The strength of the current passing through the human body is the main factor that determines the consequences of the injury. Currents of different magnitudes produce different effects on the human body.

There are three main current thresholds:

Threshold perceptible current is the lowest value of electric current that causes noticeable irritation when passing through the human body;

Threshold inaccessible current - the smallest value of electric current that causes convulsive contractions of the muscles of the arm in which the conductor is clamped, making it impossible for a person to independently free himself from the action of the current.

Threshold fibrillation (deadly) current - the lowest value of electric current that causes fibrillation of the heart when passing through the human body

Table 71 shows the threshold values ​​of current strength when it passes through the human body via the “arm-to-arm” or “arm-to-leg” path.

Current (alternating and direct) more than 5. A causes instant cardiac arrest, bypassing the state of fibrillation

Table 71: AC and DC Thresholds

The higher the voltage value, the greater the risk of electric shock. Conventionally, a voltage that is considered safe for human life does not exceed 42.V (in Ukraine, this voltage, depending on operating conditions and environment, is 36 and 12.V), at which a breakdown of human skin should not occur, which leads to a sharp decrease in the overall resistance her body; body.

The electrical resistance of the human body depends mainly on the condition of the skin and the central nervous system. For calculations, the resistance of the human body is conventionally assumed to be equal. I - 1 kOhm. When the skin is moisturized, dirty, and damaged (sweating, cuts, scratches, etc.), the applied voltage, contact area, current frequency, and time of action increase, the resistance of the human body decreases to a certain minimum value (0.5-0.7 kOhm).

The type and frequency of current passing through the human body also influence the consequences of the injury. Direct current is approximately 4-5 times safer than alternating current. However, the frequency of alternating current also leads to damage to aslids. Thus, alternating current with a frequency of 20-100 is considered the most dangerous. Hz At a frequency less than 20 or greater than 100 Hz, the danger of electric shock is noticeably reduced; a current with a frequency of up to 500 kHz cannot fatally affect a person, but very often causes burns.

The path of current through the human body? possible paths for the passage of current through the human body (current loops), their characteristics are given in Table 72. As can be seen from the table, the greatest danger is represented by the “head - hands” path (with it, the proportion of victims who lost consciousness is 92%), for it goes - "head - legs", then - "right hand - legs", and the least danger is the path "leg - leg".

Table 72. Characteristics of the most common paths for current passage through the human body

Current path	Frequency of occurrence of this current path,%	Proportion of victims who lost consciousness during action	The value of the current passing through the heart,% of the total current passing through the body
Hand - hand
Right hand - legs
Left hand - legs
Leg - leg
Chair - legs
Chairman - hands

Permissible values ​​of currents and voltages

Touch voltage is the voltage between two points in an electrical circuit that are simultaneously touched by a person

The maximum permissible values ​​of touch voltage and current strength for normal (failure-free) and emergency modes of electrical installations when current passes through the human body by "arm - arm" or "arm - legs" are regulated using. GOST 121038-82 (Table 73 12.1.038-82 (Table 7.3).

When performing work in conditions of high temperature (more than 25 ° C) and relative air humidity (more than 75%), the values ​​of Table 73 must be reduced by three times

Modern life is full of a variety of household appliances and devices that make our life much easier, making it more and more comfortable, but at the same time a whole complex of dangerous, harmful factors appears: electromagnetic fields of various frequencies, increased levels of radiation, noise, vibration, danger of mechanical injury, the presence of toxic substances, as well as, most importantly, electric current.

Electric shock called the ordered movement of electrical particles. Electric current affects a person thermal(heating of tissues when electric current flows through them), electrolytic(decomposition of blood and other body fluids), biological(excitation of living tissues of the body, accompanied by muscle spasms) actions.

When a person is exposed to electric current, electrical injuries occur: electrical burns, electrical marks, skin metallization, mechanical damage, electric arc blinding (electro-ophthalmia), electric shock, electric shock.

Electrical burn- This is damage to the surface of the body or internal organs under the influence of an electric arc or large currents passing through the human body. There are two types of burns: current (or contact) and arc.

Electrical burns are caused by the passage of current directly through the human body as a result of touching a live part. Electrical burn is a consequence of the conversion of electrical energy into thermal energy; As a rule, this is a skin burn, since human skin has many times greater electrical resistance than other body tissues.

Electrical burns occur when working on electrical installations of relatively low voltage (not higher than 1-2 kV) and are in most cases first- or second-degree burns; however, sometimes severe burns occur.

