The main damaging factors of electric current. Effects on the body. Factors Determining the Electricity Hazard How the Electric Shock Hazard is Determined

The degree of damage to a person when an electric current passes through it depends on the strength of the electric current, the type and value of the voltage, the frequency of the electric current, the path of the current through the human body, duration of action, environmental conditions, electrical resistance of the human body.

2.1. Current strength

The strength of the current is the main damaging factor. The following current thresholds are set:

1.sense current is 0.5-1.5 mA for alternating ( f \u003d 50 Hz) and 5–7 mA for direct current, with a slight tingling sensation, mild itching with alternating current and a feeling of heating of the skin in the area touching the conductive part with direct current;

2. the non-releasing current is 10 - 15 mA for alternating current and 50 - 80 mA for direct current, while hardly tolerable pain with involuntary contraction of the muscles of the forearm is characteristic, the inability to unclench the hand;

3. the fibrillation current (lethal) is 80-100 mA or more for alternating current and 300 mA for direct current, while cardiac fibrillation occurs, i.e. chaotic, fast and multi-temporal contractions of the fibers of the heart muscle (fibrils), in which the heart stops working as a pump and is unable to provide the movement of blood through the blood vessels, which entails a lack of oxygen, this, in turn, leads to cessation of breathing, due to which is death.

2.2 Duration of exposure to electric current

Analysis of experiments on animals shows a direct dependence of the duration of the passage of electric current through the body on the outcome of the lesion. This dependence is explained by the fact that with an increase in the duration of exposure to current on living tissue, the value of the current increases, the effects of exposure to electric current accumulate, and, finally, the probability of coincidence of the moments of current passage through the heart with the vulnerable phase increases. T cardiac cycle (cardiocycle).

An increase in the current value is due to a decrease in the body's resistance. The consequences of the effect of current on a living organism are expressed in dysfunctions of the central nervous system, changes in blood composition, local destruction of tissues under the influence of heat released, disruption of the heart and lungs.

Each cycle of cardiac activity consists of two periods: diastolewhen the ventricles of the heart, while in a relaxed state, fill with blood, and systoleswhen the heart, contracting, pushes blood into the arterial vessels. The most vulnerable heart becomes in phase T (0.2 s), when the contraction of the ventricles in diastole ends and they go into a relaxed state. The entire period of the cardiocycle is 0.75 - 1.0 s. Therefore, if during the phase T an electric current passes through the heart, then, as a rule, cardiac fibrillation occurs.

According to GOST 12.1.038–82, depending on the duration of the flow of electric current through the human body, the maximum permissible current values \u200b\u200bfor alternating current with a frequency of 50 Hz are set: 500 mA for 0.1 s and 50 mA for 1 s.

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

· Values \u200b\u200bof the current passing through the human body;

· Electrical resistance of a person;

· The level of stress applied to a person;

· Duration of exposure to electric current;

· Paths of current through the human body;

· Kind and frequency of electric current;

· Environmental conditions and other factors.

The electrical resistance of the human body. The human body is a conductor of electric current, but non-uniform in resistance. The greatest resistance

possesses the skin. The upper stratum corneum in a dry and uncontaminated state can be considered a dielectric, its resistance. Body resistance with dry and clean skin from 3 to 100 kOhm, internal organs 300-500 Ohm. Usually, the capacitive resistance, which is negligible, is neglected, and the human resistance is considered purely active and unchanged. 1000 Ohm is taken as the calculated value. In real conditions, human resistance is not constant and depends on a number of factors. Resistance is reduced by the following:

· Damage to the stratum corneum (cuts, scratches, etc.);

• moisturizing the skin with water or sweat;

· Pollution by harmful substances that conduct electric current;

· Increase in current and time of its passage;

· An increase in the voltage applied to the body of the current;

· An increase in the frequency of the current;

The body resistance is also influenced by the contact area and the place of contact, since the resistance of the skin is not the same in different areas.

Amount of current and voltage.

Perceptible current is an electric current that causes perceptible irritations (threshold perceptible currents) when passing through the body. Alternating current or direct current produces such sensations.

Unreleasing current is an electric current that, when passing through a person, causes irresistible convulsive contractions of the muscles of the arm in which the conductor is clamped. Threshold non-release current is AC and DC. A person cannot unclench his hand on his own, help is required.

Fibrillation current is an electrical current that causes cardiac fibrillation to pass through the body. Threshold fibrillation current is alternating current and direct current with duration of 1-2 sec. With current, instantaneous cardiac arrest occurs.

Duration of exposure. The influence of the duration of the passage of the current through the human body on the outcome of the lesion can be estimated by the empirical formula:, where is the current passing through the human body, mA; is the duration of the passage of the current, s. This formula is valid in the range of 0.1-1.0 s. It is used to determine the maximum permissible currents passing through a person along the path of an arm - a leg, necessary for calculating protective devices.

The path of the current through the human body. Possible current paths in the human body are also called current loops. The most common loops are: arm-arm, arm-legs and leg-leg. The most dangerous loops are head-arms and head-legs, but these loops are relatively rare.

Kind and frequency of electric current. For voltages of 250-300V, direct current is about 4-5 times safer than alternating current. At higher voltages, direct current is more dangerous than alternating current (50Hz). for alternating current, its frequency also plays a role. With an increase in the frequency of alternating current, the impedance of the human body decreases, therefore, the risk of injury increases. Greatest danger

represents a current with a frequency of 50 to 100 Hz; with a further increase in frequency, the risk of injury decreases and completely disappears at a frequency of 45-50 kHz. These currents keep the risk of burns. The reduction in current hazard becomes practically noticeable at a frequency of 1-2kHz.

