The widespread use of electrical energy has led to the fact that almost the entire adult population, and non-adults too, daily come into contact with various electrical installations in their lives. Like all machines and mechanisms, electrical installations, if they are malfunctioning or misused, can be a source of injury. To reduce the risk of electric shock to a person, you need to know the rules for the safe operation of electrical installations and the safety precautions for working on them.

Electric shock to a person

Electric current, passing through the human body, has thermal, chemical and biological effects. The thermal effect is manifested in the form of burns of the skin of the body, overheating of various organs, as well as ruptures of blood vessels and nerve fibers resulting from overheating. The chemical action leads to the electrolysis of blood and other solutions contained in the body, which leads to a change in their physico-chemical composition, and hence to a violation of the normal functioning of the body. The biological effect of electric current is manifested in the dangerous excitation of living cells and tissues of the body. As a result of such arousal, they may die.

There are two main types of electric shock to a person: electric shock and electric shock. An electric shock is such an action of current on the human body, as a result of which the muscles of the body begin to convulsively contract. In this case, depending on the magnitude of the current and the time of its action, a person may be conscious or unconscious, but with normal heart and breathing. In more severe cases, loss of consciousness is accompanied by disruption of the cardiovascular system, which can even lead to death. As a result of an electric shock, paralysis of the most important organs (heart, brain, etc.) is possible.

An electrical injury is such an effect of current on the body, in which the tissues of the body are damaged: skin, muscles, bones, ligaments. Of particular danger are electrical injuries in the form of burns. Such a burn appears at the point of contact of the human body with the current-carrying part of an electrical installation or an electric arc. There are also injuries such as metallization of the skin, various mechanical damage resulting from sudden involuntary movements of a person. As a result of severe forms of electric shock, a person may be in a state of clinical death: he stops breathing and blood circulation. In the absence of medical care, clinical death (imaginary) can turn into biological death. In some cases, however, with proper medical care (artificial respiration and heart massage), it is possible to revive the imaginary dead.

The immediate causes of death of a person struck by an electric current are the cessation of the heart, respiratory arrest due to paralysis of the muscles of the chest, and the so-called electric shock.

The cessation of the work of the heart is possible as a result of the direct action of an electric current on the heart muscle or reflexively due to paralysis of the nervous system. In this case, there may be a complete stop of the heart or the so-called fibrillation, in which the fibers of the heart muscle come into a state of rapid chaotic contractions. Respiratory arrest (due to paralysis of the muscles of the chest) may be the result of either the direct passage of an electric current through the chest area, or caused reflexively due to paralysis of the nervous system. Electric shock is a nervous reaction of the body to excitation by an electric current, which manifests itself in a violation of normal breathing, blood circulation and metabolism. Prolonged shock may result in death.

If the necessary medical assistance is provided, then the state of shock can be removed without further consequences for the person. The main factor determining the amount of resistance of the human body is the skin, its horny upper layer, in which there are no blood vessels. This layer has a very high resistivity and can be considered as a dielectric. The inner layers of the skin, which have blood vessels, glands and nerve endings, have a relatively low resistivity. The internal resistance of the human body is a variable value that depends on the condition of the skin (thickness, humidity) and the environment (humidity, temperature, etc.). If the stratum corneum of the skin is damaged (abrasions, scratches, etc.), the electrical resistance of the human body sharply decreases and, consequently, the current passing through the body increases. With an increase in the voltage applied to the human body, a breakdown of the stratum corneum is possible, which is why the resistance of the body drops sharply, and the magnitude of the damaging current increases.

From the foregoing, it becomes clear that many factors affect the severity of an electric shock to a person. The most unfavorable outcome of the lesion will be in cases where live parts are touched with wet hands in a damp or hot room.

The defeat of a person by electric current as a result of an electric shock can be different in severity, since a number of factors affect the degree of damage: the magnitude of the current, the duration of its passage through the body, the frequency, the path passed by the current in the human body, as well as the individual properties of the victim ( health status, age, etc.). The main factor affecting the outcome of the lesion is the magnitude of the current, which, according to Ohm's law, depends on the magnitude of the applied voltage and the resistance of the human body. The magnitude of the voltage plays an important role, since at voltages of about 100 V and above, a breakdown of the upper stratum corneum of the skin occurs, as a result of which the electrical resistance of a person decreases sharply, and the current increases.