At higher voltages, an electric arc is formed between the live part and the human body or between the live parts, which causes another type of burn - an arc burn.

An arc burn is caused by the action of an electric arc on the body, which has a high temperature (over 3500 C) and high energy. Such a burn usually occurs in high-voltage electrical installations and is severe - III or IV degree.

Electrical signs- these are gray and pale yellow spots, bruises, scratches on human skin that have been exposed to electric current. The strength of the sign corresponds to the strength of the live part touched by the person. In most cases, treatment of electrical signs ends successfully, and the affected area is completely restored.

Metallization of leather– penetration into the upper layers of the skin of the smallest particles of metal melted under the action of an electric arc. In the affected area, the skin becomes hard, rough and takes on a metallic color (for example, green from contact with copper). Work involving the possibility of an electric arc should be done with glasses, and the worker's clothing should be buttoned up.

Mechanical damage occurs as a result of mechanical movement during involuntary convulsive muscle contraction and requires long-term treatment.

Electroophthalmia is an inflammation of the outer membranes of the eyes that occurs under the influence of a powerful stream of ultraviolet rays. Such irradiation is possible when an electric arc (short circuit) is formed, which intensely emits not only visible light, but also ultraviolet and infrared rays.

Electric shock– this is the excitation of living tissues of the body by an electric current passing through them, accompanied by involuntary convulsive muscle contractions. The degree of negative impact of these phenomena on the body may vary. An electric shock can lead to disruption and even complete cessation of the activity of vital organs - the lungs and heart, and therefore to the death of the body. A person may not have external local injuries.

Depending on the outcome of the lesion, electric shocks can be divided into four degrees, each of which is characterized by certain manifestations:

I – convulsions without loss of consciousness;

II – convulsions with loss of consciousness, but with preserved breathing and heart function;

III – loss of consciousness and disturbance of cardiac activity or breathing (or both);

IV – clinical death.

Causes of death from electric shock include cardiac arrest, respiratory failure, and electrical shock.

The work of the heart can stop as a result of either the direct effect of the current on the heart muscle, or a reflex action when the heart is not in the path of the current. In both cases, cardiac arrest or fibrillation may occur, i.e. random contraction and relaxation of the muscle fibers of the heart. Fibrillation usually lasts very briefly and is followed by complete cardiac arrest. If first aid is not provided immediately, clinical death occurs.

The cessation of breathing is caused by the direct or reflex action of the current on the muscles of the chest involved in the breathing process.

Electric shock- a peculiar reaction of the nervous system in response to strong irritation by electric current. Manifested by circulatory and respiratory disorders. Shock can last from several tens of minutes to a day, after which the body dies.

The main factor determining the outcome of electric shock is the amount of current passing through the human body. According to safety precautions, electric current is classified as follows:

A current is considered safe, the long passage of which through the human body does not cause harm to it and does not cause any sensations; its value is no more than 50 μA (alternating current 50 Hz) and 100 μA direct current;

The minimum human-perceptible alternating current is about 0.6-1.5 mA (50 Hz alternating current) and 5-7 mA direct current;

The threshold non-releasing current is the minimum current of such strength that a person is no longer able to tear his hands away from the current-carrying part by force of will. For alternating current it is 10-15 mA, for direct current it is 50-80 mA;

The fibrillation threshold is a current strength of about 100 mA (50 Hz) and 300 mA direct current, the impact of which for more than 0.5 seconds is likely to cause fibrillation of the heart muscles. This threshold is also considered conditionally fatal for humans.

Direct current is less dangerous than alternating current. Voltage up to 12 V can be considered practically safe for humans in damp rooms, and up to 36 V in dry rooms. The probability of electric shock to a person depends on the climatic conditions in the room (temperature, humidity), as well as conductive dust, metal structures connected to the ground , conductive floor, etc. Danger zones– face, palm, crotch. Dangerous Paths– hand-head, hand-hand, two hands-two legs.

The severity of the lesion is increased by: alcohol intoxication, fatigue, exhaustion, chronic diseases, old age or childhood.

In accordance with the “Rules for the Construction of Consumer Electrical Installations” (PUE), all premises are divided into three classes:

· without increased danger– not hot (up to +35°C), dry (up to 60%), dust-free, with a non-conductive floor, not cluttered with equipment;

· with increased danger– have at least one increased risk factor, i.e. hot or humid (up to 75%), dusty, with a conductive floor, etc.;

· especially dangerous– have two or more high hazard factors or at least one special hazard factor, i.e. special dampness (up to 100%) or the presence of a chemically active environment.