Individual properties of a person. Physically fit and strong people are more likely to withstand electrical shocks. People suffering from diseases of the skin, cardiovascular system, lungs, nervous diseases and others are distinguished by an increased susceptibility to electric current. Such people are not allowed to work on electrical installations.

Environmental conditions.

"Rules for Electrical Installations" divide all rooms according to the danger of electric shock to people into the following classes:

1. Premises without increased danger are characterized by the absence of conditions that create an increased or special danger.

2. Premises with increased danger are characterized by the presence of one of the following conditions that create an increased danger: a) dampness (relative humidity exceeds 75% for a long time); b) high temperature (above 35); c) conductive dust; d) conductive floors (metal, earth, reinforced concrete, brick, etc.); e) the possibility of simultaneous contact of a person with the metal structures of the building, technological devices, mechanisms, etc., which have connections to the ground, on the one hand, and the metal cases of electrical equipment, on the other.

3. Particularly dangerous are characterized by the presence of one of the following conditions: a) special dampness (relative humidity is close to 100%): the ceiling, shadows, under and objects in the room are covered with moisture0; b) chemically active or organic environment (destroying insulation and live parts of electrical equipment); c) simultaneously two or more conditions of increased danger.

Measures to comply with safety regulations when performing work related to electricity

When developing measures to protect against electric shock and high-frequency radiation, it is necessary to strictly follow the safety rules for the operation of power plants, power grids

In order to prevent cases of electrical injury, it is prohibited:

carry out all kinds of work on energized power lines;

perform installation and repair work on power lines, at radio stations without protective equipment (dielectric rugs, gloves, aprons, galoshes), even if the current collectors are disconnected from the mains supply;

allow persons who do not have special training and admission to operate and carry out work on power grids, sources of electricity and electrical equipment;

turn on and off the power supply of the power lines laid in the target areas, without the order of the flight director or the head of the aviation range.

Persons working with current should be regularly conducted safety classes, which explain the inadmissibility of careless and careless handling of sources of electricity, power grids and electrical equipment.

With persons working on machine tools, sawmills, circular saws and other equipment, he organizes safety classes and a systematic test of their knowledge of safety rules.

In the workshop, in garages, at power plants, at radar stations and other facilities, instructions on observance of labor protection rules must be approved by the head of the aviation range.

Allowed to work with electrical equipment after passing tests on knowledge of labor protection.

34 Vibration

Vibration is the movement of a point or a mechanical system, in which there is an alternate increase and decrease, usually in time, of the values \u200b\u200bof some quantity that characterizes it.

By the mechanism of generation, vibrations with force, kinematic and parametric excitation are distinguished.

Forceful excitement is the excitation of vibration of the system by forcing forces and (or) moments.

Kinematic excitation is the excitation of the vibration of the system by imparting predetermined movements to any of its points, which do not depend on the state of the system.

Parametric excitation is the excitation of the vibration of a system by a change in time of one or more of its parameters (mass, moment of inertia, stiffness and resistance coefficients), which does not depend on the state of the system.

According to the method of transmission to a person, vibration is divided into 2 groups:

1. General, which acts on the body of a sitting or standing person and is estimated in octave bands f \u003d 2, 4, 8, 16, 31.5; 63 Hz.

2.Local, which is transmitted through the hands at frequencies f \u003d 8, 16, 31.5; 63, 125, 250, 500, 1000 Hz.

According to the source of origin, vibration is divided into three categories:

1. Transport (mobile vehicles on the ground).

2.Transportation and technological

(cranes, loaders).

3. Technological (jobs).

According to the duration of the action, vibration is divided into the following categories.

1. Permanent. Here, the value of the monitored parameter changes by no more than two times during the observation time;

2.Inconstant. Here, the value of the monitored parameter changes by more than 2 times during the observation time of at least 10 minutes when measured with a time constant of 1 s.

Intermittent vibration can be oscillatory, intermittent and impulsive.

T-9 ELECTRICAL SAFETY 1. The effect of electric current on a person 2. Factors that determine the danger of electric shock 3. The phenomenon of current flowing into the ground.

Classification of electrical installations and premises according to the degree of danger of shock to people in them

5. Analysis of the conditions of electric shock. Touch voltage. Step tension. First aid for electric shock

6. Safe operation of electrical installations. Protection against electric shock ( Protective grounding. Protective grounding. Protective shutdown. Protective equipment used in electrical installations).7. Requirements for those working in electrical installations. Electrical Safety Groups

Introduction

Electrical safety is a system of organizational and technical measures and means to protect people from harmful and dangerous effects of electric current and electric arc, electromagnetic field and static electricity (GOST 12.1.009).

In accordance with the requirements of the Rules for the Installation of Electrical Installations (PUE), electrical safety is ensured by: the design of electrical installations, technical methods and means of protection, organizational and technical measures.

Organizational measures include briefings and training in safe working methods, checking knowledge of safety rules and instructions, admission to work, control of work by a responsible person.

Technical measures provide for disconnecting the installation from the voltage source, removing fuses and other measures to ensure the impossibility of erroneous supply of voltage to the place of work, installation of safety signs and fencing of live parts, workstations, etc.

The effect of electric current on a person

Passing through the body, electric current causes thermal, electrolytic and biological effects.

Thermal actioncurrent causes burns of certain parts of the body, heating of blood vessels, nerves, blood, etc.

Electrolytic actioncurrent is expressed in the decomposition of blood and other organic body fluids and causes significant disturbances in their physical and chemical composition.

Biological actioncurrent manifests itself as irritation and excitement of living tissues of the body, which is accompanied by involuntary convulsive contractions of muscles, lungs and heart. As a result, various disorders and even complete cessation of the activity of the circulatory and respiratory organs may occur.

