The current at which a person can independently free himself from the electrical circuit should be considered acceptable. Its value depends on the rate of passage of current through the human body: with a duration of more than 10 s - 2 mA, and for 120 s or less - 6 mA.
The safe voltage is considered to be 36 V (for lamps of local stationary lighting, portable lamps, etc.) and 12 V (for portable lamps when working inside metal tanks, boilers). But under certain situations, even such voltages can be dangerous.
Safe voltage levels are obtained from the lighting network using step-down transformers. It is impossible to extend the use of safe voltage to all electrical devices.
Two types of current are used in production processes - 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 alternating current. The greatest danger is the 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 the directions of current flow through the human body are possible:
- - a person touches current-carrying wires (parts of equipment) with both hands, in this case there is a direction of current flow from one hand to the other, i.e. “hand-hand”, this loop is most common;
- - when touching with one hand to the source, the current path closes through both legs to the ground “arm-legs”;
- - in the event of a breakdown of the insulation of the current-carrying parts of the equipment, the hands of the worker are energized on the case, at the same time, the flow of current from the equipment case to the ground leads to the fact that the legs are energized, but with a different potential, this is how the “hands-feet” current path arises ;
- - when current drains to earth from faulty equipment, the earth nearby receives a changing voltage potential, and a person who stepped on such earth with both feet finds himself under a potential difference, i.e. each of these feet receives a different voltage potential, as a result, a step voltage arises and the leg-to-leg electrical circuit, which happens the least and is considered the least dangerous;
- - touching the head to the current-carrying parts can cause, depending on the nature of the work performed, the current path to the arms or legs - “head-arms”, “head-legs”.
All options differ in the degree of danger. The most dangerous are the options “head-arms”, “head-legs”, “arms-legs” (full loop). This is due to the fact that vital important systems body - brain, heart.
The duration of current exposure affects the final outcome of the lesion. The longer the electric current acts on the body, the more severe the consequences.
environmental conditions, human environment in the course of work activities may increase the risk of electric shock. Increase the risk of electric shock high temperature and humidity, metal or other conductive floor.
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, 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 |
The nature of the impact |
|
|
AC 50 Hz |
D.C |
|
|
The beginning of the sensation is a slight itching, tingling of the skin under the electrodes |
Not felt |
|
|
The sensation of current extends to the wrist, slightly reduces the hand |
Not felt |
|
|
Pain intensifies in the entire hand, accompanied by convulsions. Hands can usually be taken off the electrodes |
The beginning of the feeling. The impression of heating the skin under the electrode |
|
|
Violent pains and cramps in the whole arm, including the forearm. Hands are difficult to take off the electrodes |
Increased feeling of warmth |
|
|
Hardly bearable pains in the whole arm. Hands cannot be torn off from the electrodes. |
Increased feeling of warmth |
|
|
Hands are paralyzed instantly, it is impossible to tear yourself away from the electrodes. Severe pain, difficulty breathing |
An even greater increase in the sensation of heating the skin. |
|
|
Very severe pain in arms and chest. Breathing is extremely difficult. With prolonged current, respiratory paralysis or weakening of the heart's activity with loss of consciousness may occur. |
Sensation 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 muscle contraction. |
|
|
Breathing is paralyzed after a few seconds, the work of the heart is disturbed. With prolonged current flow, cardiac fibrillation may occur. |
Sensation of very strong superficial and internal heating, severe pains in the whole arm and in the chest. Difficulty breathing. Hands cannot be torn off the electrodes due to severe pain when contact is broken |
|
|
Fibrillation of the heart after 2-3 s; a few seconds later - heart failure |
Respiratory paralysis with prolonged current flow |
|
|
Same action in less time |
Fibrillation of the heart after 2-3 s; a few seconds later - respiratory paralysis |
|
|
Breathing is paralyzed immediately - in a fraction of a second. Fibrillation of the heart, as a rule, does not occur; possible temporary cardiac arrest during the current flow. With prolonged current flow (several seconds), severe burns, tissue destruction |
Why is electric current dangerous and what effect does it have on the human body?
The largest number of electrical injuries (60-70%) occurs in electrical installations up to 1000V. This is due to the wide distribution of these electrical installations and relatively low level electrical training of persons operating them.
