wire. Which will ultimately lead to a fire in the wiring.
Breakage of sockets or plugs - in places where devices are connected to the electrical network, contacts may become loose, the housing may be destroyed, fasteners will loosen, and other damage may occur, leading to excessive heating in these units. Such problems are characterized by sparking when the electrical appliance is turned on, a corresponding cracking sound, or may be felt as excessive temperature when the plug is touched.

It should also be noted that even a slight thermal effect on the insulation leads to its gradual destruction. And when a certain temperature level is reached, the dielectric properties decrease significantly due to the ionization of inclusions in the insulation, leakage current, melting and subsequent fire may occur. What to consider when turning on multiple powerful devices at the same time.

Transfer of electric current to grounded metal structures

The transition of electric current to metal grounded structures of buildings and structures that have an electrical connection to the ground (roofs, drainpipes, heating and water supply system pipes, metal beams, meshes under a layer of plaster, etc.) occurs as a result of their contact with one of live phase wires. In the event of contact between them, significant duck currents are generated, which can lead to operation of the electrical protection if it is selected correctly. In this case, the danger of electric current passing to metal structures is limited to the point where the wire touches the structure, where significant sparking and a short-term occurrence of an electric arc are possible, which can ignite nearby combustible materials.

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Lightning protection of buildings, structures, equipment and communications

If there is a transition of electric current to metal structures that do not have good grounding and a sufficiently tight connection of individual parts to each other, then large transition resistances arise along the path of the current, and periodic breakdown of the air gap or constant sparking is possible. In this case, combustion is possible both from heating of metal parts and from sparking. Heating and sparking can be so strong that individual sections of metal structures can melt. With this phenomenon, the leakage current may be insufficient to trigger even the correctly selected protection.

It is characteristic that heating of metal structures and sparking can occur not only in the place where the contact of the electric wire with parts of the building is detected, but in completely other areas where there are no electrical switches, sometimes several hundred meters away from the point of contact. Fires caused by the spread of electric current through the metal structures of buildings are characterized by the possible presence of several sources. In this case, a fire can even occur in different buildings.

The transfer of electric current to metal structures is possible:

when an overhead power line wire breaks;
in case of mechanical damage to the insulation of electrical wires laid on metal structures and communications of buildings;
when using metal structures and communications as a return wire when carrying out electric welding work;
when using metal structures and building communications as grounding;
in case of destruction of insulators or damage to the insulation of wires in metal pipe supports at the entrance to buildings, etc.

The transfer of electric current is possible not only to the metal structures of the building, but also to other electrical networks. If this transition occurs in low-current lines, it can lead to their ignition and fire. Such a transition is possible in places where lines of different voltages are laid together, upon contact or intersection, if the insulation in them is damaged.

Electrical short circuits: causes and protective measures

Short circuits

(hereinafter referred to as short circuits) arise as a result of a violation of the insulation of live parts of electrical installations. Dangerous damage to cables and wiring can occur due to excessive stretching, kinks, in places where they are connected to electric motors or control devices, during excavation work, etc. If the insulation on the cable cores is damaged, current leaks occur, which then develop into short-circuit currents. Depending on the nature of the damage inside the cable, an emergency short circuit process may increase with the accompanying powerful release of sparks and flames into the environment.

The cause of a short circuit may be the whipping of overhead power line wires under the influence of wind and metal objects being thrown onto them. The occurrence of short circuits can be caused by erroneous actions of maintenance personnel during various operational switchings, inspections and repairs of electrical equipment.

The most effective measures to prevent short circuits are the correct selection, installation and operation of electrical networks, machines and devices. The design, type of execution, installation method and insulation class of the machines, devices, devices, cables, wires and other electrical equipment used must comply with the rated parameters of the network or electrical installation (current, load, voltage), environmental conditions and the requirements of the PUE (Electrical Installation Rules). In addition, electrical protection of networks and electrical equipment must be provided. The most effective protection devices are high-speed relays and switches, automatic circuit breakers and fuses.