Any impact of an electric current is expressed in receiving two types of injury - local electrical injuries and electrical shocks.

Local electrical injury- This is a pronounced local violation of the integrity of body tissues as a result of exposure to electric current or electric arc. In most cases, electrical injuries are healed, but in severe burns, the outcome of the injury can be fatal.

There are several types local electrical injuries.

Electric burnwhich is the most common electrical injury, it can be current (or contact) and arc.

Current burnis caused by the passage of current through the human body as a result of its contact with the current-carrying part and is a consequence of the conversion of electrical energy into heat.

Burns are divided into four degrees: I - redness of the skin, II - blistering, III - necrosis of the entire thickness of the skin; IV-carbonization of tissues. The severity of the damage to the body is determined not by the degree of the burn, but by the area of \u200b\u200bthe burned surface of the body. Current burns occur at a voltage no higher than 1-2 kV and in most cases they are assigned degrees I and II. There are also severe burns.

Arc burnis a consequence of the formation of an electric arc between the live part and the human body, which causes a burn. The arc has a temperature above 3500 0 С and has a very significant energy. Arc burns are usually severe and have a grade III or IV.

Electrical signs- these are clearly defined spots of gray or pale yellow color, formed on the human skin as a result of the action of the current. Signs can also be in the form of scratches, wounds, cuts or bruises, warts, hemorrhages and calluses. As a rule, electrical signs are painless, and their treatment ends well.

Skin metallization -this is the penetration into the upper layers of the skin of the smallest particles of metal, melted under the action of an electric arc. This can happen in the event of a short circuit, disconnection of a circuit breaker under load, etc. Metallization is accompanied by skin burns caused by heated metal.

Electrophthalmia- This is eye damage caused by intense radiation from an electric arc, the spectrum of which contains ultraviolet and infrared rays that are harmful to the eyes. Mechanical damagearise as a result of sharp involuntary convulsive muscle contractions under the action of a current passing through the human body. As a result, the skin, blood vessels and nerve tissue can rupture, as well as joint dislocations and even bone fractures. Electric shock -this is the excitation of living tissues of the body by an electric current passing through it, accompanied by involuntary convulsive muscle contractions. In case of electric shocks, the outcome of the effect of current on the body can be different - from a slight, barely perceptible contraction of the muscles of the fingers of the hand to the cessation of the work of the heart or lungs, i.e. until fatal defeat. Electric shocks, depending on the outcome of the effect of the current on the body, are conventionally divided into the following four degrees: I - convulsive muscle contraction without loss of consciousness; II - convulsive muscle contraction with loss of consciousness, but preserved breathing and heart function; III- loss of consciousness and impaired cardiac activity or breathing (or both together); IV - clinical (imaginary) death - a transitional period from life to death, occurring from the moment the activity of the heart and lungs ceases.

Factors determining the risk of electric shock

The nature and consequences of exposure to an electric current on a person are determined by the electrical resistance of the human body, the voltage and duration of exposure to the electric current, depend on the path of the current through the human body, the type and frequency of the electric current, as well as on the environmental conditions and individual characteristics of the person.

The electrical resistance of the human body.The human body is a conductor of electric current, non-uniform in electrical resistance. The greatest resistance to electric current is shown by the skin, therefore the total resistance of the human body is determined mainly by the value of the resistance of the skin

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 kΩ or more, and the resistance of the inner layers of the body is only 300-500 Ohms.

In reality, the resistance of the human body is not constant. It depends on the condition of the skin, the environment, the parameters of the electrical circuit, etc. Damage to the stratum corneum (cuts, scratches, abrasions) reduces the body's resistance 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. Therefore, working with electrical installations with wet hands and in conditions that cause skin moisture, as well as at elevated temperatures, exacerbate the risk of electric shock to a person.

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

The area of \u200b\u200bcontact and the place of touch are important, since the resistance of the skin is not the same in different parts of the body. The least resistance is the skin of the face, neck, palms and arms, especially on the side facing the body (armpits, etc.). The skin on the back of the hand and soles has a resistance that is many times greater than the resistance of the skin on other parts of the body.

With an increase in the current and the time of its passage, the resistance of the human body decreases, because as a result of local heating of the skin, the vessels expand, the blood supply to this area and sweating increase.

The resistance of the human body decreases with an increase in the frequency of the current and at 10-20 kHz the outer layer of the skin practically loses its resistance to electric current.

Current and voltage... The main factor that determines one or another degree of electric shock to a person is the strength of the current passing through his body (table 9.1). With an increase in the current strength, the resistance of the human body decreases, since the local heating of the skin increases, which leads to vasodilation, an increase in the supply of blood to this area and an increase in sweating.

Table 9.1 - Threshold values \u200b\u200bof various types of current

* Instant cardiac arrest occurs at a current strength of 5 A.

The voltage applied to the human body also affects the outcome of the lesion, since it determines the value of the current passing through the person. An increase in voltage leads to a breakdown of the stratum corneum of the skin, the resistance of the skin decreases tenfold, approaching the resistance of internal tissues (300-500 ohms), respectively, the current strength increases.

Features of the effect of electric current on the human body are transmitted by the data in Table 9.2

Type and frequency of electric current. Direct current is about 4-5 times safer than alternating current. This follows from the comparison of the threshold values \u200b\u200bof tangible and non-releasing DC and AC currents. But this is true only up to voltages of 250-300 V. At higher voltages, direct current becomes more dangerous than alternating current (with a frequency of 50 Hz).

In the case of alternating current, its frequency is important. With an increase in the frequency of alternating current, the impedance of the body decreases and at 10-20 kHz, the outer layer of the skin practically loses its resistance to electric current, which also leads to an increase in the current passing through a person, and therefore, the risk of injury increases.

