The concept of static electricity is familiar to everyone from the school physics course. Static electricity occurs when charges appear on conductors and surfaces various items. They appear as a result of friction that occurs when objects come into contact.
What is static electricity?
All substances are made up of atoms. An atom has a nucleus around which there are equal numbers of electrons and protons. They are able to move from one atom to another. When moving, negative and positive ions are formed. Their imbalance leads to static. The static charge of protons and electrons in an atom is the same, but has different polarity.
Static appears in everyday life. Static discharge can occur at low currents but high voltages. In this case there is no danger to people, but the discharge is dangerous for electrical appliances. During the discharge, microprocessors, transistors and other circuit elements are damaged.
Causes of static electricity
Static occurs under the following conditions:
- contact or separation of two different materials from each other;
- sudden changes in temperature;
- radiation, UV radiation, x-rays;
- operation of paper cutting machines and cutting machines.
Static often occurs during or before a thunderstorm. Thunderclouds, when moving through moisture-saturated air, generate static electricity. The discharge occurs between the cloud and the ground, between individual clouds. The device of lightning rods helps to conduct the charge into the ground. Thunderclouds create an electrical potential on metal objects that cause mild shocks when touched. The impact is not dangerous for humans, but a powerful spark can cause some objects to ignite.
Each resident has repeatedly heard the crash that is heard when removing clothes, the impact of touching the car. This is a consequence of the appearance of statics. An electric discharge can be felt when cutting paper, combing hair, or pouring gasoline. Free charges accompany a person everywhere. Using various electrical devices increases their appearance. They occur when pouring and crushing solid products, pumping or pouring flammable liquids, when transporting them in tanks, when winding up paper, fabrics and films.
Charge appears as a result of electrical induction. On metal car bodies in dry time years, large electrical charges are created. A TV screen or computer monitor can be charged by exposure to a beam created in a cathode ray tube.
Harm and benefits of static electricity
Many scientists and inventors tried to use static charge. Cumbersome units were created, the benefits of which were low. The discovery of corona discharge by scientists turned out to be useful. It is widely used in industry. Using an electrostatic charge, complex surfaces are painted and gases are purified from impurities. This is all good, but there are also numerous problems. Electric shocks can be very powerful. They can sometimes infect humans. This happens both at home and in the workplace.
The harm of static electricity manifests itself in shocks of varying power when removing a synthetic sweater, getting out of a car, turning on and off a food processor and vacuum cleaner, laptop and microwave oven. These blows can be harmful.
Static electricity occurs, which affects the functioning of the cardiovascular and nervous systems. You should protect yourself from it. The person himself is also often a carrier of charges. When contacting the surfaces of electrical appliances, they become electrified. If this is a control and measuring device, the matter may end in its failure.
The discharge current brought by a person, with its heat, destroys connections, breaks the tracks of microcircuits, and destroys the film of field-effect transistors. As a result, the circuit becomes unusable. Most often, this does not happen immediately, but at any stage during the operation of the tool.
In factories that process paper, plastics, and textiles, materials often behave incorrectly. They stick to each other, stick to various types equipment, are pushed off, collect a lot of dust on themselves, and are wound incorrectly on reels or bobbins. The reason for this is the occurrence of static electricity. Two charges of equal polarity repel each other. Others, one of which is positively charged and the other negatively charged, attract each other. Charged materials behave the same way.
In printing plants and other places where flammable solvents are used, a fire may occur. This occurs when the operator is wearing shoes with non-conductive soles and the equipment does not have proper grounding. The ability to ignite depends on the following factors:
- discharge type;
- discharge power;
- static discharge source;
- energy;
- the presence of solvents or other flammable liquids nearby.
Discharges can be spark, hand, or sliding hand. A spark discharge comes from a person. Carpal occurs on pointed parts of equipment. Its energy is so low that it poses virtually no fire hazard. Sliding brush discharge occurs on synthetic sheets, as well as on roll materials with different charges on each side of the canvas. It poses the same danger as a spark discharge.
Lethality -- main question for safety professionals. If a person holds on to the bobbin and is himself in the tension zone, his body will also be charged. To remove the charge, be sure to touch ground or grounded equipment. Only then will the charge go into the ground. But the person will receive a strong or weak electric shock. The result is reflexive movements that sometimes lead to injury.
A long stay in a charged zone leads to a person’s irritability, decreased appetite, and poor sleep.