Thermal effects and emergency operation of incandescent lamps

Incandescent lamp device

The main causes of fires from incandescent electric lamps are:

direct contact of flammable materials with the heated lamp bulb;
the effect of thermal radiation from a lamp on combustible materials;
emission of hot drops of a spiral formed under the influence of an arc between the electrodes or one of the electrodes and a burnt filament;
contact of heated particles of the spiral with flammable materials as a result of an explosion of the bulb of an incandescent lamp.

Fires caused by incandescent lamps can be caused by:

violation of the rules for operating incandescent lamps, for example, using them in fire hazardous areas without protective glass covers;
non-compliance with the minimum permissible distances from incandescent lamps to flammable and combustible materials, the use of paper lampshades, etc.;
poor-quality energy supply (sharp fluctuations in voltage in the electrical network, which can lead to an arc or explosion of the bulb).

The degree of heating of the bulbs of electric incandescent lamps depends on the distance from the filament to the bulb and on the power of the lamp. However, lower power lamps with small bulb sizes may have a higher temperature on the surface of the bulbs than larger, more powerful lamps. For industrially produced incandescent lamps with a power of 40 to 100 W, under normal operating conditions, the temperature on the surface of the bulb is in the range of 125-240 °C. But if heat accumulates (for example, contact with any materials), it can increase by several hundred degrees and lead to the ignition of flammable materials. For example, a 100 W incandescent lamp wrapped in cotton cloth takes just 5 minutes. may have a temperature on the surface of the flask of 350 ° C and lead to fire of the fabric.

Studies have shown that cotton, cotton wool and products made from them, located at a distance of up to 30 mm from the bulb of an incandescent lamp, can ignite within one hour.

An emergency mode in incandescent lamps and, as a result, rupture of the bulbs, the occurrence of an arc, melting of the electrodes and penetration of the lamp bulbs by drops of molten metal is possible with a significant increase in voltage in the electrical network, as well as due to the low quality of incandescent lamps (design and technological factors, for example, poor contact at the connecting a tungsten filament to a nickel electrode).

If the bulb of an incandescent lamp is destroyed, hot particles of the spiral may fall out and fall on flammable materials. When an electric arc is formed inside the bulb of an incandescent lamp, the contact of hot metal particles with combustible materials is possible not only when the lamp bulb is destroyed, but also when it is penetrated by molten metal particles. Studies have shown that when nickel electrodes are melted, drops of metal in 50% of cases melt through the bulb of an incandescent lamp, leaving holes with a diameter of 1 to 3 mm. When hot drops of nickel exit the bulb of an incandescent lamp into the atmosphere, they explode, forming a stream of approximately 4,000 particles. The temperature of nickel particles ranging in size from 0.5 to 3 mm is in the range of 1500-2200 °C, which represents a high fire hazard.

Topic No. 12. Causes of fires and fires from electrical installations

EMERGENCY OPERATING MODES IN ELECTRICAL INSTALLATIONS, LEADING TO FIRE.

Emergency operating mode of an electrical installation is an operating mode accompanied by a deviation of operating parameters from the maximum permissible values, characterized by damage, failure of electrical equipment, possible interruption of power supply, or posing a threat to human life.

The most common causes of emergency operation of an electric motor are damage to its windings caused by overheating, insulation breakdown or mechanical damage to the motor.

Overheating of the electric motor windings occurs in cases of loss of one of the supply phases, a decrease in the supply voltage, too much load on the shaft or its complete stop, insufficient cooling of the windings, high frequency of motor switching on or its starting under too much load.

Insulation breakdown most often occurs when the electric motor operates in conditions of high humidity, as a result of moistening of the insulation of the electric motor windings.

A common cause of mechanical damage to an electric motor is bearing wear, which causes axial displacement of the rotor relative to the stator.

Operating electric motors in emergency mode leads to expensive repairs or premature failure.