Table 9.2 - Features of the effect of electric current on the human body

Current strength, mA	The nature of the impact
AC 50 Hz	D.C
0,6 – 1,5	The beginning of the sensation is mild itching, tingling of the skin under the electrodes	Not felt
2,0 – 4,0	The sensation of current extends to the wrist of the hand, slightly reduces the hand	Not felt
5,0 – 0,7	Pain sensations intensify throughout the hand, accompanied by convulsions; mild pain is felt throughout the arm, right down to the forearm	The beginning of the sensation. Impression of heating the skin under the electrode
8,0 – 10	Severe pain and cramps in the entire arm, including the forearm. Hands can still be torn off the electrodes	Increased heating sensation
10 – 15	Barely bearable pain in the whole arm. Hands cannot be taken off the electrodes. With an increase in the duration of the current flow, the pain increases	Increased heating sensation both under the electrodes and in adjacent areas of the skin
20 – 25	Hands are paralyzed instantly, it is impossible to tear away from the electrodes. Severe pain, difficulty breathing	An even greater increase in the sensation of heating the skin, the appearance of a sensation of internal heating Minor muscle contractions in the arms
25 – 50	Very severe pain in the arms and chest. Breathing is extremely difficult. With prolonged current, respiratory paralysis or weakening of the activity of the heart with loss of consciousness may occur.	Feeling of intense heat, pain and cramps in the hands. When the hands are taken off the electrodes, barely tolerable pain occurs as a result of convulsive muscle contraction
50 – 80	Breathing is paralyzed after a few seconds, the work of the heart is disrupted. With prolonged current flow, cardiac fibrillation may occur	Sensations of very strong superficial and internal heating, severe pain in the entire arm and in the chest area. Difficulty breathing. Hands cannot be taken off the electrodes due to severe pain when contact is broken
		Respiratory paralysis with prolonged current flow
	The same action in less time	Cardiac fibrillation after 2-3 seconds; a few seconds later - respiratory paralysis
More than 5000	Breathing is paralyzed immediately - after a split second. Cardiac fibrillation usually does not occur; possible temporary cardiac arrest during the period of current flow. With prolonged current flow (several seconds), severe burns, tissue destruction

The greatest danger is the current with a frequency of 50 to 1000 Hz. With a further increase in frequency, the risk of injury decreases and completely disappears at a frequency of 45-50 kHz. These currents are dangerous only from the point of view of burns. The decrease in the risk of electric shock with increasing frequency becomes practically noticeable at 1 - 2 kHz.

Duration of exposure to electric current.Prolonged exposure to electric current leads to severe and sometimes fatal injuries to a person.

A long-term exposure to a current of 1 mA is considered safe, with a duration of up to 30 s, a current of 6 mA is safe.

The following values \u200b\u200bof the current strength are accepted as practically permissible with a rather low probability of defeat:

The path of the current through the human body. This factor also plays a significant role in the outcome of the lesion, since the current can pass through vital organs - the heart, lungs, brain, etc.

Individual properties of a person.It has been found that physically healthy and strong people tolerate electric shocks more easily.

Individuals suffering from skin diseases, diseases of the cardiovascular system, internal secretion organs and lungs, nervous diseases, etc. are distinguished by an increased susceptibility to electric current.

Environmental conditions.The condition of the surrounding air, environment, and the surrounding environment can significantly affect the risk of electric shock.

Dampness, conductive dust, the presence of corrosive vapors and gases that have a destructive effect on the insulation of electrical installations, as well as high ambient temperatures, reduce the electrical resistance of the human body, which further increases the risk of electric shock.

The impact of current on a person is also aggravated by conductive floors and metal structures close to electrical equipment that have a connection to the ground, since when this object and the body of electrical equipment accidentally come into contact with the ground, a high current will pass through the person at the same time.

Depending on the listed conditions that increase the risk of exposure to current, the "Electrical Installation Rules" divide all rooms into four classes for the risk of electric shock to people.

The nature and consequences of exposure to an electric current depends on the following factors:

The electrical resistance of the human body;

Voltage and current values;

The duration of the action of the electric current;

Paths of current through the human body;

The type and frequency of the electric current;

Individual properties of a person;

Environmental conditions.

The electrical resistance of the human body.The strength of the current Ih passing through any part of the human body depends on the supplied voltage Upr (touch voltage) and electrical resistance Z t, provided to the current by this part of the body:

In the area between the two electrodes, the electrical resistance of the human body mainly consists of the resistances of the two thin outer layers of the skin touching the electrodes and the internal resistance of the rest of the body.

The poorly conducting outer layer of the skin adjacent to the electrode, and the inner tissue under this layer, as it were, form capacitor plates with a capacity FROM with resistance r n (Figure 7.1). From the equivalent circuit, it can be seen that in the outer layer of the skin, the current flows along two parallel paths; through the active external resistance Rн and the capacitance, the electrical resistance of which

, where Wpf - angular frequency, Hz; f - current frequency, Hz,

Fig. 7.1. Electrical circuit for replacing the resistance of the outer layer of the skin

a - electrode contact diagram; b - electrical equivalent circuit; 1 - electrode; 2 - the outer layer of the skin; 3 - the inner area of \u200b\u200bthe skin.

Then the impedance of the outer layer of the skin for alternating current is:

(7.2)

Resistance r n and capacitance C depend on the area of \u200b\u200bthe electrodes (contact area). With an increase in the contact area, rn decreases, and the capacity C increases. Therefore, an increase in the contact area leads to a decrease in the impedance of the outer layer of the skin. Experiments have shown that the internal resistance of the body r in can be considered as purely active. Thus, for the arm-to-arm current path, the total electrical resistance of the body can be represented by the equivalent circuit shown in Figure 7.2.

