The environment (natural, industrial and domestic) is fraught with potential danger of various kinds... Among them - electric shock. With the widespread use of the achievements of scientific and technological progress in production and in everyday life, the factors of this risk are increasing, although modern electrical devices are being certified in terms of safety.
The danger of electric shock at work and in everyday life appears if precautions are not taken, as well as in case of failure or malfunction of electrical equipment and household appliances. A person cannot detect voltage at a distance without special devices, it is detected only when there is a touch to live parts. In comparison with other types of industrial injuries, electrical injuries account for a small percentage, but in terms of the number of injuries with severe and especially fatal outcomes, it occupies one of the first places. In production, due to non-compliance with safety regulations, 75% of electrical injuries occur.
Act electric current on the human body.
Electricity represents an orderly movement electric charges... The current in the section of the circuit is directly proportional to the potential difference, that is, to the voltage at the ends of the section and inversely proportional to the resistance of the section of the circuit.
Having touched an energized conductor, a person includes himself in electrical circuit, if it is poorly insulated from the ground or simultaneously touches an object with a different potential value. In this case, an electric current passes through the human body.
The effect of electric current on living tissue is versatile. Passing through the human body, an electric current produces thermal, electrolytic, mechanical, biological and light effects.
With thermal action, overheating and functional disorder of organs along the path of current flow occur.
Electrolytic action current is expressed in the electrolysis of fluid in the tissues of the body, including blood, and the violation of its physical and chemical composition.
Mechanical action leads to tissue rupture, stratification, shock effect of evaporation of fluid from body tissues. The mechanical action is associated with a strong contraction of the muscles up to their rupture.
Biological action current is expressed in irritation and overexcitation of the nervous system.
Light action leads to eye damage.
The nature and depth of the effect of electric current on the human body depends on the strength and type of current, the time of its action, the path of passage through the human body, the physical and psychological state of the latter. Thus, human resistance under normal conditions with dry intact skin is hundreds of kilo-ohms, but under unfavorable conditions it can drop to 1 kilo-ohm.
A current of about 1 mA is felt. With a higher current, a person begins to feel unpleasant painful muscle contractions, and with a current of 12-15 mA, he is no longer able to control his muscular system and cannot independently break away from the current source. Such a current is called non-releasing. The action of a current above 25 mA on muscle tissue leads to paralysis of the respiratory muscles and respiratory arrest. With a further increase in current, cardiac fibrillation may occur.
Alternating current is more dangerous than direct current. What matters is what parts of the body a person touches the live part. The most dangerous are those paths in which the brain or spinal cord (head-arms, head-legs), heart and lungs (arms and legs) are affected. Any electrical work must be carried out away from grounded equipment elements (including water pipespipes and radiators) to prevent accidental touching.
Types of electric shock to the human body.
A typical case of hitting voltage is contact with one pole or phase of the current source. The voltage acting on a person is called touch voltage. Particularly dangerous are areas located on the temples, back, backs of the arms, legs, back of the head and neck.
Premises with metal, earthen floors and damp areas pose an increased danger. Especially dangerous are rooms with acid and alkali vapors in the air. Safe for life is voltage not higher than 42 V for dry rooms heated with non-conductive floors without increased danger, not higher than 36 V for rooms with increased danger (metal, earthen, brick floors, dampness, the possibility of touching grounded structural elements), not higher than 12 B for especially hazardous premises with a chemically active environment or two or more signs of premises with increased danger.
In the event that a person is near a live wire that has fallen to the ground, there is a risk of injury from step voltage. The step voltage is the voltage between two points of the current circuit, located at a step distance from one another, on which a person is simultaneously standing. Such a circuit creates a current flowing along the ground from the wire. Once in the zone of current spreading, a person must connect his legs together and, slowly, leave the danger zone so that when moving the foot of one leg does not completely go beyond the foot of the other. In case of an accidental fall, you can touch the ground with your hands, which increases the potential difference and the risk of injury.
Electric current has an internal effect on a person, leading to external injuries, electric shocks and electric shock.
Internal effects can be thermal, electrolytic and biological.
Thermal impact - this is burns, heating and damage to blood vessels, overheating of the heart, brain and other internal organs, which leads to their functional disorders.
Electrolytic exposure - this is the decomposition of organic liquid, including blood, which causes significant disturbances, both in its composition and in the tissue as a whole.
Biological impact... A normally functioning organism is characterized by certain biochemical processes and biorhythms that provide vital functions. When exposed to electric current, they are disrupted.
In addition to internal influences, electric current can cause external injuries.
Electrical burns are of two types: current and arc. Current (contact) occurs when directly touching a live part due to conversion electrical energy in heat. As a rule, this is a skin burn. the skin has many times more resistance than other body tissues. A current burn occurs when working on electrical installations with a voltage of 1 - 2 kV and is, in most cases, a 1 - 2 degree burn (skin redness and blistering).
Arc burn arises under the influence of an electric arc and, since the temperature of the arc is over 3500 ° C, such a burn is of a severe nature of 3 - 4 degrees (from charring of the skin to charring of the subcutaneous retina, muscles, blood vessels, nerves, bones). An electric arc can occur between a human body and a live part at voltages in electrical installations of more than 2 kV.