Static electricity– this is a potential supply of electrical energy generated on equipment as a result of friction and the inductive influence of strong electrical discharges. In rooms with a large amount of dust of organic origin, static discharges can form (fire and explosion hazard), and also accumulate on people when using linen and clothing made of lye, wool and artificial fibers, when moving on a non-conductive synthetic floor covering, such as linoleum, carpet etc.

To protect against electric shock when working with electrical equipment connected to the network, you must use are common And personal electrical protective equipment.

TO general Electrical protective equipment includes: fencing; grounding; grounding and disconnecting equipment bodies that may be energized; use of safe voltage 12-36 V; posters posted near dangerous places; automatic air switches (warning, prohibiting, reminding). Good insulation condition of electrical installations is one of the most important safety conditions. The importance of network insulation is to avoid the possibility of electrical short circuits causing fires, as well as to reduce energy costs due to current leakage. Protective grounding, grounding or automatic shutdown are designed to reduce voltage or completely shut down electrical installations whose housings are energized. Typically, artificial grounding devices are used: metal rods, pipes, and metal strips placed horizontally into the ground that are specially driven into the ground. For grounding, it is possible to use metal structures of buildings, metal water pipes in contact with the ground.

Individual protective equipment is divided into basic(insulating rods of all types; insulating clamps; voltage indicators; electrical clamps; dielectric gloves; manual insulating tools) and additional(dielectric galoshes; dielectric carpets and insulating supports; insulating caps, coverings and linings; ladders, insulating fiberglass stepladders).

Diagram 1. First aid algorithm for electric shock

When providing assistance, you first need to free the person from the electric current. The safest thing is to quickly remove the plugs if an accident occurs in the house. If for some reason this cannot be done, then you need to throw a rubber mat, board or thick cloth under your feet, or put rubber boots or galoshes on your feet; You can put household rubber gloves on your hands. Pull the victim away from the wire, grabbing his clothes with one hand. In the area where a high-voltage wire falls, you need to move in small steps, without spreading your legs wide. You can also try to move the victim himself away from the current source or remove the source from him. This must be done with one hand, so that even if you receive a shock, the current does not pass through the entire body of the person providing assistance.

After turning off the current (freeing the victim), it is necessary to act in accordance with the presented algorithm (Scheme 1).

Regardless of the condition of the victim, it is necessary to call a doctor and ensure complete rest and observation until he arrives. The absence of severe symptoms after the injury does not mean that the victim’s condition will not worsen subsequently (respiratory paralysis and cardiac arrest sometimes do not occur immediately, but over the next 2-3 hours).

Questions for self-control of knowledge

1. Define the concepts: “industrial environment”, “hazardous chemical substance”, “accidentally chemically hazardous substance”, “toxicity”, “toxicant”, “toxin”, “toxic process”, “harmful substance”, “resorption”, “ deposition", "elimination", "mechanism of toxic action", "luminous flux", "luminous intensity", "illumination", "brightness", "mechanical oscillations", "periodic oscillations", "oscillation amplitude", "oscillation period" , “vibration”, “sound”, “noise”, “electromagnetic field”, “ionizing radiation”, “isotopes”, “radioactivity”, “activity”, “half-life”, “static electricity”.

2. Classification of negative factors of the human environment and their brief characteristics.

3. Technosphere – as a habitat. Qualitative changes in the habitat.

4. Classification of potentially hazardous substances. The concept of poisons.

5. Routes of harmful substances entering the body and their characteristics. Deposition of harmful substances. Elimination. Biotransformation phases.

6. Classification of harmful substances by hazard class. Types of action of combined poisons.

7. The mechanism of formation and development of the toxic process at different levels of biological organization.

8. Illumination. Its qualitative and quantitative indicators. Co-factor of natural light.

9. Mechanical vibrations. Their varieties.

10. Basic characteristics and classification of vibration. The concept of vibration disease.

11. Sound. Noise and its characteristics. Noise control measures.

12. Electromagnetic fields. Standards and measures for protection from exposure to electromagnetic fields.

13. Infrared (IR) radiation. Its effect on the human body.

14. Ultraviolet radiation. Its influence on humans and use in industry.

15. Ionizing radiation. Its types and sources. Application in industry and medicine.

16. Electric current. The effect of electric current on the human body. Electrical burns. Electrical signs. Metallization of leather. Mechanical damage. Electroophthalmia.

