Electricity is a powerful and dangerous element. She faithfully serves a person, however, if you are careless or as a result of an accident, you can fall under her attack. Electric shocks occur due to contact with bare wires, switches, lamp sockets and other live household and industrial appliances, and failure to comply with safety precautions. Having received an electric shock, a person receives electrical injury, which may result in disability or even death.
What are the dangers of electric shock?
When exposed to electrical voltage, the victim always receives a shock, but its consequences can be different: from cramps of the fingers and trembling, from unpleasant sensations of heating and burning to respiratory arrest and cardiac fibrillation (unsystematic contraction) and its complete stop. In the latter case, blood stops moving through the vessels, causing the person to die. In addition, electric current is dangerous for humans, since at certain values of its strength it creates the effect of sticking to bare wires due to excessive stimulation of nerve fibers by electricity. One of the causes of death from electric shock can be mechanical injury as a result of involuntary muscle contraction. Loss of vision may occur due to the impact of the resulting electric arc on the retina.
The skin on the face, neck and back of the hands is most affected by the effects of electric current.
Note! Certain (acupuncture) points on a person’s ears and neck are extremely susceptible to electricity - if they are hit, even a weak current can kill the victim.
The passage of an electric charge through the human body leaves peculiar marks on it - the so-called. "electrical marks", which are dead skin with a yellow coating, similar to calluses.
An electrical burn causes redness of the skin at the point of contact with its source, bubbles with physiological fluid inside are inflated, areas of the body become charred and blackened, sometimes pieces of metal or fabric from clothing are literally “melted” into them. Such burn injuries are treated worse than simple thermal burns; they do not always appear immediately - the consequences can become visible after hours, days or even months (therefore, all victims are under the supervision of doctors for a long time).
The most dangerous current is that which enters the areas of the back, hands, temporal and occipital parts of the head.
The extent of health damage from electric shock depends on the direction of current movement inside the human body. As a rule, there are several “routes” for the passage of charge. The fatal path of current for a person is the path from one hand holding a bare wire to the other, because it goes through the lungs, bronchi and heart muscle and causes them to fibrillate. If the victim holds onto a source containing direct current with one hand and stands with his feet on the ground, the route is called “hand-foot”, in which case the electricity disrupts the functioning of almost all internal organs and, of course, the heart muscle. The “road” of electricity through the head to the arms or legs is also fatal: if the victim touches live elements with his head. Sometimes people experience electrical injury from the so-called. “step voltage”, when they are on the ground that receives electric direct current without grounding, it passes through the body only through the legs, the heart does not suffer.
What current values are deadly?
The depth and extent of electrical damage is influenced by three main factors:
- current frequency – variable in magnitude and direction or constant;
- current strength;
- direction of current as it passes through the human body.
According to the degree of influence on people’s health, the current is divided into:
- tangible - it only causes skin irritation to a person, a safe value is a current strength of up to 0.6 milliamps;
- non-releasing - alternating current, which, due to periodic impulses, causes a person to stick to the current source, this happens with a current strength of 0.025 amperes;
- fibrillation - causes fibrillation of internal organs, primarily the heart, which can lead to its stop; the strength of such a current exceeds 0.1 Ampere.
The body of any person resists electric current (described by Ohm’s law), its value depends on the general health of the victim at the time of the electrical injury, the degree of moisture, mental state and even the quality of shoes. Knowing the values of electrical resistance, the values of current voltage are displayed, which becomes dangerous to humans.
According to the canons of electrical safety technology, the following voltage values are considered dangerous to human life and health:
- 65 volts - for residential premises and public buildings with heating and internal humidity not exceeding 60 percent;
- 36 volts – for rooms with high humidity levels up to 75 percent (for example, basements, kitchens in canteens and restaurants, metro station lobbies);
- 12 volts – for very humid (up to 100 percent) spaces (swimming pools, baths, laundries, rooms with boilers).
Additional Information. As for the frequency of the current, its value in the range of 50-60 hertz poses a danger to life.
If the current exceeds 50 milliamps, great harm is caused to health, and at values above 100 milliamps, exposure to electricity even for a few seconds can kill a person.
Which current is more dangerous: alternating or direct?
Everyone knows that current can be alternating or constant, but not everyone understands which one is more harmful to life and health. Experts answer that the most dangerous is the variable type.
Why is alternating current more dangerous than direct current? The reason is that direct current must be three times more powerful than alternating current to be mortally dangerous to a person, since alternating current has a much stronger and faster effect on nerve endings and muscle tissue (primarily the heart). The power of direct current in many cases (with a power of up to 50 milliamps) is covered by the electrical resistance of the human body, whereas for alternating current this limit is only ten milliamps. However, when electricity reaches a voltage of 500 volts, the harm from both types of current is the same, and when it is exceeded, direct current becomes more dangerous.