Electric sign - a clear spot of gray or pale yellow color with a diameter of 1 - 5 mm on the skin. The affected area of \u200b\u200bthe skin hardens like a callus. Over time, the top layer of the affected skin peels off and it regains its original color, sensitivity and elasticity.
Skin electrometallization occurs when metal particles penetrate the skin due to its splashing and evaporation under the influence of current (when an electric arc is burning). The damaged area becomes hard and rough, its color is determined by the color of the metal that has penetrated the skin. Over time, sore skin comes off, painful sensations disappear. In case of damage to the eyes, treatment is long, complex, trauma can lead to loss of vision.
Electrophthalmia (eye damage). This is an inflammation of the outer membranes under the influence of a powerful stream of ultraviolet rays in an electric arc. It appears after 2 - 6 hours: redness and inflammation of the mucous membranes of the eyes, purulent discharge, cramps of the eyelids, partial blindness. The victim experiences a severe headache, sharp pain in the eyes, which intensifies in the light, and photophobia occurs.
Mechanical damage. They arise due to a sharp involuntary muscle contraction under the action of a current, which leads to rupture of the skin, blood vessels, nerve tissues, dislocation of joints, and bone fractures.
Electric shock is the excitation of living tissues of the body by a passing electric current, accompanied by a sharp, involuntary muscle contraction. An electric shock can lead to disruption and even complete cessation of the activity of vital organs - lungs, heart, and therefore to the death of the body. Depending on the outcome of the defeat, electric shocks are conventionally divided into 4 degrees:
1 degree - convulsive muscle contraction without loss of consciousness;
2 degree - convulsive muscle contraction with loss of consciousness;
3 degree - loss of consciousness and impaired cardiac activity;
4 degree - clinical death - the transition from life to death, which occurs at the time of cessation of the activity of the heart and lungs. All signs of life are absent: there is no breathing, the heart does not work, the pupils are dilated and do not react to light, there is no reaction to painful irritation. The duration of clinical death is determined by the time from the moment of cessation of respiration and cardiac activity until the beginning of cell death in the cerebral cortex. Under normal conditions, it is 4 - 5 minutes, and with electrical injuries - 7 - 8 minutes.
Electric shock - the reaction of the nervous system of the body in response to severe irritation with electric current. It leads to circulatory disorders, breathing, high blood pressure. Shock has two phases: excitement and inhibition. The stage of inhibition is characterized by depletion of the nervous system, increased heart rate, weak breathing, depression, complete indifference to the environment with complete preservation of consciousness. The state of shock can last from several tens of minutes to a day, after which the body dies.
The nature of the electric shock depends on the value and type of current, the path of passage, the duration of exposure, the individual characteristics of the person, the physiological state at the time of injury.
Action Email current on the human body, types of exposure, types of damage
Electrical safetyb is a system of organizational and technical measures and means to protect people from harmful and dangerous effects of electric current, electric arc and static electricity in order to reduce electrical injuries to an acceptable level of risk and below.
A distinctive feature of electric current from other industrial hazards and hazards (except for radiation) is that a person is not able to detect electric voltage remotely with his senses.
In most countries of the world, statistics of accidents due to electric shock show that the total number of injuries caused by electric current with loss of ability to work is small and amounts to approximately 0.5-1% (in the energy sector - 3-3.5%) of the total number of accidents in production. However, with a fatal outcome, such cases in production are 30-40%, and in the energy sector up to 60%. According to statistics, 75-80% of fatal electric shocks occur in installations up to 1000 V.
An electric current flows through the human body if there is a potential difference between its two points. The voltage between two points of the current circuit that a person touches at the same time is called tension of touch
The effect of electric current on the human body
Passing through the body, the electric current causes thermal, electrolytic and biological effects.
Thermal actionit is expressed in burns of certain parts of the body, heating of blood vessels and nerve fibers.
Electrolytic actionit is expressed in the decomposition of blood and other organic fluids, causing significant violations of their physical and chemical composition.
Biological actionmanifests itself in irritation and arousal of living tissue organisms, which may be accompanied by involuntary convulsive contraction of muscles, including the muscles of the heart and lungs. As a result, various disorders in the body can occur, including a violation and even complete cessation of the activity of the respiratory and circulatory organs.
The irritating effect of the current on the tissues can be direct when the current passes directly through these tissues, and reflex, that is, through the central nervous system, when the current path lies outside these organs.
All the variety of action of electric current leads to two types of injury: electrical injury and electrical shock.
Electrical injuryare clearly expressed local damage to body tissues caused by exposure to an electric current or an electric arc (electrical burns, electrical signs, skin metallization, mechanical damage).
Electric shock- This is the excitation of living tissues of the body by an electric current passing through it, accompanied by an involuntary convulsive muscle contraction.
Distinguish four degrees of electric shock:
I degree - convulsive muscle contraction without loss of consciousness;
II degree - convulsive muscle contraction with loss of consciousness, but with preserved breathing and heart function;
III degree - loss of consciousness and impaired cardiac activity or breathing (or both together);
IV degree - clinical death, that is, lack of breathing and blood circulation.