Usually a person begins to feel the irritating effect of alternating current of industrial frequency at a current value of 1-1.5 mA and direct current 5-7 mA. These currents are called threshold sensible currents. They do not pose a serious danger, and with such a current, a person can independently free himself from exposure. At alternating currents of 5-10 mA, the irritating effect of the current becomes stronger, pain in the muscles appears, accompanied by their convulsive contraction. At currents of 10-15 mA, the pain becomes difficult to bear, and the cramps of the muscles of the arms or legs become so strong that the person is not able to free himself from the action of the current. Alternating currents of 10-15 mA and above and direct currents of 50-80 mA and above are called non-release currents, and their smallest value of 10-15 mA at a power frequency voltage of 50 Hz and 50-80 mA at a constant source voltage is called a threshold non-release current.

A power frequency alternating current of 25 mA or more affects not only the muscles of the arms and legs, but also the muscles of the chest, which can lead to respiratory paralysis and cause death. A current of 50 mA at a frequency of 50 Hz causes a rapid disruption of the respiratory organs, and a current of about 100 mA or more at 50 Hz and 300 mA at a constant voltage in a short time (1-2 s) affects the heart muscle and causes its fibrillation. These currents are called fibrillation currents. When the heart fibrillates, its work as a pump for pumping blood stops. Therefore, due to a lack of oxygen in the body, breathing stops, i.e., clinical (imaginary) death occurs. Currents over 5 A cause paralysis of the heart and breathing, bypassing the stage of cardiac fibrillation. The longer the current flow through the human body, the more severe its results and the greater the likelihood of death.

The current path is of great importance in the outcome of the lesion. The defeat will be more severe if the heart, chest, brain and spinal cord are in the path of the current. The path of the current also has the significance that in different cases of contact the resistance of the human body will be different, and, consequently, the value of the current flowing through it. The most dangerous paths for the passage of current through a person are: "arm - legs", "arm - arm". The current path "leg - leg" is considered less dangerous. As statistics show, the largest number of accidents occur due to accidental touching or approaching bare, unprotected parts of electrical installations that are energized. To protect against electric shock, bare wires, busbars and other current-carrying parts are either located in inaccessible places or protected by fences. In some cases, covers, boxes, etc. are used to protect against contact.

Electric shock can occur when touching the non-current-carrying parts of the electrical installation, which are energized when the insulation is broken. In this case, the potential of the non-current-carrying part turns out to be equal to the potential of that point of the electrical circuit at which the insulation was broken. The danger of injury is aggravated by the fact that touching non-current-carrying parts under operating conditions is a normal work operation, so the injury is always unexpected. With regard to the defeat of people by electric current, the "Rules for the installation of electrical installations" distinguish:

1. Premises with increased danger, which are characterized by the presence in them of one of the following conditions that create an increased danger:
   1. dampness or conductive dust;
   2. conductive floors (metal, earthen, reinforced concrete, brick, etc.);
   3. high temperature;
   4. 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, on the other.
2. Particularly dangerous premises, which are characterized by the presence of one of the following conditions that create a particular danger:
   1. special dampness;
   2. chemically active environment;
   3. the simultaneous presence of two or more conditions of increased danger.
3. Premises without increased danger, in which there are no conditions that create increased danger and special danger.

As protective measures when touching non-current-carrying parts, protective grounding, grounding or disconnection, double insulation, low voltage, protective equipment, etc. are used.

Protective grounding is a metal connection to the ground of non-current-carrying metal parts of an electrical installation (housings of electrical machines, transformers, rheostats, lamps, devices, shield frames, metal sheaths of cables, trusses, columns, etc.). Protective grounding is used in networks with an isolated neutral point. In four wired networks with voltages up to 1000 V with a grounded neutral, protective grounding is used - the connection of non-current-carrying metal parts to a repeatedly grounded neutral wire. In the event of an insulation breakdown, a short circuit mode (emergency mode) is created, and the electrical installation is turned off by protection devices. Zeroing is not required for low power installations in residential, office, commercial heated premises with dry, poorly conductive floors.

