The main factors that determine the outcome of the lesion are:
The magnitude of current and voltage;
Duration of current exposure;
body resistance;
Loop ("path") of current;
Psychological readiness to strike.
The magnitude of the current and voltage.
Electric current, as a damaging factor, determines the degree of physiological impact on a person. Voltage should be considered only as a factor that determines the flow of a particular current under specific conditions - the greater the touch voltage, the greater the damaging current.
According to the degree of physiological impact, the following damaging currents can be distinguished:
0.8 - 1.2 mA - threshold perceptible current (that is, the smallest current value that a person begins to feel);
10 - 16 mA - threshold non-letting (chaining) current, when, due to convulsive contraction of the hands, a person cannot independently free himself from current-carrying parts;
100 mA - threshold fibrillation current; it is the estimated shock current. In this case, it must be borne in mind that the probability of being struck by such a current is 50% with a duration of its impact of at least 0.5 seconds.
It should be noted that no voltage can be considered completely safe and work without protective equipment. So, for example, a car battery has a voltage of 12-15 volts and does not cause electric shock when touched (the current through the human body is less than the threshold perceptible current). But if the battery terminals are accidentally shorted, a powerful arc occurs that can severely burn the skin or retina of the eyes; mechanical injuries are also possible (a person instinctively recoils from the arc and may fall unsuccessfully). In the same way, a person instinctively recoils when touched by a network of temporary lighting (36 Volts, the current is already felt), which threatens to fall from a height, even if the current flowing through the body is small and could not cause damage by itself.
Thus, whatever low voltage does not cancel the use of protective equipment, but only changes their nomenclature (type), for example, when working with a battery, you should use safety glasses. Work on live parts without the use of protective equipment is possible only when the voltage is completely removed!
Duration of current exposure
It has been established that electric shock is possible only when the human heart is completely at rest, when there is no compression (systole) or relaxation (diastole) of the ventricles of the heart and atria. Therefore, for a short time, the impact of the current may not coincide with the phase of complete relaxation, however, everything that increases the rate of the heart, increases the likelihood of cardiac arrest during an electric shock of any duration. Such reasons include: fatigue, agitation, hunger, thirst, fear, taking alcohol, drugs, certain drugs, smoking, illness, etc.
body resistance
The value is not constant, depends on specific conditions, varies from several hundred ohms to several megohms. With a sufficient degree of accuracy, we can assume that when exposed to a voltage of industrial frequency of 50 Hertz, the resistance of the human body is an active quantity, consisting of internal and external components. The internal resistance of all people is approximately the same and is 600 - 800 ohms. From this we can conclude that the resistance of the human body is determined mainly by the magnitude of the external resistance, and specifically, by the condition of the skin of the hands with a thickness of only 0.2 mm (primarily by its outer layer - the epidermis). There are many examples of this, here is one of them. A worker dips his middle and index fingers into an electrolytic bath and receives a fatal blow. It turned out that the cause of death was a skin cut on one of the fingers. The epidermis did not exert its protective action, and the lesion occurred in an apparently harmless current loop. Indeed, if we evaluate this fact in relative units and take the resistance of the skin as 1, then the resistance of internal tissues, bones, lymph, blood will be 0.15 - 0.20, and the resistance of nerve fibers will be only 0.025 (“nerves” are excellent conductors of electric current!) . By the way, this is why the application of electrodes to the so-called acupuncture points is dangerous. Since they are connected by nerve fibers, a striking current can occur at very low voltages. It is one of these cases described in the literature, when a person was injured at a voltage of 5 volts. The resistance of the body is not a constant value: in conditions of high humidity it decreases 12 times, in water - 25 times, sharply reduces its acceptance of alcohol. Thus, the factors of a person’s condition that significantly increase the likelihood of a fatal electric shock to a person include:
everything that increases the pace of the heart - fatigue, agitation, taking alcohol, drugs, certain drugs, smoking, illness;
anything that reduces skin resistance - sweating, cuts, drinking alcohol.
Path ("loop") of current through the human body
When investigating accidents associated with the impact of electric current, first of all it turns out which way the current flowed. A person can touch current-carrying parts (or non-current-carrying metal parts that may be energized) with a variety of parts of the body. Hence the variety of possible current paths.