Clinical ("imaginary") death- This is a transitional process from life to death, starting from the moment the activity of the heart and lungs stops. 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 cells of the cerebral cortex (4-5 minutes, and in the case of death of a healthy person from accidental causes - 7-8 minutes). Biological (true) death- This is an irreversible phenomenon characterized by the cessation of biological processes in the cells and tissues of the body and the decay of protein structures. Biological death occurs after the period of clinical death.
Thus, causes of death from electric shockthere may be cardiac arrest, respiratory arrest, and electrical shock.
Cardiac arrest or fibrillation, that is, chaotic fast and multi-temporal contractions of the fibers (fibrils) of the heart muscle, in which the heart stops working as a pump, as a result of which the blood circulation in the body stops, can occur with a direct or reflex action of an electric current.
Cessation of breathing as the root cause of death from electric current is caused by the direct or reflex action of the current on the muscles of the chest involved in the breathing process (as a result - asphyxiation or suffocation due to lack of oxygen and excess carbon dioxide in the body).
Types of electrical injuries:
- electrical burns –
Electrometallization of the skin
Electrical signs
Electric shock
Electrophthalmia
Mechanical damage
Electrical burnand arise during the thermal action of an electric current. The most dangerous are burns: arising from exposure to an electric arc, since its temperature can exceed 3000 ° C.
Electrometallization of the skin- penetration of the smallest metal particles into the skin under the influence of electric current. As a result, the skin becomes electrically conductive, that is, its resistance drops sharply.
Electrical signs- spots of gray or pale yellow color, arising from close contact with a live part (ps of which an electric current flows in working condition). The nature of electric signs has not yet been sufficiently studied.
Electrophthalmia- damage to the outer membranes of the eyes due to exposure to ultraviolet radiation from an electric arc.
Electric shock is a general damage to the human body, characterized by convulsive contractionsmuscles, violation of the nervous and cardiovascular systems of a person. Electrical shocks are often fatal.
Mechanical damage(tissue ruptures, fractures) occur with convulsive muscle contraction, as well as as a result of falls when exposed to electric current.
The nature of the electric shock and its consequences depend on the value and type of current, the path of its passage, the duration of exposure, the individual physiological characteristics of a person and his condition at the time of injury.
Electric shock- this is a severe neuro-reflex reaction of the body in response to strong electrical irritation, accompanied by dangerous disorders of blood circulation, respiration, metabolism, etc. This condition can last from several minutes to a day.
Basically, the value and type of current determine the nature of the lesion. In electrical installations up to 500 V, alternating current of industrial frequency (50 Hz) is more dangerous to humans than direct current. This is due to complex biological processes occurring in the cells of the human body. As the frequency of the current increases, the risk of injury decreases. At a frequency of the order of several hundred kilohertz, electrical shocks are not observed. Currents, depending on their value according to their effect on the human body, are divided into tangible, not letting goand fibrillatory.Perceptible currents- currents that cause tangible irritation when passing through the body. The person begins to feel the impact alternating current (50 Hz) at values \u200b\u200bfrom 0.5 to 1.5 mA and direct current - from 5 to 7 mA. Within these values, slight trembling of the fingers, tingling, heating of the skin (with constant current) are observed. Such currents are called threshold sensible currents.
Non-releasing currentscause convulsive contraction of the arm muscles. The smallest current value at which a person cannot independently tear off his hands from live parts is called threshold non-release current... For alternating current, this value lies in the range from 10 to 15 mA, for direct current - 50 to 80 mA. With a further increase in current, damage to the cardiovascular system begins. Breathing becomes difficult, and then stops, the work of the heart changes.
fibrillation currentscause fibrillation of the heart - flutter or arrhythmic contraction and relaxation of the heart muscle. As a result of fibrillation, blood from the heart does not flow to the vital organs and, first of all, the blood supply to the brain is disrupted. The human brain, deprived of blood supply, lives for 5 to 8 minutes, and then dies, so in this case it is very important to provide first aid to the victim quickly and in a timely manner. Fibrillation current values \u200b\u200brange from 80 to 5000 mA
Factors affecting the outcome of the defeat El. shocked
The outcome of the impact of electric current on the human body depends on a number of factors, the main of which are: the electrical resistance of the human body; the magnitude of the electric current; the duration of its effect on the body; the magnitude of the stress acting on the body; type and frequency of current; the path of current flow in the body; psychophysiological state of the organism, its individual properties; condition and characteristics environment (air temperature, humidity, gas and dustiness of the air), etc.
Current strengthI.Currents:
0,6 – 1,5 mA: there is a sensation (change), not felt (constant)
5 - 7mA: convulsionsin the hands of (change), there is a feeling (constant)
20 -25mA: threshold, not letting go - the hands are paralyzed, it is impossible to tear off the equipment, slowing of breathing (changes), slight muscle contraction (constant)
50 - 80mA: fibrillation - arrhythmic contraction or relaxation of the heart muscles
|
At AC 50 Hz |
At constant current |
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Feeling, slight trembling of fingers |
Not felt |
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Hand cramps |
Sensation, skin heating Increased heating |
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Hands are difficult, but you can still tear them off the electrodes; severe pain in the hands and forearms |
Increased heating |
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Hands are paralyzed, it is impossible to tear them off the electrodes, breathing is difficult |
Minor muscle contraction |
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Stop breathing. Onset of cardiac fibrillation |
Strong heat; contraction of the arm muscles; difficulty breathing |
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Cessation of breathing and cardiac activity (with an exposure duration of more than 3 s) |
Stop breathing |
Duration of exposure to current on the human bodyis one of the main factors. The shorter the exposure time, the less the hazard.