Dust from production premises removed by ventilation. It accumulates in pipes and can ignite from a static spark discharge.
How to remove static electricity from a person
The simplest means of protection against it is to ground the equipment. In production conditions, screens and other devices are used for this purpose. IN liquid substances special solvents and additives are used. Antistatic solutions are actively used. These are substances with low molecular weight. The molecules in the antistatic agent move easily and react with moisture in the air. Due to this characteristic, static is removed from a person.
If the operator's shoes have non-conductive soles, he must always touch the ground. Then the flow of static current into the ground cannot be stopped, but the person will receive a strong or weak shock. We feel the effect of static current after walking on carpets and rugs. Drivers getting out of the car receive electric shocks. It’s easy to get rid of this problem: just touch the door with your hand while sitting still. The charge will flow into the ground.
Ionization helps a lot. This is done using an antistatic strip. It has many needles made of special alloys. Under the influence of a current of 4-7 kV, the air around is decomposed into ions. Air knives are also used. They are an antistatic strip through which air is blown and cleans the surface. Static charges are actively formed when liquids with dielectric properties are splashed. Therefore, to reduce the effect of electrons, a falling jet should not be allowed.
It is advisable to use antistatic linoleum on the floor and clean it more often using household chemicals. At enterprises involved in the processing of fabrics or paper, the problem of getting rid of static is solved by wetting the materials. Increasing humidity prevents harmful electricity from accumulating.
To remove static, you must:
- humidify the air in the room;
- treat carpets and rugs with antistatic agents;
- wipe the seats in the car and in the rooms with antistatic wipes;
- moisturize your skin more often;
- refuse synthetic clothing;
- wear shoes with leather soles;
- prevent the appearance of static on laundry after washing.
Indoor flowers, a boiling kettle, and special devices moisturize the atmosphere well. Antistatic compounds are sold in household chemical stores. They are sprayed over the carpet surface. You can make an antistatic agent yourself. To do this, take fabric softener (1 cap) and pour it into a bottle. Then the container is filled clean water, which is sprayed over the surface of the carpet. Wipes moistened with an antistatic agent neutralize charges on seat upholstery.
Moisturize the skin with lotion after a shower. Hands are wiped several times a day. You should change your clothes to natural ones. If it is charging, treat it with antistatic agents. It is recommended to wear shoes with leather soles or walk around the house barefoot. Before washing, it is advisable to sprinkle ¼ cup of baking soda on your clothes. It removes electrical discharges and softens the fabric. When rinsing clothes, you can add vinegar (¼ cup) to the machine. It is better to dry clothes in the fresh air.
All of the above measures help neutralize static problems.
Electrostatic charges arise on the surfaces of some rubbing materials, both liquid and solid, as a result of a complex process of contact electrification. Electrification occurs when two dielectric or dielectric and conductive materials rub together if the latter is insulated.
The intensity of the formation of electric charges is determined by the difference in the electrical properties of materials, 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 and lead to a spark when a person exits the car - a discharge through the person to the ground.
In production, in various technological processes, large electrical charges are also formed, the potentials of which can reach tens of kilovolts, for example, during grinding, pouring and pneumatic transportation of solid materials, during transfusion, pumping through pipelines, transporting dielectric liquids (gasoline, kerosene, etc.) in tanks. When the rubber conveyor belt slips relative to the rollers or the belt drive 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 (cars, etc.) isolated from the ground, in dry weather under the influence of an electric FIELD high voltage lines power lines or thunderclouds can generate significant electrical charges.
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 not large, and they are 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 a sudden movement of the hand, a fall from a height, or ending up in a dangerous production area.
The greatest danger of electrostatic charges is that the spark discharge can have sufficient energy to ignite a flammable or explosive mixture. The spark that occurs when electrostatic charges are discharged is common cause fires and explosions. At a voltage of 3 kV, a spark discharge can cause ignition of almost all steam and gas-air mixtures; at 5 kV - ignition of most combustible dusts.
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 living 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 in normal conditions everyday life
To protect against static electricity use:
- a method that eliminates or reduces the intensity of the formation of static electricity charges;
- a method that eliminates forming charges.
First method is most effective and is carried out through the selection of pairs of materials for machine elements that interact with each other with friction. Another way to neutralize static electricity charges is to mix materials that, when interacting with elements of equipment, are charged differently. For example, when a material consisting of 40% nylon and 60% dacron is rubbed against a chrome-plated surface, no electrolysis is observed.