The most likely are the following:
"right arm - legs" (20% of lesions);
"left arm - legs" (17%);
"both arms - legs" (12%);
"head - legs" (5%);
"hand - hand" (40%);
"leg - leg" (6%).
All loops, except the last one, are called "large" or "full" loops, the current captures the region of the heart and they are the most dangerous. In these cases, 8-12 percent of the total current flows through the heart.
The "leg - leg" loop is called "small", only 0.4% of the total current flows through the heart. This loop occurs when a person finds himself in the current spreading zone, falling under the step voltage. Stepping is the voltage between two points of the earth, due to the spreading of current in the earth, while simultaneously touching them with the feet of a person. Moreover, the wider the step, the more current flows through the legs. Such a current path does not pose a direct danger to life, however, under its action a person may fall and the current flow path will become life-threatening.
Send your good work in the knowledge base is simple. Use the form below
Students, graduate students, young scientists who use the knowledge base in their studies and work will be very grateful to you.
Posted on http://www.allbest.ru/
Ministry of Transport of the Russian Federation
Federal State Budgetary Educational Institution of Higher Professional Education
"Ulyanovsk Higher Aviation School of Civil Aviation (Institute)"
Department of Search and Rescue Flight Support
abstract
Factors that determine the risk of electric shock
Compiled by: Dyachenko M.V.
Head: Shurekov V.V.
Ulyanovsk 2014
Introduction
1. The effect of electric current on the human body
1.1 Types of electric shock
1.2 Electric shock
1.3 Electrical resistance of the human body
1.4 Main factors influencing the outcome of electric shock
2. Conditions and causes under which electric shock occurs
3. Measures to ensure electrical safety in production
3.1 Organizational safeguards
3.2 Organizational and technical protection measures
Conclusion
List of used literature
ATconducting
Russia is a working country. According to statistics for January 2014, the economically active population in Russia is 52% (74.6 million people). Percentage of unemployed as of January 2014 is 5.6%, i.e. 4.2 million people Thus, there are 70.4 million people working in our country.
It is noteworthy that almost all professions today in one way or another come into contact with the use of electricity.
Electric current poses a serious danger to human life, so the task of ensuring electrical safety is very, very serious.
Electrical safety is a system of organizational and technical measures and means that ensure the protection of people from the harmful and dangerous effects of electric current, electric arc, electromagnetic field and static electricity.
Distinguish between direct and alternating current. Today, the use of alternating current with a frequency of 50 Hz to 300 GHz is common.
Let's analyze this range in more detail:
1. Industrial frequency current, 50 Hz, is used in industrial and household electrification systems.
2. Low frequency current, 3-300 kHz - in radio broadcasting, during melting, welding, heat treatment of metals.
3. Medium frequency current, 0.3-3.0 MHz - in broadcasting, with inductive heating of metals and other materials.
4. High frequency current, 3.0-30 MHz - in radio broadcasting, television, medicine, when welding polymers.
5. Very high frequency current, 30-300 MHz - in radio broadcasting, television, in medicine, when welding polymers.
6. Ultra-high frequency current, 0.3-3.0 GHz - in radar, in multichannel radio communications, in radio astronomy during sterilization and cooking, etc.
7. Ultra-high frequency current. 3-30 GHz
8. Extreme high frequency current, 30-300 GHz.
In this work, I will consider the factors that determine the danger of electric shock and the main causes of electrical injuries, as well as measures to prevent and prevent them.
1. Influence of electric currenton the human body
1.1 Views electric shock
Passing through the body, an electric current produces 3 types of effects : thermal, electrolytic and biological.
Thermal the effect is manifested in burns of external and internal parts of the body, heating of blood vessels and blood, etc., which causes serious functional disorders in them.
electrolytic the action is expressed in the decomposition of blood and other organic fluids, thereby causing significant violations of their physico-chemical compositions and tissue as a whole.
biological the action is expressed in irritation and excitation of the living tissues of the body, which may be accompanied by involuntary convulsive muscle contractions, including the muscles of the heart and lungs. In this case, various disorders can occur in the body, including mechanical damage to tissues, as well as a violation and even complete cessation of the activity of the respiratory and circulatory organs.