Our body is a conductor of electrical current, which, passing through it, has a detrimental effect on human health and can be fatal. The danger is posed not only by the strength, type and power of the current, but also by the duration of exposure and the route through the body. Current resistance depends on many conditions, so different countries have different standards that determine the safe voltage of electric current.
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A current at which a person can independently free himself from the electrical circuit should be considered acceptable. Its value depends on the speed of current passage through the human body: with a duration of action of more than 10 s - 2 mA, and with 120 s or less - 6 mA.
Safe voltage is considered to be 36 V (for local stationary lighting lamps, portable lamps, etc.) and 12 V (for portable lamps when working inside metal tanks, boilers). But in certain situations, such tensions can pose a danger.
Safe voltage levels are obtained from the lighting network using step-down transformers. It is not possible to extend the use of safe voltage to all electrical devices.
In production processes, two types of current are used - direct and alternating. They have different effects on the body at voltages up to 500 V. The danger of injury from direct current is less than from alternating current. The greatest danger is represented by current with a frequency of 50 Hz, which is standard for domestic electrical networks.
The path along which the electric current passes through the human body largely determines the degree of damage to the body. The following options for directions of current movement through the human body are possible:
- - a person touches live wires (parts of equipment) with both hands, in this case the direction of movement of the current appears from one hand to the other, i.e. “hand-to-hand”, this loop occurs most often;
- - when one hand touches the source, the current path is closed through both legs to the ground “arm-legs”;
- - when the insulation of current-carrying parts of the equipment breaks down on the body, the hands of the worker become energized; at the same time, the flow of current from the equipment body to the ground leads to the legs being energized, but with a different potential, so an “arms-legs” current path arises ;
- - when current flows to the ground from faulty equipment, the ground nearby receives a changing voltage potential, and a person who steps on such ground with both feet finds himself under a potential difference, i.e., each of these legs receives a different voltage potential, resulting in a step voltage and a “leg-to-leg” electrical circuit, which occurs least often and is considered the least dangerous;
- - touching the head to live parts can cause, depending on the nature of the work performed, a current path to the arms or legs - “head-arms”, “head-legs”.
All options differ in the degree of danger. The most dangerous options are “head-arms”, “head-legs”, “arms-legs” (full loop). This is explained by the fact that vital systems of the body - the brain, the heart - fall into the affected area.
The duration of current exposure affects the final outcome of the lesion. The longer the electric current affects the body, the more severe the consequences.
Environmental conditions surrounding a person during work activities can increase the risk of electric shock. High temperature and humidity, metal or other conductive floors increase the risk of electric shock.
According to the degree of danger of electric shock to a person, all premises are divided into three classes: without increased danger, with increased danger, and especially dangerous.
The nature of the impact of alternating and direct current on the human body is presented in Table 1.
Table 1 - The nature of the impact of alternating and direct current on the human body
|
Meaning |
Nature of impact |
|
|
AC 50 Hz |
D.C |
|
|
The beginning of the sensation - mild itching, tingling of the skin under the electrodes |
Not felt |
|
|
The sensation of current spreads to the wrist, slightly cramps the hand |
Not felt |
|
|
The pain intensifies throughout the entire hand and is accompanied by cramps. Hands, as a rule, can be removed from the electrodes |
The beginning of the sensation. The impression of skin heating under the electrode |
|
|
Severe pain and cramps in the entire arm, including the forearm. It is difficult to take your hands off the electrodes |
Increased feeling of heating |
|
|
Barely bearable pain in the whole arm. It is impossible to take your hands off the electrodes. |
Increased feeling of heating |
|
|
The hands are instantly paralyzed; it is impossible to tear yourself away from the electrodes. Severe pain, difficulty breathing |
An even greater sensation of skin heating. |
|
|
Very severe pain in the arms and chest. Breathing is extremely difficult. With prolonged current, respiratory paralysis or weakening of heart activity with loss of consciousness may occur |
Feeling of intense heating, pain and cramps in the hands. When you remove your hands from the electrodes, barely bearable pain occurs as a result of convulsive muscle contractions |
|
|
Breathing becomes paralyzed within a few seconds, and heart function is disrupted. With prolonged current flow, cardiac fibrillation may occur. |
A feeling of very strong superficial and internal heating, severe pain in the entire arm and in the chest area. Difficulty breathing. It is impossible to take your hands off the electrodes due to severe pain when contact is broken |
|
|
Heart fibrillation after 2-3 s; after a few more seconds - cardiac paralysis |
Respiratory paralysis due to prolonged current flow |
|
|
Same action in less time |
Heart fibrillation after 2-3 s; after a few more seconds - respiratory paralysis |
|
|
Breathing is paralyzed immediately - within a split second. Cardiac fibrillation, as a rule, does not occur; Temporary cardiac arrest is possible during the current flow. If the current flows for a long time (several seconds), severe burns and tissue destruction |
11.1 . Hazardous properties.