An analysis of fires that occur during the operation of electrical installations shows that their most common causes are:

— short circuits in electrical wiring and electrical equipment;

- ignition of flammable materials located in close proximity to electrical receivers, turned on for a long time and left without

supervision;

— current overloads of electrical wiring and electrical equipment;

— high transition resistances at contact connections;

— the appearance of stress on building structures and technological equipment;

- rupture of electric lamp bulbs and contact of hot particles of the filament with flammable materials, etc.

Short circuits

Short circuits occur as a result of failure of the insulation of live parts of electrical installations.

Dangerous damage to cables and wiring can occur due to excessive stretching, kinks, in places where they are connected to electric motors or control devices, during excavation work, etc. If the insulation on the cable cores is damaged, current leaks occur, which then develop into short circuit currents. Depending on the nature of the damage inside the cable, an emergency short circuit process may increase with the accompanying powerful release of sparks and flames into the environment.

Since many types of electrical equipment are not moisture- and dust-proof, industrial dust (especially conductive dust), chemically active substances and moisture penetrate into their shell and settle on the surface of electrical insulating parts and materials. Some heating parts of electrical equipment cool down when stopped, so water condensation often falls on them. All this can lead to damage and waterlogging of the insulation and cause excessive leakage currents, arcing short circuits, flashovers or short circuits of both insulated windings and other live parts.

The insulation of electrical installations can be damaged when exposed to high temperature or flame during a fire, due to overvoltage as a result of primary or secondary lightning, voltage transition from installations above 1000 V to installations up to 1000 V, etc.

The cause of a short circuit may be the lashing of overhead power line wires under the influence of wind and metal objects being thrown onto them. A short circuit can be caused by erroneous actions of maintenance personnel during various operational switchings, inspections and repairs of electrical equipment.

Short circuit prevention

The most effective prevention of short circuits is the correct selection, installation and operation of electrical networks, machines and devices. The design, type of execution, installation method and insulation class of the machines, devices, devices, cables, wires and other electrical equipment used must comply with the rated parameters of the network or electrical installation (current, load, voltage), environmental conditions and the requirements of the PUE (Electrical Installation Rules). Regular inspections, repairs, scheduled preventive and maintenance tests of electrical equipment in explosive installations, both upon acceptance and during operation, should be especially strictly observed. In addition, electrical protection of networks and electrical equipment must be provided. The main purpose of electrical protection is that the power supply to wiring damaged anywhere must be stopped before a dangerous development of the accident occurs. The most effective protection devices are high-speed relays and switches, automatic circuit breakers and fuses.

Overload

An overload is an emergency mode in which currents arise in the conductors of electrical networks, machines and devices that for a long time exceed the values allowed by the standards.

One of the types of transformation of electrical energy is its conversion into heat. Electric current in the conductors of electrical networks, machines and devices releases heat, which is dissipated in the surrounding space. Conductors can reach dangerous temperatures. Thus, for bare copper, aluminum and steel wires of overhead lines, the maximum permissible temperature should not exceed 70°C.

This is explained by the fact that with increasing temperature, oxidative processes intensify and oxides with high resistance are formed on the wires (especially in contact connections); The contact resistance increases, and therefore the heat generated in it. As the connection temperature increases, oxidation increases, and this can lead to complete destruction of the wire contact.

Overheating of insulated conductors, especially those with flammable insulation, is very dangerous, leading to accelerated wear (aging). Insulation aging is assessed in relative units. The unit is taken to be aging corresponding to operation at a temperature allowed by the standards for this type of insulation. For calculations, the experimentally established “eight-degree rule” is usually used. According to this rule, a prolonged increase in the temperature of the conductor above the permissible value for every 8 ° C leads to twice the wear of its insulation.

Experiments have shown that the service life of insulation in electric motors when heated to 100°C will be 10 - 15 years, and at 150°C it is reduced to l.5 - 2 months.

Aging of insulation is characterized by a decrease in its elasticity and mechanical strength. Severely aged insulation, under the influence of vibration during the operation of transformers, generators, electric motors, etc., begins to crack and break. The consequence of this may be an electrical breakdown of the insulation and damage to the electrical installation, and in the presence of combustible insulation and a fire and explosive environment - a fire or even an explosion.