If the current is not letting go, but still does not disrupt breathing and the work of the heart, a quick shutdown saves the victim, who could not free himself. With prolonged exposure to current, the resistance of the human body decreases and the current increases to a value that can cause respiratory arrest or even cardiac fibrillation.
Cessation of breathing does not occur instantly, but after a few seconds, and the more current through a person, the less this time. Timely shutdown of the victim prevents the cessation of the respiratory muscles.
Thus, the shorter the duration of the action of the current on a person, the less the probability of the coincidence of the time during which the current passes through the heart with phase T.
Current path in the human body... The most dangerous is the passage of current through the respiratory muscles and heart. Thus, it was noted that 3.3% of the total current passes through the heart along the "hand-arm" path, "left arm - legs" - 3.7%, "right arm - legs" - 6.7%, "leg - leg "- 0.4%," head - legs "- 6.8%," head - hands "- 7%. According to statistics, disability for three days or more was observed with the current path "hand - arm" in 83% of cases, "left arm - legs" - in 80%, "right arm - legs" - 87%, "leg - leg" - in 15% of cases.
Thus, the current path affects the outcome of the lesion; the current in the human body does not necessarily follow the shortest path, which is explained by the large difference in the specific resistance of various tissues (bone, muscle, fat, etc.).
The smallest current through the heart passes with the current path along the lower leg-leg loop. However, one should not draw conclusions from this about the low danger of the lower loop (action of step voltage). Usually, if the current is large enough, it causes leg cramps, and the person falls, after which the current can already pass through the chest, that is, through the respiratory muscles and heart. Most dangerousis a path through the brain and spinal cord, heart, lungs
Type and frequency of current... It has been established that alternating current with a frequency of 50-60 Hz is more dangerous than direct current. since the same influences are caused by higher values \u200b\u200bof direct current than alternating one. However, even a small direct current (below the sensation threshold), when the circuit is quickly broken, gives very sharp blows, sometimes causing muscle cramps in the arms.
Many researchers claim that the most dangerous is alternating current with a frequency of 50-60 Hz. Danger of current action decreases with increasing frequency, but a current with a frequency of 500 Hz is no less dangerous than 50 Hz.
Human body resistanceunstable and depends on many factors - skin condition, size and density of contact, applied voltage and time of exposure to current.
Usually, when analyzing the danger of electrical networks and in calculations, it is customary to consider the resistance of the human body to be active and equal to 1 kOhm.
The nature of the lesion also depends on the duration of the current. With prolonged exposure to current, the heating of the skin increases, the skin is moistened due to perspiration, its resistance decreases and the current passing through the human body increases sharply.
The nature of the lesion is also determined by the individual physiological characteristics of a person. If the person is physically healthy, then the electrocution will be less severe. With diseases of the cardiovascular system, skin, nervous system, with alcohol intoxication, electrical injury can be extremely serious even with small acting currents.
The psychophysiological preparedness of the employee for the impact has an important influence on the outcome of the lesion. If a person is attentive, focused when performing work, prepared for the fact that he may be exposed to electric current, then the injury may be less severe.
ENVIRONMENTAL PARAMETERS: temperature, humidity, dust
Physiological characteristics of the body at the time of injury
The applied voltage dependence is directly proportional
Phenomenon when current flows into the ground
The foot-to-foot path is least dangerous... Most often, such a path occurs when a person falls under the influence of the so-called step stress, that is, between points on the earth's surface that are at a step distance from each other. 
If there is a short circuit to the ground of any circuit - an accidental electrical connection of a live part directly to the ground or through a metal structure, then an electric current will spread along the ground, called earth fault current.The ground potential will change from maximum to zero value with distance from the fault location,
since the ground resists the earth fault current.
Fig. 1 Turning on a person to step tension
If a person enters the zone of current spreading, then a potential difference will exist between his feet, which will cause the current to flow along the leg-to-leg path. The effect of the current can cause muscle contraction in the legs and the person may fall. A fall will cause a new, more dangerous circuit of current to flow through the heart and lungs.
In fig. 3.1 shows the formation of step voltage and shows the potential distribution curve on the earth's surface. At a distance of 20 m from the short circuit, the potential can be considered equal to zero. Figure: 3.1. Turning on a person to step tension
The value of the current passing through the human body depends on the applied voltage and body resistance. The higher the voltage, the more current flows through the person
(I 2 - the path of passage is more dangerous and more current is higher)
Touch and step voltages
Step voltage - voltage on the surface of the earth between points located at a step distance from each other.
Touch voltage - the potential difference between two points of electrical whose chains are simultaneously touched by a person.