The study of the mechanism of electric shock showed that electric current causes a general reflex reaction in the body from the central and peripheral nervous systems, as well as from the cardiovascular system. This leads to disruption of the normal functioning of the heart or to respiratory arrest and is a symptom of electric shock. In other words, when exposed to current, the functions of vital organs are disrupted, and various outcomes are possible.
The effect of electric current on a person is complex and varied: it can be thermal (burn), mechanical (rupture of tissue and bones), chemical (electrolysis). But the most important thing is that the current acts biologically, disrupting the processes with which the viability of living matter is associated. Biocurrents arise in muscle tissue, especially during contraction of the heart muscle, in the central and peripheral nervous system and in other tissues. In case of electrical injuries, when electric currents from electrical installations and other sources penetrate the body, the biological balance is disturbed and pathological phenomena occur, leading to various outcomes.
The passage of an electric current (i.e., a flow of electrons) through biological tissues causes the ionization of their atoms, changes the membrane potential of the cells and tissues themselves: This leads to changes in the strength and voltage of biocurrents; the normal functioning of tissues is disrupted, either excitation or inhibition of the central nervous system occurs.
Thus, the development of electrical injury is possible not only due to the ionization of atoms and molecules of tissues from the passage of electric currents, but also due to changes in the potentials of cells in organ tissues. The biological consequence of this is metabolic disorders that can lead to death.
The thermal effect is expressed in burns of individual parts of the body, heating of blood vessels, nerves, etc.
Burns occur due to the thermal effects of current and the formation of an electric arc. The amount of heat in calories released in the conductor is expressed by the ratio: Q = 0.24×I 2 ×R×t, cal,
where 0.24 is a coefficient showing how much heat is generated in 1 second by a current of 1 A passing through a conductor with resistance t ohm ;
I - current flowing in A;
t - time in sec ;
R - conductor resistance in ohms .
All other things being equal, the greater the resistance R at the point of contact, the greater the current I and the longer the time of exposure to the current t, the more heat is generated and the stronger the burn. According to this, burns can be superficial or deep, accompanied by damage not only to the skin, but also to subcutaneous tissue, fat, muscles, nerves and bones. In the latter cases, as experience shows, the healing of the burn is very slow.
Due to significant skin resistance, predominantly superficial burns are observed (70-80%). However, at high current frequencies, internal burns can occur, even without noticeable damage to the skin surface.
Burns with a severe outcome are observed mainly when a person comes into contact (directly or through an electric arc) with live parts of installations with voltages above 1000 V.
The electrolytic effect is expressed in the decomposition of blood and other organic liquids, causing significant disturbances in their physical and chemical composition.
All this variety of actions of electric current leads to two types of damage: electrical injuries and electric shocks.
Electrical injuries - These are clearly defined local damage to body tissues caused by exposure to electric current or electric arc.
Depending on the pathological processes that occur during electric shock, according to the proposal of Academician of the USSR Academy of Medical Sciences G. A. Frenkel, the following classification of electrical injuries according to their severity has been adopted:
Electrical trauma - I degree - the presence of convulsive muscle contractions without loss of consciousness;
Electrical injuries of the second degree - convulsive muscle contraction and loss of consciousness;
Electrical injuries of the third degree - loss of consciousness and dysfunction of cardiac activity or breathing (possibly both);
Electrical injury of the IV degree - clinical death.
According to the nature of the manifestation, the following electrical injuries are distinguished: electrical burns, electrical marks, metallization of the skin and mechanical damage.
Electrical burns can be caused by the flow of current directly through the human body, as well as by the effect of an electric arc on the body. In the first case, a burn occurs as a result of the conversion of electrical energy into thermal energy and is a relatively mild injury (redness of the skin, formation of blisters). Burns caused by an electric arc are usually severe (necrosis of the affected area of skin and charring of tissue).
Contact burns develop as a result of the complex electrical and thermal effects of current and cause profound pathological changes in blood vessels, nerves, and ionized tissues.
Electric arc burns occur under various conditions of exposure to the light (ultraviolet) and thermal (infrared) influence of the electric arc, as well as during the phenomena of a two-phase short circuit or a single-phase short circuit to ground.
Electric arc burns, so-called ophthalmia, often occur during electric arc welding. Ophthalmia is usually observed in persons who are near or near the production site of electric arc welding and do not have protective masks or shields with special protective glasses.