Fig. 7.2. Electrical circuit for replacing the resistance of the human body: 1 - electrode; 2 - the outer layer of the skin; r r, r vk - internal resistance of arms and body.

With an increase in the frequency of the current due to a decrease in Xc, the resistance of the human body decreases and at high frequencies (more than 10 kHz) it practically becomes equal to the internal resistance rv. The dependence of the resistance of the human body on frequency is shown in Fig. 7.3.

There is a nonlinear relationship between the current flowing through the human body and the voltage applied to it: with increasing voltage, the current increases faster. This is mainly due to the nonlinearity of the electrical resistance of the human body. So, when the voltage on the electrodes is 40 ... 45 V, significant electric field strengths arise in the outer layer of the skin, at which a breakdown of the outer layer occurs completely or partially, which reduces the total resistance of the human body (Fig. 7.4.) At a voltage of 127 ... 220 V it is practically falls to the value of the internal resistance of the body. The internal resistance of the body is considered active. Its value depends on the length of the transverse size of the part of the body through which the current flows.

As a calculated value at an alternating current of industrial frequency, the active resistance of the human body is taken equal to 1000 0m.

In actual conditions, the resistance of the human body is not constant. 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 body's resistance to 500 ... 700 Ohm, which increases the risk of electric shock to a person.

Moisturizing the skin with water or sweat has the same effect. Thus, working with electrical installations with wet hands or in conditions that cause skin moisture, as well as at elevated temperatures that cause increased sweating, exacerbates the risk of electric shock to a person.

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

The body resistance is influenced by the area of \u200b\u200bcontacts, as well as the place of contact, since the skin resistance of the same person is not the same in different parts of the body. The least resistance is possessed by the skin of the face, neck, arms in the area above the palms and especially on the side facing the body, armpits, the 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 vasodilation, to an increase in the supply of blood to this area and an increase in sweating.

The resistance of the human body depends on the sex and age of people: in women this resistance is less than in men, in children it is less than in adults, in young people it is less than in the elderly. This is due to the thickness and degree of coarsening of the upper layer of the skin. A short-term (several minutes) decrease in the resistance of the human body (by 20 ... 50%) causes external, unexpected physical irritations: painful (blows, injections), light and sound.

The magnitude of the voltage and current. The main factor determining the outcome of electric shock is the strength of the current passing through the human body (Table 7.1)

The voltage applied to the human body also affects the outcome of the defeat, but only insofar as it determines the value of the current passing through the person.

Table 7.1

The nature of the impact of the current

Current passing through the human body, mA	AC (50 Hz) current	D.C
0,5 … 1,5	Onset of sensations: slight itching, tingling of the skin	Not felt
2 … 4	The sensation extends to the wrist; slightly tightens the muscles.	Not felt
5 … 7	Pain sensations increase throughout the hand; convulsions; mild pain in the entire arm up to the forearm	The beginning of sensations; weak heating of the skin under the electrodes
8 … 10	Severe pain and cramps in the entire arm, including the forearm. It is difficult to take your hands off the electrodes.	Increased sensation.
10 … 15	Barely bearable pain in the whole arm. Hands cannot be taken off the electrodes. With an increase in the duration of the current flow, the pain intensifies.	Significant heating under the electrodes and in the adjacent skin area.
20 … 25	Severe pain. Hands are paralyzed instantly, it is impossible to tear them off the electrodes. Breathing is difficult.	Feeling of internal heating, slight contraction of the arm muscles.
25 … 50	Very severe pain in the arms and chest. Breathing is extremely difficult. Prolonged exposure may cause respiratory arrest or weakening of cardiac activity with loss of consciousness	Strong heat, pain and cramps in the arms. Severe pain occurs when the hands are taken off the electrodes.
50 … 80	Breathing is paralyzed after a few seconds, the work of the heart is disrupted. Prolonged exposure may cause cardiac fibrillation	Very strong surface and internal heating. Severe pain in the arm and chest. Hands cannot be taken off the electrodes due to severe pain.
80 … 100	Cardiac fibrillation in 2 ... 3 s .; a few seconds later, breathing stops.	The same action is more pronounced. With prolonged action, respiratory arrest.
	The same action in less time.	Cardiac fibrillation in 2 ... 3 s .; after a few more seconds, breathing stops.

From the above table, the following current threshold values \u200b\u200bcan be distinguished:

About u t i m y t o k - an electric current that causes palpable irritations when passing through the body. Perceptible irritations are called by an alternating current with a force of 0.6 ... 1.5 mA and a constant current with a force of 5 ... 7 mA. The indicated values \u200b\u200bare threshold sensible currents; the region of tangible currents begins with them.

Not pus - an electric current that, when passing through a person, causes irresistible convulsive contractions of the muscles of the arm in which the conductor is clamped. The threshold non-release current is 10 ... 15 mA AC and 50 ... 60 mA DC. With such a current, a person can no longer unclench the hand on his own, in which the current-carrying part is clamped, and becomes, as it were, chained to it.

F i b r i l l c i o n y t o k - an electric current that causes cardiac fibrillation when passing through the body. Threshold fibrillation current is 100 mA AC and 300 mA DC with a duration of 1 ... 2 s along the path "hand-hand" or "hand-feet". The fibrillation current can reach 5 A. A current greater than 5 A does not cause cardiac fibrillation. With such currents, instantaneous cardiac arrest occurs.

Threshold (smallest) values \u200b\u200bof perceptible, non-releasing and fibrillation currents are random values, the normalized values \u200b\u200bof which are determined by the distribution law and its parameters. The numerical values \u200b\u200bof the currents correspond to a certain probability of the occurrence of a given biological reaction.

