Introduction
The electrical saturation of modern production creates an electrical hazard, the source of which can be electrical networks, electrified equipment and tools, computer and organizational equipment that runs on electricity. This determines the relevance of the problem of electrical safety - the elimination of electrical injuries.
Electrical safety is a system of organizational and technical measures and means that protect people from the harmful and dangerous effects of electric current, electric arc, electromagnetic field and static electricity.
In comparison with other types of industrial injuries, electrical injuries make up a small percentage, however, it occupies one of the first places in terms of the number of injuries with a severe and especially fatal outcome.
An analysis of industrial injuries in the meat industry shows that, on average, about 18% of all severe and fatal cases occur as a result of electric shock.
The largest number of electrical injuries (60-70%) occurs at work on electrical installations with voltage up to 1000 V. This is due to the wide distribution of such installations and relatively low level training of persons operating them. There are much fewer electrical installations over 1000 V in operation and they are serviced by specially trained personnel, which causes a smaller number of electrical injuries.
1. The effect of electric current on the human body
Electric current, passing through the human body, has a biological, electrolytic, thermal and mechanical effect.
The biological effect of current is manifested in irritation and excitation of tissues and organs. This results in seizures skeletal muscle, which can lead to respiratory arrest, avulsion fractures and dislocations of the limbs, spasm of the vocal cords.
The electrolytic effect of the current is manifested in the electrolysis (decomposition) of liquids, including blood, and also significantly changes the functional state of cells.
The thermal effect of electric current leads to burns of the skin, as well as the death of subcutaneous tissues, up to charring. The mechanical action of the current is manifested in the stratification of tissues and even the separation of body parts.
There are two main types of damage to the body: electrical trauma and electrical shock. Often both types of damage accompany each other. However, they are different and should be considered separately.
Electrical injuries are clearly defined local violations of the integrity of body tissues caused by exposure to electric current or an electric arc. Usually these are superficial injuries, that is, lesions of the skin, and sometimes other soft tissues, as well as ligaments and bones.
The danger of electrical injuries and the complexity of their treatment are determined by the nature and degree of tissue damage, as well as the body's response to this damage. Usually, injuries are cured, and the victim's working capacity is restored completely or partially.
Sometimes (usually with severe burns) a person dies. In such cases, the immediate cause of death is not electric current, but local damage to the body caused by current.
Typical types of electrical injuries are electrical burns, electrical signs, skin plating, electrophthalmia and mechanical damage.
Electrical burns are the most common electrical injury. They make up 60-65%, and 1/3 of them are accompanied by other electrical injuries.
There are burns: current (contact) and arc.
Contact electrical burns, i.e. tissue damage at the points of entry, exit and on the path of electric current flow occurs as a result of human contact with the current-carrying part. These burns occur during the operation of electrical installations of relatively low voltage (not higher than 1-2 kV), they are relatively light.
An arc burn is caused by the action of an electric arc that creates a high temperature. Arc burn occurs when working in electrical installations of various voltages, often the result of accidental short circuits in installations from 1000 V to 10 kV or erroneous operations of personnel. The defeat arises from a change in the electric arc or clothing caught fire from it.
There may also be combined lesions (contact electric burn and thermal burn from the flame of an electric arc or ignited clothing, electric burn in combination with various mechanical damage, electric burn simultaneously with thermal burn and mechanical injury).
Electrical signs are clearly defined patches of gray or pale yellow color on the surface of the skin of a person exposed to current. Signs are round or oval with a depression in the center. They come in the form of scratches, small wounds or bruises, warts, skin hemorrhages, and calluses. Sometimes their shape corresponds to the shape of the current-carrying part that the victim touched, and also resembles the shape of wrinkles.
In most cases, electrical signs are painless, and their treatment ends safely: over time, the top layer of the skin and the affected area acquire their original color, elasticity and sensitivity. Signs occur in about 20% of those affected by the current.
Metallization of the skin is the penetration into its upper layers of particles of metal that has melted under the action of an electric arc. This is possible in case of short circuits, trips of disconnectors and knife switches under load, etc.
The affected area has a rough surface, the color of which is determined by the color of the metal compounds that have fallen under the skin: green - in contact with copper, gray - with aluminum, blue-green - with brass, yellow-gray - with lead. Usually, over time, the diseased skin disappears and the affected area becomes normal. At the same time, all the painful sensations associated with this injury also disappear.
Metallization of the skin is observed in approximately one in ten of the victims. Moreover, in most cases, simultaneously with metallization, an electric arc burn occurs, which almost always causes more severe injuries.
Electrophthalmia - inflammation of the outer membranes of the eyes as a result of exposure to a powerful stream ultraviolet rays causing chemical changes in the cells of the body. Such exposure is possible in the presence of an electric arc (for example, during a short circuit), which is a source of intense radiation not only of visible light, but also of ultraviolet and infrared rays. Electrophthalmia occurs relatively rarely (in 1-2% of victims), most often during electric welding.
Mechanical damage are the result of sharp, involuntary convulsive muscle contractions under the influence of a current passing through a person. As a result, ruptures of the skin, blood vessels and nervous tissue can occur, as well as dislocations of the joints and even bone fractures. These injuries are usually serious injuries that require long-term treatment. Fortunately, they rarely occur - no more than 3% of those affected by the current.
Electric shock is the excitation of living tissues by an electric current passing through the body, accompanied by involuntary convulsive muscle contractions.
Depending on the outcome of the negative impact of current on the body, electric shocks can be conditionally divided into the following four degrees:
I - convulsive muscle contraction without loss of consciousness;
II - convulsive muscle contraction with loss of consciousness, but with preserved breathing and heart function;
III - loss of consciousness and impaired cardiac activity or respiration (or both);
IV - clinical death, that is, the lack of breathing and blood circulation.
Clinical (or "imaginary") death is a transitional period from life to death, occurring from the moment of cessation of activity and lungs. In a person who is in a state clinical death, there are no signs of life, he does not breathe, his heart does not work, pain stimuli do not cause any reactions, the pupils of the eyes are dilated and do not react to light. However, during this period, life in the body has not yet completely died out, because its tissues do not die immediately and the functions of various organs do not immediately die out.