The cause of overload may be incorrect calculation of conductors during design. If the cross-section of the conductors is underestimated, then when all the provided electrical receivers are turned on, an overload occurs. Overload may occur due to the additional inclusion of electrical receivers for which the network conductors are not designed.

Overload prevention

To avoid overload or its consequences, during design it is necessary to correctly select the cross-sections of network conductors according to the permissible current, as well as

electric motors and control devices.

During the operation of electrical networks, you cannot turn on additional electrical receivers if the network is not designed for this.

When operating machines and devices, they should not be allowed to heat up to temperatures exceeding the maximum permissible.

To protect electrical installations from overload currents, the most effective are circuit breakers, thermal relays of magnetic starters and fuses.

Transient resistances Transient resistances are called resistances at places where current passes from one contact surface to another through the areas of their actual contact. In such a contact connection, a certain amount of heat is released per unit time, proportional to the square of the current and the resistance of the areas of actual contact.

The amount of heat generated can be so significant that the transition resistances become very hot. Consequently, if heated contacts come into contact with flammable materials, they may ignite, and contact of these places with explosive concentrations of flammable dusts, gases and vapors of flammable liquids will cause an explosion.

Prevention of fires from contact resistances

To increase the areas of actual contact between the contacts, it is necessary to increase their compression forces by using elastic contacts or special steel springs. If the contact planes are pressed against each other with some force, the small tubercles in the places where the planes touch will be somewhat crushed, while the sizes of the contacting main areas will increase and new additional contact areas will appear. The contact resistance will decrease, and the heating of the contact device will also decrease.

To remove heat from points of contact and dissipate it into the environment, contacts with sufficient mass and cooling surface are required. Particular attention should be paid to the places where the wires are connected and connected to the contacts of the input devices of electrical receivers. At the removable ends, for convenience and reliability of contact, tips of various shapes and special clamps are used, which is especially important for aluminum wires. To ensure reliable contact, spring washers and sides are also provided to prevent aluminum from spreading. In areas subject to vibration, spring washers or locknuts must be used on any conductors. All contact connections must be accessible for inspection - they are systematically monitored during operation.

There are several ways to connect wires; the main ones are soldering, welding, mechanical connection under pressure (pressure testing). Soldering requires a heat source with a temperature sufficient to heat the connecting wires and melt the additional metal (tin or tin-lead solders). When soldering insulated wires, precautions should be taken to avoid damaging the insulation.

Wire welding (electric and flame) provides reliable electrical contact (which is especially important for aluminum wires), but it is a complex operation that requires a lot of experience. Connecting wires by soldering and welding is not allowed in rooms with an explosive atmosphere.

The most common method currently used is to connect wires using mechanical crimping using special pliers and a hydraulic press. This method provides good electrical contact, does not require a heat source and scarce solders, and is allowed in areas with explosive atmospheres.

The cores of wires and cables at junctions and branches must have the same insulation as in entire areas of these wires and cables. To reduce the effect of oxidation on contact resistance, opening contacts are designed in such a way that their opening and closing are accompanied by sliding (friction) of one contact over the other. In this case, the thin film of oxides is destroyed, removed from the area where the contacts actually touch, and self-cleaning of the contacts occurs.

Contacts made of copper, brass and bronze are protected from oxidation by tinning with a thin layer of tin or an alloy of tin and lead. Tinning of copper contacts is especially effective in outdoor installations, in damp rooms or those containing active gases and vapors and at air temperatures above 60°C. During operation, it is necessary to systematically ensure that the contacts of devices, machines, etc. fit tightly and with sufficient force to each other. A protective lubricant plays a significant role, protecting the contact surface from rapid oxidation.

Conclusion on the issue: Operation of electric motors in emergency mode leads to expensive repairs or premature failure.

HAZARD OF STATIC ELECTRICITY.

Electrostatic charges arise on the surfaces of some materials, both liquid and solid, as a result of a complex process of contact electrolysis.