Causes:
touching non-insulated live parts;
touching metal non-current-carrying parts of electrical equipment that are energized;
touching non-metallic objects that are energized;
electric shock of step voltage or touch voltage;
defeat through the arc.
The degree of dangerous or harmful effects of electric current on a person depends on the parameters of the current flowing through the human body, the duration of exposure, environmental conditions and the state of the body (mass, physical condition).
For women, the threshold current values are 1.5 times lower than for men.
The resistance of the human body is from 0.8 to 100 kOhm. Depends on the condition of the skin (wet, dry, clean or dirty).
The air environment in many cases affects the numerical values of the damaging parameters of the electrical circuit in which a person finds himself. This: Atmosphere pressure, temperature, humidity, season, altitude, electrical magnetic field, constantly acting on a person (value electric field- 120 - 150 V / m, and even more in the thunderstorm and pre-storm period).
Currents in the frequency range from 5 to 500 Hz are almost equally dangerous. With a further increase in the frequency, the values of the threshold currents increase. A noticeable decrease in the risk of human injury - at a frequency above 1000 Hz (but the effect of the electric field increases).
Direct current up to 110 V is less dangerous than alternating current. Threshold values of direct current are 3 - 4 times higher than at a frequency of 50 Hz. 150 - 600 V - the danger is about the same.
Over 600 V - alternating current is more dangerous. It is explained by physiological processes of influence on a living cell.
The degree of danger is affected by the composition of the air, the nature of the environment (room category).
The human body reacts to following currents:
Response current, mA DC current, mA
Tangible (itching and heat) 0.6 - 1.5 5 - 7
Not letting go 8 - 10
Respiratory contraction 25 - 50
muscles, suffocation
fibrillatory
(cardiac arrest) 80 - 100 (50 - 200)
Suffocation, paralysis, severe More than 500
burn, death.
Electric current, electric arc, electromagnetic or electrostatic field can produce a damaging effect of a different nature:
Thermal - heating of tissues, burns;
Electrolytic - decomposition of tissues, blood;
Biological - irritation and excitation of living tissues, muscle contraction;
Light - the effect of an electric arc on the eyes, skin;
Mechanical - damage as a result of convulsive muscle contraction or falling from a height (ruptures of the skin, blood vessels, dislocations, fractures).
There are the following electrical injury:
electrical burns,
electrical signs,
electrometallization of the skin,
Electric shock.
electrical burns There are four degrees, like any other burns. They arise as a result of heating body tissues with a current of more than 1A. There are superficial and internal:
1st degree - redness, swelling of the skin;
2nd degree - water bubbles;
3rd degree - necrosis of the deep layers of the skin;
4th degree - charring of the skin, damage to muscles, tendons and bones.
electrical signs- spots of gray or pale yellow color in the form of corns on the surface of the skin as a result of thermal exposure at the point of contact with current-carrying parts. They are painless and go away with time.
Leather plating- penetration into the upper layers of the skin of the smallest particles of molten or splashed (as a result of an arc) metal. Color grey. The skin becomes rough and painful. It passes over time. Metallization of the eyes is a great danger. Inflammation of the eyes (electrophthalmia) - as a result of exposure to ultraviolet rays of an electric arc.
electric shock- electrocution of the body as a whole, causing a violation of physiological processes in it. Manifested in convulsive muscle contraction:
without loss of consciousness;
with loss of consciousness without violation of the functions of cardiac activity and respiration;
with loss of consciousness and dysfunction of cardiac activity and respiration;
clinical death.
Can thus lead to death by asphyxiation (respiratory muscle spasms), cardiac arrest or fibrillation. Clinical death lasting more than 10 minutes leads to irreversible consequences.
Electromagnetic field has harmful biological and electrolytic effects.
The most dangerous is the electrical component of the electromagnetic field. Failure of natural processes in the body, since dipole molecules (water) line up along the field lines of force.
On outdoor switchgear and overhead lines with a voltage of 330 kV and above, if the strength of the email. fields greater than 5 kV/m, the use of protective equipment is extremely important.
At E< 5 кВ/м ограничений при работе в электроустановках нет.
The possibility of human exposure to electrical discharges should be excluded.