To reduce the difference φ 2 -φ 1, you need to leave the spreading zone in small steps
Classification of premises according to the degree of danger of electric shock
Electrical installationsare called installations in which electrical energy is produced, converted, distributed and consumed. Electrical installations include generators and electric motors, transformers and rectifiers, wire, radio and television communication equipment, etc.
The safety of work in electrical installations depends on the electrical circuit and parameters of the electrical installation, rated voltage, environment and operating conditions. From the point of view of ensuring safety, all electrical installations according to the PUE are divided into installations up to 1000 V and installations above 1000 V. Since installations above 1000 V are more dangerous, then more stringent requirements are imposed on protective measures.
Electrical installations can be located indoors and outdoors. Environmental conditions have a significant impact on the state of insulation of an electrical installation, on
is the resistance of the human body, and therefore the safe one? service personnel. According to the degree of electrical safety, working conditions are divided into three categories: with increased danger of "electric shock" to people; especially dangerous; without increased danger.
Conditions with increased dangercharacterized by the presence of one of the following features: - conductive bases (reinforced concrete, earth, metal, brick);
Conductive dust that worsens the cooling conditions and insulation, but does not cause a fire hazard;
Dampness (relative humidity exceeding 75%);
Temperature long exceeding + 35 ° С;
Possibility of simultaneous contact of a person to grounded metal structures, on the one hand, and to the metal cases of electrical equipment, on the other.
To reduce the risk of electric shock under these conditions, it is recommended to use a low voltage (no more than 42 V).
Extremely hazardous conditionscharacterized by the presence of one of the following features:
special dampness (relative humidity close to 100%);
chemically active environment that destroys insulation and current-carrying parts of electrical equipment;
at least two signs with increased danger.
In conditions without increased danger, the above signs are absent
1. The effect of electric current on the human body
When operating and repairing electrical equipment and networks, a person may be in the sphere of action electric field or direct contact with live electrical current lines. As a result of the passage of current through a person, a violation of his vital functions can occur.
The danger of electric shock is aggravated by the fact that, firstly, the current has no external signs and, as a rule, a person without special devices cannot detect the danger threatening him in advance; secondly, the impact of current on a person in most cases leads to serious disorders of the most important vital systems, such as the central nervous, cardiovascular and respiratory systems, which increases the severity of the lesion; thirdly, alternating current is capable of causing intense muscle cramps, leading to a non-letting effect, in which a person cannot independently free himself from the effects of the current; fourthly, the effect of the current causes a sharp withdrawal reaction in a person, and in some cases loss of consciousness, which, when working at a height, can lead to injury as a result of a fall.
Electric current passing through the human body can have biological, thermal, mechanical and chemical effects. The biological effect consists in the ability of an electric current to irritate and excite the living tissues of the body, heat - in the ability to cause burns to the body, mechanical - to lead to tissue rupture, and chemical - to blood electrolysis.
Exposure to electric current on the human body can cause electrical injury. Electrical injury is an injury caused by electric shock or electric arc. Conventionally, electrical injuries are divided into local and general. With local electrical injuries, local damage to the body occurs, expressed in the appearance of electrical burns, electrical signs, metallization of the skin, mechanical damage and electrophthalmia (inflammation of the outer membranes of the eyes). General electrical injuries, or electrical shocks, lead to damage to the whole body, which is expressed in the violation or complete cessation of the activity of the most vital organs and systems - the lungs (respiration), the heart (blood circulation).
The nature of the effect of electric current on a person and the severity of the injury to the victim depends on many factors.
It is possible to evaluate the danger of exposure to electric current on a person by the response of the body. With increasing current, three qualitatively different responses are clearly visible. This is primarily a sensation, a more convulsive muscle contraction (non-letting go for alternating current and painful effect of constant) and, finally, cardiac fibrillation. Electric currents that cause an appropriate response are subdivided into tangible, non-releasing, and fibrillatory.
2. Factors Determining the Outcome of Electric Shock
Factors affecting the outcome of electric shock include:
the magnitude of the current, the magnitude of the voltage, the time of action, the type and frequency of the current, the path of the circuit, the human resistance, the environment, the factor of attention.
2.1. Amount of current
By the magnitude of the current, currents are divided into:
Imperceptible (0.6 - 1.6mA);
Felt (3mA);
Releasing (6mA);
Non-releasing (10-15mA);
Suffocating (25-50mA);
Fibrillation (100-200mA);
Thermal influences (5A and higher).
2.2. The magnitude of the voltage and
2.3. Time of action
According to GOST 12.1.038-82 SSBT “Maximum allowable voltages and currents. Electrical safety". The factors of the magnitude of the voltage and the time of exposure to electric current are given in table. one.
Table 1
With short-term exposure (0.1-0.5 s), a current of about 100 mA does not cause cardiac fibrillation. If you increase the duration of exposure to 1 s, then the same current can be fatal. With a decrease in the duration of exposure, the value of currents permissible for a person increases significantly. When the exposure time changes from 1 to 0.1 s, the permissible current increases 16 times.
In addition, reducing the duration of exposure to electric current reduces the risk of injury to a person based on some features of the heart. The duration of one period of the cardiocycle (Fig. 2.1.) Is 0075-0.85 s.