“Electrolysis occurs when two dielectric materials, or a dielectric and a conductive material, rub against each other if the latter is insulated. When two dielectric materials are separated, electrical charges are separated, with the material having a higher dielectric constant being charged positively, and the material having a lower dielectric constant being charged negatively. The more the dielectric properties of materials differ, the more intense the separation and accumulation of charges occurs. No charges are formed on contacting materials with the same dielectric properties (dielectric constant).

The intensity of the formation of electric charges is determined by the difference in the electrical properties of materials in materials of electrical properties, as well as by the force and speed of friction. The greater the force and speed of friction and the greater the difference in electrical properties, the more intense the formation of electrical charges.

For example, electrostatic charges are formed on the body of a car moving in dry weather if the tire rubber has good insulating properties. As a result, an electrical voltage arises between the body and the ground, which can reach 10 kV (kilovolts) and lead to a spark when a person exits the car - a discharge through a person to the ground. Charges can arise during grinding, pouring and pneumatic transportation of solid materials, during transfusion, pumping through pipelines, transporting dielectric liquids (gasoline, kerosene) in tanks, when processing dielectric materials (hard rubber, plexiglass), when winding fabrics, paper, film (for example, polyethylene). When the rubber conveyor belt slips relative to the rollers or the drive belt relative to the pulley, electrical charges with a potential of up to 45 kV can arise.

In addition to friction, the cause of the formation of static charges is electrical induction, as a result of which bodies isolated from the ground in an external electric field acquire an electric charge. Induction electrolysis of electrically conductive objects is especially great. For example, on metal objects (car, etc.) isolated from the ground, in dry weather, under the influence of the electric field of high-voltage power lines or thunderclouds, significant electrical charges can form.

On monitor and TV screens, positive charges accumulate under the action of an electron beam created by a cathode ray tube.

Dangerous and harmful factors of static electricity

When a person touches an object carrying an electric charge, the latter is discharged through the human body. The magnitudes of the currents arising during discharge are small and very short-lived. Therefore, electrical injuries do not occur. However, the discharge, as a rule, causes a reflexive movement of a person, which in some cases can lead to sudden movement and a person falling from a height.

In addition, when a charge with a high electric potential is formed, an electric field of increased intensity is created around them, which is harmful to humans. When a person stays in such a field for a long time, functional changes are observed in the central nervous, cardiovascular and other systems.

“People working in an area exposed to an electrostatic field have a variety of complaints: irritability, headaches, sleep disturbances, loss of appetite, etc. They are characterized by peculiar “phobias” caused by the fear of the expected discharge. The tendency to “phobias” is usually combined with increased emotional excitability.”

The beneficial effect on well-being of removing excess electrostatic charge from the human body (grounding, walking barefoot) has also been established.

The greatest danger of electrostatic charges is that the spark discharge can have sufficient energy to ignite a flammable or explosive mixture. A spark that occurs when electrostatic charges are discharged is a common cause of fires and explosions.

Thus, removing dust from dielectric material from a room using exhaust ventilation can lead to the accumulation of electrostatic charges and dust deposits in flues. The appearance of a spark discharge in this case can lead to ignition or explosion of dust. There are known cases of very serious accidents at enterprises as a result of explosions in ventilation systems.

When flammable liquids are transported, pumped through pipelines, drained from a tank, or due to splashing of the liquid, electrostatic charges accumulate and a spark may occur that ignites the liquid.

Static electricity poses the greatest danger in production and transport, especially in the presence of fire and explosive mixtures, dust and vapors of flammable liquids.

In everyday conditions (for example, when walking on a carpet), small charges accumulate, and the energy of the resulting spark discharges is not enough to initiate a fire under normal living conditions.

Anti-static electricity

Permissible levels of electrostatic field strength are established in GOST 12.1.045-84. "Electrostatic fields. Permissible levels at workplaces and requirements for monitoring.” Permissible field strength levels depend on the time spent at workplaces. The maximum permissible level of electrostatic field strength is 60 kV/m per 1 hour.