The strength and induction of the magnetic field are also limited. So the allowable residence time is no more than 1 hour at H = 1600 A / m or B = 200 μT; no more than 8 hours at H = 80 A/m or B = 100 µT.
Why is electric current dangerous and what effect does it have on the human body? - concept and types. Classification and features of the category "How dangerous is electric current and what effect does it have on the human body?" 2017, 2018.
11.1 . dangerous properties.
The study of the mechanism of electrodamage showed that the 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.
Action electric current on a person is complex and diverse: it can be thermal (burn), mechanical (rupture of tissues and bones), chemical (electrolysis). But the most important thing is that the current acts biologically, violating the processes with which the viability of living matter is associated. In muscle tissue, especially during contraction of the heart muscle, in the central and peripheral nervous system and in other tissues, biocurrents arise. With 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 membrane potential cells and tissues themselves: This leads to changes in the strength and tension 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 the cells of the tissues of organs. 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 the 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 sec by a current of 1 A passing through a conductor with resistance t ohm ;
I - flowing current 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 released and the stronger the burn. In accordance with this, burns can be superficial or deep, accompanied by lesions not only of the skin, but also of the subcutaneous tissue, fat, muscles, nerves and bones. In the latter cases, as experience shows, the healing of the burn is very slow.
Due to the significant resistance of the skin, predominantly superficial burns (70-80%) are observed. However, at a high frequency of current, burns of an internal nature 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 current-carrying parts of installations with a voltage above 1000 V.
The electrolytic action is expressed in the decomposition of blood and other organic liquids, causing significant disturbances in their physico-chemical compositions.
All this variety of actions of electric current leads to two types of damage: electric injuries and electric shocks.
Electrical injury - these are clearly defined local damage to body tissues caused by exposure to electric current or an electric arc.
Depending on the pathological processes that occur during electric shock, according to the proposal of Academician of the Academy of Medical Sciences of the USSR G. A. Frenkel, the following classification of electrical injuries according to their severity was adopted:
Electrical injury - I degree - the presence of convulsive muscle contractions without loss of consciousness;
Electrical injury II degree - convulsive muscle contraction and loss of consciousness;
electrical injury III degree- loss of consciousness and dysfunction of cardiac activity or respiration (possibly both);
Electrical injury IV degree - clinical death.
According to the nature of the manifestation, the following electrical injuries are distinguished: electrical burns, electrical signs, skin metallization and mechanical damage.
electrical burns can be caused by the flow of current directly through the human body, as well as the impact of an electric arc on the body. In the first case, the burn occurs as a result of the transformation electrical energy into heat and is a relatively mild injury (reddening of the skin, blistering). Burns caused by an electric arc are usually severe (necrosis of the affected area of the skin and charring of tissues).
Contact burns develop as a result of the complex electrical and thermal effects of current and cause deep pathological changes in blood vessels, nerves, and ionized tissues.
Electric arc burns occur in various conditions being in the sphere of light (ultraviolet) and thermal (infrared) influence of an electric arc, as well as in the event of a two-phase short circuit or a single-phase short circuit “to ground”.
Electric arc burns, the so-called ophthalmia, often occur during electric arc welding. Ophthalmia is usually observed in persons who are near or near the place 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 who has been exposed to current.
Electric signs are painless and their treatment ends, as a rule, safely.
Leather plating - 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 diseased skin disappears, the affected area acquires a normal appearance and painful sensations disappear.
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. Mechanical damage occurs very rarely.
Electric shock - this is the excitation of the living tissues of the body by an electric current passing through it, accompanied by involuntary convulsive muscle contractions.
Clinical ("imaginary") death - the transitional process from life to death that occurs from the moment the activity of the heart and lungs ceases.
A person who is in a state of clinical death lacks all 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. 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 all at once and the functions of various organs do not immediately die out. At the first moment, metabolic processes continue in almost all tissues, although at a very low level and sharply different from the usual ones, but sufficient to maintain minimal vital activity. These circumstances make it possible, by influencing the more stable vital functions of the organism, to restore fading or just extinct functions, i.e., to revive a dying organism.
The cells of the cerebral cortex, which are very sensitive to oxygen starvation, are the first to begin to die, with the activity of which consciousness and thinking are associated. 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 (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 the period of clinical death.