Electrical signs - These are clearly defined spots of gray or pale yellow color with a diameter of 1-5 mm on the surface of the skin of a person exposed to current.
Electric signs are painless and their treatment usually ends successfully.
Metallization of leather - This is the penetration into the upper layers of the skin of the smallest particles of metal melted under the action of an electric arc. Usually, over time, the sore skin goes away, the affected area takes on a normal appearance and the pain disappears.
Mechanical damage are the result of sharp involuntary convulsive muscle contractions under the influence of current passing through a person. As a result, ruptures of the skin, blood vessels and nerve tissue can occur, as well as joint dislocations and even bone fractures. Mechanical damage occurs very rarely.
Electric shock - This is the excitation of living tissues of the body by an electric current passing through it, accompanied by involuntary convulsive muscle contractions.
Clinical (“imaginary”) death - the transition process from life to death, which occurs from the moment the activity of the heart and lungs ceases.
A person in a state of clinical death lacks all signs of life: he does not breathe, his heart does not work, painful stimuli do not cause any reactions, the pupils of the eyes are dilated. do not respond to light. However, during this period, life in the body has not yet completely died out, because its tissues do not all die at once and the functions of various organs do not immediately fade away. At the first moment, metabolic processes continue in almost all tissues, although at a very low level and sharply different from normal ones, but sufficient to maintain minimal vital activity. These circumstances make it possible, by influencing the more persistent vital functions of the body, to restore fading or just extinct functions, that is, to revive a dying organism.
The cells of the cerebral cortex, which are very sensitive to oxygen starvation and whose activities are associated with consciousness and thinking, begin to die first. Therefore, the duration of clinical death is determined by the time from the moment of cessation of cardiac activity and respiration until the beginning of the death of cells in the cerebral cortex; in most cases it is 4 - 5 minutes, and in the case of the death of a healthy person from an accidental cause, for example from an electric current, it is 7 - 8 minutes.
Biological (true) death - 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 a period of clinical death.
In addition, as a result of the thermal, chemical and physical action of the current, physicochemical processes simultaneously occur in the body, for example, the formation of “bone beads”, tissue ruptures of bones, electrolysis, etc. Contact electrical injuries affect the entire body.
Depending on the conditions of development and the nature of electrical injuries, contact electrical injuries, contact electrical burns, and electric arc burns are distinguished.
Contact electrical injuries occur either through contact with live parts of electrical installations that are normally energized, or through contact with structural parts that accidentally become energized due to insulation damage. They can also occur through contacts with any “ground”, soil or with individual well-grounded objects, as well as with objects located in the zone of current flow and carrying potential.
11.2 . Quantification of electrical hazards.
The body's response to the action of electric current is natural and depends on the type and magnitude of the current flowing through the human body, the duration of exposure and the path of the current.
The different nature of the reaction of individual organs to the influence of current depends not only on the parameters of the current, but is mainly determined by the nature of the electrical excitation characteristic of the tissues of these organs. The greatest excitability during electrical stimulation is characteristic of the nervous and muscle tissues of the body. Electrical excitation of nerve and muscle tissue can occur with direct electrical stimulation at a voltage of several hundredths of a volt. This phenomenon has been used for quite a long time in electrodiagnostics as a method for identifying changes in the reactions of muscles and motor nerves in certain diseases of the nervous and muscular systems.
It is known that there are certain points on the surface and inside the human body that correspond to the most electrically excitable points for each nerve and muscle.
The reaction of the neuromuscular system and individual nerves to electrical stimulation is subject to a certain pattern, which is expressed in sequential contraction of muscles when they are directly irritated or irritated by nerves by closing and opening the positive and negative poles of direct current.
Due to the relatively high resistance of the skin, significantly greater voltage will be required to electrically excite the tissue. The protective ability of the skin can explain why, at different contact locations at the same voltage, in some cases people died from electric current, and in others they were “safely injured.”
According to the physiological law of electrical excitation of biological tissue, excited tissue reacts to electrical stimulation, i.e., to the influence of electric current, only at the moment the current increases or decreases. Accordingly, the most dangerous is alternating current of industrial frequency 50-60 Hz (sinusoidal), which changes in magnitude and direction over time and has a continuous irritating effect on tissues and organs; Each period of current is, as it were, an independent irritating impulse. The alternating current frequency of 50 Hz is perceived differently by individual tissues and organs. For example, skeletal muscles are capable of reproducing the same frequency of stimulation and responding to it with a normal contraction; for the heart muscle, the limit of frequency of stimulation of which does not exceed five to six times per second, stimulation with a current of 50 Hz is excessive and disrupts the normal functioning of this organ.
Direct current, as one that does not change over time in magnitude and direction, is felt at the moments when a circuit in which a person is connected is turned on and off from the power source. Usually its effect is thermal. It can cause electrolytic excitation of tissue only at a relatively large value. At low voltages, direct current does not pose the same danger when touched by hand that occurs with alternating current.