The use of protective equipment for workers is mandatory in cases where the actual levels of electrostatic field strength at workplaces exceed 60 kV/m.

When choosing means of protection against static electricity, the features of technological processes, the physico-chemical properties of the material being processed, the microclimate of the premises, etc. must be taken into account, which determines a differentiated approach to the development of protective measures.

Protection against static electricity is carried out in two ways:

• reducing the intensity of the formation of electrical charges;

• eliminating the resulting charges of static electricity.

Reducing the intensity of the formation of electrical charges is achieved by reducing the speed and force of friction, differences in the dielectric properties of materials and increasing their electrical conductivity. Reducing the friction force is achieved by lubrication, reducing the roughness and contact area of the interacting surfaces. Friction rates are limited by reducing the speed of processing and transporting materials.

Since static electricity charges are formed during splashing, spraying and spraying of dielectric liquids, it is desirable to eliminate these processes or at least limit them. For example, “filling tanks with dielectric liquids by a freely falling stream is not allowed. The drain hose must be lowered below the liquid level or, in extreme cases, the stream must be directed along the wall to avoid splashing.”

Since the intensity of charge formation is higher, the lower the electrical conductivity of the material, it is desirable to use, if possible, materials with higher electrical conductivity or increase their electrical conductivity by introducing electrically conductive (antistatic) additives. So, to cover floors you need to use antistatic linoleum; it is advisable to periodically carry out antistatic treatment of carpets, carpet materials, synthetic fabrics and materials using household chemicals.

It is preferable to make contacting objects and substances from the same material, since in this case contact electrolysis will not occur. For example, it is advisable to store polyethylene powder in polyethylene barrels, and pour and transport it through polyethylene hoses and pipelines. If this is not possible, then materials that are similar in their dielectric properties are used. For example, electrification in a fluoroplastic-polyethylene pair is less than in a fluoroplastic-ebonite pair.

Thus, to protect against static electricity, it is necessary to use weakly electrified or non-electrified materials, eliminate or limit friction, spraying, splashing, and splashing of dielectric liquids.

“Elimination of static electricity charges is achieved primarily by grounding equipment housings. Grounding for the removal of static electricity can be combined with protective grounding of electrical equipment. If grounding is used only to remove static electricity, then its electrical resistance can be significantly greater than for the protective resistance of electrical equipment (up to 100 Ohms). Even a thin wire is enough for electrical charges to constantly flow into the ground.”

To remove static electricity from the car body, an electrically conductive strip is used - “antistatic”, attached to the bottom of the car. If, when leaving the car, you notice that the body is “sparking,” discharge the body by touching it with a metal object, for example, the ignition key. It is not dangerous for humans. Be sure to do this if you are going to fill your car with gasoline.

Airplanes are equipped with metal cables attached to the landing gear and fuselage bottoms, which allows static charges generated during flight to be removed from the body during landing.

To remove electrical charges, the protective screens of computer monitors are grounded. Gasoline tankers are equipped with grounding switches in the form of circuits that are constantly in contact with the ground when the vehicle is moving. When draining gasoline into tanks at a gas station, the tanker vehicle and the gasoline drain system must be additionally grounded.

Moist air has sufficient electrical conductivity for the resulting electrical charges to flow through it. Therefore, in a humid air environment, practically no electrostatic charges are formed, and air humidification is one of the simplest and most common methods of combating static electricity.

Another common method for eliminating electrostatic charges is air ionization. The ions generated during operation of the ionizer neutralize static electricity charges. Thus, household air ionizers not only improve the aeroionic composition of the indoor air, but also eliminate electrostatic charges formed in dry air on carpets, synthetic carpets, and clothing. In production, special powerful air ionizers of various designs are used, but electric ionizers are the most common.

Antistatic shoes, antistatic gowns, grounding bracelets to protect hands and other means that provide electrostatic grounding of the human body can be used as personal protective equipment.

Conclusion on the issue: When a person touches an object carrying an electric charge, the latter is discharged through the human body.

LIGHTNING PROTECTION OF BUILDINGS AND STRUCTURES.