In each cardiocycle, there is a period of systole, when the ventricles of the heart contract (peak QRS) and push blood into the arterial vessels.
Phase T corresponds to the end of the contraction of the ventricles and they go into a relaxed state. During diastole, the ventricles are filled with blood. Phase P corresponds to atrial contraction. It has been established that the heart is most sensitive to the effects of electric current during the T phase of the cardiocycle. In order for cardiac fibrillation to occur, it is necessary to coincide in time with the action of the current with the phase T, the duration of which is 0.15-0.2 s. With a reduction in the duration of exposure to electric current, the likelihood of such a coincidence becomes less, and therefore, the risk of cardiac fibrillation decreases. If the time of current passing through a person does not coincide with the T phase, currents significantly exceeding the threshold values \u200b\u200bwill not cause cardiac fibrillation.
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Value | The nature of the impact |
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| AC 50 Hz | D.C |
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The beginning of the sensation is mild itching, tingling of the skin under the electrodes |
Not felt |
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The sensation of current spreads to the wrist, slightly reduces the hand |
Not felt |
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Pain sensations intensify throughout the hand, accompanied by convulsions; mild pain is felt throughout the arm, up to the forearm. Hands can usually be lifted off the electrodes |
The beginning of the sensation. Impression of heating the skin under the electrode |
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Severe pain and cramps in the entire arm, including the forearm. Hands are difficult, but in most cases you can still tear them off the electrodes |
Increased heating sensation |
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Barely bearable pain in the whole arm. In many cases, the hands cannot be removed from the electrodes. With an increase in duration, the flow of pain current increases |
Increased heating sensation both under the electrodes and in adjacent skin areas |
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Hands are paralyzed instantly, it is impossible to tear off the electrodes. Severe pain, difficulty breathing |
An even greater increase in the sensation of heating the skin, the emergence of a sensation of internal heating. Minor muscle contractions in the arms |
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Very severe pain in the arms and chest. Breathing is extremely difficult. With prolonged current, respiratory paralysis or weakening of the heart with loss of consciousness may occur |
Feeling of intense heat, pain and cramps in the hands. When the hands are taken off the electrodes, barely tolerable pain occurs as a result of convulsive muscle contraction |
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Breathing is paralyzed after a few seconds, the work of the heart is disrupted. With prolonged current flow, cardiac fibrillation may occur. |
Sensation of very strong superficial and internal heating, severe pain in the entire arm and chest. Difficulty breathing. Hands cannot be taken off the electrodes due to severe pain when contact is broken |
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Cardiac fibrillation after 2-3 seconds; a few seconds later - heart paralysis |
Respiratory paralysis with prolonged current flow |
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The same action in less time |
Cardiac fibrillation after 2-3 seconds; in a few seconds - respiratory paralysis |
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Breathing is paralyzed immediately - after a split second. Cardiac fibrillation usually does not occur; possible temporary cardiac arrest during the period of current flow. With prolonged current flow (several seconds), severe burns, tissue destruction |
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There are the following types of effects on the human body:
1. Thermal (manifests itself in the form of burns of individual parts of the body, heating of blood vessels, nerves, blood, plasma and other organic substrates of the body.)
2. Electrolytic (characterized by the decomposition of blood and other organic body fluids, as a result of which the composition and their physical and chemical properties change.
3. Biological (currentmanifests itself in the form of irritation and excitement of living tissues of the body, which is accompanied by involuntary convulsive contractions of muscles, including the lungs and heart. As a result of such excitement, a violation may occur, and even a complete cessation of the activity of the respiratory and circulatory organs.
There are 2 types of electric shock:
2) Local
The general includes:
1) electric shock - damage to the human body caused by the excitation of living tissues of the body with an electric current and accompanied by convulsive muscle contractions.
Depending on the outcome of the defeat, electric shocks are conventionally divided into four degrees, characterized by:
I - convulsive muscle contraction, without loss of consciousness;
II - convulsive muscle contraction, with loss of consciousness, but preservation of breathing and heart function;
III - loss of consciousness and impaired cardiac activity or breathing (or both together);
IV - clinical death, i.e., lack of breathing and blood circulation.
Deaths from electric shock can be caused by cardiac arrest, respiratory failure, and electrical shock.
Local include:
1) electric burn(current, arc) burnsare the most common electrical injuries. They are of two types - current (contact) and arc.
Current burnoccurs when an electric current passes through the human body as a result of contact with a live part of the equipment and is a consequence of the conversion of electrical energy into heat.
Arc burns occur when exposed to higher voltages.
2) electricalmarks are clearly outlined spots of gray or pale yellow color on the surface of human skin at the point of contact with live parts of the equipment. Signs are also in the form of scratches, wounds, cuts or bruises, warts, skin hemorrhages and calluses. In most cases, electrical signs are painless and their treatment ends well.
3) 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. This can happen when short circuits, disconnection of circuit breakers under load, etc. Metallization is accompanied by skin burns caused by heated metal. Over time, the affected skin peels off, the area takes on a normal appearance, and the painful sensations disappear.