Protective shutdown - automatic shutdown of the electrical installation by the protection system in the event of a danger of electric shock to a person. Since in the event of damage to the electrical installation, the values ​​\u200b\u200bof some quantities change (hull voltage relative to earth, earth fault current, etc.), if these changes are perceived by sensitive sensors, the protection devices will work and turn off the electrical installation.

Double is understood as additional, in addition to the main, insulation, which protects a person from metal non-current-carrying parts that may accidentally become energized. The most reliable double insulation is provided by enclosures made of insulating material. Usually they carry the entire mechanical part. This method of protection is most often used in electrical equipment of low power (electrified hand tools, household appliances and hand-held electric lamps).

In rooms with increased danger and especially dangerous, even with simultaneous contact of a person with current-carrying parts of different phases or poles, low voltage (12 and 36 V) is used. The source of such voltage is batteries of galvanic cells, batteries, rectifiers, frequency converters and transformers (the use of autotransformers as a source of low voltage is prohibited). Since the power of these sources is insignificant, the scope of low voltages is limited to hand tools, hand and machine lamps of local lighting.

An important factor in ensuring safety is knowledge of the device and rules for the operation of electrical installations, maintenance of electrical equipment in good condition, serviceability of alarms and interlocks, and availability of fire extinguishing equipment.

If, despite all the measures taken, a person is still injured by an electric current, then the salvation of the victim in most cases depends on the speed of releasing him from the action of the current, as well as on the speed and correctness of providing first aid to the victim.

It may turn out that the victim himself is not able to free himself from the action of the electric current. In this case, he must immediately be helped, taking precautions so as not to find himself in the position of the victim. It is necessary to turn off the installation with the nearest switch or interrupt the current circuit by cutting the wire with a knife, wire cutters, an ax, etc. If the victim lies on the ground or on a conductive floor, isolate him from the ground by slipping a wooden board or plywood under him.

After the release of the victim from the action of electric current, he must immediately receive first aid in accordance with his condition. If the victim has not lost consciousness and can move independently, take him to a room convenient for rest, calm him down, give him a drink of water, offer him to lie down. If at the same time the victim had any injuries (bruises, cuts, dislocations of joints, bone fractures, etc.), then provide appropriate assistance on the spot, and, if necessary, refer to a medical center or call a doctor.

If, after being released from the electric current, the victim is unconscious, but breathes normally and a pulse is heard, you should immediately call a doctor, and before he arrives, provide assistance on the spot - bring the victim to consciousness: give a sniff of ammonia, provide fresh air. If, after being released from the action of an electric current, the victim is in a serious condition, that is, he does not breathe or breathes heavily, intermittently, then, having called a doctor, it is necessary, without wasting a minute, to start artificial respiration. Before starting artificial respiration:

1. without wasting a second, release the victim from tight clothing - unbutton the collar, untie the scarf, remove the belt, etc .;
2. open the victim's mouth if it is convulsively compressed;
3. quickly free the victim's mouth from foreign objects, remove dentures.

After that, you can start performing artificial respiration using the mouth-to-mouth method. The air injection technique is as follows. The victim lies on his back, under the shoulder blades - a roller of clothing. His head is thrown back, for which they put one hand under the neck, and with the other hand they press on the crown. This ensures the departure of the root of the tongue from the back wall of the larynx and the restoration of airway patency. In this position of the head, the mouth usually opens. If there is mucus in the mouth, then it is wiped with a handkerchief or the edge of a shirt stretched over the index finger, they check if there are any foreign objects in the mouth (dentures, mouthpiece, etc.) that must be removed. After that, they start blowing air. The person providing assistance takes a deep breath, tightly (possibly through gauze or a handkerchief) presses his mouth to the victim's mouth and blows air with force.

During the blowing of air, you should close the nose of the victim with your fingers in order to fully ensure the flow of all the blown air into the lungs. If it is impossible to completely cover the victim's mouth, air should be blown into the nose (while his mouth should be closed). Air is blown in every 5-6 seconds, which corresponds to a respiratory rate of 10-12 times per minute. After each blow, the mouth and nose of the victim are released for the free exit of air from the lungs.