The cells of the brain, which are associated with consciousness and thinking, are the first to die, which are very sensitive to oxygen starvation. Therefore, the duration of clinical death is determined by the time from the moment of cessation of cardiac activity and respiration until the onset of death of the cells of the cerebral cortex; in most cases, it is 4-5 minutes, and when a healthy person dies from an accidental cause, for example, from an electric current, it is 7-8 minutes.
Biological (or true) death is an irreversible phenomenon characterized by the cessation of biological processes in the cells and tissues of the body and the breakdown of protein structures; it occurs after the period of clinical death.
Causes of electrocution death include cardiac arrest, respiratory arrest, and electrical shock.
The cessation of cardiac activity is a consequence of the effect of current on the heart muscle. Such an effect can be direct, when the current flows directly in the region of the heart, and reflex, that is, through the central nervous system, when the current path lies outside this area. In both cases, cardiac arrest can occur or its fibrillation occurs, that is, chaotically fast and different-time contractions of the fibers (fibrils) of the heart muscle, in which the heart stops working as a pump, as a result of which blood circulation in the body stops.
The cessation of breathing as the root cause of death from electric current is caused by a direct or reflex effect of the current on the muscles of the chest involved in the breathing process. A person begins to experience breathing difficulties already at a current of 20-25 mA (50 Hz), which increases with increasing current. With prolonged exposure to current, asphyxia can occur - suffocation as a result of a lack of oxygen and an excess of carbon dioxide in the body.
Electric shock is a kind of severe neuro-reflex reaction of the body in response to strong irritation with electric current, accompanied by dangerous disorders of blood circulation, respiration, metabolism, etc.
The state of shock lasts from several tens of minutes to a day. After that, either the death of the body may occur as a result of the complete extinction of vital functions or complete recovery as a result of timely active therapeutic intervention.
2. Factors affecting the outcome of human electric shock
The severity of electric shock depends on a number of factors: the value of the current strength, electrical resistance the human body and the duration of the current flow through it, the path of the current, the type and frequency of the current, the individual properties of a person and conditions environment,
The strength of the current is the main factor that determines one or another degree of damage to a person (path: hand-hand, hand-feet).
Fibrillation is called chaotic and multi-temporal contractions of the fibers of the heart muscle, completely disrupting its work as a pump. (For women, the current threshold values are 1.5 times less than for men).
DC current is about 4-5 times safer alternating current frequency 50 Hz. However, this is typical for relatively low voltages (up to 250-300 V). At higher voltages, the danger of direct current increases.
In the voltage range of 400-600 V, the danger of direct current is almost equal to the danger of alternating current with a frequency of 50 Hz, and at a voltage of more than 600 V, direct current is more dangerous than alternating current.
The electrical resistance of the human body with dry, clean and intact skin at a voltage of 15-20 V is in the range from 3,000 to 100,000 ohms, and sometimes more.
When the upper layer of the skin is removed, the resistance decreases to 500-700 ohms. When the skin is completely removed, the resistance of the internal tissues of the body is only 300-500 ohms.
When calculating, the resistance of the human body is taken equal to 1000 ohms. If there are various injuries on the skin (abrasions, cuts, abrasions), the
its electrical resistance in these places. The electrical resistance of the human body decreases with an increase in current and the duration of its passage due to increased local heating of the skin, which leads to vasodilation, and, consequently, to an increase in the supply of blood to this area and an increase in sweating.
With an increase in the voltage applied to the human body, the resistance of the skin decreases, and, consequently, the total resistance of the body, which approaches its lowest value of 300-500 ohms. This is due to the breakdown of the stratum corneum of the skin, an increase in the current passing through it, and other factors.
The resistance of the human body depends on the sex and age of people: in women this resistance is less than in men, in children it is less than in adults, in young people it is less than in the elderly. This is due to the thickness and degree of coarsening of the upper layer of the skin. A short-term (for several minutes) decrease in the resistance of the human body (20-50%) causes external, unexpected physical irritations: pain (blows, injections), light and sound.
The electrical resistance is also affected by the type of current and its frequency. At frequencies of 10-20 kHz, the upper layer of the skin practically loses its resistance to electric current.
In addition, there are particularly vulnerable areas of the body to the action of electric current. These are the so-called acupuncture zones (the area of the face, palms, etc.) with an area of 2-3 mm2. Their electrical resistance is always less than the electrical resistance of the zones lying outside the acupuncture zones.
The duration of the current flow through the human body greatly affects the outcome of the lesion due to the fact that over time the resistance of the human skin decreases, and heart damage becomes more likely.
The path of the current through the human body is also essential. The greatest danger arises with the direct passage of current through the vital organs.
Statistics show that the number of injuries with loss of consciousness during the passage of current along the path " right hand-legs" make up 87%; along the “leg-leg” path - 15%, The most characteristic current circuits through a person are: arm-legs, arm-arm, arm-torso (respectively 56.7; 12.2 and 9.8% of injuries). But the most dangerous are those current circuits in which both hands are involved - both legs, left hand legs, arm-arm, head-legs.
The type and frequency of the current also affect the degree of damage. The most dangerous is alternating current with a frequency of 20 to 1000 Hz. Alternating current is more dangerous than direct current, but this is typical only for voltages up to 250-300 V; at high voltages, direct current becomes more dangerous. With an increase in the frequency of the alternating current passing through the human body, the impedance of the body decreases, and the passing current increases. However, a decrease in resistance is possible only within frequencies from 0 to 50-60 Hz.
A further increase in the frequency of the current is accompanied by a decrease in the danger of damage, which completely disappears at a frequency of 450-500 kHz. But these currents can cause burns both when an electric arc occurs, and when they pass directly through the human body. The decrease in the risk of electric shock with increasing frequency is practically noticeable at a frequency of 1000-2000 Hz.
The individual properties of a person and the state of the environment also have a noticeable effect on the severity of the lesion.
3. Conditions and causes of electric shock
The defeat of a person by electric current or electric arc can occur in the following cases:
with a single-phase (single) touch of a person isolated from the earth to non-insulated live parts of electrical installations that are energized;
when a person simultaneously touches two non-insulated parts of electrical installations that are energized;
when approaching a person who is not isolated from the ground, at a dangerous distance to current-carrying parts of electrical installations that are not protected by insulation, which are energized;
when a person who is not isolated from the ground touches non-current-carrying metal parts (cases) of electrical installations that are energized due to a short circuit on the case;
under the action of atmospheric electricity during a lightning discharge;
as a result of the action of an electric arc;
when releasing another energized person.