4) Electrophthalmia- eye damage caused by intense radiation of an electric arc, the spectrum of which contains ultraviolet and infrared rays harmful to the eyes. In addition, splashes of molten metal may enter the eyes. Protection against electrophthalmia is achieved by wearing UV-resistant goggles and eye protection from molten metal splashes.
5) mechanical damage- arise as a result of sharp involuntary convulsive muscle contractions under the action of a current passing through the human body. As a result, ruptures of the skin, blood vessels and nerve tissue can occur, as well as joint dislocations and even bone fractures. This type of injury should include bruises, fractures caused by a person falling from a height, hitting objects as a result of involuntary movements or loss of consciousness when exposed to current. Mechanical injuries are usually serious injuries requiring long-term treatment
Factors affecting the severity of an electric shock. Types of electric shock. Human body resistance.
Electrical installations pose a great potential danger to humans, since during operation, it is possible to touch live parts.
The peculiarity of electric shock is:
the absence of external signs of impending danger that a person could detect in advance: see, hear, smell, etc. In most cases, a person is connected to the electrical network either with his hands ("hand-arm" current path), or with his hand and feet (current path "hand-feet"). The current passing through this leads to serious damage to the central nervous system and vital organs such as the heart and lungs.
the severity of the outcome of electrical injuries. Temporary disability due to electrical injuries, as a rule, is prolonged. So, with a defeat in networks with a voltage of 220/380 V, it averages 30 days. In general, electrical injuries account for 12-16% of all cases of industrial injuries with fatal outcomes.
currents of industrial frequency 10-25 mA can cause intense muscle cramps, resulting in a non-releasing effect, that is, "chaining" a person to live parts, in which the victim cannot free himself from the influence of electric current. Long-term flow of such a current can lead to serious consequences.
the impact of the current on a person causes a sharp withdrawal reaction, and in some cases, loss of consciousness. When working at heights, it could cause a person to fall. As a result, there is a danger of mechanical injury, which is caused by the effect of current.
a specific danger of electric shock lies in the fact that the live parts of electrical installations, which are energized as a result of insulation damage, do not give any signals that would warn a person of the danger. The reaction of a person to an electric current occurs only when the latter flows through the human body.
Effects of current on the human body
Passing through the human body, an electric current has a thermal, chemical, mechanical and biological effect on it.
The thermal effect of the current manifests itself in burns of individual parts of the body, heating of tissues and biological media, which causes functional disorders in them. The chemical effect is expressed in the decomposition of organic liquid, blood and manifests itself in a change in their physical and chemical composition; mechanical rupture of muscle tissue; biological is the ability of the current to irritate and excite the living tissues of the body.
Any of the following effects of current can cause injury. An injury caused by exposure to an electric current or an electric arc is called electrical injury (GOST 12.1.009-76).
Types of electric shock
In practice, electrical injuries are conventionally divided into local and general. Local electrical injuries cause local damage to the body - an electric burn, an electric sign, metallization of the skin with particles of metal melted under the action of an electric arc, mechanical damage caused by involuntary muscle contractions under the influence of current, and electrophthalmia (inflammation of the outer membranes of the eyes under the influence of an electric arc).
General electrical injuries, often called electrical shock, cause disruption of the normal functioning of the most vital organs and systems of the body or lead to damage to the whole body.
Factors Affecting the Severity of an Electric Shock
These factors include: strength, duration of exposure to current, its type (constant, variable), pathways, as well as environmental factors, etc.
Current strength and duration of exposure. An increase in current strength leads to qualitative changes in its impact on the human body. With an increase in current strength, three qualitatively different responses are clearly manifested - the body's reactions: sensation, convulsive muscle contraction (non-letting go for alternating current and painful effect for direct current) and cardiac fibrillation. Electric currents that cause the corresponding response of the human body are called perceptible, non-releasing and fibrillatory, and their minimum values \u200b\u200bare usually called threshold.
Experimental studies have shown that a person feels the effect of an alternating current of industrial frequency with a strength of 0.6-1.5 mA and a direct current of 5-7 mA. These currents do not pose a serious danger to the human body, and since under their influence, a person's independent release is possible, their long-term flow through the human body is permissible.
In those cases when the damaging effect of the alternating current becomes so strong that the person is not able to free himself from the contact, the possibility arises for a long flow of current through the human body. Such currents are called non-letting ones, their prolonged exposure can lead to difficulty and impairment of breathing. The numerical values \u200b\u200bof the strength of the non-letting current are not the same for different people and are in the range from 6 to 20 mA. Exposure to direct current does not lead to a non-releasing effect, but causes strong pain sensations that occur in various people with a current strength of 15-80 mA.
When a current flows of several tenths of an ampere, there is a danger of disruption of the heart. Fibrillation of the heart may occur, i.e., irregular, uncoordinated contractions of the fibers of the heart muscle. In this case, the heart is not able to carry out blood circulation. Fibrillation usually lasts a few minutes, followed by complete cardiac arrest. The process of cardiac fibrillation is irreversible, and the current that caused it is fatal. As shown by experimental studies carried out on animals, the threshold fibrillation currents depend on the body weight, the duration of the current flow and its path.
Current path.