In addition, as a result of the thermal, chemical and physical effects of current in the body, physical and chemical processes occur simultaneously, 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, electric arc burns are distinguished.
Contact electrical injuries occur either in contact with current-carrying parts of electrical installations that are normally energized, or in contact with structural parts that accidentally become energized due to damage to the insulation. They can also occur through contacts with any "earth", soil or with separate well-grounded objects, as well as with objects located in the current flow zone and carrying potential.
11.2 . Quantification of the danger of electric current.
The body's response to the action of an 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 action of current depends not only on the parameters of the current, but is mainly due to the nature of the electrical excitation inherent in the tissues of these organs. The greatest excitability during electrical stimulation is characteristic of the nervous and muscular tissues of the body. Electrical excitation of nerve and muscle tissues can occur with direct electrical stimulation at a voltage of several hundredths of a volt. This phenomenon has long been used in electrodiagnostics as a method to detect 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 apparatus and individual nerves to electrical stimulation obeys a certain pattern, which is expressed in the sequential contraction of the muscles during their direct irritation or irritation of the nerves by closing and opening the positive and negative poles of direct current.
Due to the relatively high resistance of the skin, much more voltage is required to electrically excite the tissue. The protective ability of the skin can explain why, at different locations of contact at the same voltage, in some cases people died from electric current, while in others they turned out to be “safely injured”.
According to the physiological law of electrical excitation of biological tissue, the excited tissue responds to electrical stimulation, i.e., to the action of an electric current, only at the moment the current increases or decreases. Accordingly, the most dangerous is the alternating current of industrial frequency of 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 the current is, as it were, an independent irritating impulse. Frequency alternating current 50 Hz is perceived differently by individual tissues and organs. For example, skeletal muscles are able to reproduce the same frequency of stimulation and respond to it with a normal contraction, for the heart muscle, the frequency limit 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.
A direct current, as not changing in time in magnitude and direction, is felt at the moments of switching on and off from the power source of the circuit into which the person is connected. 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 danger when touched by the hand that occurs with alternating current.
Thus, the physiological irritating effect of the current on the body in terms of the nature, intensity and consequences of the lesion 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 the equipment; currents that cause cardiac fibrillation; producing a blockade of the nervous system and neutralizing the effect of currents that cause shock.
Threshold currents - currents that cause the first sensations of current exposure: tingling, twitching in the fingers, burning, slight muscle contractions The value of threshold currents (from tenths to 3-4 mA) depends on the voltage, the condition of the skin surface and individual sensitivity to current to the perception of small currents is observed in women.
Releasing currents are considered, during the passage of which a person retains the ability to independently free himself from contact with parts under voltage.
Currents holding in contact with energized live parts are larger in magnitude than releasing currents and can cause cardiac fibrillation, i.e. flutter of the ventricles of the heart. At the same time, the muscles of the heart do not contract in coordination, but shudder, contract inconsistently, 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 and even less is sufficient for the occurrence of cardiac fibrillation.
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 application 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, defibrillation of the heart is possible at currents of 1-2 A of industrial frequency; passing directly through the heart of such a current delays the fibrillation of the heart; when the current is turned off, the heart resumes its normal coordinated beating.
The characteristic is given without taking into account the touch voltage and installation voltage, duration of exposure, current frequency, current path, therefore these data are not sufficient for the development of various kinds of specific protective devices: switches, fuses, interlocks, alarms, etc.
Striking currents should not be considered abstractly from voltage. Voltage is not only the initial factor that determines the value of the current flowing through it with the existing resistance of the human body, but also the factor causing skin ionization at the point of contact, and hence the change in the resistance of the human body, in connection with which 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, it changes only with a change 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 given data show that the least resistance to the current is provided by the liquid components of the body and tissues saturated with liquids.
Relatively good conductors are muscles, subcutaneous tissue and gray matter of the brain. Adipose tissue, due to the blood vessels in it, can be a good conductor, despite the fact that adipose tissue itself is a poor conductor. Dry skin has the most resistance. The current enters the body through the pores and channels of the sweat glands of the skin, the presence and intensity of which mainly determine the conductivity of the skin.
