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

There are three main current thresholds:

Threshold perceptible current - the smallest value of an electric current that causes perceptible irritation when passing through the human body;

Threshold non-letting current - the smallest value of the 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 release the action of the th current

Threshold fibrillation (deadly) current - the smallest value of an electric current that causes cardiac fibrillation when passing through the human body

Table 71 shows the threshold values ​​of the current strength when it passes through the human body by the "hand - arm" or "arm - legs"

Current (alternating and constant) 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. It is considered conventionally safe for human life that the voltage does not exceed 42. V (in Ukraine, such a voltage, depending on the working conditions and the environment, is 36 and 12. V), at which a breakdown of the human skin should not occur, which leads to a sharp decrease in the total resistance her "bodies; tila.

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 taken equal. I am 1 kΩ. When the skin is moistened, dirty and scared (sweating, cuts, scratches, etc.), an increase in the applied voltage, contact area, current frequency and time of its action, the resistance of the human body decreases to a certain minimum value (0.5-0.7 kOmm).

The type and frequency of the current passing through the human body also affects the consequences of the injury. Direct current is about 4-5 times safer than alternating current. However, the frequency of the alternating current also leads to severe lesions. So, the most dangerous is the alternating current with a frequency of 20-100. Hz. At a frequency lower than 20 or higher than 100 Hz, the risk of electric shock is noticeably reduced. The current with a frequency of 500 kHz cannot fatally hit a person, but very often causes burns.

The path of the current through the human body? possible ways 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 most dangerous path is the "head-hands" path (with it, the proportion of patients who lost consciousness is 92%), followed by - "head - legs", then - " right hand- legs ", and the least danger is the path" leg - leg "to become the path" leg - leg ".

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

Current path	The 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 arm - legs
Left arm - legs
Leg - leg
Chair - legs
Chair - hands

Permissible values ​​of currents and voltages

Touch voltage is the voltage between two points in an electrical circuit that a person touches at the same time.

The maximum permissible values ​​of touch voltage and current strength for normal (trouble-free) and emergency modes of electrical installations when current passes through the human body by "hand-hand" or "p hand-feet" are regulated by. 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 humidity (more than 75%), the values ​​of table 73 must be reduced by three times

1. Maximum permissible values ​​of touch voltages and currents

1.1. The limit values ​​for touch voltages and currents are established for current paths from one hand to the other and from hand to feet.

(Modified edition, Amendment N 1).

1.2. Touch voltages and currents flowing through the human body during the normal (non-emergency) mode of the electrical installation should not exceed the values ​​indicated in table. one .

Table 1

Notes:

1. Touch voltages and currents are given for a duration of exposure no more than 10 minutes per day and are set based on the reaction of sensation.

2. Touch voltages and currents for persons performing work in conditions of high temperatures (above 25 ° C) and humidity (relative humidity above 75%) should be reduced by three times.

1.3. The maximum permissible values ​​of touch voltages and currents during emergency operation of industrial electrical installations with voltages up to 1000 V with a solidly grounded or insulated neutral and above 1000 V with an isolated neutral should not exceed the values ​​indicated in table. 2.

table 2

Current type	Normalized May magnitude	Maximum permissible values, no more, with the duration of exposure to current t, s
		0,01- 0,08	0,1	0,2	0,3	0,4	0,5	0,6	0,7	0,8	0,9	1,0	St. 1,0
Variable 50 Hz	U, B I, mA	550 650	340 400	160 190	135 160	120 140	105 125	95 105	85 90	75 75	70 65	60 50	20 6
Variable 400 Hz	U, B I, mA	650	500	500	330	250	200	170	140	130	110	100	36 8
Constant	U, B I, mA	650	500	400	350	300	250	240	230	220	210	200	40 15
Rectified full-wave	U_sample, V I_ampl, mA	650	500	400	300	270	230	220	210	200	190	180	-
Rectified half-wave	U_sample, V I_ampl, mA	650	500	400	300	250	200	190	180	170	160	150	-

Note. The maximum permissible values ​​of touch voltages and currents flowing through the human body with an exposure duration of more than 1 s are given in table. 2 correspond to releasing (alternating) and non-painful (direct) currents.

1.4. The maximum permissible values ​​of touch voltages in emergency mode of industrial electrical installations with a current frequency of 50 Hz, a voltage above 1000 V, with a dead neutral ground should not exceed the values ​​indicated in table. 3.