Distinguish two main types of damage to the body: electrical injuries and electrical shocks. Often both types of damage accompany each other. However, they are different and should be considered separately.
electrical injury- these are clearly expressed local violations of the integrity of body tissues caused by exposure to electric current or an electric arc. Usually these are superficial injuries, that is, lesions of the skin, and sometimes other soft tissues, as well as ligaments and bones.
The danger of electrical injuries and the complexity of their treatment are determined by the nature and degree of tissue damage, as well as the body's response to this damage.
Usually, injuries are cured and the victim's working capacity is restored completely or partially. Sometimes (usually with severe burns) a person dies. In such cases, the immediate cause of death is not electric current, but local damage to the body caused by current. Typical types of electrical injuries are electrical burns, electrical signs, skin plating and mechanical damage.
Electrical burn- the most common electrical injury: burns occur in most of the victims of electric current (60-65%), and a third of them are accompanied by other injuries - signs, metallization of the skin and mechanical damage.
Depending on the conditions of occurrence, they differ three types of burns:
current, or contact arising from the passage of current directly through the human body as a result of human contact with the current-carrying part; this type of burn occurs in electrical installations of relatively low voltage - not higher than 1-2 kV and is, as a rule, a skin burn, that is, external damage;
arc, due to the impact on the human body of an electric arc, but without the passage of current through the human body; usually these burns are the result of accidental short circuits in electrical installations 220-6000 V, for example, when working under voltage on panels and assemblies, when performing measurements with portable devices, etc .;
mixed, which is the result of the action of both of these factors simultaneously, that is, the action of an electric arc and the passage of current through the human body; this burn occurs, as a rule, in higher voltage installations - above 1000 V. In this case, an arc is formed between the current-carrying part and a person, and a current that is usually of great importance (several amperes and even tens of amperes) passes through the human body. In this case, the lesions are severe and often end in the death of the victim, and the severity of the injury increases with the voltage of the electrical installation.
Electric signs, also called current signs or electric marks, are clearly defined spots of gray or pale yellow color on the surface of the skin of a person who has been exposed to current. Often signs are round or oval with a depression in the center; the sizes of signs are 1-5 mm. The affected area of the skin hardens like a callus. As a rule, electric signs are painless and their treatment ends successfully: over time, the top layer of the skin comes off and the affected area acquires its original color, elasticity and sensitivity. Signs occur quite often - in about 20% of those affected by the current.
Leather plating- penetration into the skin of the smallest particles of metal melted under the action of an electric arc. This phenomenon occurs during short circuits, disconnectors and knife switches under load, etc. The affected area of the skin has a rough, hard surface. Sometimes there is reddening of the skin caused by a burn, due to the heat brought into the skin by the metal. The victim feels skin tension on the affected area from the presence of a foreign body in it, and in some cases experiences pain from burns.
Usually, over time, the diseased skin disappears and the affected area becomes normal. At the same time, all the painful sensations associated with this injury also disappear.
Metallization of the skin is observed in approximately one in ten of the victims. Moreover, in most cases, simultaneously with metallization, an electric arc burn occurs, which almost always causes more severe injuries.
Mechanical damage is the result of sharp, involuntary convulsive muscle contractions under the influence of a current passing through a person. As a result, ruptures of the skin, blood vessels and nervous tissue can occur, as well as dislocations of the joints and even bone fractures.
1.2 electric shock
electric shock- this is the excitation of living tissues by an electric current passing through the body, accompanied by involuntary convulsive muscle contractions. Depending on the outcome of the negative impact of current on the body, electric shocks can be conditionally divided into the following four degrees:
1. convulsive muscle contraction without loss of consciousness;
2. convulsive muscle contraction with loss of consciousness, but with preserved breathing and heart function;
3. loss of consciousness and impaired cardiac activity or breathing (or both);
4. Clinical death, that is, lack of breathing and circulation.
Clinical (or “imaginary”) death is a transitional period from life to death, occurring from the moment of cessation of activity and lungs. A person who is in a state of clinical death lacks all signs of life, he does not breathe, his heart does not work, pain stimuli do not cause any reactions, the pupils of the eyes are dilated and do not react to light. However, during this period, life in the body has not yet completely died out, because its tissues do not die immediately and the functions of various organs do not immediately die out. These circumstances make it possible to restore the fading or just extinct functions of the organism, that is, to revive the dying organism.