Bone has a lot of 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 a conductive part under voltage), the total resistance of the body is determined mainly by the active resistance of the outer skin cover.
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 the 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.
Of great interest are the results of studies conducted by A.P. Kiselev and L.K. Meshcheryakov in different time, by definition of the resistance of the human body under conditions different options current paths.
In 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 the impact of 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 the current flow through the human body affects the outcome of the lesion due to the fact that over time the current increases sharply due to a decrease in the resistance of the body and the accumulation of negative effects of the 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 of less than 20 or more than 1000 Hz, the danger of electric shock is markedly reduced.
With direct current, the threshold perceptible current rises to 6-7 mA, and the threshold non-release current - up to 50-70 mA. Currents with frequencies above 500,000 Hz do not irritate tissues and therefore do not cause electric shock. However, they remain dangerous under the conditions of thermal burns.
The individual properties of a person - the state of health, readiness to work in an electrical installation and other factors are also important for the outcome of the lesion. Therefore, the maintenance of electrical installations is entrusted to persons who have passed medical checkup And special education.
Not every current kills
But any current can kill.
Austrian scientist S. Jellinek
Every year, up to 30 thousand people die from electric shock.
Students take turns giving examples of accidents with teenagers.
Leading: Dry statistics of numbers, and behind it someone's tears, the collapse of hopes and lives.
We cannot imagine our life without a refrigerator, TV, computer. Electricity has firmly entered our lives, has become commonplace. And over time we become careless, forgetting about the danger that electric current poses. You guys have started practice in the welding workshop, and not far off summer holidays and lots of free time. Therefore, our today's briefing lesson will affect both industrial and domestic electrical safety, and it is called "This useful and dangerous electric current" (the name appears on the screen). Guys, you should have prepared electrical safety questions in advance, the answers to which you would like to know. Have you prepared? Well done! Who wants to start first?
1. Is household electricity dangerous?
The strength of the current that flows in the wires of our apartments is 5-10A, which is deadly. Already at J = 0.1-0.15A, a person cannot independently break away from the electrical wire. An example of a dangerous situation in everyday life is given.
2. What are the consequences of the action of electric current on the body? (in the process of answering, pictures appear on the screen - explanations follow).
The current passes through the human body, affects the central nervous system, thereby disrupting breathing and cardiac activity. Electric shock can result in electrical burns, mechanical injury due to muscle contraction, and electric arc blindness. The most vulnerable parts of the human body are described below.
3. What factors affect the extent of electrical damage? .
The amount of current passing through the human body depends on the resistance of the human body. The lower the resistance of the body, the higher the current. The resistance decreases from: high voltage, skin condition, exposure time, O2 content in the air, high air t. The danger of electric shock depends on the path of current in the human body. The most dangerous is the current loop, the path of which lies through the heart. The leading cause of death from electric shock is fibrillation (rhythm disturbance) hearts.
4. What should I do if someone gets electrocuted?
Immediately, not forgetting about your own safety, provide assistance, first of all, freeing the victim from the effects of electricity. Having quickly de-energized the victim, you can turn off the power source; throwing off the wires from the victim with any non-conductive object; cutting or interrupting the wires on different levels; dragging the victim by the clothes. Only then can you start helping.
5. Why is it dangerous to be near a broken wire lying on the ground?
The earth, being a conductor of electric current, becomes, as it were, a continuation of the wire. The path of the current is not interrupted, and it spreads over the ground. An electric shock occurs when the feet touch two points of the ground, the wider the step, the more likely the shock. A dangerous zone is formed around a broken wire lying on the ground within a radius of 8-10 m. When entering the step voltage zone, a person is in danger, even if he does not touch the wire. You can leave the danger zone in steps without taking your feet off the ground and without creating a gap between your feet.
Leading: You have many questions, all of which you will receive answers over time. And now I want to ask you - where does the danger of electric shock lie in wait for you?
The children take turns listing the objects of danger.
Well done! In order to avoid an accident, each of you should know that the places of possible electric shock are marked with special warning signs. It is unacceptable to neglect them, and even more so to remove them.
Leading: Your profession is electrically dangerous, but do you know the main causes of electrical injuries?
Answer.
Leading: Guys, listen carefully to the production situation and determine the reasons for the accident with the worker.