A decrease in the intensity of the formation of electrostatic charges is facilitated by a decrease in the force and speed of friction and the roughness of interacting surfaces. For this purpose, when transporting flammable liquids with high electrical resistivity through pipelines (for example, gasoline, kerosene, etc.), maximum pumping speeds are regulated. Pouring such liquids into containers with a jet freely falling onto the surface of the liquid is not allowed: the drain hose is buried under the surface of the liquid being drained.
The main method of implementation second method is the grounding of electrically conductive parts of technological equipment to discharge the resulting charges of static electricity into the ground. For this purpose, you can use conventional protective grounding designed to protect against damage. electric shock. If it is impossible to ground the elements of machines and equipment, electrically conductive coatings (antistatic agents) are applied to their surface, and fabric materials (for example, filters) are subjected to special impregnation, which increases their electrical conductivity. Grounding of gas ducts is extremely important ventilation systems, through which dusty air is transported.
To increase the intensity of static charges draining from machine elements, the air in the room where they are installed is humidified.
The daily activity of any person is connected with his movement in space. At the same time, he not only walks, but also travels by transport.
During any movement, a redistribution of static charges occurs, changing the balance of internal equilibrium between the atoms and electrons of each substance. It is associated with the process of electrification, the formation of static electricity.
U solids The distribution of charges occurs due to the movement of electrons, and in liquids and gases - both electrons and charged ions. All of them together create a potential difference.
Reasons for the formation of static electricity
The most common examples of the manifestation of static forces are explained in school during the first physics lessons, when they rub glass and ebonite rods on woolen fabric and demonstrate the attraction of small pieces of paper to them.
There is also known experience in deflecting a thin stream of water under the influence of static charges concentrated on an ebonite rod.
In everyday life, static electricity manifests itself most often:
when wearing woolen or synthetic clothing;
walking in shoes with rubber soles or woolen socks on carpets and linoleum;
using plastic items.
The situation is aggravated by:
dry indoor air;
reinforced concrete walls from which multi-storey buildings are made.
How is a static charge created?
Typically, the physical body contains an equal number of positive and negative particles, due to which a balance is created in it, ensuring its neutral state. When it is violated, the body acquires an electrical charge of a certain sign.
Static means a state of rest when the body does not move. Polarization can occur inside its substance - the movement of charges from one part to another or their transfer from a nearby object.
Electrification of substances occurs due to the acquisition, removal or separation of charges when:
interaction of materials due to friction or rotation forces;
sudden temperature change;
irradiation different ways;
dividing or cutting physical bodies.
They are distributed over the surface of an object or at a distance from it of several interatomic distances. For ungrounded bodies they spread over the area of the contact layer, and for those connected to the ground loop they flow onto it.
The acquisition of static charges by the body and their drainage occur simultaneously. Electrification is ensured when the body receives a greater energy potential than it expends into the external environment.
A practical conclusion follows from this provision: to protect the body from static electricity, it is necessary to remove the acquired charges from it to the ground circuit.
Methods for assessing static electricity
Physical substances, based on their ability to form electrical charges of different signs when interacting with other bodies by friction, are characterized on the scale of the triboelectric effect. Some of them are shown in the picture.
The following facts can be cited as an example of their interaction:
walking in wool socks or rubber-soled shoes on dry carpet can charge human body up to 5÷-6 kV;
the body of a car driving on a dry road acquires a potential of up to 10 kV;
the drive belt rotating the pulley is charged up to 25 kV.
As we can see, the potential of static electricity reaches very high values even in domestic conditions. But he does not cause us much harm because he does not have high power, and its discharge passes through the high resistance of the contact pads and is measured in fractions of a milliampere or a little more.
In addition, it is significantly reduced by air humidity. Its effect on the amount of body stress upon contact with various materials is shown in the graph.
From his analysis, the conclusion follows: in a humid environment, static electricity manifests itself less. Therefore, various air humidifiers are used to combat it.
In nature, static electricity can reach enormous values. When clouds move over long distances, significant potentials accumulate between them, which manifest themselves as lightning, the energy of which is enough to split a century-old tree along the trunk or burn down a residential building.
When static electricity is discharged in everyday life, we feel “tingling” in our fingers, see sparks emanating from woolen items, and feel a decrease in vigor and efficiency. The current to which our body is exposed in everyday life has a negative effect on our well-being and condition. nervous system, but it does not cause obvious, visible damage.