The following causes of electrical injuries can be distinguished:
Technical reasons - non-compliance of electrical installations, protective equipment and devices with safety requirements and conditions of use, associated with defects in design documentation, manufacturing, installation and repair;
malfunctions of installations, protective equipment and devices that occur during operation.
Organizational and technical reasons - non-compliance with technical safety measures at the stage of operation (maintenance) of electrical installations; untimely replacement of faulty or obsolete equipment and the use of installations that have not been put into operation in the prescribed manner (including home-made ones).
Organizational reasons - non-fulfillment or incorrect fulfillment of organizational security measures, inconsistency of the work performed with the task.
Organizational and social reasons:
overtime work (including work to eliminate the consequences of accidents);
inconsistency of the work of the specialty;
violation of labor discipline;
admission to work on electrical installations of persons under 18 years of age;
attraction to work of persons who have not been issued an order for employment in an organization;
admission to work of persons with medical contraindications.
When considering the causes, it is necessary to take into account the so-called human factors. These include both psychophysiological, personal factors (a person’s lack of individual qualities necessary for this work, a violation of his psychological state, etc.), and socio-psychological (unsatisfactory psychological climate in the team, living conditions, etc.).
4. Measures for protection against electric shock
According to requirements normative documents, the safety of electrical installations is ensured by the following main measures:
1) inaccessibility of live parts;
2) proper, and in some cases increased (double) insulation;
3) grounding or grounding of electrical equipment cases and elements of electrical installations that may be energized;
4) reliable and fast automatic protective shutdown;
5) the use of low voltages (42 V and below) to power portable current collectors;
6) protective separation of circuits;
7) blocking, warning signaling, inscriptions and posters;
8) the use of protective equipment and devices;
9) carrying out scheduled preventive repairs and preventive testing of electrical equipment, apparatus and networks in operation;
10) carrying out a number of organizational activities (special training, certification and re-certification of electrical personnel, briefings, etc.).
To ensure electrical safety at the enterprises of the meat and dairy industry, the following technical methods and means of protection are used: protective grounding, zeroing, the use of low voltages, winding insulation control, personal protective equipment and safety devices, protective shutdown devices.
Protective earth is an intentional electrical connection to earth or its equivalent to non-current-carrying metal parts that may be energized. It protects against electric shock when touching the metal cases of the equipment, metal structures electrical installations that, due to a violation of electrical insulation, are energized.
The essence of protection lies in the fact that during a short circuit, the current passes through both parallel branches and is distributed between them in inverse proportion to their resistances. Since the resistance of the person-to-ground circuit is many times greater than the resistance of the body-to-ground circuit, the current flowing through the person is reduced.
Depending on the location of the grounding conductor relative to the equipment to be grounded, remote and contour grounding devices are distinguished.
Remote grounding switches are located at some distance from the equipment, while the grounded enclosures of electrical installations are on the ground with zero potential, and a person, touching the enclosure, is under the full voltage of the grounding conductor.
Low voltage - a voltage not exceeding 42 V, used to reduce the risk of electric shock. Small AC voltages are obtained using step-down transformers. It is used when working with portable power tools, when using portable lamps during installation, dismantling and repair of equipment, as well as in remote control circuits.
Isolation of the workplace is a set of measures to prevent the occurrence of a human-ground current circuit and increase the value of the transient resistance in this circuit. This protective measure is applied in cases of increased risk of electric shock and usually in combination with an isolating transformer.
There are the following types of insulation:
working - electrical insulation of the current-carrying parts of the electrical installation, ensuring its normal operation and protection against electric shock;
additional - electrical insulation provided in addition to the working insulation to protect against electric shock in case of damage to the working insulation;
double - electrical insulation, consisting of working and additional insulation. Double insulation consists in one electrical receiver of two stages of insulation independent of each other (for example, covering electrical equipment with a layer of insulating material - paint, film, varnish, enamel, etc.). The use of double insulation is most rational when, in addition to the working electrical insulation of current-carrying parts, the body of the power receiver is made of insulating material (plastic, fiberglass).
Protective shutdown is a fast-acting protection that provides automatic shutdown of an electrical installation in the event of a danger of electric shock in it.
Devices (apparatus) for protective shutdown with respect to the reliability of operation must meet special technical requirements. Personal protective equipment is divided into insulating, auxiliary and enclosing.
Insulating protective equipment provides electrical isolation of a person from current-carrying parts and earth. They are divided into basic (insulated gloves, tools with insulated handles) and additional (insulated galoshes, rugs, coasters)
Auxiliary ones include glasses, gas masks, masks designed to protect against light, thermal and mechanical influences.
Fencing includes portable shields, cages, insulating pads, portable grounding and posters. They are intended mainly for temporary fencing of current-carrying parts, which may be touched by workers.
5. Rendering PP in case of electric shock
All personnel servicing electrical installations must be trained annually in the methods of release from electric current, performing artificial respiration and external heart massage. Classes are conducted by competent medical personnel with training of practical actions on simulators. The head of the enterprise is responsible for the organization of training.
If a person touches live parts under voltage with his hand, then this causes an involuntary convulsive contraction of the muscles of the hand, after which he is no longer able to free himself from live parts. Therefore, the first action of the person providing assistance is the immediate shutdown of the electrical installation, which the victim touches. Shutdown is carried out using switches, knife switches, turning out plugs and other methods. If the victim is at a height, then when turning off the installation, make sure that he does not fall.
If it is difficult to turn off the installation, then it is necessary to free the victim, using all means of protection, so as not to be energized yourself.
At voltages up to 1000 V, you can use a dry board or stick to free the victim from the wire that has fallen on him. You can also pull on dry clothes, while avoiding touching metal parts and open areas the victim's body; it is necessary to act with one hand, holding the other behind the back. It is most reliable for the assisting person to use dielectric gloves and rubber mats when releasing the victim. After releasing the victim from the action of electric current, it is necessary to assess the condition of the victim in order to provide appropriate first aid.