In the absence of a pulse, artificial respiration should be continued and, at the same time, an external heart massage should be started. External cardiac massage supports blood circulation in both stopped and fibrillating hearts. It is well known that such a massage can lead to the resumption of independent normal activity of the heart. The caregiver puts both hands on top of each other, palms down, on the lower part of the victim's sternum. Rhythmically 60-80 times per minute press on the lower part of the sternum vertically down. During the clinical death of a person, the chest becomes very mobile due to loss of muscle tone, which allows the lower end of the sternum to be displaced by 3-4 cm during massage. The heart is thus squeezed and blood is squeezed out of it into the blood vessels. After each pressure, hands should be taken away from the sternum so that the chest is completely straightened, and the heart is filled with blood. It is best to revive the victim together, alternately performing external heart massage and artificial respiration.

The effect of electric current on the human body. The current strength in the circuit section is directly proportional to the potential difference, that is, the voltage at the ends of the section and inversely proportional to the resistance of the circuit section. The action of electric current on living tissue is versatile. During thermal action, overheating and functional disorder of organs in the path of current flow occurs.

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66. The effect of electric current on the human body.Hidden danger of defeat. External (local) defeat, electric shock (internal defeat) . Factors on which the extent of damage depends.

Electric current is an ordered movement of electric charges.The current strength in the circuit section is directly proportional to the potential difference,that is, the voltage at the ends of the section and is inversely proportional to the resistance of the circuit section.

Touching the conductorunder tension,a person includes himself in an electrical circuit,if it is poorly isolated from the ground or touches an object with a different potential value at the same time.In this case, an electric current passes through the human body.

The action of electric current on living tissue is versatile.Passing through the human bodyelectric current produces thermal,electrolytic, mechanical, biological and light effects.

During thermal action, overheating and functional disorder of organs in the path of current flow occurs.

The electrolytic effect of the current is expressed in the electrolysis of fluid in the tissues of the body,including blood,and violation of its physico-chemical composition.

Mechanical action leads to tissue rupture, bundle, shock action of evaporation of fluid from the tissues of the body.The mechanical action is associated with a strong contraction of the muscles up to their rupture.

The biological effect of the current is expressed in irritation and overexcitation of the nervous system.

Light exposure causes damage to the eyes.

The nature and depth of the impact of electric current on the human body depends on the strength and type of current,time of its actionpathways through the human bodyphysical and psychological state of the latter. So, human resistance under normal conditions with dry intact skin is hundreds of kiloohms,but under adverse conditions can drop to 1 kiloohm.

The perceptible current is about 1 mA. With a higher current, a person begins to feel unpleasant painful muscle contractions, and at a current of 12-15 mA is no longer able to control its muscular system and cannot independently break away from the current source.Such a current is called non-letting.The action of the current over 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 permanent. What matters iswhat parts of the body a person touches the current-carrying part.The most dangerous pathsin which the brain or spinal cord is affected(head-arms, head-legs), heart and lungs (arms-legs). Any electrical work must be carried out away from grounded equipment. (including water pipes,pipes and radiators) , to prevent accidental contact with them.

Local electrical injury – pronounced local violation of the integrity of body tissues,including bone tissuecaused by the action of an electric current or an electric arc.Most often, these are superficial lesions, i.e. skin lesions, and sometimes other soft tissues,as well as ligaments and bones.

The danger of local injuries and the complexity of their treatment depend on the place,the nature and extent of tissue damage,as well as from the reaction of the body to excitation. Usually, local injuries are cured and the victim's working capacity is restored completely or partially.In rare cases (usually in severe burns) the person dies.In this case, the direct cause of death is not electric current,and local damage to the body,caused by current.

Typical local electrical injuries – electrical burns,electrical signs,leather plating,mechanical damage and electrophthalmia.

As stated, approximately 75% cases of electric shock to people is accompanied by the occurrence of local electrical injuries (electrical burns; electrical signs; metallization of the skin; mechanical damage to the skin; electrophthalmia; mixed injuries, i.e. burns with other local injuries) .

electric shock – is the excitation of living tissues by electric current,passing through the bodyaccompanied by involuntary muscle contractions.

Depending on pathological processes,arising from electric shock, accepted, conditional categorize common electrical injuries as follows:

- electric shocks of the 1st degree – the presence of convulsive muscle contraction without loss of consciousness;

- electric shocks II degree – convulsive muscle contractions,accompanied by loss of consciousness;

- electrical shock III degree – loss of consciousness and dysfunction of the heart or breathing (possibly both);

- electric shocks IV degree - clinical death.