17. Electric shock, electric shock.

18. Classes of premises in accordance with the “Rules for the construction of electrical installations of consumers”. The concept of static electricity.

19. General and individual electrical protective equipment.

20. First aid algorithm for electric shock.

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How does electric current affect a person?

Electrical injuries

Electric current strikes a person suddenly. The passage of current through the human body causes electrical injuries of various types: electric shock, burns, electrical marks.

An electric shock is an electric shock that causes shock, i.e., a kind of severe reaction of the body to a strong irritant - an electric current.

The outcome of shock varies. In severe cases, shock is accompanied by circulatory and respiratory disorders. Cardiac fibrillation is possible, i.e., instead of simultaneous rhythmic (about once per second) contraction of the heart muscle, chaotic twitching of its individual fibers - fibrils - occurs. This stops the heart from working normally, blood flow stops, and death can occur.

Electric shock to a person at voltages up to 1000 V is in most cases accompanied by an electric shock.

Burns occur when exposed to significant current (about 1 A or more) or from an electric arc. Thus, when approaching live parts with a voltage above 1000 V, a spark discharge appears at an unacceptably short distance between the live part and the human body, and then an electric arc, which causes a severe burn. In case of accidental contact with a live part with a voltage of up to 1000 V, the current passing through the human body heats the tissue to 60-70°C. This causes the protein to clot. Electrical burns are difficult to heal. They cover a large surface of the body and penetrate deeply.

Electrical signs (marks) are necrosis of the skin in the form of a yellow callus with a gray border at the point of entry and exit of the current. If the lesion penetrates deeply, the body tissues gradually die.

The nature of the impact of alternating electric current, depending on its magnitude, is given in Table. 1

From the table 1 it follows that a current of more than 15 mA is dangerous for a person, at which a person cannot free himself. A current of 50 mA causes severe damage. A current of 100 mA, acting for more than 1-2 s, is deadly.

Factors influencing the outcome of the lesion

The amount of electric current passing through the human body, and therefore the outcome of the injury, depends on many circumstances.

The most dangerous is alternating current with a frequency of 50-500 Hz. Most people retain the ability to independently free themselves from currents of this frequency at very small values ​​(9-10 mA). Direct current is also dangerous, but it is possible to free yourself from it at somewhat higher values ​​(20-25 mA).

The magnitude of the current depends on the voltage of the electrical installation and on the resistance of all elements of the circuit through which the current flows, including the resistance of the human body. Body resistance consists of active and capacitive resistance of the skin and internal organs . Dry, intact skin has a resistance of about 100,000 ohms, wet skin has a resistance of about 1000 ohms, and the resistance of internal tissues (with the stratum corneum removed) is approximately 500-1000 ohms. The skin of the face and armpits has the least resistance.

The resistance of the human body is a nonlinear quantity. It sharply, disproportionately decreases with an increase in the voltage applied to the body, an increase in the time of exposure to current, with an unsatisfactory physical and mental state, with large and dense contact with the current-carrying part, etc. From Fig. 1 it follows that with an increase in the voltage applied to the body from 0 to 140 V, the resistance of the body nonlinearly drops from tens of thousands to 800 Ohms (curve 1). Accordingly, the current passing through the body increases (curve 2).

The resistance of the human body (Ohm) is approximately determined by the formula

Z people = U pr / I people

Where U pr- voltage drop across the human body resistance - V.

In calculations for electrical safety, it is (also approximately) taken equal to:

Z person = 1000 Ohm

The most dangerous path of current is through the heart, brain, and lungs. Characteristic paths: palm - feet, palm - palm, foot - foot. However, fatal injury is also possible when the current passes along a path that does not seem to affect vital organs, for example through the lower leg to the foot. This phenomenon is explained by the fact that current in the body flows along the path of least resistance (nerves, blood), and not in a straight line - through tissues with greater resistance (muscles, fat).

It has been established that the outcome of electric shock depends on the physical and mental state of a person . If he is hungry, tired, intoxicated or unhealthy, then the likelihood of a serious defeat increases. Women, teenagers, and men with poor health are able to withstand significantly lower currents (within 6 mA) than healthy men (12-15 mA).