If the victim is conscious, breathing and pulse are stable, then it is necessary to lay him on the bed; unfasten clothes; create a tributary fresh air; create complete peace by observing the breath and pulse. In no case should the victim be allowed to move, as deterioration may occur. Only a doctor can decide what to do next. If the victim breathes very rarely and convulsively, but his pulse is felt, it is necessary to immediately start artificial respiration.
If the victim has no consciousness, breathing, pulse, pupils are dilated, then we can assume that he is in a state of clinical death. In this case, it is urgent to start reviving the body with the help of artificial respiration according to the “mouth-to-mouth” method and external heart massage. If, within only 5-6 minutes after the cessation of cardiac activity, one does not begin to revive the body of the victim, then without oxygen in the air, brain cells die and death passes from clinical to biological; the process becomes irreversible. Therefore, the five-minute time limit is a critical factor in animating.
With the help of an indirect heart massage in combination with artificial respiration, anyone can bring the victim back to life or time will be won before the arrival of the resuscitation team.
Conclusion
The development of technology changes the working conditions of a person, but does not make them safer, on the contrary, in the process of operating new equipment, previously unknown dangerous factors often appear.
Modern production is unthinkable without the widespread use of the electric power industry. Perhaps there is no such professional activity where electric current would not be used.
Negative consequences for human health during operation technological equipment, have now put forward the provision of industrial safety in production as one of the most acute technical and socio-economic problems. The worst consequence of an electric shock is death. Fortunately, it happens quite rarely in this case.
To prevent electrical shock and ensure electrical safety in production, the following are used: insulation of wires and other components of electrical circuits, instruments and machines; protective grounding; zeroing, emergency power off; personal protective equipment and some other measures.
Unfortunately, the widespread aging of production assets, dilapidation of premises has a negative impact on the quality of electrical wiring. Breakdowns in electrical wiring lead not only to electric shocks, but are also one of the main causes of fires.
The effect of electricity on the human body
In this section, we will try to correct a very common error in electronics textbooks related to ignoring or insufficiently detailed disclosure of the topic of electrical safety. If you are reading this article, then you are engaged or are going to engage in practical work with electricity, and the topic of safety is of paramount importance to you. Those authors, editors and publishers who for some reason do not include this topic in their writings are depriving the reader of vital information.
Most of us have experienced some form of electrical shock resulting in pain or injury. Basically, such an experience is limited to tingling or painful shock due to the discharge of static electricity. When working with electrical diagrams, which produce more power at loads, pain is the least significant result of an electric shock.
The passage of an electric current through a material that has some kind of resistance leads to the dissipation of energy in the form of heat. This is the most basic form of the effect of electricity on living tissue: under the influence of current, it heats up. If allocated a large number of heat, the fabric can simply be burned. In fact, the effect of electric shock is similar to that of exposure to open flames or other sources of high temperatures, but in addition, electricity can burn tissue under the skin of a person, and even his internal organs.
Even more dangerous is the effect of electric current on the human nervous system. The "nervous system" is the body's network of specialized cells called "nerve cells" or "neurons" that process and conduct a vast array of signals that control all bodily functions. The brain, spinal cord and sensory-motor organs function as a whole in the body, allowing it to feel, move, react, think and remember.
Nerve cells interact with each other according to the principle of "transformation": they create electrical signals (very small voltages and currents) in response to the introduction of certain chemical compounds called neurotransmitters , and release these neurotransmitters when stimulated by electrical signals. If through man will pass an electric current of sufficient magnitude, then under its influence the tiny electrical impulses generated by neurons will be repeatedly exceeded, which will lead to overload of the nervous system and blocking of reflexes and muscle control signals. At the same time, the latter will involuntarily decrease, and a person will not be able to do anything about it.
A particularly dangerous situation can arise if a person touches a live wire with his hand. The muscles of the forearm, which are responsible for clenching the fingers, are much better developed than the muscles responsible for unclenching the fingers, therefore, when an electric current is applied to both muscle groups, the clenching muscles will win and clench the fingers into a fist. At the same time, if the wire is located on the side of the palm, then the fingers will clasp it, exacerbating the current situation. A person will no longer be able to release the wire on his own.
Medically, involuntary muscle contraction is called numbness . There is only one way to get an electric shocked person out of a state of stupor: to stop the passage of current through him.
Even after the cessation of exposure to electric current, a person will not be able to regain control over his muscles for some time until the balance of neurotransmitters is normalized. Devices such as "stun guns" are built on this principle, which, with the help of a high-voltage pulse, can incapacitate a person for some time (up to several minutes).
Electric current can affect not only the muscles of the skeleton, but also the muscles of the diaphragm and the heart. To disrupt the work of the heart and cause arrhythmia a small amount of current is sufficient. In this case, the normal heartbeat will be replaced by "flutter", which will not be able to provide effective pumping of blood to the vital organs of the body. If the current through the body is strong enough, then death will occur from suffocation or from cardiac arrest. Strange as it may seem, but to restore the heartbeat, doctors also use a powerful electric current applied to a person’s chest.
And the last thing we will consider in this article is the dangers inherent in electrical networks. common use. While our initial electrical circuit research will focus solely on direct current (DC), most modern household appliances use alternating current (AC) for power. The technical reasons for preferring alternating current to direct current in power systems are beyond the scope of this article, but the inherent dangers of each type of electrical energy are very important in terms of safety.
The nature of the impact of alternating current on the human body largely depends on its frequency. in Russia, USA and European countries low frequency alternating current (50 - 60 Hz) is used. Such a current is more dangerous than high frequency alternating current, and 3-5 times more dangerous than direct current of the same voltage. The impact of low frequency alternating current leads to a prolonged contraction of the muscles, which will not allow you to remove the hand that has squeezed the wire from this wire. Exposure to direct current will cause a single convulsive muscle contraction, after which the affected person will be able to move away from the current source.
Alternating current is more likely to cause an arrhythmia of the heart, while direct current can stop it. After the effect of the current on the body stops, then the stopped heart has a better chance of restoring a normal heartbeat than a heart with an arrhythmia (fluttering). Therefore, defibrillators used by emergency physicians use a direct current shock that stops the arrhythmia and gives the heart a chance to recover.