Factors affecting the severity of electric shock

These factors include: force, duration of current exposure, its gender (constant, variable), paths,as well as environmental factors, etc.

The strength of the current and the duration of exposure.An increase in current strength leads to qualitative changes in its impact on the human body.As the current strength increases, three qualitatively different responses are clearly manifested.- body reactions: feeling, spasmodic muscle contraction (non-letting go for AC and pain effect for DC) and cardiac fibrillation.electric currents,causing a corresponding response of the human body,received the names of tangible,non-releasing and fibrillatory,and their minimum values ​​are usually called threshold.

Experimental studies have shownthat a person feels the effects of alternating current industrial frequency with a force of 0.6 -.5 mA and DC power 5 - mA. These currents do not pose a serious danger to the human body,and since, under their influence, the independent liberation of a person is possible,then their long-term flow through the human body is permissible.

In those cases when the damaging effect of alternating current becomes so strong,that a person is not able to free himself from contact,there is a possibility of a long flow of current through the human body.Such currents are called non-letting currents,prolonged exposure to them can lead to difficulty and impaired breathing.The numerical values ​​​​of the strength of the non-letting current are not the same for different people and are in the range from 6 to 20 mA. The impact of direct current does not lead to a non-release effect,and causes severe painwhich in different people occur with the strength of the current 15 - mA.

With the flow of a current of a few tenths of an ampere, there is a danger of disruption of the heart.Cardiac fibrillation may occur i.e. disorderly, uncoordinated contractions of the fibers of the heart muscle.In this case, the heart is not able to carry out blood circulation.fibrillation continues. usually, A couple of minutes,followed by complete cardiac arrest.The process of cardiac fibrillation is irreversible, and the current that caused it, is fatal.As experimental studies show,carried out on animalsthreshold fibrillation currents depend on body weight,the duration of the current flow and its path.

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Electrical injuries occur in everyday life and at work quite often, because people are surrounded by a large number of devices. To avoid electric shock, it is necessary to know as much as possible what an electrical injury is, why it happens and what are the safety rules when working with various devices.

The concept of electrical injury

Electrical injury is the damage to organs and systems of the body under the influence of electric current. For the first time, the death of a person from electric current was recorded in French Lyon, where a carpenter who was hit by an alternator died. According to statistics, in modern Russia more than 30 thousand people die from such injuries every year. No one is immune from this danger because electricity surrounds people everywhere. Most often, young men suffer from electric shock.

The human body is the best conductor of electrical energy. A person receives an electric shock when interacting with live parts of faulty equipment or as a result of non-compliance with safety precautions. It is painful to feel a shock of more than 1 mA.

You can also suffer without touching the current-carrying elements, for example, in the event of a current leakage or a breakdown of the air gap when an electric arc is formed.

The severity of the injuries depends on the nature of the current, the power of the discharge, the time of exposure, the place of contact, the individual characteristics of the victim (health, age, body moisture).

Electric shock is one of the most dangerous injuries, because with them a fatal outcome is often possible. Electrical injury occurs in many situations:

Types of electric shock

The classification of electric shock is based on the nature and extent of its impact on the human body. Depending on this, there are:

Main symptoms

If a person was electrocuted in front of relatives or colleagues, then there can be no doubt about the diagnosis. The victim must be immediately sent to a medical facility. If the misfortune happened when the wounded man was alone, then determine whether there was an electric shock, possible by the following signs:

Particular attention should be paid to affected children. Signs of electric shock are heavy breathing, seizures, extreme pallor, lethargy, or hyperactivity.

Help for the victim

Witnesses of the incident should first of all move the victim to a safe distance from the source of energy. If a person grabbed a bare wire, and his hands cramped, then it is necessary to break the electrical circuit. First of all, you need to take care of the safety of those who came to the rescue. Be sure to wear rubber gloves and boots, and turn off the breaker. The wire can be laid aside with a wooden stick. If the victim's clothing is wet, do not touch it with bare hands.

Having dragged a person to a safe place, you need to understand what state he is in: whether a pulse can be felt, whether the heart is working.

If the victim is conscious, they ask him for his name, age and other data to understand that he has not lost his memory. A patient who has received an electrical injury should be taken to the hospital as soon as possible. The duration of recovery after an accident depends on the severity of the lesion and on how correctly and quickly resuscitation was carried out.