The cells of the brain, which are associated with consciousness and thinking, are the first to die, which are very sensitive to oxygen starvation. Therefore, the duration of clinical death is determined by the time from the moment of cessation of cardiac activity and respiration until the onset of death of the cells of the cerebral cortex; in most cases, it is 4-5 minutes, and when a healthy person dies from an accidental cause, for example, from an electric current, it is 7-8 minutes.
Biological (or true) death- an irreversible phenomenon characterized by the cessation of biological processes in the cells and tissues of the body and the breakdown of protein structures; it occurs after the period of clinical death.
Cause of death from electric shock there may be a cessation of the heart, cessation of breathing and electric shock.
The cessation of cardiac activity is a consequence of the effect of current on the heart muscle. Such an effect can be direct, when the current flows directly in the region of the heart, and reflex, that is, through the central nervous system, when the current path lies outside this area. In both cases, cardiac arrest can occur or its fibrillation occurs, that is, chaotically fast and different-time contractions of the fibers (fibrils) of the heart muscle, in which the heart stops working as a pump, as a result of which blood circulation in the body stops.
Cessation of breathing as the root cause of death from electric current is caused by the direct or reflex effect of the current on the muscles of the chest involved in the breathing process. A person begins to experience breathing difficulties already at a current of 20-25 mA (50 Hz), which increases with increasing current. With prolonged action of the current, asphyxia can occur - asphyxiation as a result of a lack of oxygen and an excess of carbon dioxide in the body.
electric shock- a kind of severe neuro-reflex reaction of the body in response to strong irritation with an electric current, accompanied by dangerous disorders of blood circulation, respiration, metabolism, etc. The state of shock lasts from several tens of minutes to a day. After that, either the death of the body may occur as a result of the complete extinction of vital functions or complete recovery as a result of timely active therapeutic intervention.
1.3 Electricalwhat is the resistance of the human body
The human body is a conductor of electric current. Different tissues of the body provide different resistance to current: skin, bones, adipose tissue - large, and muscle tissue, blood, and especially the spinal cord and brain - small. The skin has a very high resistivity, which is the main factor determining the resistance of the entire human body.
The resistance of the human body with dry, clean and intact skin (measured at voltages up to 15-20 V) ranges from about 3000 to 100,000 ohms, and sometimes more.
Usually, with alternating current of industrial frequency, only the active resistance of the human body is taken into account and taken equal to 1000 ohms. In reality, this resistance is a variable value that has a non-linear dependence on many factors, including skin condition, electrical circuit parameters, physiological factors and environmental conditions.
Skin condition- greatly affects the magnitude of the resistance of the human body. Thus, damage to the stratum corneum, including cuts, scratches, abrasions and other microtraumas, can reduce the total resistance of the body to a value close to the value of internal resistance, which certainly increases the risk of electric shock to a person. Moisturizing the skin with water or sweat, as well as contamination of the skin with conductive dust or dirt, has the same effect.
Since the skin resistance of the same person is not the same in different parts of the body, the place where the contacts are applied, as well as their area, affects the resistance as a whole. The magnitude of the current and the duration of its passage through the body have a direct effect on the total resistance: with an increase in the current and the time it passes, the resistance decreases, since this increases the local heating of the skin, which leads to the expansion of its vessels, and consequently to an increase in the supply of blood to this area and an increase in perspiration.
1.4 Main factors, owaffecting the outcome of electric shock
The magnitude of the electric current passing through the human body is the main factor determining the outcome of the lesion. At the same time, the duration of the current exposure, its frequency, as well as some other factors are of great importance. The resistance of the human body and the amount of voltage applied to it also affect the outcome of the lesion, but only insofar as they determine the amount of current passing through the person.
A person begins to feel the effect of the current passing through him small value: 0.6-1.5 mA at alternating current with a frequency of 50 Hz and 5-7 mA at direct current. This current is called the sensible current threshold or sensible threshold current. Large currents cause muscle cramps and unpleasant painful sensations, which increase with increasing current and spread to larger areas of the body. At 10-15 mA, the pain becomes barely bearable, and the cramps of the muscles of the hands are so significant that a person is not able to overcome them; as a result, he cannot unclench the hand in which the current-carrying part is clamped, he cannot throw the wire away from himself, etc., that is, he is not able to break contact with the current-carrying part on his own and becomes, as it were, chained to it. Larger currents produce the same effect. All these currents are called non-release currents, and the smallest of them - 10-15 mA at a frequency of 50 Hz (and 50-80 mA at direct current) is called the non-release current threshold or threshold non-release current.