The duration of exposure is one of the main factors influencing the outcome of the lesion. The heart cycle is approximately 1 s. The cycle has a phase T, equal to 0.1 s, when the heart muscle is relaxed and it is most vulnerable to current: fibrillation may occur. The shorter the current exposure time (less than 0.1 s), the lower the likelihood of fibrillation. Prolonged (several seconds) exposure to current leads to a serious outcome: body resistance decreases, and the damage current increases.

The mechanism of the effect of electric current on a person is complex. On the one hand, in high-voltage installations there were cases when short-term (hundredths of a second) exposure to a current of several amperes did not lead to death. On the other hand, it has been established that death is possible at a voltage of 12-36 V, when a current of several milliamps is applied. This occurs as a result of touching a live part with the most vulnerable part of the body - the back of the hand, cheek, neck, shin, shoulder.

Considering the danger of electrical installations with voltages both up to 1000 and above 1000 V, everyone working must firmly remember that they must not touch live parts, regardless of what voltage they are under, they must not come close to live parts in high-voltage installations, they must not touch them unnecessarily to metal structures of switchgears, power transmission line supports, to equipment casings that may become energized when live parts are shorted to them.

Ground faults in electrical installations are usually cleared by the main relay protection in a fraction of a second. Therefore, electrical safety devices (grounding, etc.) can be calculated based on large values ​​of permissible current. In this case, a current that does not cause fibrillation in 99.5% of experimental animals whose body and heart weight is close to human is considered acceptable. The permissible values ​​of current and touch voltage obtained in laboratory tests are given in table. 2

From the table 3-2 it follows that currents of more than 65 mA and voltages of more than 65 V are allowed for less than 1 s.

To properly design methods and means of protecting people from electric shock, it is necessary to know the permissible levels of touch voltages and the values ​​of currents flowing through the human body.

Touch voltage is the voltage between two points in a current circuit that are simultaneously touched by a person. The maximum permissible values ​​of touch voltages U PD and currents I PD flowing through the human body along the “arm-arm” or “arm-legs” path under normal (non-emergency) electrical installation mode, according to GOST 12.1.038-82* are given in table. 1.

In emergency mode of industrial and household appliances and electrical installations with voltage up to 1000 V with any neutral mode, the maximum permissible values ​​of U PD and I PD should not exceed the values ​​​​given in table. 2. Emergency mode means that the electrical installation is faulty and dangerous situations may occur, leading to electrical injuries.

When the duration of exposure is more than 1 s, the values ​​of U PD and I PD correspond to releasing values ​​for alternating current and conditionally non-painful values ​​for direct current.

Table 1

Maximum permissible values ​​of touch voltages and currents

in normal operation of the electrical installation

Note. Touch voltages and currents for persons working in conditions of high temperatures (above 25 °C) and humidity (relative humidity more than 75%) must be reduced by 3 times.

table 2

Maximum permissible values ​​of touch voltage

and currents in emergency operation of an electrical installation

Duration of electric current, s	Production electrical installations		Appliances, electrical installations

4. Electrical resistance of the human body

The value of current through the human body greatly influences the severity of electrical injuries. In turn, the current itself, according to Ohm’s law, is determined by the resistance of the human body and the voltage applied to it, i.e. tension of touch.

The conductivity of living tissues is determined not only by physical properties, but also by the most complex biochemical and biophysical processes inherent only to living matter. Therefore, the resistance of the human body is a complex variable that has a nonlinear dependence on many factors, including the condition of the skin, the environment, the central nervous system, and physiological factors. In practice, the resistance of the human body is understood as the modulus of its complex resistance.

The electrical resistance of various tissues and fluids of the human body is not the same: skin, bones, adipose tissue, tendons have a relatively high resistance, and muscle tissue, blood, lymph, nerve fibers, spinal cord and brain have low resistance.

The resistance of the human body, i.e. The resistance between two electrodes placed on the surface of the body is mainly determined by the resistance of the skin. The skin consists of two main layers: the outer (epidermis) and the inner (dermis).

The epidermis can be conventionally represented as consisting of a stratum corneum and a germinal layer. The stratum corneum is composed of dead keratinized cells, lacks blood vessels and nerves, and is therefore a layer of nonliving tissue. The thickness of this layer ranges from 0.05 - 0.2 mm. In a dry and uncontaminated state, the stratum corneum can be considered as a porous dielectric, penetrated by many ducts of the sebaceous and sweat glands and having a high resistivity. The germinal layer is adjacent to the stratum corneum and consists mainly of living cells. The electrical resistance of this layer, due to the presence of dying and keratinizing cells in it, can be several times higher than the resistance of the inner layer of the skin (dermis) and internal tissues of the body, although compared to the resistance of the stratum corneum it is small.