Consequences of trauma

With a strong electric shock, a lethal outcome is not ruled out. Survivors after such an injury are usually in a coma. The victim is diagnosed with unstable work of the heart and respiratory system, convulsions, mechanical damage, hypovolemic shock, renal failure.

The consequences of electric shock can affect the work of almost any organ in the human body. Electrical trauma provokes malfunctions in the work of the heart and blood vessels, exacerbates chronic diseases (for example, stomach and duodenal ulcers), causes pulmonary edema, loss of vision and hearing. With a contraction of the heart muscle, a heart attack is not ruled out.

No one can prevent malfunctions in the operation of electrical appliances. But in order not to get serious injuries, you must follow the safety rules. In this case, the risk is significantly reduced.

Electric current has a negative impact on a person and is a dangerous production factor. In this case, the following types of electrical injuries are possible:
- electric burn;
- electric signs - appear in places of contact of a person with current-carrying parts;
- metallization of the skin - penetration into the skin of the smallest particles of metal;
- electrophthalmia - inflammation of the outer membranes of the eyes;
- electric shock - electrical injury caused by the reaction of the nervous system to irritation with an electric current.
The main causes of electric shock are:
- violation of the rules for the technical operation of electrical installations; touching live parts;
- touching metal non-current-carrying parts that are energized due to a faulty insulation or grounding devices.
In dry rooms, voltage over 42 V is dangerous for human life, in damp and especially humid rooms, in boilers, steel and reinforced concrete tanks, wells and on the ground - over 12 V.
If a person is energized, then an electric current flows through his body. The effect of electric current on a person depends on many factors: on the type of current (alternating or direct); with alternating current - on its frequency; on the magnitude of the current (or voltage); current flow duration; from the path of current through the human body; physical and mental state of a person.
The most dangerous for humans is alternating current with a frequency of 50 - 500 Hz. The ability to self-liberate from the current of this frequency in most people is preserved at a very small current value (up to 10 mA). The amount of current passing through an energized person depends on the magnitude of the installation voltage and the resistance of all circuit elements through which the current flows.
The resistance of the human body is made up of external resistance - the resistance of the skin - and the resistance of internal organs. Dry human skin has a resistance of about 100,000 ohms, wet - about 1000 ohms, and the resistance of internal organs - about 500 - 1000 ohms. However, the design resistance is assumed to be 1000 ohms.

It is known that when a current flows, the resistance of the skin drops, and the cells of the internal organs are reborn, so the longer a person is under the influence of current, the stronger and more serious the consequences of the lesion.
A fatal electric shock to a person can occur as a result of the cessation of the heart or respiratory arrest. With prolonged action of current (from several seconds to several minutes), simultaneous cessation of the work of the heart and respiratory organs is possible. As a result of exposure to the heart of an electric current with a frequency of 50 Hz, a chaotic contraction of individual fibers of the heart muscle occurs, the so-called fibrillation. With the onset of fibrillation, the work of the heart stops, which leads to a cessation of blood flow and a rapid onset of death. Currently, a current of 100 mA, acting on a person from 1 to 2 s, is taken as the magnitude of the current that causes death. The degree of current impact on the human body is given in the table.
A person is exposed to the greatest danger when the current passes through vital organs (heart, lungs) or cells of the central nervous system. However, death is possible at low voltages (12 - 36 V) as a result of contact of current-carrying parts with the most vulnerable parts of the body - the back of the hand, cheek, neck, shin, shoulder.
If you turn off the electric current, then the normal functioning of the heart itself will not be restored. However, the cessation of visible signs of life - respiratory movement and heartbeat - does not yet mean the actual onset of death. Firstly, such phenomena are accompanied by a severe form of shock, and secondly, even with the cessation of breathing and heartbeat, that is, with the onset of the so-called clinical death, a person can still be saved by artificial respiration and chest compressions, if they start immediately. In a healthy person, the period of clinical death lasts up to 7-8 minutes.