A current of 25-50 mA at a frequency of 50 Hz affects the muscles not only of the arms, but also of the body, including the muscles of the chest, as a result of which breathing becomes very difficult. Prolonged exposure to this current can cause cessation of breathing, after which, after some time, death by suffocation will occur. Currents over 50 mA up to 100 mA at 50 Hz disrupt the functioning of the lungs and heart even faster. However, in this case, as with lower currents, the lungs are affected first in time and then the heart.
Alternating current from 100 mA to 5 A at a frequency of 50 Hz and direct current from 300 mA to 5 A act directly on the heart muscle, which is very life-threatening, because after 1-2 seconds from the moment this current circuit is closed, fibrillation can occur through a person. At the same time, blood circulation stops and a lack of oxygen occurs in the body, which, in turn, leads to the cessation of breathing, that is, death occurs. These currents are called fibrillation currents, and the smallest of them is called the threshold fibrillation current.
A current of more than 5 A, as a rule, does not cause cardiac fibrillation. With such currents immediate cardiac arrest, bypassing the state of fibrillation, as well as respiratory paralysis. If the action of the current was short-term (up to 1-2 s) and did not cause damage to the heart (as a result of heating, burns, etc.), then after the current is turned off, the heart, as a rule, resumes normal activity on its own. Breathing about this does not recover on its own and immediate assistance is required to the victim in the form of artificial respiration.
The duration of the passage of current through a living organism significantly affects the outcome of the lesion: the longer the action of the current, the greater the likelihood of a severe injury or death. This dependence is explained by the fact that with an increase in the time of current exposure to living tissue, the value of this current increases, the probability of the coincidence of the moment of passage of the current through the heart with the vulnerable phase T of the cardiac cycle (0.2 s) increases.
The current path in the body of the victim plays a significant role in the outcome of the lesion.. If vital organs - the heart, respiratory organs, brain - are on the path of the current, then the danger of injury is very high, since the current acts directly on these organs. When the current passes through other paths, the effect on the vital organs can only be a reflex, due to which the likelihood of a severe injury is sharply reduced. Since the resistance of the skin in different parts of the body is different, the influence of the current path on the outcome of the lesion also depends on the place where the current-carrying paths are applied to the body of the victim.
There are a lot of possible current paths in the human body; the most common are the following: right arm - legs, left arm - legs, arm - arm and leg - leg. The danger of one or another current path can be assessed by the severity of the lesion, as well as by the value of the current flowing through the heart, with a given loop.
It is known that the value of the current passing through the human heart (as a percentage of the total current passing through the body) is 6.7% during the path right arm - legs; left arm - legs - 3.7%; hand - hand - 3.3%; leg - leg - 0.4%.
Thus, the most dangerous path is the right hand - legs, and the least dangerous - the leg - leg path.
Direct current, as practice shows, is about 4-5 times safer than alternating current of industrial frequency (50 Hz). However, this is true for relatively small voltages - up to 250-300 V. At higher voltages, the danger of direct current increases.
Individual properties of a person play a significant role in the outcome of the lesion. It has been established that healthy and physically strong people tolerate electric shocks more easily than sick and weak people. Persons suffering from a number of diseases, primarily diseases of the skin, cardiovascular system, internal secretion organs, lungs, nervous diseases, etc., have an increased susceptibility to electric current.
2. Termsand reasonsat which electric shock occurs
Causes of electric shock:
§ touching live parts, bare wires, contacts of electrical appliances, knife switches, lamp sockets, live fuses;
§ touching parts of electrical equipment, metal structures of structures, etc., which are not in their usual state, but as a result of damage (breakdown) of the insulation that are energized:
§ being near the junction with the ground of a broken wire of the power supply network;
§ being in close proximity to live parts that are energized above 1000 V;
§ touching a live part and a wet wall or metal structure connected to the ground;
§ simultaneous contact with two wires or other live parts that are energized;
§ inconsistent and erroneous actions of personnel (powering the installation where people work; leaving the installation energized without supervision; admission to work on disconnected electrical equipment without checking for the absence of voltage, etc.).