Lightning is a giant electrical spark discharge in the atmosphere that can usually occur during a thunderstorm, resulting in a bright flash of light and accompanying thunder. The current in a lightning discharge reaches 10-300 thousand amperes, the voltage ranges from tens of millions to billions of volts.

Lightning is an electrical discharge several kilometers long, developing between a thundercloud and the ground or any ground structure, between differently charged parts of the cloud or neighboring clouds.

Lightning is a serious threat to human life. A person or animal being struck by lightning often occurs in open spaces, since the electric current travels along the shortest path “thundercloud-ground”. Often lightning strikes trees and transformer installations on the railway, causing them to catch fire. It is impossible to be struck by ordinary linear lightning inside a building, but there is an opinion that so-called ball lightning can penetrate through cracks and open windows. Normal lightning is dangerous for television and radio antennas located on the roofs of high-rise buildings, as well as for network equipment.

Depending on the charge that lightning delivers to the ground, negative and positive lightning are distinguished.

Throughout Russia, approximately 90% of lightning is negative and 10% is positive)

Types of lightning: Downward (damage to ground objects), Ascending (damage to high-rise structures) Intercloud (damage to aircraft)

Lightning Formation: The development process of ground lightning consists of several stages.

At the first stage, in the zone where the electric field reaches a critical value, impact ionization begins, created initially by free charges, always present in small quantities in the air, which, under the influence of the electric field, acquire

significant speeds towards the ground and, colliding with the molecules that make up the air, ionize them.

Thus, electron avalanches arise, turning into threads of electric discharges - streamers, which are well-conducting channels, which, merging, give rise to a bright thermally ionized channel with high conductivity - a stepped lightning leader. The movement of the leader towards the earth's surface occurs in steps of several tens of meters at a speed of ~ 50,000 kilometers per second, after which its movement stops for several tens of microseconds, and the glow greatly weakens; then, in the subsequent stage, the leader again advances several tens of meters. A bright glow covers all the steps passed; then a stop and weakening of the glow follows again. These processes are repeated as the leader moves to the surface of the earth at an average speed of 200,000 meters per second.

As the leader moves toward the ground, the field intensity at its end increases and under its action, a response streamer is ejected from objects protruding on the surface of the Earth, connecting to the leader.

In the final stage, a reverse (from bottom to top), or main, lightning discharge follows along the channel ionized by the leader, characterized by currents from tens to hundreds of thousands of amperes, a brightness noticeably exceeding the brightness of the leader, and a high speed of progress, initially reaching ~ 100,000 kilometers per second , and at the end decreasing to ~ 10,000 kilometers per second. The channel temperature during the main discharge can exceed 20000-30000 °C. The length of the lightning channel can be from 1 to 10 km, the diameter can be several centimeters.

Lightning Hazard

1. Direct hit. Thermal effects (overheating, penetration of metal surfaces; ignition of fire and explosive mixtures).

Mechanical (shock wave propagating from the lightning channel; electrodynamic forces acting

on conductors, local destruction of solid non-combustible material, splitting of wooden structures and trees).

Electrical (electric shock to people or animals; the appearance of overvoltages on elements of an object struck by lightning)

2. Secondary impact is associated with the effect of close discharges on the object of the electromagnetic field.

Electrostatic induction manifests itself in the form of overvoltage that occurs on the metal structures of an object and depends on the lightning current, the distance to the strike site and the resistance of the ground electrode. In the absence of a proper grounding system, overvoltage can reach hundreds of kilovolts and create a danger of injury to people and overlaps between different parts of the facility.

Another type of dangerous impact of lightning is high potential drift. It is an overvoltage that occurs on communications during direct and close lightning strikes and spreads in the form of a wave impinging on the object.

Means and methods of lightning protection

Lightning protection is a set of measures aimed at preventing a direct lightning strike on an object or eliminating the dangerous consequences associated with a direct lightning strike; This complex also includes protective equipment that protects the object from the secondary effects of lightning and the introduction of high potential.