The currents permissible for a person are assessed according to three criteria of electrical safety.

First criterion- tangible current. As the first criterion for an alternating current with a frequency of 50 Hz, a current I \u003d 0.6 mA is adopted, which does not cause disturbances in the activity of the body. The permissible duration of the flow of such a current through a person is no more than 10 minutes.

Second criterion - releasing current. As the second criterion of electrical safety, a current I \u003d 6 mA is adopted, when it flows through a person, the probability of letting go is 99.5%. The duration of exposure to such a current is limited by the protective reaction of the person himself.

Third criterion- non-fibrillation current. This is an industrial frequency current, which does not cause fibrillation of the heart in a person weighing 50 kg with a certain margin of 50 mA under prolonged exposure for 1 ... 3 s.

Thus, the magnitude of the current has a significant effect on the degree of damage to a person. With the same duration of current flow through a person, the nature of the impact changes significantly from sensation (0.6 ... 1.6 mA) to non-letting go (6 ... 24 mA) and cardiac fibrillation (more than 50 mA).

The duration of the action of the electric current. The duration of the passage of the current through the human body has a significant effect on the outcome of the lesion. Prolonged exposure to current leads to severe and sometimes fatal injuries.

With short-term exposure (0.1 ... 0.5 s), a current of about 100 mA does not cause cardiac fibrillation. If the duration of exposure is increased to 1 s, then the same current can be fatal. With a decrease in the duration of exposure, the values \u200b\u200bof currents permissible for a person increase significantly. So, when the exposure time changes from 1 to 0.1 s, the permissible current will increase approximately 16 times.

In addition, reducing the duration of exposure to electric current reduces the risk of injury to a person based on some features of the heart.

Electrocardiogram diagram

The duration of one period of the cardiocycle (Fig. 7.5.) Is 0.75 ... 0.85 s. In each cardiocycle, there is a period of systole, when the ventricles of the heart contract (peak QRS) and push blood into the arterial vessels. Phase T corresponds to the end of the contraction of the ventricles and they go into a relaxed state.

During diastole, the ventricles are filled with blood. Phase P corresponds to atrial contraction. It was found that the heart is most sensitive to the effects of electric current during the T phase of the cardiocycle. In order for cardiac fibrillation to occur, it is necessary to coincide in time with the action of the current with the phase T, the duration of which is 0.15 ... 0.2 s. With a reduction in the duration of exposure to electric current, the likelihood of such a coincidence becomes less, and therefore, the risk of cardiac fibrillation decreases.

If the time of current passing through a person does not coincide with the T phase, currents significantly exceeding the threshold values \u200b\u200bwill not cause cardiac fibrillation.

The influence of the duration of the passage of current through the human body on the outcome of the lesion can be estimated by the empirical formula

I h \u003d 50 / t (7.3)

where I h is the current passing through the human body, mA; t - duration of current passage, s.

This formula is valid for 0.1 ... 1.0 s. It is used to determine the maximum permissible currents passing through a person along the hand-to-foot path, which are necessary for calculating protective devices.

Paths of current through the human body. The path of the current in the human body depends on which parts of the body the victim touches the live parts, its influence on the outcome of the lesion is also manifested because the resistance of the skin in different parts of the body is not the same.

The most dangerous is the passage of current through the respiratory muscles and the heart. So it was noted that on the path "hand - hand" 3.3% of the total current passes through the heart, "left arm - legs" - 3.7%, "right arm - legs" - 6.7%, "leg - leg" - 0.4%, "head - legs" - 6.8%, "head - hands" - 7%.

According to statistics, disability for three days or more was observed with the current path "hand - arm" in 83% of cases, "left arm - legs" - 80%, "right arm - legs" - 87%, "leg - leg" - in 15% of cases.

Thus, the current path affects the outcome of the lesion; the current in the body does not necessarily follow the shortest path, which is explained by the large difference in the specific resistance of various tissues (bone, muscle, fat, etc.).

The smallest current through the heart passes with the current path along the lower leg-leg loop. However, one should not draw conclusions from this about the low danger of the lower loop (the action of the step voltage). Usually, if the current is large enough, it causes leg cramps, and the person falls, after which the current passes through the chest, i.e. through the respiratory muscles and heart.

Kind and frequency of the current. It has been found that alternating current is more dangerous than direct current. This also follows from table. 7.1., Since the same influences are caused by higher values \u200b\u200bof direct current than alternating one. However, this is typical for relatively low voltages (up to 250 ... 300 V). It is considered that a voltage of 120 V DC under the same conditions is equivalent in hazard to a voltage of 40 V AC power frequency. At higher voltages, the danger of direct current increases.

In the range of voltages 400 ... 600 V, the danger of direct current is practically equal to the danger of alternating current with a frequency of 50 Hz, and at a voltage of more than 600 V, direct current is more dangerous than alternating current. When exposed to constant voltage, especially sharp painful sensations arise at the moment of closing and opening of an electrical circuit.

Studies have shown that the most unfavorable for humans are currents of industrial frequency (50 Hz). With an increase in frequency (from 50 Hz to 0), the values \u200b\u200bof the non-releasing current increase (Fig. 7.6.) And at a frequency equal to zero (direct current - pain effect), they become approximately 3 times larger.

Fig. 7.6. Dependence of non-releasing current on frequency:

1 - for 0.5% of the subjects; 2 - for 99.5% of the subjects

With increasing frequency (more than 50 Hz), the values \u200b\u200bof the non-releasing current increase. A further increase in the frequency of the current is accompanied by a decrease in the risk of injury, which completely disappears at a frequency of 45 ... 50 kHz. But these currents can cause burns both when an electric arc occurs, and when they pass directly through the human body. A decrease in the risk of electric shock with increasing frequency is practically noticeable at a frequency of 1000 ... 2000 Hz.