A person falls under the influence of electric current when accidentally touching the current-carrying parts of the electrical installation or approaching an unacceptably close distance, if an emergency mode occurs in the electrical installation; in case of non-compliance of the parameters of the electrical installation with the norms, as well as in case of violation of the safety regulations and operation of electrical installations.
Table 1. Statistical data on the causes of people getting under voltage
Reason for defeat |
% of all electrical injuries |
|
Touching open live parts under voltage |
||
Touching the conductive parts of the equipment that are energized as a result of damage to the insulation |
||
Touching live parts covered with insulation that has lost its properties; touching current-carrying parts with objects with low electrical resistance |
||
Contact with floors, walls, structural elements, soil that are energized due to an emergency ground fault |
||
Defeat through an electric arc |
electric shock factor
When considering the conditions for the emergence of an electrical circuit through the human body, direct contact of a person with current-carrying parts and indirect contact are distinguished. Direct contact occurs, as a rule, as a result of a violation of safety regulations and the operation of electrical installations, and indirect contact occurs when the insulation on the equipment case is broken.
Short circuit to the body - an accidental electrical connection of the current-carrying part with metal non-current-carrying parts of the electrical installation. Earth fault - an accidental electrical connection of a current-carrying part with the earth or non-current-carrying conductive structures or objects that are not isolated from the earth.
Current passes through the human body when a person touches two points at the same time, between which there is a voltage. The magnitude of the striking current depends on which parts of the electrical installation a person touches, that is, on the conditions of the defeat.
The following lesion conditions may be observed:
bipolar contact with live parts
With a bipolar touch to current-carrying parts, a person simultaneously touches with parts of the body (for example, hands) the current-carrying parts of the equipment.
single-pole contact to live parts
The current circuit through the human body in a network with an isolated neutral closes through the ground and conduction existing between the phases of the network and the ground. In a network with a grounded neutral, the current is closed through a person, earth and neutral ground. Thus, with a unipolar touch, one of the touch points is the point of the ground (earth).
touching grounded non-current-carrying parts that are energized
When touching grounded equipment that is energized, a person is in the current spreading zone, that is, in the zone, each point of which has a certain electrical potential, due to the flow of ground fault current through the ground electrode.
touch voltage
In all cases of electric shock to a person, voltage is applied to the entire circuit of a person, which includes resistance: body, shoes, floor or ground on which a person stands, etc. That part of the voltage that falls on the human body in this circuit is called the touch voltage.
step voltage impact
If a person is near a grounding conductor from which current flows into the ground or near a place of an accidental earth fault, then part of this current can branch off and pass through the person’s legs. The potential difference between the feet at a step distance in the current spreading zone is called step voltage. The step voltage is defined as the voltage between two points of the ground in the current spreading zone, located one from the other at a step distance, on which the feet of a walking person simultaneously rest. The step voltage is greater, the closer a person is to the grounding conductor and the longer the length of his step. From here, measures to prevent damage by step voltage are obvious - eliminating the possibility of people staying in the current spreading zone and removing a person from the zone in which a dangerous potential arose, in small steps.
3. Measures to ensure electrical safetyin production
3.1 Organizational safeguards
briefing
The purpose of the briefing is to provide employees with the knowledge necessary for the correct and safe performance of their professional duties.
There are the following types.
induction training
initial briefing
periodic (repeated).
Safety
Safety is a system of technical means and methods of work that ensure the safety of working conditions. This is one of the most important measures in the field of labor protection.
Proper organization of the workplace
A workplace is a zone of application of labor of a certain employee or group of workers (teams).
Mode of work and rest
The optimal mode of work and rest is such an alternation of periods of work with periods of rest, in which the greatest efficiency of human activity and a good state of health are achieved.
The optimal mode of work and rest is achieved:
pauses in work and breaks;
changing forms of work and environmental conditions;
maintaining a certain pace and rhythm of work;
elimination of monotony and inactivity;
removal of neuropsychic stress by rest in the personnel rest rooms;
using the psychological impact of color, music and technical aesthetics.