The lesion will be more severe if the heart, chest, brain and spinal cord are in the path of the current. In the practice of servicing electrical installations, the current flowing through the body of a person who has come under voltage usually follows the "hand-hand" or "hand-foot" path. However, it can also flow along other paths, for example, "head-legs", "back-arms", "leg-leg", etc. The degree of damage in these cases depends on which human organs will be affected by the current, and also from the strength of the current passing directly through the heart. Thus, when current flows along the leg-to-leg path, 0.4% of the total current passes through the heart, and 3.3% passes through the hand-arm path. The strength of the non-releasing current along the hand-hand path is approximately 2 times less than that along the right hand-foot path.
Current type
Power frequency current is most unfavorable. With an increase in frequency (more than 50 Hz), the values \u200b\u200bof the perceptible and non-releasing current increase. With a decrease in frequency from 50 Hz to 0, the values \u200b\u200bof the non-releasing current also increase and at a frequency of zero (direct current), they become approximately 3 times larger.
The values \u200b\u200bof the fibrillation current at frequencies of 50-100 Hz are equal. With an increase in frequency to 200 Hz, the strength of the fibrillation current increases by about 2 times, and up to 400 Hz - by almost 3.5 times. Increasing the frequency of the supply voltage of electrical installations is used as one of the electrical safety measures.
Environment.
Air humidity and temperature, availability of grounded metal structures and floors, conductive dust have an additional effect on electrical safety conditions.
The degree of electric shock is largely dependent on the density and area of \u200b\u200bhuman contact with live parts. In humid rooms with high temperatures or in outdoor electrical installations, unfavorable conditions develop, in which the area of \u200b\u200bhuman contact with live parts increases. The presence of grounded metal structures and floors creates an increased risk of injury due to the fact that a person is almost constantly connected to one pole (ground) of an electrical installation. In this case, any touch of a person to live parts immediately leads to a two-pole connection to the electrical circuit. Conductive dust also creates conditions for electrical contact with both live parts and the ground.
Electrical resistance of the human body
The strength of the current Ich passing through any part of the human body depends on the supplied voltage Upr (touch voltage) and electrical resistance zt of the current provided by this part of the body,
Ich \u003d Upr / zt
In the area between the two electrodes, the electrical resistance of the human body mainly consists of the resistances of the two thin outer layers of the skin touching the electrodes and the internal resistance of the rest of the body.
The poorly conducting current outer layer of the skin adjacent to the electrode and the inner tissue under this layer, as it were, form capacitor plates with capacitance C with resistance rн. In the outer layer of the skin, the current flows along two parallel paths: through the active external resistance rн and capacitance C, (Figure 1) whose electrical resistance
where ω \u003d 2nf - angular frequency, Hz; f - current frequency, Hz.
Then the total resistance of the outer layer of the skin for alternating current zn \u003d rn xc / √ rn2 + xc 2
Resistance rн and capacitance C depend on the area of \u200b\u200bthe electrodes (contact area). With an increase in the contact area, rн decreases; and the capacity C is increased. Therefore, an increase in the contact area leads to a decrease in the impedance of the outer layer of the skin.
Experiments have shown that the internal resistance of the human body rv can be considered as purely active. Thus, for the hand-arm current path, the total electrical resistance of the body can be represented by the equivalent circuit shown in Figure 2. With an increase in the current frequency, due to a decrease in xc, the resistance of the human body decreases and at high frequencies (more than 10 kHz) it practically becomes equal to the internal resistance rv. The dependence of the resistance of the human body on frequency is shown in Figure 3.
There is a nonlinear relationship between the current flowing through the human body and the voltage applied to it: with increasing voltage, the current increases faster. This is mainly due to the nonlinearity of the electrical resistance of the human body. So, when the voltage on the electrodes is 40-45 V, significant electric field strengths arise in the outer layer of the skin, at which a breakdown of the outer layer occurs completely or partially, which reduces the total resistance of the human body (Figure 4). At a voltage of 127-220 V, it practically drops to the value of the internal resistance of the body. This resistance is assumed to be 1 kOhm.
Knowing the permissible values \u200b\u200b\\ u200b \\ u200bof the currents for various durations of exposure and the impedance of the human body zt, you can determine the permissible touch voltage
Upr add \u003d Iadd zt
where Idop is the permissible current strength.
Figure 1. Electrical circuit for replacing the resistance of the outer layer of the skin:
a - diagram of the contact of the electrode with the human body; b - electrical equivalent circuit;
1 - electrode; 2- the outer layer of the skin; 3- inner skin area
Figure 2: The electrical equivalent circuit of the resistance of the human body
1 - electrode, 2 - external resistance of the skin of the hands, rvr, rvk - internal resistance of the hands and body
Figure 3: Dependence of the resistance of the human body zt on the frequency of the current f
Figure 4: Dependence of human body resistance zt on voltage. U
List of references
Buralev Yu.V., Pavlova E.I. Life safety in transport: Textbook. for universities. Moscow: Transport, 1999.
V. A. Devisilov Occupational Safety and Health. - M .: Forum-INFRA-M, 2003
Klochkova E.A. Labor protection in railway transport. - M .: Route, 2004
Neim and L. A. Life safety: theory, questions and answers: Textbook. pos. M .: Vuzovskaya kniga, 1997.142 p.