Manufacturers of measuring industrial equipment produce devices that allow you to accurately determine the voltage value of accumulated static charges both on equipment housings and on the human body.
How to protect yourself from static electricity at home
Each of us must understand the processes that create static discharges that pose a threat to our body. They should be known and limited. For this purpose, various educational events are held, including popular television programs for the population.
On them available means shows ways to create static voltage, principles of its measurement and methods of performing preventive measures.
For example, given the triboelectric effect, it is best to use combs made of natural wood, and not metal or plastic, as most people do. Wood has neutral properties and does not form charges when rubbed against hair.
To remove static potential from the car body when driving on a dry road, use special antistatic tapes attached to the bottom. Various types of them are widely available on sale.
If there is no such protection on the car, then the voltage potential can be removed by briefly grounding the body through a metal object, for example, a car ignition key. It is especially important to perform this procedure before refueling.
When a static charge accumulates on clothing made of synthetic materials, it can be removed by treating the vapors from a special canister containing “Antistatic” composition. In general, it is better to use such fabrics less and wear natural materials made from linen or cotton.
Shoes with rubberized soles also contribute to the accumulation of charges. It is enough to put antistatic insoles in it from natural materials how the harmful effects on the body will be reduced.
The influence of dry air, characteristic of city apartments in winter time, has already been discussed. Special humidifiers or even small pieces of dampened cloth placed on the battery improve the situation and reduce the formation of static electricity. But regular execution wet cleaning indoors allows you to promptly remove electrified particles and dust. This is one of the best ways protection.
Household electrical devices During operation, static charges also accumulate on the body. A potential equalization system connected to the general grounding loop of the building is designed to reduce their impact. Even a simple acrylic bathtub or an old cast-iron structure with the same insert is susceptible to static and requires protection in a similar way.
How to protect against static electricity in production
Factors that reduce the performance of electronic equipment
Discharges that occur during the manufacture of semiconductor materials can cause great harm, disrupt the electrical characteristics of devices, or even disable them.
In production conditions, the discharge can be random and depend on a number of different factors:
the size of the formed capacity;
energy potential;
electrical resistance of contacts;
type of transient processes;
other accidents.
In this case, at the initial moment of about ten nanoseconds, the discharge current increases to a maximum, and then it decreases within 100÷300 ns.
The nature of the occurrence of a static discharge on a semiconductor device through the operator’s body is shown in the picture.
The magnitude of the current is influenced by: the charge capacity accumulated by a person, the resistance of his body and contact pads.
During the production of electrical equipment, a static discharge can be created without operator participation due to the formation of contacts through grounded surfaces.
In this case, the discharge current is affected by the charge capacity accumulated by the device body and the resistance of the formed contact pads. In this case, the semiconductor is initially simultaneously affected by the induced high voltage potential and the discharge current.
Due to this complex effect, damage can be:
1. obvious, when the performance of the elements is reduced to such an extent that they become unsuitable for use;
2. hidden - due to a reduction in output parameters, sometimes even falling within the established factory characteristics.
The second type of malfunction is difficult to detect: they most often result in loss of performance during operation.
An example of such damage from the action of high static voltage is demonstrated by graphs of the deviation of the current-voltage characteristics in relation to the KD522D diode and the integrated circuit BIS KR1005VI1.
The brown line numbered 1 shows the parameters of semiconductor devices before testing with increased voltage, and curves numbered 2 and 3 show their decrease under the influence of an increased induced potential. In case #3 it has a greater impact.
Damage may be caused by:
excessive induced voltage, which breaks through the dielectric layer of semiconductor devices or disrupts the structure of the crystal;
high density of flowing current, causing high temperatures, leading to melting of materials and burning of the oxide layer;
tests, electrical and thermal training.
Hidden damage may not affect performance immediately, but after several months or even years of operation.
Methods for implementing protection against static electricity in production
Depending on the type of industrial equipment, one of the following methods of maintaining operability or a combination of them is used:
1. eliminating the formation of electrostatic charges;
2. blocking their entry into the workplace;
3. increasing the resistance of devices and components to the action of discharges.
Methods No. 1 and No. 2 allow you to protect a large group of different devices in a complex, and No. 3 is used for individual devices.