Use of personal protective equipment
Personal protective equipment is designed to protect the body, respiratory organs, vision, hearing, head, face and hands from injuries and exposure to adverse production factors.
Electrical protective equipment is divided into basic and additional ..
Main electrical protective equipment for work in electrical installations with voltage above 1 kV: insulating rods, insulating and electrical clamps, voltage indicators.
Additional: dielectric gloves, boots, carpets and caps; individual shielding kits, insulating stands and pads; portable grounding; protective devices; posters and safety signs.
Basic electrical protective equipment for work in electrical installations with voltage up to 1 kV: insulating rods, insulating and electrical pliers, voltage indicators, dielectric gloves, bench and assembly tools with insulating handles.
Additional: dielectric galoshes and carpets, portable grounding, insulating stands and pads, protective devices, posters and safety signs.
Use of warning posters and safety signs
When working in electrical installations, there is a risk of loss of orientation by workers.
Recruitment
Safety regulations provide for the selection of personnel for the maintenance of existing electrical installations for health reasons.
3.2 Organizational and technical protection measures
Insulation and fencing of live parts of electrical equipment
To exclude the possibility of touching or dangerously approaching uninsulated live parts, the inaccessibility of the latter must be ensured by fencing or locating live parts at an inaccessible height or in an inaccessible place.
Applying locks
Interlocks are used to ensure that bare live parts are inaccessible. They are used in electrical installations in which work is often carried out on protected current-carrying parts (test benches, installations for testing insulation with increased voltage, etc.).
Portable earthing switches
These are temporary grounding switches that are designed to protect against electric shock to personnel working on disconnected current-carrying parts of an electrical installation, in case of accidental voltage on these parts.
Protective isolation
Ш working - electrical insulation of the current-carrying parts of the electrical installation, ensuring its normal operation and protection against electric shock;
Ш additional - electrical insulation provided in addition to the working insulation to protect against electric shock in case of damage to the working insulation;
Ш double - electrical insulation, consisting of working and additional insulation.
Workplace isolation.
Conclusion
Perhaps there is no such professional activity where electric current would not be used. Even a teacher often resorts to electrical appliances (tape recorder, projector, lighting lamps) - what can we say about other professions.
In addition, it should be noted a serious danger to human health, which is an electric current. Its effect on the body, which is a conductor with a resistance of about 1000 ohms, is manifested by contact (often accidental) of any part of his body with live components of the electrical circuit. This effect directly depends on the characteristics of the current (strength and voltage) in the circuit, as well as on the physical and neuropsychic state of a person.
With an electric shock, we can talk about the severity of the damaging current: safe releasing, irritating, non-releasing and deadly currents.
In addition to touching the live parts of the equipment or bare wires, the cause of electric shock can be the so-called step voltage.
The most terrible consequence of an electric shock is death. Fortunately, it happens quite rarely in this case.
To prevent electrical shock and ensure electrical safety in production, the following is used: insulation of wires and other components of electrical circuits, instruments and machines; protective grounding; zeroing, emergency power off; personal protective equipment and some other measures.
Unfortunately, the widespread aging of production assets, dilapidation of premises has a negative impact on the quality of electrical wiring. Breakdowns in electrical wiring lead not only to electric shocks, but are also one of the main causes of fires.
List of used literature
1. V.E. Anofrikov, S.A. Bobok, M.N. Dudko, G.D. Elistratov Life Safety: Textbook for High Schools, SUM. Moscow, CJSC “Finstatinform”, 1999. 156p.
2. Ed. B.A. Knyazevsky Occupational safety. Moscow, Higher School, 1972., 67p.
3. V.I.Rusin, G.G.Orlov, N.M.Nedelko et al. Occupational safety in construction. Engineering solutions: Handbook, Kyiv, "Budivelnyk", 1990, 45p.