Now we know that electric currents are dangerous and interaction with them must be avoided. In subsequent articles in this section, we will look at what currents enter and leave the human body, and we will study precautions when working with electricity.
Short review:
Electric current can cause deep and severe burns in the human body due to the dissipation of power through the electrical resistance of the body.
torpor - this is a situation in which the muscles of a person inadvertently contract due to the passage of an external electric current through his body.
The muscles of the diaphragm (lungs) and the heart are also affected by the electrical current. To disrupt the work of the heart and cause arrhythmia a small amount of current is sufficient.
Alternating current is more likely to cause an arrhythmia of the heart, while direct current can stop it.
Flowing through the human body, the electric current causes thermal, electrochemical and biological effects.
The thermal effect of the current is manifested in the heating and burns of individual parts of the body; electrochemical in the decomposition of blood and other organic fluids; the biological effect of the current is associated with irritation and excitation of the living tissues of the body, which is accompanied by involuntary convulsive contractions of the muscles, including the muscles of the lungs and the muscles of the heart, and can cause a cessation of the activity of the circulatory and respiratory organs.
These current actions can lead to two types of injury: electrical injury and electrical shock.
Electrical injuries include electrical burns, electrical signs, electroplating of the skin, electrophthalmia, and mechanical damage.
The cause of electrical burns can be the action of an electric arc (arc burn) or the passage of current through the human body as a result of its contact with a live part (current burn). A current burn is, as a rule, a skin burn at the point of contact of the body with a current-carrying part due to the conversion of electrical energy into heat. Since human skin has many times more resistance than other tissues of the body, most of the heat is generated in it. Current burns occur in electrical installations, mainly with voltages up to 1000 V.
Arc burn is caused by the impact on the body of an electric arc, which is created during a discharge in the event that a person approaches live parts that are energized above 1000 V, or during short circuits in electrical installations
voltage up to 1000 V. A high temperature electric arc can cause extensive burns to the body and lead to death.
Electric signs, also called current signs or electric marks, are dead spots on the skin of a person who has been exposed to current. In most cases, electrical signs are painless and treatable.
The electrometallization of the skin is due to the penetration into its upper layers of the smallest particles of metal, which have melted under the influence of an electric arc. Subsequently, the damaged area is restored and acquires a normal appearance, painful sensations disappear. Cases of eye damage can be very dangerous, often leading to loss of vision. Therefore, work in which such cases are possible must be carried out with protective glasses. At the same time, the clothes of the worker must be fastened with all buttons, the collar is closed, and the sleeves are lowered and fastened at the wrists.
Often, simultaneously with the metallization of the skin, an electric arc burn is possible.
Electrophthalmia is an inflammation of the outer membranes of the eyes resulting from exposure to a stream of ultraviolet rays. Such exposure is possible when an electric arc occurs, for example, during short circuits, which is a source of intense radiation not only of visible light, but also of ultraviolet and infrared rays.
The prevention of electrophthalmia during the maintenance of electrical installations is ensured by the use of special goggles, which simultaneously protect the eyes from splashes of molten metal.
Mechanical damage occurs as a result of sharp involuntary convulsive muscle contractions under the influence of current. This can lead to falls from a height, dislocations of joints, fractures, etc.
Electric shocks refer to the type of damage that occurs when exposed to low currents (on the order of several hundred milliamps) and voltages up to 1000 V. With electric shocks, the outcome of current exposure to a person can vary from a slight, barely perceptible convulsive contraction of the muscles of the fingers to a fatal injury, associated with the cessation of the work of the heart or respiratory organs.
The degree of electric shock during electric shocks is characterized by its threshold value. The following currents are characteristic: threshold perceptible, threshold non-letting, threshold fibrillation.
Threshold sensible current smallest value perceptible current, causing perceptible irritations when passing through the human body.
The threshold non-release current is the smallest value of the non-release current that, when passing through a person, causes irresistible convulsive contractions of the muscles of the hand in which the conductor is clamped.
Threshold fibrillation current is the smallest value of fibrillation current that causes fibrillation of the heart when passing through the body.
As will be shown below, the current flowing through a person varies widely and depends on numerous physical and physiological phenomena that are difficult to take into account. Unlike past years, at present, in electrical safety engineering, the opinion that it is inappropriate to standardize dangerous and safe threshold values of voltage and current in industry and in everyday life prevails.
Table 1. The nature of the impact of electric current on the human body
Current value, mA | AC, 50 Hz | D.C |
The beginning of a feeling of mild itching, tingling of the skin under the electrodes | Not felt |
|
The feeling of current spreads. and on the wrist, slightly reduces the hand | Not felt |
|
Pain intensifies in the entire hand, accompanied by convulsions; weak pains are felt in the whole arm, up to the forearm. Hands can usually be taken off the electrodes | Beginning to feel the impression of heating the skin under the electrode |
|
Violent pains and cramps in the whole arm, including the forearm. Hands are difficult, but still can be torn off the electrodes | Increased feeling of warmth |
|
Hardly bearable pains in the whole arm. In many cases, the hands cannot be removed from the electrodes. With an increase in the duration of the flow of the current, the pain intensifies. | An even greater increase in the sensation of heating both under the electrodes and in the adjacent areas of the skin |
|
Hands are paralyzed instantly, it is impossible to tear them off the electrodes. Severe pain, difficulty breathing | An even greater increase in the sensation of heating the skin. Slight contractions of the ARM MUSCLE |
|
Very severe pain in arms and chest. With prolonged current, respiratory paralysis or weakening of the heart's activity with loss of consciousness may occur. | Sensations of intense heat, pain and cramps in the arms. When the hands are separated from the electrodes, hardly tolerable pain occurs as a result of convulsive contraction of the muscles of the hands. |
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Breathing is paralyzed after a few seconds, the work of the heart is disturbed. With prolonged current flow, cardiac fibrillation may occur. | Sensations of very strong heating, severe pain in the entire chest area. Difficulty breathing. Hands cannot be taken off the electrodes |
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Cardiac fibrillation after 23 s, after a few seconds, cardiac paralysis | Respiratory paralysis with prolonged current flow |
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Same action in less time | Fibrillation of the heart after 23 seconds, after a few seconds, respiratory paralysis |
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Breathing is paralyzed immediately after a few seconds. Fibrillation of the heart, as a rule, does not occur. Temporary cardiac arrest during the current flow is possible. With prolonged current flow (several seconds), severe burns, tissue destruction | Increased feeling of warmth |
The main factors affecting the outcome of a person's electric shock are as follows.