The nature of the impact of current on the human body

current strength, mA	Alternating current	D.C
Up to 1	Not felt
1 - 8	Feeling painless. Muscle control is not lost. Independent release from contact with live parts possible	mild itching
8 - 15	Feelings are painful. Muscle control has not yet been lost and independent release from the action of the current is possible	Feeling warm
20 - 50	The sensations of current are very painful. Strong muscle contractions. Breathing is difficult. It is impossible to free yourself from the action of the current	Contraction of the arm muscles
50 - 100	Possible cardiac fibrillation leading to immediate death	Respiratory paralysis
100 - 200	The occurrence of cardiac fibrillation

It has been established that at the time of electric shock, the physical and mental state of a person is of great importance. If a person is hungry, tired, drunk or unwell, then the resistance of his body decreases, i.e., the likelihood of a severe injury increases. By following the safety rules, i.e. with careful and careful work, the likelihood of electric shock is reduced.
Sometimes a misleading idea is created about the safety of touching live parts with voltage up to 220 V, based on the facts when a person, having touched live parts, was not injured. Indeed, such cases are possible if the touched person was well isolated from the ground, was in a dry room. But in practice, under operating conditions, there are always a number of unfavorable circumstances that increase the risk of contact. These include dampness, high room temperature, wet body skin, conductive floors (metal, earthen, reinforced concrete, brick), wooden floors that are wet or contaminated with an emulsion with metal shavings. A person accustomed to touching current-carrying parts with impunity in favorable conditions can be mortally struck in the presence of one of the unfavorable factors. Statistics show that the number of accidents, including fatal ones, at voltages from 120 to 380 V is more than half of all accidents.

The nature and consequences of human exposure to electric current depend on the following factors:

The value of the current passing through the human body,

human electrical resistance,

The level of stress applied to a person,

Duration of exposure to electric current,

Current paths through the human body

Type and frequency of electric current,

environmental conditions and other factors.

Electrical resistance of the human body.

The human body is a conductor of electric current, however, inhomogeneous in electrical resistance. The greatest resistance to electric current is provided by the skin, therefore the resistance of the human body is determined mainly by the resistance of the skin.

The skin is made up of two main layers: the outer layer, the epidermis, and the inner layer, the dermis. The outer layer - the epidermis, in turn, has several layers, of which the thickest upper layer is called the stratum corneum. The stratum corneum in a dry, uncontaminated state can be considered as a dielectric: its volume resistivity reaches 10 5 - 10 6 Ohm m, which is thousands of times higher than the resistance of other skin layers, the resistance of the dermis is insignificant: it is many times less than the resistance of the stratum corneum.

The resistance of the human body with dry, clean and intact skin (measured at a voltage of 15-20 V) ranges from 3 to 100 kOhm or more, and the resistance of the inner layers of the body is only 300-500 Ohm.

As a calculated value for alternating current of industrial frequency, the resistance of the human body is used, equal to 1000 ohms.

Under actual conditions, the resistance of the human body is not a constant value. It depends on a number of factors, including the condition of the skin, the state of the environment, the parameters of the electrical circuit, etc.

Damage to the stratum corneum (cuts, scratches, abrasions, etc.) reduces the resistance of the body to 500-700 ohms, which increases the risk of electric shock to a person. Moisturizing the skin with water or sweat has the same effect.

Contamination of the skin with harmful substances that conduct electric current well (dust, scale, etc.) leads to a decrease in its resistance.

The resistance of the body is also influenced by the area of ​​contacts, as well as the place of contact, since in the same person the resistance of the skin is not the same in different parts of the body. The skin of the face, neck, hands in the area above the palms has the least resistance, and especially on the side facing the torso, armpits, back of the hand, etc. The skin of the palms and soles has a resistance that is many times greater than the resistance of the skin of other parts of the body.

With an increase in the current and the time of its passage, the resistance of the human body decreases, since this increases the local heating of the skin, which leads to the expansion of its vessels, to an increase in the supply of this area with blood and an increase in sweating.

With an increase in the voltage applied to the human body, the skin resistance decreases tenfold, approaching the resistance of internal tissues (300-500 ohms). This is due to an electrical breakdown of the stratum corneum of the skin, an increase in the current passing through the skin.

With an increase in the current frequency, the resistance of the body will decrease, and at 10-20 kHz, the outer layer of the skin practically loses its resistance to electric current.