Example: A team of locksmiths was installing blower pipelines. During operation, the welding transformer windings closed and the voltage from the high side entered the welding circuit. At this time, the electric welder touched the welding wire at the place where the insulation was broken, and received a fatal injury.
Lack of grounding of the secondary winding and the use of a cable with broken insulation led to an accident.
Work was carried out to connect the fire water supply to the previously installed valve. The electric welder put welding wires that were twisted into the fire well. The places of the twists were "insulated" by the material from the mittens. In the process of work, the electric welder stepped on a welding twist of wires lying on a fire-fighting water pipe, while the welding wire closed on the pipe (due to a violation of the insulation of the twist). The overalls of the electric welder caught fire, which was facilitated by its severe pollution, and he received severe burns, despite timely assistance.
The guys determine its cause.
Leading: And now attention! Situational task, carefully read and choose the correct answers and arrange them in order of priority.
Correct answers: 5, 7, 2, 1 or 5, 9, 2, 1
To finally consolidate the acquired knowledge, we offer test tasks.
TESTS
I. What is the best way to drop an electric wire from an unconscious person in your apartment:
- Dry-handled mop brought from the bathroom.
- With a dry house slipper taken from your foot.
- With a dry wooden stick brought from the yard.
- A thick magazine or book lying nearby on the table.
II. The sequence of actions when providing the first medical care to an electric shock victim lying unconscious in a bath:
- Drain the water from the bath.
- Enter the bathroom and turn off all electrical appliances from the network.
- Turn off the electricity in the entire apartment.
- Assess the condition and start cardiopulmonary resuscitation.
- Call the ambulance team.
III. The sequence of actions when providing first aid to an injured person lying unconscious under an electric wire of city lighting on a lawn near a walking path:
- Discard the wire with any non-conductive object.
- Assess the condition of the victim and, in the absence of a pulse on the carotid artery, strike the chest.
- Pull the victim 3-4 meters away from the wire lying on the ground and place him on a footpath free of grass.
- Pull the victim 3-4 meters away from the wire lying on the ground away from the footpath that people can walk on.
- Quickly run up to the victim or approach with large steps.
- Carefully approach with a goose step.
- Ask others to call an ambulance.
Correct answers: I (2, 4); II (3, 2, 1, 4, 5); III (6, 1, 4, 2, 7)
But before we part with you - 5 commandments - how to avoid electric shock.
This concludes our briefing lesson, I hope each of those present received useful information, be healthy!
- O.N. Kulikov “Labor protection in the production of welding works”, M .: “Academy”, 2005.
- Journal "Library of an engineer for labor protection", No. 11, 2006.
- Materials of the "Republican seminar on the prevention of children's electrical injuries", 2006.
Chapter 14
FROM THE ACTION OF ELECTRIC CURRENT
ELECTRIC SHOCK HAZARD
The effect of electric current on the human body. Electric current is currently used in all areas of human activity: production, everyday life, medicine, etc., as an energy source that is convenient to transport and use. With all the advantages of using electricity, the danger of electricity to humans cannot be ignored.
The effect of electric current on living tissue unlike other factors, it has a peculiar and versatile character. Passing through the body, the electric current produces thermal, electrolytic, mechanical (dynamic) and biological effects.
thermal action manifests itself in the heating of tissues up to burns of individual parts of the body, heating to a high temperature of blood vessels, nerves, heart, brain and other organs that are on the path of the current, which causes serious functional disorders in them.
Electrolytic action causes decomposition of blood and plasma, which is accompanied by significant violations of their physico-chemical composition.
Mechanical (dynamic) action current is expressed in stratification, rupture and other similar damage to various tissues of the body: muscle tissue, walls of blood vessels, vessels of the lung tissue.
Biological action It is expressed in irritation and excitation of the living tissues of the body, which may be accompanied by involuntary convulsive muscle contractions, including the muscles of the heart and lungs, as well as in violation of internal bioelectrical processes that occur in a normally functioning organism and are closely related to its vital functions.
These actions are conditionally reduced to two main types of injuries: local electrical injuries and electric shocks.
Local electrical injuries - these are clearly defined local violations of the integrity of body tissues caused by exposure to electric current or an electric arc. Typical types of local electrical injuries - electrical burns, electrical signs and marks, skin metallization, electrophthalmia and mechanical damage.