The path of current in the human body.
The path of current in the human body affects the lesion in different ways. For some time now, great importance has been attached to this issue, since the analysis of accidents made it possible to establish their dependence on the type of the so-called current loop, that is, on the path of the current through the human body. The most common are the following four loops: right arm leg, left arm leg, arm arm, leg leg. In most cases, the current circuit occurs along the path of the right hand leg. The most common and, as a rule, accompanied by severe damage is the current path (current loop) arm arm, when the current passes through the vital organs, in particular through the heart.
Accident analyzes show that approximately 55% of all electrical shocks occur along two main routes: from the hand or hands to the feet and from one hand to the other hand. However, fatal injuries account for half of the reported figure of accidents.
The danger is determined not by whether current flows or does not flow through the region of the heart, but by what part of the body a person touches current-carrying parts. The most vulnerable places human body are the back of the hand, neck, temple; front of the leg, shoulder. The formation of an electrical circuit through vulnerabilities leads to death even at very low currents and voltages.
Electrical resistance of the human body.
The electrical resistance of the circuit through which the current passes through the human body consists of the electrical resistance of active and inductive wires; electrical resistance of machines, devices or devices that are connected in series with the human body; electrical resistance of the transitional contact between the current-carrying parts of the equipment that the person has touched; own electrical resistance of the human body.
The resistance of the human body is a complex set of biophysical, biochemical and other phenomena. It is usually divided into two parts: the resistance of the skin and blood vessels and the resistance of the nerves. Upper layer skin has a noticeable resistance compared to the resistance internal organs. The presence of sweat glands in the skin greatly changes its electrical resistance. There is very little nerve resistance. It is this component of the total resistance that plays the most significant role in current conduction, and therefore, in the outcome of electrical injury. The electrical resistance of a living organism is influenced by big number factors. In this case, the condition of the skin is essential: damage to the stratum corneum (pores, scratches, abrasions and other microtraumas); moisturizing with water or sweat; pollution by various substances, and especially those that conduct electricity well (metal or coal dust, scale, etc.).
The resistance of the human body, i.e., the resistance between two electrodes applied to the surface of the body, can be conditionally considered to consist of three resistances connected in series: two resistances of the outer (horny) layer of the skin and one, called the internal resistance of the body, which includes the resistance of the inner layer of the skin and resistance of the internal tissues of the body. In general, these resistances have active and capacitive components.
In practical calculations, it is necessary to know and evaluate the numerical values of the resistance of the human electrical circuit between two electrodes applied to the body. Type of current and voltage. Studies (see Table 1), the practice of operating electrical installations show that direct current, compared with alternating current of the same values, is less dangerous for humans. This is explained primarily by the fact that due to the presence of a capacitive component in the electrical resistance of the human body, the current density, and hence the field strength in the tissues, will be greater at equal voltages in the case of an alternating current shock than in a direct shock. The significant circumstance also affects that with alternating current the striking amplitude voltage can be 1.4 times greater than the effective voltage. And finally, the probability of an electrical circuit forming through vulnerable places with alternating current is greater than with direct current, because alternating current networks cover an incomparably greater number of installations, and the most diverse ones, while direct current networks have more limited and specialized applications.
What has been said about the relative danger of damage by direct and alternating currents is true only for small voltages of the order of 250 - 300 V. At higher voltages, direct current is more dangerous than alternating current with a frequency of 50 Hz, due to the possibility of throwing the victim from live parts under high voltage , which is extremely rare with similar alternating current injuries. A thrown person can receive a mechanical injury, as a result of which (for example, when falling) a fatal outcome is not ruled out.
In general, it should be noted that the question of the relative danger to humans of alternating and direct current needs further study, which will expand our understanding of the biophysics of electrical injury.
The voltage applied to the electrical circuit leads to the transformation of electrical phenomena into other phenomena, the impact of which on the human body directly causes one or another outcome of the lesion. It has been and is believed that the outcome of an electric shock depends on the mains voltage: the higher this voltage, the more dangerous the consequences of an electrical injury. In statistical reporting, electrical injuries are recorded with a subdivision according to the values of the mains voltage. On the same basis, data are analyzed and electrical injuries are classified, research and experiments are carried out. Meanwhile, such a study of electrical injury does not always give a correct idea of this damaging factor.
Our Regulations divide all installations with voltages below and above 1000 V. In installations with voltages above 1000 V, burns caused by the passage of electric current are the main cause of fatal injuries. In installations below 1000 V, the main cause of damage is due to the direct action of the current. Statistics show that fatal electrical injuries occur mainly in installations up to 1000 V.
Fatal injuries also occur at low voltages (65, 36, 24, 12 V). Their analysis shows that they are caused not only by the fibrillation current, which cannot be obtained at these voltages. Damage from 12 to 65 V can be fatal only under special circumstances, for example, if the electrical circuit occurs through places vulnerable to current, if environmental conditions are unfavorable. There are also other causes of death, which are still not well understood.
Summarizing what has been said regarding the absence of a direct relationship between the outcome of a lesion and voltage, current, we state that it is impossible to normalize with high accuracy in industry (and in everyday life) dangerous and safe threshold values of current and voltage.
The duration of the existence of an electrical circuit through the human body.
The outcome of electric shock is related to the time factor. When analyzing accidents, much attention is paid to this parameter, especially if we take into account the presence of contradictions in the assessment of the dangerous (and safe) time for the passage of current through a person. On the one hand, there are lesions with a severe outcome even at low currents and a very short duration of current passage through a person (fractions of a second), on the other hand, cases with a favorable outcome (excluding burns) with a lesion duration of several seconds or more.
Due to the above contradictions, it is not possible to strictly substantiate the dependence of the outcome of the lesion on the duration of the existence of the electrical circuit.