The dermis consists of connective tissue fibers that form a thick, strong, elastic mesh. This layer contains blood and lymphatic vessels, nerve endings, hair roots, as well as sweat and sebaceous glands, the excretory ducts of which extend to the surface of the skin, penetrating the epidermis. The electrical resistance of the dermis, which is living tissue, is low.

The total resistance of the human body is the sum of the resistances of the tissues located in the path of current flow. The main physiological factor that determines the value of the total resistance of the human body is the condition of the skin in the current circuit. With dry, clean and intact skin, the resistance of the human body, measured at a voltage of 15 - 20 V, ranges from units to tens of kOhms. If the stratum corneum is scraped off in the area of ​​the skin where the electrodes are applied, the body resistance will drop to 1 - 5 kOhm, and when the entire epidermis is removed - to 500 - 700 Ohm. If the skin under the electrodes is completely removed, the resistance of the internal tissues will be measured, which is 300 - 500 Ohms.

For an approximate analysis of the processes of current flow along the “hand-to-hand” path through two identical electrodes, a simplified version of the equivalent circuit diagram of the flow of electric current through the human body can be used (Fig. 1).

Rice. 1. Human body resistance equivalent circuit

In Fig. 1 is indicated: 1 – electrodes; 2 – epidermis; 3 – internal tissues and organs of the human body, including the dermis; İ h – current flowing through the human body; Ů h – voltage applied to the electrodes; R Н – active resistance of the epidermis; C H is the capacity of a conventional capacitor, the plates of which are the electrode and the well-conducting tissues of the human body located under the epidermis, and the dielectric is the epidermis itself; R VN – active resistance of internal tissues, including the dermis.

From the diagram in Fig. 1 it follows that the complex resistance of the human body is determined by the relation

where Z Н = (jС Н) -1 = -jХ Н – complex resistance of capacitance С Н;

Х Н – module Z Н; f , f – frequency of alternating current.

In what follows, by the resistance of the human body we mean the module of its complex resistance:

. (1)

At high frequencies (more than 50 kHz) Х Н =1/(C Н)<< R ВН, и сопротивления R Н оказываются практически закороченными ма­лыми сопротивлениями емкостей C Н. Поэтому на высоких частотах со­противление тела человека z h в приближенно равно сопротивлению его внутренних тканей: R ВН z h в. (2)

With direct current in steady state, capacitances are infinitely large (at 
0 X N

). Therefore, the resistance of the human body to direct current

R h = 2R H + R VN. (3)

From expressions (2) and (3) we can determine

R Н = (R h -z h в)/2. (4)

Based on expressions (1) – (4), you can obtain a formula for calculating the value of capacitance Cn:

, (5)

where z hf is the modulus of complex resistance of the body at frequency f;

C H has the dimension μF; z hf , R h and R HV – kOhm; f - kHz.

Expressions (2) – (5) allow us to determine the parameters of the equivalent circuit (Fig. 1) based on the results of experimental measurements.

The electrical resistance of the human body depends on a number of factors. Damage to the stratum corneum of the skin can reduce the resistance of the human body to the value of its internal resistance. Moisturizing the skin can reduce its resistance by 30 – 50%. Moisture that gets on the skin dissolves minerals and fatty acids located on its surface, removed from the body along with sweat and fatty secretions, becomes more electrically conductive, improves contact between the skin and the electrodes, and penetrates the excretory ducts of the sweat and fat glands. When the skin is moisturized for a long time, its outer layer loosens, becomes saturated with moisture and its resistance can decrease even more.

When a person is briefly exposed to thermal radiation or elevated ambient temperature, the resistance of the human body decreases due to the reflex expansion of blood vessels. With longer exposure, sweating occurs, as a result of which the skin's resistance decreases.

With an increase in the area of ​​the electrodes, the resistance of the outer layer of skin R H decreases, the capacitance C H increases, and the resistance of the human body decreases. At frequencies above 20 kHz, the indicated influence of the electrode area is practically lost.

The resistance of the human body also depends on the location of application of the electrodes, which is explained by the different thickness of the stratum corneum of the skin, the uneven distribution of sweat glands on the surface of the body, and the unequal degree of blood filling of the skin vessels.