Thus, the physiological irritating effect of current on the body in terms of the nature, intensity and consequences of the damage depends on the type and magnitude of the current flowing through the human body, duration and other factors.
There are currents: threshold; letting go; keeping in contact with live parts of equipment; currents causing cardiac fibrillation; producing a blockade of the nervous system and neutralizing the effect of currents that cause shock.
Threshold currents are currents that cause the first sensations of the influence of current: tingling, twitching in the fingers, burning, small muscle contractions. The magnitude of threshold currents (from tenths to 3-4 mA) depends on the magnitude of the voltage, the condition of the skin surface and individual sensitivity to the current. The greatest sensitivity. to the perception of small currents is observed in women.
Release currents are considered to be currents, during the passage of which a person retains the ability to independently free himself from contact with live parts.
Currents holding in contact with live parts that are energized are greater in magnitude than releasing currents and can cause cardiac fibrillation, i.e., fluttering of the ventricles of the heart. In this case, the heart muscles do not contract in coordination, but shudder, contract uncoordinated, and the work of the heart becomes arrhythmic. The heart no longer acts as an efficient pump; blood circulation stops, leading to death. It has been established that the current value of 0.1-0.5 A or even less is sufficient to cause cardiac fibrillation.
The currents that cause blockade or partial paralysis of the nervous system are equal to several amperes. As a result of the blockade of the nervous system, breathing stops. In such cases, immediate use of artificial respiration is required.
Currents that prevent shock bring the muscles of the fibrillating heart to a state of rest, i.e. defibrillation. According to the literature, cardiac defibrillation is possible at currents of 1-2 A of industrial frequency; passing such a current directly through the heart delays cardiac fibrillation; When the current is turned off, the heart resumes its normal, coordinated beating.
The characteristics are given without taking into account touch voltage and installation voltage, duration of exposure, current frequency, current path, therefore this data is not sufficient for the development of various types of specific protective devices: switches, fuses, interlocks, alarms, etc.
Striking currents should not be considered in abstraction from voltage. Voltage is not only the initial factor that determines, given the existing resistance of the human body, the amount of current flowing through it, but also a factor causing ionization of the skin at the point of contact, and therefore a change in the resistance of the human body, and therefore the total resistance decreases and may turn out to be equal to the internal resistance.
Internal resistance. Research has established that the resistance of internal tissues and organs does not depend on the magnitude of the applied voltage, changes only with changes in body temperature and on average can be taken equal to 500-1000 Ohms.
Cerebrospinal fluid..... 55.5
Brain nervous tissue....... 2500.0
Blood serum……………. 71.1
Adipose tissue……………… 5000.0
Muscle tissue…………… 151.0
Dry skin …………. over -330×10 3
Blood.....…………………….. 185.0
Liver.....………………………… 1250.0
Bone without periosteum…….. 200×10 6
The data presented show that the least resistance to current is provided by the liquid components of the body and tissues soaked in liquids.
Relatively good conductors are muscles, subcutaneous tissue and gray matter of the brain. Adipose tissue, due to the blood vessels it contains, can be a good conductor, despite the fact that adipose tissue itself is a poor conductor. Dry skin has the greatest resistance. The current enters the body through the pores and channels of the sweat glands of the skin, the presence and intensity of action of which mainly determines the conductivity of the skin.
Bone tissue has great resistance. The resistance of bones without periosteum is the highest, reaching hundreds of megohms.
According to the results of experiments conducted by L.K. Meshcheryakov at voltages not exceeding 30-40 V, and with a small surface of the electrodes (i.e., contact of the human body with the live part), the total resistance of the body is determined mainly by the active resistance of the external skin.
Increasing the contact surface reduces the external resistance. At voltages above 40 V, an increase in voltage significantly reduces the external resistance, and the total resistance at voltages of 110-220 V drops to the values of its internal resistance. It must also be taken into account that resistance is highly dependent on the striking voltage. This voltage, acting on the skin and causing ionization, reduces the resistance of the skin, and the resistance decreases accordingly.
The results of research conducted by A.P. are of great interest. Kiselev and L.K. Meshcheryakov at different times, to determine the resistance of the human body under different current path options.
In table Table 1.7 shows the characteristics of the internal resistance of the human body for different current paths and for different sizes of the electrode surface, which were carried out at low voltages in the frequency range from 50 Hz to 12-20 kHz. On average, the internal resistance of the body is 600-800 Ohms.
The outcome of exposure to electric current depends on a number of factors, including the electrical resistance of the human body, the magnitude and duration of the current flowing through it, the type and frequency of the current, and the individual properties of the person.