Frequency effect
From the above formula for the impedance of the human body, it follows that as the frequency of the alternating current increases, the resistance decreases, which leads to an increase in current and an increase in the risk of injury. However, practice shows that this conclusion is valid only within certain frequencies. For a long time it was believed that in the low-frequency region, the 50-period current has the greatest danger. With a further increase in frequency within 50 - 400 Hz, the current remains approximately the same. Further increase in frequency reduces the risk of injury. But whether it is harmful or not harmful to the human body, there is no affirmative answer yet.
A comparative danger for a person of a rectified current is noted. The presence of frequency components in it aggravates the outcome of an electrical injury. So far, this is a little-studied section of electrical safety.
Environmental impact.
The environment in many cases can influence the electric shock to a person. The factors of this influence include atmospheric pressure, temperature, humidity, electrical or magnetic field and etc.
An increase in air temperature affects sweating in a person, as a result of which the electrical resistance of his body decreases and the risk of electric shock increases.
Similar phenomena are also associated with high humidity. Here, there is a decrease not only in electrical resistance, but also in the overall resistance of the body to electric current.
The influence of these two factors of temperature and humidity is recorded in the regulatory documents.
The third atmospheric factor is the pressure of the surrounding air, which also affects the sensitivity to electric current. As the pressure increases, the risk of injury decreases. For example, statistics show that no fatal or severe electrical injuries were recorded during underwater electric welding, although cases of contact between divers working underwater with current-carrying elements and contacts have been repeatedly noted.
The reverse picture was established for low atmospheric pressure, which is especially significant in connection with the electrification of mountainous regions. It has been experimentally proven that low atmospheric pressure increases the danger of electric current for living organisms.
Medico-biological properties of a person
An analysis of accidents in case of electric shock shows that the outcome of the injury is associated with the medical and biological characteristics of a person, his state of health. physically healthy and tough people easier to tolerate electrical injuries than ball and weak. People suffering from skin diseases, cardiovascular, nervous diseases are more susceptible to electric current.
Therefore, the safety regulations for the operation of electrical installations provide for the medical selection of personnel for the maintenance of electrical installations. Selection is carried out upon admission to work, periodic examinations within the time limits established by the Ministry of Health in accordance with the list of diseases and disorders that prevent admission to work. The selection also has another goal: to prevent people with diseases from servicing electrical installations that may interfere with their production work or cause erroneous actions that are dangerous for others (indistinguishability of the color of the signal due to visual impairment, the inability to give a clear command due to a sore throat or stuttering, etc.).
In addition, safety regulations do not allow persons under the age of 18 and who do not have certain knowledge in the field of electrical safety to service electrical installations, corresponding to the scope and conditions of the work they perform.
It is known that a person is unable to determine the presence of dangerous voltage with his organs, and the physiological processes constantly occurring in the body are incompatible with the flow of electric current through his body.
There are four types of current exposure:
Thermal;
- electrolytic;
- dynamic;
- biological.
thermal effect- on the body, after contact with electricity, burns of arbitrary shape appear. When overheated, the organs that are in the path of the electric current temporarily lose their functionality. As a result of the lesion, both the brain and the circulatory or nervous system which leads to serious problems.
electrolytic effect- damage to the blood and lymph in the body, which leads to their splitting and changes in the physico-chemical composition.
dynamic, or as it is also called mechanical, the impact causes damage to the structure of body tissues (including muscle, lung tissues, walls of blood vessels) in the form of delamination, lacerations, in some cases even tears. Mutilation contributes to overheating of the blood and tissue fluid with instantaneous release of steam, similar to an explosion.
Biological impact affects the muscular system and living tissues, leads to its temporary dysfunction. As a result, involuntary spasmodic muscle contractions may occur. This action, even of a temporary nature, can adversely affect the functioning of the heart or respiratory system, and death is not ruled out.
Types of electrical injuries:
Local character, when certain parts of the body are violated;
- general defeat - injuries were caused by electric shock to the entire body.
The ratios of electrical injuries, according to static studies, were distributed as follows:
20% - local manifestations;
- 25% - total damage to the body;
- 55% - mixed lesions.
Most often, accidents occur with both types of injuries, however, they should be considered as separate, as they have significant differences.
Electrical injuries of a local nature. Damage to the body is associated with violations of the integrity of body tissues. More often the skin is injured, but there are cases of harm to the ligaments or bones.
The degree of danger of injury depends on the condition and location of the damaged tissue. In most cases, they are cured with full restoration of the functionality of the affected part of the body.
About 75% of accidents from electric shock have a local damage zone and occur with the following frequency:
Electrical burns - ≈40%;
- electrical signs - ≈7%;
- metallization of the skin - ≈3%;
- mechanical damage - ≈0.5%
- cases of electrophthalmia - ≈1.5%;
- mixed injuries - ≈23%.
electrical burns. Tissue damage occurs from the thermal influence of an electric current, occurs frequently, is divided into:
Current or contact, arising from the contact of a person with current-carrying equipment;
- arc, due to the action of an electric arc.
Current burns are typical for electrical devices with voltages up to 2 kV. Electrical objects of higher voltage form an electric arc.
The complexity of the burn depends on the power of the current and the duration of its passage. The skin burns quickly due to greater resistance than the internal tissues. At increased frequencies, the currents penetrate deep into the body, affecting the internal organs.
Arc burns occur during the operation of the ED with different voltages. Moreover, sources up to 6 kV can form an arc in case of an accidental short circuit. More high voltages break through the resistance of air insulation between a person and electrical equipment while reducing the safe gap to live parts.
electrical signs. These are oval-shaped spots of pale yellow or gray color located on the surface of the body. They are about 1-5 mm in size. They are easy to treat and do not bring much discomfort to a person.
It is damage to the skin by small particles of molten metal that penetrate into the upper layers of the skin from the arc during short circuits.
The most dangerous injury is damage to the eye area. To prevent it, during work related to breaking circuits and the simultaneous formation of an electric arc, the employee must use special goggles, and completely cover the body with overalls.
Mechanical damage. Most common when working in electrical installations up to 1000 V under long-term exposure to electric current.
Manifested as involuntary muscle spasms, which can lead to rupture of the skin, nerve tissue or blood vessels. There are cases with dislocation of the joints and fracture of the bones.
Electrophthalmia. Eye damage is associated with inflammatory processes of the outer shell (conjunctiva and cornea) from exposure to a strong light flux of the ultraviolet spectrum of an electric arc.