High efficiency of maintaining the operability of the equipment is achieved by placing it inside a Faraday cage - a space fenced on all sides with fine mesh metal mesh connected to the ground loop. Externals do not penetrate inside it electric fields, and static magnetic is present.
Cables with a shielded sheath work on this principle.
Static protection is classified according to the principles of execution into:
physical and mechanical;
chemical;
structural and technological.
The first two methods allow you to prevent or reduce the formation of static charges and increase the rate of their drainage. The third technique protects devices from the effects of charges, but it does not affect their drainage.
The drainage of discharges can be improved by:
creation of coronation;
increasing the conductivity of materials on which charges accumulate.
These issues are resolved:
air ionization;
increasing working surfaces;
selection of materials with the best volume conductivity.
Due to their implementation, lines prepared in advance are created to drain static charges onto the ground loop, preventing them from reaching the working elements of devices. It is taken into account that the total electrical resistance of the created path should not exceed 10 Ohms.
If materials have high resistance, then protection is performed in other ways. Otherwise, charges begin to accumulate on the surface, which can be discharged upon contact with the ground.
An example of complex electrostatic protection of a workplace for an operator involved in the maintenance and adjustment of electronic devices is shown in the picture.
The table surface is connected to the ground loop through a connecting conductor and a conductive mat using special terminals. The operator works in special clothing, wears shoes with conductive soles and sits on a chair with a special seat. All these measures make it possible to efficiently discharge accumulated charges to the ground.
Working air ionizers regulate humidity and reduce the potential of static electricity. When using them, it is taken into account that the increased content of water vapor in the air negatively affects human health. Therefore, they try to maintain it at a level of about 40%.
Also effective way There may be regular ventilation of the room or the use of a ventilation system in it, when the air passes through filters, is ionized and mixed, thus ensuring the neutralization of emerging charges.
To reduce the potential accumulated by the human body, bracelets can be used to complement a set of antistatic clothing and shoes. They consist of a conductive strip that is attached to the arm using a buckle. The latter is connected to the ground wire.
With this method, the current flowing through human body. Its value should not exceed one milliamp. Larger values may cause pain and electrical injuries.
As the charge flows to the ground, it is important to ensure that it leaves at a rate of one second. For this purpose, floor coverings with low electrical resistance are used.
When working with semiconductor boards and electronic components, protection against damage by static electricity is also provided by:
forced shunting of the terminals of electronic boards and units during checks;
using tools and soldering irons with grounded working heads.
Containers with flammable liquids located on vehicles are grounded using a metal circuit. Even the fuselage of the aircraft is equipped with metal cables, which act as protection against static electricity during landing.
Permissible levels of electrostatic field strength are established in GOST 12.1.045-84. "Electrostatic fields. Permissible levels at workplaces and requirements for control." 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 following features must be taken into account: technological processes, physicochemical characteristics the material being processed, the microclimate of the premises, etc., 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 to remove static electricity can be combined with protective grounding 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, use an electrically conductive strip - “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. Special powerful air ionizers are used in production various designs, but the most common are electric ionizers.
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.
Static electricity is a set of phenomena associated with the emergence, preservation and relaxation of a free electric charge on the surface and in the volume of dielectric and semiconductor substances, materials, products or on insulated conductors.
The emergence of static electricity charges occurs during deformation, crushing of substances, relative movement of two bodies in contact, layers of liquid and bulk materials, during intense mixing, crystallization, and also due to induction.
The nervous, cardiovascular, neurohumoral and other systems of the body are most sensitive to electrostatic fields. This necessitates hygienic regulation of the maximum permissible intensity of the electrostatic field.
An electrostatic field is characterized by a intensity determined by the ratio of the force acting in the field on a point electric charge to the magnitude of this charge. The unit of voltage measurement is volts per meter. The permissible level of electrostatic field strength is 60 kV/m. If the field strength exceeds this value, appropriate protective measures must be applied.
The widespread use of dielectric materials and organic compounds (polymers, paper, solid and liquid hydrocarbons, petroleum products, etc.) in all areas of economic activity is inevitably accompanied by the formation of static electricity charges, which not only complicate technological processes, but also often cause fires and explosions causing great material damage. This often leads to death.