The duration of current flow through the human body affects the outcome of the injury due to the fact that over time the current increases sharply due to a decrease in body resistance and the accumulation of negative consequences of the influence of current on the body.
The type and frequency of the current largely determine the degree of damage. The most dangerous is alternating current with a frequency of 20 to 1000 Hz. At a frequency less than 20 or more than 1000 Hz, the risk of electric shock is noticeably reduced.
With constant current, the threshold sensible current rises to 6-7 mA, and the threshold non-releasing current - to 50-70 mA. Currents with frequencies above 500,000 Hz do not irritate tissue and therefore do not cause electric shock. However, they remain dangerous due to thermal burns.
Individual characteristics of a person - health status, readiness to work in an electrical installation and other factors also play a role in the outcome of the injury. Therefore, the maintenance of electrical installations is entrusted to persons who have undergone a medical examination and special training.
Almost all workplaces where there is electrical equipment (portable electrical receivers) under voltage are considered dangerous. In each such place, the danger of electric shock to a person cannot be considered excluded. Approximately 70% of accidents involving electric shock occur during the victim's professional activity. According to long-term statistical data in general occupational injuries electrical injuries are about 2%, and in fatal cases - about 20%.
EFFECT OF ELECTRIC CURRENT ON HUMAN
The human body is a conductor of electric current. Electric current has significant features that distinguish it from other harmful and dangerous production factors.
The first feature of electric current is that it has no color, smell, or sound, and therefore a person cannot use his own senses to determine the presence of electric current.
The second feature of electric current is that you can get an electrical injury without direct contact with live parts (for example, when moving along the ground (conductive floor) near a damaged electrical installation, electrical receiver (in case of a short circuit to the ground, floor), as well as through electric arc, lightning strike
The third feature of electric current is that, passing through the human body, the electric current exerts its effect not only at the points of contact and on the path through the body, but also causes a reflex effect, disrupting the normal activity of individual organs and systems of the human body (nervous, cardiac, vascularity, respiratory organs, etc.)
Electric current, passing through the human body, has biological, electrochemical, thermal and mechanical effects.
Biological the effect of the current manifests itself in irritation and excitation of tissues and organs. As a result, skeletal muscle spasms are observed, which can lead to respiratory arrest, avulsion fractures, dislocations of the limbs, and spasm of the vocal cords.
Electrolytic the effect of the current is manifested in electrolysis(decomposition) of liquids, including blood, and also significantly changes the functional state of cells.
Thermal the action leads to burns of the skin, as well as death of the underlying tissues, up to charring.
Mechanical the effect of the current manifests itself in tissue separation and even separation of body parts.
Typical types of local electrical injuries - electrical burns, electrical signs, skin metallization, electroophthalmia and mechanical damage.
The most common electrical injuries are electrical burns. According to the depth of damage, all burns are divided into four degrees:
The first is redness and swelling of the skin;
The second is water bubbles;
The third is necrosis of the superficial and deep layers of the skin;
The fourth is charring of the skin, damage to muscles, tendons and bones.
Metallization of leather- penetration into it by particles of metal melted under the action of an electric arc.
Electroophthalmia- inflammation of the outer membranes of the eyes as a result of exposure to a powerful stream of ultraviolet rays. Occurs most often during electric welding work.
Mechanical damage occurs as a result of sharp, involuntary, convulsive muscle contractions under the influence of current passing through the human body. This may result in ruptures of the skin, blood vessels and nerve tissue, as well as dislocations of joints and broken bones.
The variety of effects of electric current on the body leads to various electrical injuries. Conventionally, all electrical injuries can be divided into local and general.
TO local electrical injuries include local damage to the body or pronounced local damage to the integrity of body tissue, including bone tissue, caused by exposure to electric current or electric arc.
The most common local injuries include electrical burns, electrical marks, skin metallization, mechanical injuries, and electroophthalmia.
Electrical burn (skin) occurs, as a rule, in electrical installations up to 1000 V. At higher voltages, an electric arc or spark occurs, which causes an electrical arc burn.
Electrical burn area of the body is a consequence of the conversion of the energy of the electric current passing through this area into heat. This burn is determined by the magnitude of the current, the time it takes to pass through, and the resistance of the area of the body exposed to the current. The maximum amount of heat is generated at the point of contact of the conductor with the skin. Therefore, basically an electric burn is a skin burn. However, electrical burns can also damage subcutaneous tissues. With high frequency currents, internal organs are most susceptible to electrical burns.
An electric arc causes extensive burns to the human body. In this case, the defeat is severe and often ends in the death of the victim.