Individual properties of a person. It has been found that physically healthy and strong people are easier to tolerate electrical shocks.

Individuals suffering from skin diseases, cardiovascular diseases, organs of internal secretion, lungs, nervous diseases, etc. are distinguished by an increased susceptibility to electric current.

Safety regulations for the operation of electrical installations provide for the selection of personnel to service existing electrical installations for health reasons. For this purpose, a medical examination of persons is carried out upon admission to work and periodically once every two years in accordance with the list of diseases and disorders that prevent admission to service of existing electrical installations.

Environmental conditions. Air humidity and temperature, the presence of grounded metal structures and floors, conductive dust have an additional effect on the electrical safety conditions. The degree of electric shock largely depends on the density and area of \u200b\u200bhuman contact with live parts. In humid rooms with high temperatures or outdoor electrical installations, unfavorable conditions develop, in which the area of \u200b\u200bhuman contact with live parts increases. The presence of grounded metal structures and floors creates an increased risk of injury due to the fact that a person is almost constantly connected to one pole (ground) of an electrical installation. In this case, any touch of a person to live parts immediately leads to a two-pole connection to the electrical circuit. Conductive dust also creates conditions for electrical contact with both live parts and the ground.

Depending on the presence of the listed conditions that increase the risk of exposure to current on a person, all premises for the risk of injury to people by electric shock are divided into the following classes: without increased danger, with increased danger, especially dangerous.

Premises without increased dangercharacterized by the absence of conditions that create an increased or special danger.

Premises with increased dangercharacterized by the presence in them of one of the following conditions that create an increased danger:

Dampness (relative humidity of air for a long time exceeds 75%) or conductive dust;

Conductive floors (metal, earth, reinforced concrete, brick, etc.);

High temperature (above +35 0 С);

Possibilities of simultaneous touch of a person to the metal structures of buildings, technological devices, mechanisms, etc., having connections to the ground, on the one hand, and to the metal cases of electrical equipment, on the other.

Particularly dangerous premises characterized by the presence of one of the following conditions that create a particular hazard:

Particular dampness (the relative humidity of the air is close to 100%: the ceiling, walls, floor and objects in the room are covered with moisture);

Chemically active or organic environment (destroying insulation and live parts of electrical equipment);

Simultaneously two or more conditions of increased danger.

Effects on the body. Factors determining the risk of electric shock

With an increase in the power-to-weight ratio of chemical enterprises, the number of people in contact with electrical equipment, instrumentation, lighting devices, etc. increases. Since almost everyone working at enterprises with electrical installations operating at voltages up to 1000 V can contact, the possibility of electric shock increases. especially if the electrical equipment is faulty or is operated in violation of the "Electrical Installation Rules" (PUE).

In addition, the danger of electric shock differs from other industrial hazards (toxic substances, heated surfaces, noise, etc.) in that a person is not able to detect it remotely without special measuring devices.

As for installations operating at voltages above 1000 V, then, as a rule, either they are fenced, or people with special training work with them.

When passing through the human body, an electric current has the following types of effects:

1. thermal - burns, heating of blood vessels, nerves;
2. electrolytic - decomposition of blood and lymphatic fluid, i.e. significant change in their physical and chemical properties;
3. biological - irritation and excitement of living tissues of the body, accompanied by involuntary convulsions of the muscles of the body, heart, lungs, which leads to disruption or complete cessation of the activity of individual organs, respiratory and circulatory systems.

These effects lead to two types of injury: electrical injuries - clearly expressed local damage to the body (burns, electrical signs, metallization of the skin, mechanical damage, electrophthalmia) and electric shock - electrical injury caused by the reflex action of an electric current, i.e. action on the central nervous system, as a result of which paralysis of the affected organs may occur.

Injury statistics show that of all registered cases of electric shock with loss of working capacity for more than 3 days, as well as with a fatal outcome, 19% are electrical injuries, 26% are electric shocks and 55% are mixed injuries.

Electrical, or contact, burn - the result of the thermal effect of current at the point of contact with non-insulated live parts; can be surface (typical for currents of industrial frequency up to 100 Hz) or internal (for currents with a frequency of tens and hundreds of kHz). The amount of heat released in human tissue, in this case, is determined by the Joule-Lenz law (in J)

Q \u003d IчRчt, (8.1)

where Ich is the strength of the current passing through the human body. AND; Rh is the resistance of the human body. Ohm; t - current flow time, s.

There are four degrees of burns: I - redness of the skin, II - the formation of blisters on the surface of the skin, III - charring of the skin, IV - charring of the subcutaneous tissue, muscles. Electrical burns should not be equated with thermal burns, for example, burns with an electric blast, the temperature in the channel of which can reach 4000 ° C (they are typical for installations with voltages above 1000 V).

Electric signs - clearly defined spots of gray or pale yellow color with a diameter of 1 mm; specific damage caused, according to many researchers, by mechanical and chemical exposure to current; occur on contact with live parts, are painless and disappear over time.

Skin metallization - damage to a skin area as a result of the penetration of the smallest particles of molten metal into it. Over time, the affected skin comes off, the area takes on a normal appearance and the painful sensations disappear.

Mechanical damage is the result of sharp, involuntary, convulsive muscle contractions under the action of a current, as a result of which ruptures of the skin, blood vessels, nerves, and joint dislocations are possible.