Table 3

1.5. The maximum permissible values ​​of touch voltages and currents in emergency mode of household electrical installations with a voltage of up to 1000 V and a frequency of 50 Hz should not exceed the values ​​indicated in table. 4 .

Table 4

Note. The values ​​for touch voltages and currents are set for people weighing 15 kg or more.

1.3-1.5. (Modified edition, Amendment N 1).

1.6. Protection of a person from the effects of touch voltages and currents is ensured by the design of electrical installations, technical ways and protective equipment, organizational and technical measures for

Depending on the duration of human exposure

table 2

Current type	Standardized value.	Duration of exposure to current t, s
0,01-0,08	0,1	0,2	0,3	0,4	0,5	0,6	0,7	0,8	0,9	1,0
Variable (50Hz)	I
U
Constant	I
U

The permissible values ​​of the touch voltage and the current passing through the human body are used to develop a set of protective measures and determine the parameters of protective devices at which it is still possible to ensure safety. Sometimes the term "safe current" is used, which does not make sense, since a current of any magnitude has some effect on the human body. So, electric current 0.02 - 0.07mA, 50 Hz causes pain in certain points on the human body. Therefore, it is legitimate to apply the concept of "permissible current". The value of the permissible current should be set on the basis of those threshold current values ​​at which a real danger appears. So, in hazardous working conditions (height, near moving or rotating parts, etc.), when a person in the process of work is forced to have constant contact with parts that are energized, the long-term permissible current should be taken below the sensation threshold, no more 0.5mA... When working in normal (safe) conditions, the threshold of non-tolerant current should be taken as the long-term permissible current in case of accidental contact, 10mA, since exceeding this current value threatens with real danger.

Current frequency

It has been established that the capacitive component is also included in the resistance of the human body:

Therefore, an increase in the frequency of the applied voltage is accompanied by a decrease in the impedance of the body and an increase in the current passing through the person. With an increase in the current passing through the human body, the danger of injury increases, which means that an increase in frequency should lead to an increase in such a danger.

However, this assumption is valid only in the frequency range from 0 before 50 Hz... In the frequency range from 0 before 50 Hz with decreasing frequency, the value of the non-releasing current increases and at a frequency equal to zero (direct current) it becomes approximately 3 times larger (see Fig. 2).

An increase in frequency, above this range, despite an increase in the current passing through the human body, is accompanied by a decrease in the risk of injury, which completely disappears at a frequency 450-500 kHz, i.e. such currents cannot strike a person. However, in this case, the risk of burns remains when current passes through the human body and when an electric arc occurs.

For the risk of injury, the reciprocal of the non-letting current at a given frequency, expressed as a percentage, is taken. The danger is taken as 100%. 50 Hz as the highest in the entire frequency scale.

Then the danger of injury at the desired frequency is determined from the expression

where, are non-releasing currents at 50 Hz and the desired frequency f, mA.

Simplified, the change in the danger of current with a change in frequency can be explained by the nature of the irritating effect of the current on cells of living tissue.

If a constant voltage is applied to a cell of living tissue, then in the intracellular substance, which can be considered as an electrolyte, electrolytic dissociation occurs, as a result of which the decomposition of molecules into positive and negative ions will occur. These ions will begin to move towards the cell wall, positive ions to the negative electrode, and the negative to the positive. This phenomenon will cause disruption of the normal state of the cell and the natural biochemical processes occurring in it.

With alternating current, the ions will move, following the change in the polarity of the electrodes.

It can be assumed that in the frequency range from 0 before 50 Hz, a greater disturbance of the natural state of the cell causes a current in which the ion makes from one to several "full" runs per unit of time inside the cell membrane. For a dangerous state, presumably, it is possible to consider either one "full" run of ions, or the maximum number of "complete" runs that occur at the frequency 50 Hz... Since ions, as material particles, have a certain speed of movement in the electrolyte, then at a certain frequency (obviously 50 Hz) the ion will not have time to reach the cell membrane during the polarity reversal. Such a situation would correspond, presumably, to a lesser disturbance of the normal state of the cell. With a further increase in frequency, the path length of the ions will decrease and a moment may come when the movement of ions stops, and, consequently, there will be no dangerous violation of the state of the cell. This situation occurs at frequencies above 450-500 kHz.