A means of protection against direct lightning strikes is a lightning rod - a device designed for direct contact with the lightning channel and discharging its current into the ground.

Lightning rods are divided into free-standing ones, which ensure the spread of lightning current bypassing the object, and installed on the object itself.

In this case, the current spreads along controlled paths so that there is a low probability of injury to people (animals), explosion or fire.

The lightning rod consists of the following elements:

lightning rod, support, down conductor and grounding conductor. However, in practice they can form a single structure, for example, a metal mast or truss of a building is an air terminal, a support and a down conductor at the same time.

Based on the type of lightning rod, lightning rods are divided into rod (vertical), cable (horizontal extended) and meshes consisting of longitudinal and transverse horizontal electrodes connected at intersections. Rod and cable lightning rods can be either free-standing or installed on site; lightning protection grids are laid on the non-metallic roof of protected buildings and structures. However, laying nets is rational only on buildings with horizontal roofs, where any part of them is equally likely to be struck by lightning.

In all possible cases, nearby high structures must be used as free-standing lightning rods, and structural elements of buildings and structures, such as metal roofing, trusses, metal and reinforced concrete columns and foundations, as lightning rods, down conductors and grounding conductors. Protection against the thermal effects of a direct lightning strike is carried out by properly selecting the cross-sections of lightning rods and down conductors, the thickness of the casings of outdoor installations, the melting and penetration of which cannot occur under the above parameters of lightning current, transferred charge and temperature in the channel.

Protection against secondary effects of lightning is ensured by the following measures. From electrostatic induction and the introduction of high potential - by limiting overvoltages induced on equipment, metal structures and input communications, by connecting them to ground electrodes of certain designs; from electromagnetic induction by limiting the area of open circuits inside buildings by placing jumpers in places where metal communications come together. To avoid sparking at the junctions of extended metal communications, low transition resistances of no more than 0.03 Ohm are ensured; for example, in flanged pipeline connections, this requirement is met by tightening six bolts on each flange.

In the IEC 1024-1-1 standard “Lightning protection of structures. Part 1. General provisions. Section 1. Guide A - Selection of protection levels (categories) for lightning protection systems" establishes four categories of lightning protection with the effectiveness of protection systems accordingly:

Category I - 98%

Category II - 95%

III category - 90%

IV category - 80%.

Conclusion on the issue: In accordance with the classification of buildings and structures adopted in Russia according to the conditions for protecting them from the effects of lightning, depending on the degree of danger of lightning damage and the choice of necessary protective measures, all buildings and structures are divided into categories.

Conclusion on the topic: Lightning is a serious threat to human life. Therefore, lightning protection of buildings and structures in the modern world is a necessary condition for a security system.

Emergency operation of fluorescent lamps

Fluorescent lamp

Fire safety of fluorescent lamps means the practical impossibility of fire, both the lamp itself and its environment, which must be ensured by the design of the lamp, the choice of components and materials with temperature characteristics corresponding to the thermal operating conditions of the lamp. In this case, fire safety characteristics are the compliance of the temperature on the main elements of the lighting device with acceptable values, both in operating and emergency modes of its operation.

Let's consider the possible reasons for the appearance of high temperatures on fluorescent lamps with standard electromagnetic ballasts (ballasts). From the point of view of the physical process of producing light, fluorescent lamps convert a larger portion of electricity into visible light radiation than incandescent lamps. However, under certain conditions associated with malfunctions of the ballasts of fluorescent lamps, their strong heating is possible (in some cases up to 190-200 ° C), resulting in softening and leakage of the filling compound, leading to the ignition of the polymer diffusers of the fluorescent lamp.

Starters pose a certain fire hazard, because... inside some of them there are flammable materials (paper capacitor, cardboard gaskets, etc.).

An example of a fire caused by emergency operation of a fluorescent lamp control gear is a fire that occurred on March 26, 2012 in kindergarten No. 262 of OJSC Omsk. As a result of the emergency operation of the control gear, the diffuser of the light device caught fire, it collapsed onto the floor and the subsequent fire of the floor covering.