Electrical signs current effects are sharply defined spots of gray or pale yellow color on the surface of the human body. They are usually round or oval in shape and measure 1-5 mm with a depression in the center. The affected area of skin hardens like a callus. The top layer of skin becomes necrotic. The surface of the sign is dry, not inflamed. Electrical signs are painless. Over time, the top layer of skin peels off and the affected area regains its original color, elasticity and sensitivity.
Metallization of leather- penetration into the upper layers of the skin by particles of metal melted under the action of an electric arc. Such cases occur during short circuits and switches tripping under load. In this case, splashes of molten metal, under the influence of dynamic forces and heat flow, scatter in all directions at high speed. Since the melted particles have a high temperature, but a small reserve of heat, they are not able to burn through clothing and usually affect exposed parts of the body - the face, hands.
The affected area of skin has a rough surface. The victim feels pain from burns in the affected area and experiences skin tension from the presence of a foreign body in it. Damage to the eyes from molten metal is especially dangerous. Therefore, work such as removing and replacing fuses must be carried out wearing safety glasses.
With direct current, metallization of the skin is also possible as a result of electrolysis, which occurs during close and relatively long-term contact with a live part that is energized. In this case, metal particles are carried into the skin by an electric current, which simultaneously decomposes the organic liquid in the tissues and forms basic and acidic ions in it.
Mechanical damage are the result of sharp involuntary convulsive muscle contractions under the influence of current passing through the human body. As a result, ruptures of tendons, skin, blood vessels and nerve tissue can occur. Joint dislocations and even bone fractures may also occur. Mechanical damage caused by convulsive muscle contraction occurs mainly in installations up to 1000 V when a person is under voltage for a long time.
Electroophthalmia occurs as a result of exposure to a stream of ultraviolet rays (electric arc) on the membrane of the eyes, as a result of which their outer shell becomes inflamed. Electroophthalmia develops 4-8 hours after irradiation. In this case, redness and inflammation of the skin of the face and mucous membranes of the eyelids, lacrimation, purulent discharge from the eyes, spasms of the eyelids and partial loss of vision occur. The victim experiences a headache and sharp pain in the eyes, aggravated by light. In severe cases, the transparency of the cornea is impaired. Prevention of electroophthalmia when servicing electrical installations is ensured by the use of safety glasses or shields with regular glass.
General electrical injuries arise when living tissues of the body are excited by an electric current flowing through it and manifest themselves in involuntary convulsive contractions of the muscles of the body. In this case, the entire body is at risk of damage due to disruption of the normal functioning of its various organs and systems, including the heart, lungs, central nervous system, etc. General electrical injuries include electrical shocks.
Electric shock- this is the stimulation of body tissues by an electric current passing through it, accompanied by muscle contraction.
Depending on the outcome of the impact of current on the human body, electric shocks can be divided into the following five degrees:
I - convulsive, barely noticeable muscle contraction;
II - convulsive muscle contraction, accompanied by severe pain, without loss of consciousness;
III - convulsive muscle contraction with loss of consciousness, but with preserved breathing and heart function;
IV - loss of consciousness and disturbance of cardiac activity and breathing;
V - lack of breathing and cardiac arrest (clinical death).
An electric shock may not lead to the death of a person, but may cause disorders in the body that may appear after a few hours or days (the appearance of cardiac arrhythmia, angina pectoris, absent-mindedness, weakening of memory and attention).
There are two main stages of death: clinical and biological.
Clinical death (sudden death)- a short-term transitional state from life to death, occurring from the moment the activity of the heart and lungs ceases. A person in a state of clinical death lacks all signs of life: there is no breathing, the heart does not work, painful stimuli do not cause a reaction in the body, the pupils of the eyes are sharply dilated and do not react to light. However, during this period, life in the body has not yet completely died out, i.e. tissues and cells do not immediately undergo decay and remain viable. The brain cells that are very sensitive to oxygen starvation begin to die first. After some time (4-6 minutes), multiple decay of brain cells occurs, which leads to irreversible destruction and practically eliminates the possibility of reviving the body. However, if before the end of this period the victim is provided with first medical aid, then the progression of death can be stopped and the person’s life can be saved.
Biological death- an irreversible phenomenon that is characterized by the cessation of biological processes in the cells and tissues of the body and the breakdown of protein structures. Biological death occurs after clinical death (7-8 minutes).
Causes of death from electric current can be: cessation of cardiac function, respiratory arrest and electric shock.
The effect of current on the heart muscle can be direct, when the current passes directly through the heart area, and reflex, that is, through the central nervous system. In both cases, cardiac arrest or fibrillation may occur. Cardiac fibrillation is a chaotic multi-temporal contraction of the fibers of the heart muscle, in which the heart is unable to pump blood through the vessels. Currents less than 50 mA and more than 5 A with a frequency of 50 Hz, as a rule, do not cause cardiac fibrillation.