Current paths

In the practice of operating electrical installations when a person is turned on electrical circuit current flows through it, as a rule, along the path "hand - feet" or "hand - hand". However, there are a lot of possible paths of current in the human body. The degree of damage in these cases depends on which vital organs (heart, lungs, brain) of a person are exposed to the current, as well as on the magnitude of the current directly affecting these organs and, in particular, the heart.

Typical current paths (current loops) in the human body are shown in Fig. 3.

The current is distributed throughout the entire volume of the body, but most of it passes along the path least resistance- along blood and lymphatic vessels, nerve trunks and branches.

In this case, the path of least resistance does not have to be the shortest between the electrodes. Measurements have shown that the value of the human body's resistance to electric current at different current loops is different:

- "hand - hand" - 1360 Ohm;

- "hand - legs" - 970 Ohm;

- "hands-legs" - 670 Ohm.

The danger of various current loops can be assessed using the data in Table 3.

The most dangerous are the head-arms, head-legs, when the current can pass through the brain and spinal cord. However, these loops are relatively rare. The next in danger is the path of the right hand - legs, when the greatest current flows through the heart along the longitudinal axis.

Despite the small value of the current flowing through the human heart during the leg-to-leg loop at a step voltage equal to 80-120V, cramps of the leg muscles occur, the person falls and, touching the ground with his hand, falls under great stress, since the current loop will now be "arms - legs" ("arm - leg"), which can lead to electric shock.

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How does electric current affect a person?

Electrical injury

An electric current strikes a person suddenly. The passage of current through the human body causes electrical injuries of a different nature: electric shock, burns, electrical marks.

An electric shock is called an electric shock, in which a shock occurs, that is, a kind of severe reaction of the body to a strong stimulus - an electric current.

The outcome of the shock is different. In severe cases, shock is accompanied by circulatory and respiratory disorders. Fibrillation of the heart is possible, that is, instead of a simultaneous rhythmic (about 1 time per second) contraction of the heart muscle, there is a chaotic twitching of its individual fibers - fibrils. This stops the normal functioning of the heart, blood flow stops, and death can occur.

A person's shock with a voltage of up to 1000 V is in most cases accompanied by an electric shock.

Burns occur when exposed to a significant current (about 1 BUT and more) or from an electric arc. So, when approaching live parts with a voltage higher than 1000 V, a spark discharge appears at an unacceptably small 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 tissues to 60-70 ° C. This causes the protein to clot. Electric burns are difficult to heal. They cover a large surface of the body and penetrate deeply.

Electrical signs (marks) are skin necrosis 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, then the body tissues gradually die off.

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

From table. 1 it follows that a current of more than 15 mA is dangerous for a person, at which a person cannot free himself on his own. A current of 50 mA causes severe damage. A current of 100 mA for more than 1-2 seconds is lethal.

Factors affecting the outcome of the lesion

The magnitude of the electric current passing through the human body, and therefore, the outcome of the lesion 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 get rid of it on your own at somewhat large values ​​(20-25 mA).

The magnitude of the current depends on the voltage of the electrical installation and on the resistances of all elements of the circuit through which the current flows, including the resistance of the human body. Body resistance is composed of active and capacitive resistance skin and internal organs . Dry, intact skin has a resistance of about 100,000 ohms, wet skin - about 1,000 ohms, and the resistance of internal tissues (with the stratum corneum removed) is about 500-1000 ohms. The least resistance is the skin of the face and armpits.

The resistance of the human body is non-linear. It sharply, disproportionately decreases with an increase in the voltage applied to the body, an increase in the time of exposure to the current, with an unsatisfactory physical and mental state, with a large and close 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 falls nonlinearly from tens of thousands to 800 Ohm (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 person = U pr / I person

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

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

Z people = 1000 Ohm

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

It has been established that the outcome of electric shock depends on the physical and mental state of the person. . If he is hungry, tired, drunk, or not healthy, then the likelihood of a serious injury increases. Women, adolescents, 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 affecting the outcome of the lesion. The heart cycle is approximately 1 s. There is a phase in the cycle 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 exposure time of the current (less than 0.1 s), the less the likelihood of fibrillation. Prolonged (several seconds) exposure to the current leads to a difficult outcome: the body's resistance decreases, and the lesion current increases.