The passage of current through the human body is accompanied by local heating of the skin and an irritating effect, which causes a reflex dilatation of skin vessels and, accordingly, increased blood supply and increased sweating, which, in turn, leads to a decrease in skin resistance in a given place. At low voltages (20 -30 V) in 1 - 2 minutes, the resistance of the skin under the electrodes can decrease by 10 - 40% (on average by 25%).

An increase in voltage applied to the human body causes a decrease in its resistance. At voltages of tens of volts, this occurs due to reflex reactions of the body in response to the irritating effect of the current (increased supply of blood vessels to the skin, sweating). When the voltage increases to 100 V and above, local and then continuous electrical breakdowns of the stratum corneum under the electrodes occur. For this reason, at voltages of about 200 V and higher, the resistance of the human body is almost equal to the resistance of the internal tissues R VN.

When making an approximate assessment of the risk of electric shock, the resistance of the human body is taken to be 1 kOhm (R h = 1 kOhm). The exact value of design resistances when developing, calculating and testing protective measures in electrical installations is selected in accordance with GOST 12.038-82*.

Permissible long-term currents for wires with rubber or polyvinyl chloride insulation, cords with rubber insulation and cables with rubber or plastic insulation in lead, polyvinyl chloride and rubber sheaths are given in Table. 1.3.4-1.3.11. They are accepted for temperatures: cores +65, ambient air +25 and ground + 15°C.

When determining the number of wires laid in one pipe (or cores of a stranded conductor), the neutral working conductor of a four-wire three-phase current system, as well as grounding and neutral protective conductors are not taken into account.

Permissible long-term currents for wires and cables laid in boxes, as well as in trays in bundles, must be accepted: for wires - according to table. 1.3.4 and 1.3.5 as for wires laid in pipes, for cables - according to table. 1.3.6-1.3.8 as for cables laid in the air. If the number of simultaneously loaded wires is more than four, laid in pipes, boxes, and also in trays in bundles, the currents for the wires should be taken according to the table. 1.3.4 and 1.3.5 as for wires laid openly (in the air), with the introduction of reduction factors of 0.68 for 5 and 6; 0.63 for 7-9 and 0.6 for 10-12 conductors.

For secondary circuit wires, reduction factors are not introduced.

Table 1.3.4. Permissible continuous current for wires and cords with rubber and polyvinyl chloride insulation with copper conductors

	Current, A, for wires laid in one pipe
	open	two single-core	three single-core	four single-core	one two-wire	one three-wire
0,5	11	-	-	-	-	-
0,75	15	-	-	-	-	-
1	17	16	15	14	15	14
1,2	20	18	16	15	16	14,5
1,5	23	19	17	16	18	15
2	26	24	22	20	23	19
2,5	30	27	25	25	25	21
3	34	32	28	26	28	24
4	41	38	35	30	32	27
5	46	42	39	34	37	31
6	50	46	42	40	40	34
8	62	54	51	46	48	43
10	80	70	60	50	55	50
16	100	85	80	75	80	70
25	140	115	100	90	100	85
35	170	135	125	115	125	100
50	215	185	170	150	160	135
70	270	225	210	185	195	175
95	330	275	255	225	245	215
120	385	315	290	260	295	250
150	440	360	330	-	-	-
185	510	-	-	-	-	-
240	605	-	-	-	-	-
300	695	-	-	-	-	-
400	830	-	-	-	-	-

Table 1.3.5. Permissible continuous current for rubber and polyvinyl chloride insulated wires with aluminum conductors

Cross-section of current-carrying conductor, mm 2	Current, A, for wires laid in one pipe
	open	two single-core	three single-core	four single-core	one two-wire	one three-wire
2	21	19	18	15	17	14
2,5	24	20	19	19	19	16
3	27	24	22	21	22	18
4	32	28	28	23	25	21
5	36	32	30	27	28	24
6	39	36	32	30	31	26
8	46	43	40	37	38	32
10	60	50	47	39	42	38
16	75	60	60	55	60	55
25	105	85	80	70	75	65
35	130	100	95	85	95	75
50	165	140	130	120	125	105
70	210	175	165	140	150	135
95	255	215	200	175	190	165
120	295	245	220	200	230	190
150	340	275	255	-	-	-
185	390	-	-	-	-	-
240	465	-	-	-	-	-
300	535	-	-	-	-	-
400	645	-	-	-	-	-