An electric shock is observed with prolonged exposure to a current of low strength (up to several hundred milliamperes) and, as a rule, at a voltage of up to 1000 V. There are four degrees of shock: I - convulsive muscle contraction without loss of consciousness; II - the same, but with loss of consciousness; III - loss of consciousness, impaired cardiac activity and breathing; IV - clinical death, i.e. lack of blood circulation and respiration.

The severity of electrical injuries depends on a number of factors: the strength of the flowing current, the path of its passage, the magnitude and type of voltage, the electrical resistance of the human body, the duration of the current flow, as well as the health and individual characteristics of a person, the environment, etc.

The magnitude of the current flowing through the human body is the main factor on which the outcome of the lesion depends. The smallest value of the perceptible current, which depends on the type of current, the state of the person, the type of its inclusion in the circuit, is called the threshold perceptible current. For an industrial frequency of 50 Hz, its value is on average 1 mA.

With an increase in the current strength to 10 ... 15 mA, painful cramps occur in the muscles of the hands, so a person is not able to control their action and independently free himself from the conductor (electrode) clamped in his hand. The magnitude of the current 10 mA is called the threshold non-release current.

With a current strength of 25 ... 50 mA, a strong contraction of the respiratory muscles of the chest occurs, breathing becomes difficult or stops. The likelihood of damage to the respiratory system largely depends on the time the current flows through the body.

A further increase in the current value up to 100 mA can cause fibrillation of the ventricles of the heart, in which their chaotic contraction occurs and blood circulation is disturbed or completely stopped, i.e., clinical death occurs. The danger of fibrillation lies in the fact that the human heart cannot independently get out of this state and restore its activity: urgent first aid is needed - artificial respiration and external (indirect) heart massage.

Otherwise, after 5 ... 6 minutes, neurons of the cerebral cortex begin to die, and clinical death turns into biological. As a result, both in our country and abroad, a current of 100 mA is considered fatal.

The path of its passage in the human body ("loop" of current) has a significant impact on the outcome of an electric shock. In the literature, 15 paths are described, however, the most probable paths of current flow are as follows: hand - arm (up to 40%), right arm - legs (up to 20%), leg - leg. In this case, from 0.4 to 7% of the total current flows through the human heart.

Individual characteristics of the organism - for example, health, physical development, weight, readiness to work with electrical installations ("attention factor") - also affect the outcome of the lesion. It has been established that people with increased excitability, diseases of the cardiovascular system, endocrine organs have increased sensitivity to the action of electric current.

The type and frequency of the current are essential in case of injury. It has been established that alternating current of industrial frequency 50 ... 60 Hz is 4 ... 5 times more dangerous than direct current. Currents with a frequency of 400 ... 500 kHz ns have an irritating effect on tissues and ns cause an electric shock. However, these currents have a thermal effect.

A very significant effect on the magnitude of the current passing through the human body is exerted by the total electrical resistance of his body, which, with dry intact skin, can fluctuate over a very wide range: from 103 to 105 ohms, and sometimes even more.

It is a non-linear quantity and depends on a number of factors: the condition of the skin (dry, wet, clean, damaged), the density and area of \u200b\u200bcontact with live parts, the strength of the passing current and applied voltage, the time of exposure to the current.

The upper stratum corneum (epidermis) with a thickness of 0.05 ... 0.2 mm, consisting of dead cells filled with air, has the greatest electrical resistance in the human body. When the stratum corneum is removed, the resistance of internal organs vital for a person does not exceed 800 ... 1000 Ohm. Therefore, when calculating the conditions of electrical safety of a person, its total electrical resistance Rh is taken to be 1000 Ohm.

Knowing the electrical resistance of the human body and the range of currents that are dangerous for it, it is possible to determine the range of dangerous voltages. So, for the regulated values \u200b\u200bof the threshold non-releasing current of 10 mA and Rh \u003d 1000 Ohm, the safe voltage will be Uwithout \u003d RhIch \u003d 10 V.

The environment and the situation in the room can enhance or weaken the effect of electric current, since they significantly affect the resistance of the human body, the insulation of live parts. In accordance with this, there is a certain classification of premises for the danger of electric shock. Industrial and utility premises are divided into three classes: 1 - without increased danger, 2 - with increased danger; 3 - especially dangerous.

Premises without increased danger are dry (relative humidity does not exceed 60%), dust-free premises with normal temperature and insulating floors (parquet, linoleum, etc.). These may include office premises, quality control rooms, small laboratories, some warehouse premises for storing solid polymer materials and finished products.

Premises with increased danger include: damp, in which the relative humidity of the air for a long time exceeds 75%, but does not reach 100%; hot, in which the air temperature for a long time exceeds 30 ° C; dusty, in which conductive process dust is emitted in an amount sufficient to penetrate under the casing of electrical equipment, settle on the wires, which will create an electrical circuit for the leakage of dangerous currents (dust may also be non-conductive); rooms with conductive floors - metal, earth, reinforced concrete, brick, xylolite, etc. (eliminate transitional resistance between man and earth); premises in which simultaneous contact is possible, on the one hand, to the housings of technological equipment, metal structures of buildings, etc., which are connected to the ground, and, on the other hand, to the metal housings of electrical equipment or live parts. These include areas of injection molding machines, warehouses for storage and areas for hanging ingredients with electrical conductivity (for example, hanging of carbon black), etc.

Particularly dangerous rooms include: especially damp rooms in which the relative humidity of the air is close to 100%, and in such rooms the walls, pop, ceiling and objects in them are covered with moisture: with a chemically active environment, where, according to production conditions, gases, vapors are contained or deposits are formed that destroy insulation or live parts of electrical equipment; premises in which there are simultaneously two or more factors of increased danger.

Such premises include areas for the impregnation of polymer materials, dry cleaning of molds, galvanic shops for plastics metallization, glue shops, showers, etc.