The magnitude of the current. The main factor that determines the outcome of an electric shock is the strength of the current passing through the human body. The nature of the impact of current on a person, depending on the strength and type of current, is given in Table 7.1

Table 7.1.

The nature of the impact of current on a person (current path arm - leg, voltage 220 V)

	AC, 50 Hz	direct current
	The beginning of the sensation, slight trembling of the fingers	No sensations
	The beginning of pain	No sensations
	The beginning of cramps in the hands	Itching, feeling of warmth
	Cramps in the hands, it is difficult, but you can break away from the electrodes	Increased feeling of warmth
	Severe cramps and pain, persistent current, difficulty breathing
	Respiratory paralysis	Hand cramps, difficulty breathing
		Respiratory paralysis with prolonged current flow
	Same, less time	Cardiac fibrillation under the action of current for 2-3 s, respiratory paralysis

Perceptible current is an electric current that causes perceptible irritations when passing through the body. Perceptible irritations are caused by an alternating current of 0.6-1.5 A and a constant current of 5-7 A. The indicated values ​​\u200b\u200bare threshold perceptible currents; the region of perceptible currents begins with them.

Continuous current- an electric current that, when passing through a person, causes irresistible convulsive contractions of the muscles of the hand in which the conductor is clamped. The threshold hold current is 10-15mA AC and 50-60mA DC. With such a current, a person can no longer independently open his hand, in which the current-carrying part is clamped and turns out to be, as it were, chained to it.

fibrillation current- an electric current that causes fibrillation of the heart when passing through the body. The threshold fibrillation current is 100 mA AC and 300 mA DC with an exposure duration of 1-2 s. along the way hand-foot or hand-hand. The fibrillation current can reach 5A. A current greater than 5A does not cause cardiac fibrillation. With such currents, instantaneous cardiac arrest occurs.

Duration of exposure to electric current . The duration of the passage of current through the human body has a significant impact on the outcome of the lesion. The danger of electric shock due to cardiac fibrillation depends on which phase of the cardiac cycle coincides with the passage of current through the region of the heart. If the duration of current passage is equal to or exceeds the cardiocycle time (0.75-1s), then the current “meets” with all phases of the heart (including the most vulnerable), which is very dangerous for the body. If the current exposure time is less than the duration of the cardiocycle by 0.5 s or more, then the probability of the coincidence of the moment of current passage with the most vulnerable phase of the heart, and, consequently, the risk of damage is sharply reduced. This circumstance is used in high-speed residual current devices, where the response time is less than 0.2 s.

The path of current through the human body. It plays a significant role in the outcome of the lesion, since the current can pass through vital organs: the heart, lungs, brain, etc. The influence of the current path on the outcome of the lesion is also determined by the resistance of the skin in various parts of the body.

There are a lot of possible current paths in the human body, which are also called current loops. The most common current loops are: arm-arm, arm-leg, leg-leg. The most dangerous are head-arm and head-leg loops.

Type and frequency of electric current . Direct current is about 4-5 times safer than alternating current. This provision is valid only for voltages up to 250-300V. At higher voltages, direct current is more dangerous than alternating current (with a frequency of 50 Hz).

As the frequency of the alternating current increases, the impedance of the body decreases, which leads to an increase in the current passing through the person, therefore, the danger of injury increases.

Conditions of the external environment. Dampness, conductive dust, caustic vapors and gases that destroy the insulation of electrical installations, as well as high ambient temperature, lower the electrical resistance of the human body, which further increases the risk of electric shock.

Depending on the presence of the listed conditions that increase the danger of electric shock to a person, all premises are divided according to the danger of electric shock to a person into the following classes: (Table 7.2.)

Table 7.2.

Classification of premises according to the danger of electric shock

Electric current safety criteria. When designing, calculating and monitoring protective systems, they are guided by the permissible current values ​​for a given path of its flow and the duration of exposure in accordance with GOST 12.1.038-82.

With prolonged exposure, the allowable current is assumed to be 1 mA. With exposure duration up to 30 s - 6 mA. When exposed to 1 s or less, the values ​​of the currents are given in Table 7.3. However, they cannot be considered as providing complete safety, and are accepted as practically acceptable with a fairly low probability of damage.

Table 7.3.

Practically admissible current values

These currents are considered acceptable for the most probable paths of their flow in the human body: hand-hand, hand-foot, and foot-foot.