Cessation of breathing usually occurs as a result of the direct effect of current on the chest muscles involved in the breathing process.
Electric shock- a kind of severe neuro-reflex reaction of the body in response to excessive irritation by electric current, accompanied by deep disorders of blood circulation, breathing, metabolism, etc. In case of shock, immediately after exposure to electric current, the victim enters a short-term phase of excitation, when he reacts sharply to the pain that has arisen, his blood pressure rises. This is followed by a phase of inhibition and exhaustion of the nervous system, when blood pressure drops sharply, the pulse drops and quickens, breathing weakens, and depression occurs. The state of shock lasts from several tens of minutes to a day. After this, either the death of a person or recovery may occur as a result of active therapeutic intervention.
The outcome of the effect of current on the human body depends on the value and duration of the passage of the current through his body, the type and frequency of the current, the individual properties of the person, his psychophysiological state, the resistance of the human body, voltage and other factors.
FACTORS AFFECTING THE DEGREE OF ELECTRIC SHOCK
The severity of electric shock depends on a number of factors: the magnitude of the force, type (type) and frequency of the electric current, the duration of its exposure and the path through the person, environmental conditions, the electrical resistance of the human body and its individual properties.
Current strength
To characterize the effects of electric current on a person, three criteria have been established:
Threshold perceptible current is the lowest value of electric current that causes noticeable irritation when passing through the human body. A person begins to feel a small current (0.6-1.5 mA with alternating current with a frequency of 50 Hz and 5-7 mA with direct current) - a slight trembling of the hands occurs;
Threshold non-releasing current - the lowest value of electric current (10-15 mA at a frequency of 50 Hz and 50-80 mA at constant current), at which a person is unable to overcome muscle cramps and cannot unclench the hand in which the conductor is clamped, or break contact with live part;
Threshold fibrillation current is the lowest value of current (from 100 mA to 5 A at a frequency of 50 Hz and from 300 mA to 5 A at constant current), causing cardiac fibrillation when passing through the human body - chaotic and multi-temporal contractions of the fibers of the heart muscle, which can cause it to stop
Type of current
The maximum permissible value of direct current is 3-4 times higher than the permissible value of alternating current, but only at a voltage not higher than 260-300 V. At higher voltage values, direct current is more dangerous for humans due to its electrolytic action; it also affects human cardiac activity.
Electric current frequency
The frequency of electric current accepted in the energy industry (50 Hz) poses a great danger of convulsions and fibrillation of the ventricles of the heart. Fibrillation is not a muscular response but is caused by repetitive stimulation with a maximum sensitivity of 10 Hz. In addition, production uses electric current of other (not 50 Hz) frequencies. The danger of current action decreases with increasing frequency, but this does not mean that a current with a frequency of 500 Hz is less dangerous than a 50 Hz current.
Current duration
The severity of the injury depends on the duration of the electric current. The time it takes for the electrical current to pass is critical in determining the extent of the injury.
With prolonged exposure to electric current, the resistance of the skin (due to sweating) at the points of contact and internal organs decreases due to electrical processes, and the likelihood of current passing during a particularly dangerous period of the cardiac cycle (phase T of cardiac muscle relaxation) increases. A person can withstand a deadly alternating current of 100 mA if the duration of the current does not exceed 0.5 s.
Path of electric current through the human body
The most important condition for electric shock to a person is the path of this current. If vital organs (heart, lungs, brain) are in the path of the current, then the danger of fatal injury is very high. If the current passes through other paths, then its effect on vital organs can only be reflexive. At the same time, although the danger of fatal injury remains, its likelihood is sharply reduced.
There are an innumerable number of possible paths for current passage in the human body. However, the following can be considered typical:
hand - hand;
hand leg;
leg - leg;
head - hand;
head - leg.
The most dangerous are the “head-arm” and “head-leg” loops, when the current can pass not only through the heart, but also through the brain and spinal cord.
Human body resistance
The electrical conductivity of different tissues of the body is not the same. Cerebrospinal fluid, blood serum and lymph have the highest electrical conductivity, followed by whole blood and muscle tissue. Internal organs that have a dense protein base, brain matter, and adipose tissue conduct electricity poorly. The skin and, mainly, its top layer (epidermis) have the greatest resistance.
The resistance of the human body depends on the gender and age of people: in women this resistance is less than in men, in children it is less than in adults. This is explained by the thickness and degree of coarsening of the top layer of skin.
Areas of the body with the least resistance (i.e. more vulnerable):
Lateral surfaces of the neck, temples;
Back of the hand, the surface of the palm between the thumb and index finger;
Hand in the area above the hand:
Shoulder, back;
Forefoot:
Acupuncture points located in different places of the body.