Electric shock - This is the excitation of living tissues of the body by an electric current passing through it. It can cause convulsive contraction of muscles without loss of consciousness, with loss of consciousness, without damage or with damage to the work of the heart and respiratory system, as well as clinical death. Clinical or imaginary death - a short-term transitional state from life to death, occurring from the moment the activity of the heart and lungs ceases. The signs of clinical death are as follows: cardiac arrest and, as a result, the absence of a pulse, lack of breathing, the skin is bluish-pale, the pupils of the eyes are sharply dilated (due to oxygen starvation of the cerebral cortex) and do not react to light, pain irritations do not cause any reactions in the victim. 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.
Factors that determine the outcome of electric shock. In general, the degree of electric shock is determined by the amount of absorbed electrical energy in organs, tissues and systems when an electrical circuit occurs through the human body.
The nature of the impact and the severity of human injury depends on many interrelated factors, such as the strength of the current, the duration of the current, the resistance of the human body, the path of passage, the type (constant, rectified, variable) and frequency of the current, the “attention factor”, the individual properties of the victim and factors environment.
With the increase current strength three qualitatively different responses of the body are clearly manifested: an unpleasant sensation, convulsive contraction of the muscles and fibrillation of the heart. Electric currents that cause an appropriate reaction are divided into tangible, non-release and fibrillation, and their minimum values are usually called threshold.
as show experimental studies, a person begins to feel the flow of an alternating current with a frequency of 50 Hz through him with a power of the order of 0.6 - 1.5 mA. A perceptible current does not cause disturbances in the body's activity, therefore, its long-term flow through the human body under production conditions is permissible.
If a person who has been energized is able to independently overcome the effect of convulsions and free himself from contact with conductors, then such a current is called releasing. In cases where a person cannot free himself from contact, there is a danger of prolonged convulsions. The currents that cause such a reaction of the body are called non-letting currents. The threshold values of non-releasing alternating currents at a frequency of 50 Hz lie within 10 - 15 mA. At 25–50 mA, the action of the current also extends to the muscles of the chest, which leads to difficulty and even cessation of breathing. When exposed to this current for several minutes, death may occur due to the cessation of lung function. There is a dependence of the threshold non-release currents on the weight of a person and his age. So, with an increase in weight from 50 to 80 kg, the value of the threshold current increases by 1.4 - 2 times.
A current of 50–80 mA affects the respiratory and cardiovascular systems. At 100 mA for 2–3 seconds, cardiac fibrillation occurs, which consists in random chaotic contraction and relaxation of the muscle fibers of the heart (fibrils). It stops, circulation stops. This current is called fibrillation.
Duration of current flow Through the human body, it affects the resistance of the skin, as a result of which, with an increase in the time of current exposure to living tissue, its value increases, and the consequences of current exposure to the body increase.
Permissible currents for a person are evaluated according to three electrical safety criteria. First criterion - a perceptible current that does not cause disturbances in the activity of the body and is allowed for a long (no more than 10 minutes per day) flow through the human body in the normal (non-emergency) mode of the electrical installation. For alternating current with a frequency of 50 Hz, its strength is 0.3 mA, and for direct - 1 mA. The releasing current is taken as the second criterion. Its effect on a person is permissible with a flow duration of more than 1 second. The release current for AC is 6 mA, for DC - 15 mA. The third criterion is the fibrillation current, which does not exceed the threshold fibrillation current and is short-term (up to 1 s). The maximum allowable values of alternating currents with a frequency of 50 Hz and contact voltages during emergency operation of industrial electrical installations with voltage up to 1000 V, depending on the duration of exposure, should not exceed the values \u200b\u200bspecified in GOST 12.1.038-82 as amended. dated 01.07.88 and given in Table 14.1.
Table 14.1
Maximum permissible values of contact voltage U etc. and currents I h,
flowing through the human body, in emergency mode
industrial electrical installations with voltage up to 1000 V
| t, With | Duration of current exposure t, With | Maximum permissible values, no more | |||
| U pr, V | I h, mA | U pr, V | I h, mA | ||
| 0,01-0,08 | 0,6 | ||||
| 0,1 | 0,7 | ||||
| 0,2 | 0,8 | ||||
| 0,3 | 0,9 | ||||
| 0,4 | 1,0 | ||||
| 0,5 | Over 1.0 |
Knowledge of the norms for permissible values of contact voltage and currents through the human body is necessary when developing methods and means of protecting people, when assessing electrical safety conditions in existing electrical installations, and when investigating electrical injuries.