The mechanism of the effect of electric current on a person is complex. On the one hand, in high-voltage installations, there have been cases when a 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 the live part of the most vulnerable part of the body - the back of the hand, cheek, neck, lower leg, shoulder.

Given the danger of electrical installations with voltages both up to 1000 and above 1000 V, each worker must firmly remember that one cannot touch live parts, no matter what voltage they are at, one cannot come close to live parts in high-voltage installations, one cannot touch unnecessarily to metal structures of switchgears, supports of power transmission lines, to equipment cases that may be energized when current-carrying parts are short-circuited on them.

Earth faults in electrical installations, as a rule, are disconnected 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 the permissible current. In this case, a current that does not cause fibrillation in 99.5% of experimental animals, the body and heart weight of which is close to that of a human, is considered acceptable. The permissible values ​​of the current and touch voltages obtained in laboratory studies are given in table. 2

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

When using the data below the limits of the maximum permissible values ​​of currents and voltages of contact, the following considerations must be kept in mind.

1. The product of the ventricular fibrillation current threshold value and the human body resistance value can give the ventricular fibrillation voltage threshold value, but it must be borne in mind that these values ​​are not independent. In fact, a relatively small proportion of people have high resistance body and low ventricular fibrillation current threshold, while most people have low body resistance and high ventricular fibrillation current threshold.

Therefore, the product of the values ​​of the resistance of the human body and the threshold values ​​of the ventricular fibrillation current, which have the same probability, will give the threshold values ​​of the voltages of ventricular fibrillation related to a non-existent person.

1. Even if the threshold values ​​of the current and the value of the resistance of the body were mutually independent, then a simple multiplication of their values, which have the same probability, would give the value threshold voltage, which has a lower probability compared to the probability of each of the two variable values.
2. The ventricular fibrillation current thresholds given in Publication IEC-479 were derived from dog studies. More recent studies indicate that the human heart has a higher ventricular fibrillation current threshold than the dog's heart, and therefore the published thresholds can be considered conservative values.

Non-emergency mode of electrical installation

The maximum permissible values ​​of touch voltages and currents passing through the human body are used in the design of electrical installations of direct and alternating current with a frequency of 50 and 400 Hz. The maximum permissible values ​​of touch voltages and currents are established for the paths of current from o, diy hand to the other and from hand to feet.
Touch voltage and current passing through the human body, with a duration of exposure no more than 10 minutes. per day should not exceed the values ​​given in table. 1. Data table. 1. relate to electrical installations of all voltage classes, both with insulated and grounded neutral.

Table 1. Maximum permissible values ​​of touch voltages and currents passing through the human body in non-emergency mode
electrical installations

Current type
Variable. 50 Hz
Variable, 400 Hz
Constant

Emergency electrical installation

Touch voltages and currents passing through a person during emergency operation of electrical installations with voltages up to 1 kV with grounded or isolated neutral and above 1 kV with isolated neutral should not exceed the values ​​given in table. 2.
Touch voltages and currents passing through a person during emergency operation of electrical installations with voltages above 1 kV with an effectively grounded neutral should not exceed the values ​​given in table. 3.
To control the standardized values ​​of touch voltages and currents, voltages and currents should be measured in places where the highest values ​​of the controlled quantities can be expected.
When measuring touch voltages and currents, the resistance to current spreading from a person's feet into the ground should be simulated by a metal flat plate with a contact surface area of ​​625 cm2. The plate should be pressed to the ground with a mass of at least 50 kg.
Measurements should be made under conditions corresponding to the highest values ​​of touch voltages and currents passing through the human body.
* Touch voltages and currents for persons performing work in conditions of high temperatures (over 25 ° C) and humidity (relative humidity over 75%) should be reduced by 3 times.

Table 2 . Normalized values ​​of touch voltage and currents passing through a person for electrical installations with voltage up to 1 kV with grounded and insulated neutral and above 1 kV with insulated neutral

Current type

Standardized value

Duration of exposure to current /, s

Variable

Variable

current, 400 Hz

Constant

Rectified

full-wave current

Rectified

half-wave current

Table 3. Normalized values ​​of touch voltage and currents passing through a person for electrical installations with a voltage above 1 kV and a frequency of 50 Hz with an effectively grounded neutral

Standardized value

Duration of exposure to current t, s