For protection, you need to use goggles or a mask with colored special glasses.
electric shock. The rapid, almost instantaneous formation of a current circuit in the body affects living tissues, leads to muscle cramps, disrupts the functioning of all organs, especially the nervous system, heart and lungs. The degree of electric shock is determined by five stages:
1. Light contractions of individual muscles;
2. Muscle cramps that create pain, in which the victim is conscious;
3. Convulsive contractions of the muscles, causing loss of consciousness, when the heart and lungs continue to function;
4. The victim is unconscious, the rhythm / work of the heart and / or breathing is disturbed;
5. Lethal outcome.
The consequences of an electric shock on the human body depend on a number of factors:
The duration and magnitude of the damaging electric current;
- frequency and type of current;
- flow paths;
- individual capabilities of the affected organism.
Fibrillation. The fibers of the heart muscle (fibrils) under the influence of an alternating current with a frequency of 50 Hz, exceeding 50 mA, begin chaotic contractions. After a few seconds, the pumping of blood stops completely. The body's blood flow stops.
The current path through the heart is most often created by contacts between the hands or the leg and the hand. Smaller 50 mA and larger 5 A currents do not cause fibrillation of the heart muscle in humans.
electric shock. An electric shock is hard to perceive by the body, a reaction of a neuro-reflex nature occurs. The respiratory and nervous systems, blood circulation, internal organs are affected.
After exposure to current, the phase of the so-called excitation of the body begins: pain appears, blood pressure increases.
Then the body goes into a phase of inhibition: blood pressure decreases, the pulse is disturbed, the respiratory and nervous systems weaken, depression sets in. The duration of this state can vary from several minutes to days.
The damaging effect of electric current on the human body is commonly called electrical injury. It should be taken into account that this type of industrial injuries is characterized by a large number of outcomes with severe and even fatal consequences. Below is a graph showing the percentages between them.
As statistics show, the largest percentage of electrical injuries (from 60 to 70%) falls on the operation of electrical equipment up to 1000 volts. This indicator is explained both by the prevalence of installations of this class and by the poor training of working personnel.
In most cases, electrical injury is associated with a violation of safety standards and ignorance of the elementary laws of electrical engineering. For example, electrical safety does not allow the use of foam fire extinguishers as primary funds fire extinguishing electrical equipment.
Occupational safety requires that everyone who works with electrical equipment must undergo electrical safety training. Where it is told about the danger of electric current, what measures should be taken in case of electrical injuries, as well as ways to provide the necessary assistance in these cases.
Note that the number of electrical injuries is significantly lower among those servicing electrical equipment with voltages above 1000V, which indicates a good training of such specialists.
Factors affecting the outcome of electric shock
There are several dominant reasons on which the nature of damage during electric shock depends:
Types of impact
An electric current with a strength of 0.5 to 1.5 mA is considered the minimum for human perception, when this threshold value is exceeded, a feeling of discomfort begins to appear, which is expressed in an involuntary contraction of muscle tissue.
At 15 mA or more, control over the muscular system is completely lost. In this state, it is not possible to break away from the electric source without outside help, therefore this threshold value of the electric current strength is called unreleased.
When the strength of the electric current exceeds 25 mA, paralysis of the muscles responsible for the functioning of the respiratory system occurs, which threatens to suffocate. If this threshold is significantly exceeded, fibrillation (failure of the heart rhythm) occurs.
Video: the effect of electric current on the human body
Below is a table that shows the allowable voltage, current and time of their exposure.
Electrical injuries can produce the following types of effects:
- thermal, burns of varying degrees appear, which can disrupt the functioning of both blood vessels and internal organs. Let us note that the thermal manifestation of the action of an electric current is observed in most electrical injuries;
- the effect of an electrolytic nature causes a change in the physical and chemical composition of tissues, due to the breakdown of blood and other body fluids;
- physiological, leads to convulsive contractions of muscle tissue. Note that the biological effect of the electric current also disrupts the work of other important organs, such as the heart and lungs.
Types of electrical injuries
The impact of electric current causes the following characteristic damage:
- electrical burns may occur due to the passage of an electric current or be caused by an electric arc. Note that such electrical injuries are most common (about 60%);
- the appearance on the skin of oval spots of gray or yellow color in places where the electric current passes. The dead layer of the skin becomes coarse, after some time such a formation, called an electric sign, disappears on its own;
- the penetration of small particles of metal (melted from short circuit or electric arc) into the skin. This type of injury is called skin plating. The affected areas are characterized by a dark metallic shade, touching it causes pain;
- light action, causes electrophthalmia ( inflammatory process eye shell) due to the ultraviolet radiation characteristic of the electric arc. For protection, it is enough to use special glasses or a mask;
- mechanical impact (electric shock) occurs due to involuntary contraction of muscle tissue, as a result of this, a rupture of the skin or other organs may occur.
Note that of all the electrical injuries described above, the consequences of an electric shock are the most dangerous, they are divided according to the degree of impact:
- cause contractions of muscle tissue, while the victim does not lose consciousness;
- convulsive contractions of muscle tissue, accompanied by loss of consciousness, the circulatory and respiratory systems continue to function;
- there is paralysis of the respiratory system and a violation of the heart rhythm;
- the onset of clinical death (no breathing, heart stops).
Step voltage
Given the frequent cases of damage from step voltage, it makes sense to tell more about the mechanism of its action. A break in a power line, or a violation of the integrity of the insulation in a cable laid underground, leads to the formation of a dangerous zone around the conductor, in which the current "spreads".
If you enter this zone, you can be exposed to the step voltage, its value depends on the potential difference between the places where a person touches the ground. The figure clearly shows how this happens.
The figure shows:
- 1 - electrical wiring;
- 2 - the place where the broken wire fell;
- 3 - a person who has fallen into the zone of electric current spreading;
- U 1 and U 2 are the potentials at the points where the feet touch the ground.
The step voltage (V W) is determined by the following expression: U 1 -U 2 (V).
As can be seen from the formula, the greater the distance between the feet, the greater the potential difference and the higher Vsh. That is, when you get to the area where the "spreading" of the electric current occurs, you can not take big steps to get out of it.
How to act when assisting with electrical injuries
First aid for electric shock consists in a certain sequence of actions:
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