Static electricity- this is a set of phenomena associated with the emergence, conservation and relaxation of a free electric charge on the surface or volume of dielectrics, or on insulated conductors (GOST 12.1.018). The formation and accumulation of charges on the processed material is associated with two the following conditions:
♦ the presence of surface contact, as a result of which a double electric layer is created, the appearance of which is associated with the transition of electrons in elementary donor-acceptor acts on the contact surface. The sign of the charge determines the unequal affinity of the surface material for electrons;
♦ at least one of the contacting surfaces must be made of dielectric material.
The main factors influencing the electrification of substances are their electrical properties and the rate of separation of surfaces. It has been experimentally established that the more intensively the process is carried out, i.e. The higher the detachment speed, the more charge remains on the surface.
The following ways of charging objects are known: direct contact with electrified materials, inductive and mixed charging.
Pure contact charging of surfaces includes, for example, electrification when pumping hydrocarbon fuels and solvents through pipelines. It is known that pipelines made of transparent dielectric material even glow when pumping liquids.
Along with contact, inductive charging of conductive objects and service personnel often occurs in the electric field of a moving flat electrified material.
Mixed charging is observed when electrified material enters any containers isolated from the ground. This type of charging is most often found when pouring flammable liquids into containers, when feeding rubber adhesives, fabrics, films in mobile containers, carts, etc. The formation of static electricity charges upon contact of a liquid body with a solid or one solid
one body with another largely depends on the density of contact of the rubbing surfaces, their physical state, speed and coefficient of friction, pressure in the contact zone, microclimate environment, the presence of external electric fields, etc.
Charges of static electricity can also accumulate on the human body (when working or coming into contact with electrified materials and products). The high surface resistance of human tissue makes it difficult for charges to drain, and a person can remain under high potential for a long time.
The main danger when electrifying various materials is the possibility of a spark discharge, both from a dielectric electrified surface and from an insulated conductive object.
Ignition of combustible mixtures by spark discharges of static electricity can occur if the energy released in the discharge is higher than the minimum ignition energy of the combustible mixture.
Along with fire hazard Static electricity also poses a danger to workers.
Light “pricks” when working with highly electrified materials have a harmful effect on the psyche of workers and in certain situations can contribute to injuries on technological equipment. Strong spark discharges, which occur, for example, when packaging granular materials, can lead to pain. Unpleasant sensations caused by static electricity can cause the development of neurasthenia, headaches, poor sleep, irritability, tingling in the heart, etc. In addition, with the constant passage of small electrification currents through the human body, adverse physiological changes in the body are possible, leading to occupational diseases. Systematic exposure to an electrostatic field of increased intensity can cause functional changes in the central nervous, cardiovascular and other systems of the body.
The use of artificial or synthetic fabrics for clothing also leads to the accumulation of static electricity charges on a person.
Static electricity also greatly affects the course of technological processes for obtaining and processing materials and the quality of products. At high charge densities, electrical breakdown of thin polymer films for electrical and radio engineering purposes may occur, which leads to defective products. Particularly damaging is caused by the adhesion of dust to polymer films caused by electrostatic attraction.
Electrification complicates processes such as sifting, drying, pneumatic transport, printing, transporting polymers, dielectric liquids, molding synthetic fibers, films, etc., automatic dosing of fine materials, since they stick to the walls of technological equipment and stick together.
When organizing production, processes accompanied by intensive generation of static electricity charges should be avoided. To do this, it is necessary to correctly select the friction surfaces and the speed of movement of substances, materials, devices, avoid the processes of splashing, crushing, atomization, purify flammable gases and liquids from impurities, etc.
Effective method reducing the intensity of static electricity generation is contact pair method. Most structural materials in terms of dielectric constant are located in triboelectric series in such a sequence that any of them acquires a negative charge upon contact with the material that follows in the series and a positive charge with the previous one. Moreover, with increasing distance in the row between two materials, the absolute value of the charge arising between them increases.
In accordance with GOST 12.4.124, collective and individual protective equipment is used.
Collective protection against static electricity according to the principle of action is divided into the following types: grounding devices, neutralizers, humidifying devices, anti-electrostatic substances, shielding devices.
Grounding refers to the basic methods of protection against static electricity and is the intentional electrical connection to ground or its equivalent of metallic non-live parts that may become energized. It is the simplest, but necessary means protection due to the fact that the energy of a spark discharge from conductive ungrounded elements of technological equipment is many times higher than the energy of a discharge from dielectrics.