Table 1.3.6. Permissible continuous current for wires with copper conductors with rubber insulation in metal protective sheaths and cables with copper conductors with rubber insulation in lead, polyvinyl chloride, nayrite or rubber sheaths, armored and unarmored

	Current *, A, for wires and cables
	single-core	two-wire		three-wire
	when laying
	in the air	in the air	in the ground	in the air	in the ground
1,5	23	19	33	19	27
2,5	30	27	44	25	38
4	41	38	55	35	49
6	50	50	70	42	60
10	80	70	105	55	90
16	100	90	135	75	115
25	140	115	175	95	150
35	170	140	210	120	180
50	215	175	265	145	225
70	270	215	320	180	275
95	325	260	385	220	330
120	385	300	445	260	385
150	440	350	505	305	435
185	510	405	570	350	500
240	605	-	-	-	-
* Currents apply to wires and cables both with and without a neutral core.

Table 1.3.7. Permissible continuous current for cables with aluminum conductors with rubber or plastic insulation in lead, polyvinyl chloride and rubber sheaths, armored and unarmored

Conductor cross-section, mm2	Current, A, for cables
	single-core	two-wire		three-wire
	when laying
	in the air	in the air	in the ground	in the air	in the ground
2,5	23	21	34	19	29
4	31	29	42	27	38
6	38	38	55	32	46
10	60	55	80	42	70
16	75	70	105	60	90
25	105	90	135	75	115
35	130	105	160	90	140
50	165	135	205	110	175
70	210	165	245	140	210
95	250	200	295	170	255
120	295	230	340	200	295
150	340	270	390	235	335
185	390	310	440	270	385
240	465	-	-	-	-

Note. Permissible continuous currents for four-core cables with plastic insulation for voltages up to 1 kV can be selected according to table. 1.3.7, as for three-core cables, but with a coefficient of 0.92.

Table 1.3.8. Permissible continuous current for portable light and medium hose cords, portable heavy duty hose cables, mine flexible hose cables, floodlight cables and portable wires with copper conductors

Conductor cross-section, mm2	Current *, A, for cords, wires and cables
	single-core	two-wire	three-wire
0,5	-	12	-
0,75	-	16	14
1,0	-	18	16
1,5	-	23	20
2,5	40	33	28
4	50	43	36
6	. 65	55	45
10	90	75	60
16	120	95	80
25	160	125	105
35	190	150	130
50	235	185	160
70	290	235	200

________________

* Currents apply to cords, wires and cables with and without a neutral core.

Table 1.3.9. Permissible continuous current for portable hose cables with copper conductors and rubber insulation for peat enterprises

__________________

Table 1.3.10. Permissible continuous current for hose cables with copper conductors and rubber insulation for mobile electrical receivers

__________________

* Currents refer to cables with and without a neutral core.

Table 1.3.11. Permissible continuous current for wires with copper conductors with rubber insulation for electrified transport 1.3 and 4 kV

Conductor cross-section, mm 2	Current, A	Conductor cross-section, mm 2	Current, A	Conductor cross-section, mm 2	Current, A
1	20	16	115	120	390
1,5	25	25	150	150	445
2,5	40	35	185	185	505
4	50	50	230	240	590
6	65	70	285	300	670
10	90	95	340	350	745

Table 1.3.12. Reduction factor for wires and cables laid in boxes

Laying method	Number of laid wires and cables		Reducing factor for wires supplying groups of electrical receivers and individual receivers with a utilization factor of more than 0.7
	single-core	stranded	separate electrical receivers with a utilization factor of up to 0.7	groups of electrical receivers and individual receivers with a utilization factor of more than 0.7
Multilayered and in bunches. . .	-	Up to 4	1,0	-
	2	5-6	0,85	-
	3-9	7-9	0,75	-
	10-11	10-11	0,7	-
	12-14	12-14	0,65	-
	15-18	15-18	0,6	-
Single layer	2-4	2-4	-	0,67
	5	5	-	0,6

1.3.11

Permissible long-term currents for wires laid in trays for single-row installation (not in bundles) should be taken as for wires laid in the air.

Permissible long-term currents for wires and cables laid in boxes should be taken according to table. 1.3.4-1.3.7 as for single wires and cables laid openly (in the air), using the reduction factors indicated in table. 1.3.12.

When choosing reduction factors, control and reserve wires and cables are not taken into account.