Electrical resistance of the human body is a variable that depends on the touch voltage, the condition of the skin, the parameters of the electrical circuit, physiological factors and the state of the environment.
Complete electrical resistance The human body has active and capacitive components and consists of the resistance of the skin and the resistance of internal tissues.
The upper layer of the skin, called the epidermis, and consisting mainly of dead keratinized cells, has a high resistance, which determines the overall resistance of the human body. The resistance of the lower layers (dermis) and internal human tissues is insignificant (300–500 ohms). With dry, clean and intact skin, the resistance of the human body, measured at voltages up to 15–20 V, ranges from (3–100) × 10 3 Ohm. When moistened, as well as when it is damaged (under the contacts), the resistance of the body turns out to be the smallest - about 500 Ohm, i.e. reaches a value equal to the resistance of the internal tissues of the body. For approximate calculations, the resistance of the human body is considered purely active and equal to 1 kOhm at touch voltages above 50 V, 6 kOhm at touch voltages less than 50 V.
The inclusion of vital organs of a person in the zone of action of the current increases the possibility of a severe outcome. The most dangerous are the loops when the head and spinal cord. A fatal outcome is possible even at low voltages (12 V), if the current passes through the biologically active points of the body on the neck, temples, lower leg, shoulders, back and other places of the human body.
At voltages up to 500 V, alternating current is more dangerous; with a further increase in voltage, the danger of direct current increases rapidly.
When the frequency of the alternating current changes from zero to 100 Hz, the danger of injury at the same voltage increases, reaching a maximum in the range of 50 - 60 Hz, at a frequency of 200 Hz, the risk of fibrillation is reduced by 2 times, at a frequency of 400 Hz - more than 3 times.
Currents above 500,000 Hz do not cause electric shock, but they can cause thermal burns.
A certain influence on the outcome of the lesion has a physical and psychological condition person. Fatigue, depressed mental state, alcohol consumption, a number of diseases increase the risk of exposure to electric current. Therefore, a list of diseases has been defined, in the presence of which work in existing electrical installations is not allowed. Maintenance of electrical installations is entrusted to employees who have undergone a medical examination and special training. Great importance has an "attention factor" that reduces the danger of current.
In general, people in the room are less at risk. However, if this is a production room, then the presence of dampness, conductive dust, many types of electrical equipment, and aggressive environments increase the risk of electric shock.
Classification of premises according to the danger of electric shock. In accordance with the Electrical Installation Rules (PUE), all premises are divided into three classes in relation to the danger of electric shock to people: without increased danger, with increased danger, especially dangerous.
Premises without increased danger - these are dry, dust-free rooms with normal air temperature and with insulating (for example, wooden) floors, i.e. in which there are no conditions that create an increased or special danger. Such premises include office premises, tool rooms, laboratories, etc.
High risk areas characterized by the presence in them of one of the following conditions that create an increased danger: dampness (relative humidity exceeds 75% for a long time) or conductive dust (coal, metal, etc.); conductive floors (metal, earthen, reinforced concrete, brick, etc.); high temperature (air temperature above +35 C); the possibility of a person simultaneously touching the metal structures of buildings connected to the ground, technological devices, mechanisms, etc., on the one hand, and to the metal cases of electrical equipment - with another.
Examples of premises with increased danger are stairwells of buildings with conductive floors, storage rooms that are not heated, etc.
Particularly dangerous premises, characterized by the presence of one of the following conditions that create a particular danger: extreme dampness (relative humidity close to 100%); chemically active or organic environment that destroys insulation and current-carrying parts of electrical equipment; two or more high-risk conditions at the same time.
Particularly dangerous premises are most of the industrial premises, including all equipment repair shops, workshops, etc.
Territories of location of outdoor electrical installations (in the open air or under a canopy) in relation to the danger of electric shock to people are equated to especially dangerous premises