GOST 12.4.124 prescribes that grounding must be used on all electrically conductive elements of process equipment and other objects where electrostatic charges may arise or accumulate, regardless of the use of other means of protection against static electricity. It is also necessary to ground metal ventilation ducts and thermal insulation casings of devices and pipelines located in workshops, outdoor installations, overpasses, and channels. Moreover, the indicated technological lines must be continuous throughout electrical circuit, which is connected to the ground loop at least at two points.
Special attention It is necessary to pay attention to the grounding of mobile objects or rotating elements of equipment that do not have constant contact with the ground. For example, mobile containers into which electrifying materials are poured or poured must be installed on grounded bases before filling or connected to the ground electrode with a special conductor before the hatch is opened.
Neutralization of charges static electricity is produced in cases where it is not possible to reduce the intensity of its formation by technological and other means. Neutralizers are used for this purpose. various types:
· corona discharge (induction and high-voltage);
· radioisotope with α- and β-emitting sources;
· combined, combining corona and radioisotope in one design
neutralizers;
· creating a flow of ionized air.
The simplest to implement are induction neutralizers. In most cases, they are a body or a rod with grounded spark gaps attached to them, which are needles, strings, brushes. These neutralizers use electric field, created by the electrified material itself.
To reduce the intensity of electrification of liquids, use strings or needle neutralizers, which, by increasing the conductivity of the medium, promote the flow of generated charges onto the grounded walls of pipelines (equipment) or the neutralizer housing.
IN high-voltage neutralizers Corona and sliding discharges, in contrast to induction ones, use a high voltage of up to 5 kV, supplied to the spark gap from an external power source. However, the need to use high voltage does not allow their use in explosive areas and industries.
In hazardous areas of all classes it is recommended to use radioisotope neutralizers based on α-emitting (plutonium-238, -239) HP type and β-emitting (tritium) LTSE type sources. These neutralizers are small-sized, simple in design and maintenance, have a long service life and are radiation safe. Their use in industry does not require approval from sanitary authorities.
In cases where the material (film, fabric, tape, sheet, etc.) is electrified with high intensity or moves at high speed and the use of radioisotope neutralizers does not ensure the neutralization of static electricity, combined induction radioisotope neutralizers type NRI. They are a combination of radioisotope and induction (needle) neutralizers or explosion-proof induction, high-voltage (DC and AC), high-frequency neutralizers.
Very promising are pneumoelectric neutralizers brands VEN-0.5 and VEN-1.0 and pneumoradioisotopic PRIN brands, in which ionized air or any gas is directed towards the electrified material. Such neutralizers not only have an increased range of action (up to 1 m), but also ensure the neutralization of volumetric charges in pneumatic transport systems, fluidized bed apparatus, in bunkers, as well as the neutralization of static electricity on the surfaces of products of complex shape. Devices for supplying ionized air in this case to explosive premises must have a grounded metal screen along its entire length.
In some cases it is effective to use radiation neutralizers static electricity, which provide ionization of a material or environment under the influence of ultraviolet, laser, thermal, electromagnetic and other types of radiation.
To reduce the specific volumetric electrical resistance, various soluble substances are introduced into dielectric liquids and solutions of polymers (adhesives). anti-electrostatic additives (antistatics), in particular, metal salts of variable valence, higher carboxylic acids, naphthenic and synthetic fatty acids. Such additives include Sigbol, ASP-1, ASP-2, as well as additives based on chromium, cobalt, copper oleates, naphthenates of these metals, chromium salts and FFA, etc. Abroad, additives developed by Ecco and Shell (ASA-3 additive) have found the greatest use.
Electrical resistance of solids polymer materials(plastics, rubbers, plastics, etc.) can be reduced by introducing various electrically conductive materials into their composition (carbon black, powders, etc.).
In explosive industries, in order to prevent dangerous spark discharges of static electricity that occur on the human body during contact or inductive charging with electrified materials or clothing items, it is necessary to ensure that these charges are buried in the ground. Non-conductive coatings include asphalt, rubber, linoleum, etc. Conductive coatings include concrete, foam concrete, xylolite, etc. Grounded platforms and working platforms, door handles, handrails of stairs, handles of devices, machines, mechanisms, devices are additional funds removal of charges from the human body.
TO individual means Protection against static electricity includes special electrostatic shoes and clothing.
In some cases, continuous removal of static electricity charges from a person’s hands can be carried out using special grounded bracelets and rings. At the same time, they must provide electrical resistance in the human-earth circuit and freedom of movement of the hands.