4. Ed. B.A. Knyazevsky Occupational safety in the energy sector, Moscow, Energoatomizdat, 1985, 200pp.
5. Under the total. ed. I.S. Ivanova Countries of the World: A Handbook, Moscow, Respublika, 1999, 143 pp.
Hosted on Allbest.ru
Similar Documents
Types of electric shock. Electrical resistance of the human body. The main factors affecting the outcome of electric shock. Safety criteria for electric current. Organizational measures to ensure electrical safety in production.
abstract, added 04/20/2011
The magnitude of the current and its effect on the body, the electrical resistance of the human body. Degrees of electric shocks, their characteristics. Causes of death from electric current. Electrical safety rules and methods of protection against electric shock.
abstract, added 09/16/2012
The essence and significance of electrical safety, legal requirements for its provision. Features of the action of electric current on the human body. Analysis of factors influencing the outcome of electric shock. Ways to protect against this type of damage.
test, added 12/21/2010
Risk of electric shock to persons. The influence of electric current on the human body, the main parameters of the electric current on the degree of human damage. Conditions for electric shock. Danger due to short circuit of current conductors to earth.
abstract, added 03/24/2009
Types of electric shock, electrical resistance of the human body, the main factors affecting the outcome of electric shock. Types of protection against the danger of electric shock and the principle of their operation, electrical safety measures.
test, added 09/01/2009
The concept and features of electrical injuries. The effect of electric current on a person. Environmental factors, electrical and non-electrical nature, affecting the danger of electric shock to a person. Methods for the safe operation of electrical installations.
abstract, added 02/22/2011
Types of electric shock. The main factors affecting the outcome of electric shock. The main measures of protection against damage. Classification of premises according to the danger of electric shock. Protective ground. Zeroing. Protective equipment. First aid to a person.
report, added 04/09/2005
The main causes of electrical injuries. Factors that determine the degree of influence of electric current on a person. Conditions for electric shock. Danger due to short circuit of current conductors to earth. Classification of working conditions according to the degree of electrical hazard.
tutorial, added 05/01/2010
Types of damage to the human body by electric current. Factors that determine the outcome of exposure to electricity. The main ways to ensure electrical safety. Assistance to victims of electric shock. Safe voltage, its meanings.
presentation, added 09/17/2013
Electrical injury at work and at home. The effect of electric current on the human body. Electrical injury. Conditions for electric shock. Technical methods and means of electrical safety. Optimization of protection in distribution networks.
The nature and consequences of human exposure to electric current depend on the following factors:
The value of the current passing through the human body;
electrical resistance of a person;
The level of voltage applied to a person;
duration of exposure to electric current;
current paths through the human body;
type and frequency of electric current;
environmental conditions and other factors.
Electrical resistance of the human body. The human body is a conductor of electric current, but inhomogeneous in resistance. The greatest resistance
skin has. The upper horny layer of the skin 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. The capacitive reactance, which is negligible, is usually neglected and the human resistance is considered to be purely active and unchanging. 1000 ohms are taken as the calculated value. In real conditions, human resistance is not a constant value 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 with harmful substances that conduct electric current;
increase in current and time of its passage;
an increase in the voltage of the current applied to the body;
an increase in the frequency of the current;
The body resistance is also influenced by the area of contact and the place of contact, since the resistance of the skin in different areas is not the same.
The magnitude of the current and voltage.
Perceptible current - an electric current that causes perceptible irritations when passing through the body (threshold perceptible currents). Alternating current by force or direct current cause such sensations.
Non-letting current - an electric current that, when passing through a person, causes irresistible convulsive contractions of the muscles of the hand in which the conductor is clamped. The threshold non-release current is AC and DC. A person cannot unclench his hand on his own, help is required.
Fibrillation current - an electric current that causes the passage through the body of the fibrillation of the heart. The threshold fibrillation current is alternating current and direct current with a duration of 1-2s. With a current of more than an instantaneous cardiac arrest.
Duration of exposure. 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: , where is the current passing through the human body, mA; is the duration of the current passing, s This formula is valid within 0.1-1.0s. It is used to determine the maximum allowable currents passing through a person along the path of the hand - foot, necessary for the calculation of protective devices.
The path of current through the human body. The possible current paths in the human body are also called current loops. The most common loops are: hand-hand, hand-legs and leg-leg. The most dangerous loops are head-arms and head-legs, but these loops are relatively rare.
Type 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 (with a frequency of 50 Hz). for alternating current, its frequency also plays a role. With an increase in the frequency of the alternating current, the impedance of the human body decreases, hence the danger of injury increases. greatest danger
represents a current with a frequency of 50 to 100 Hz; with a further increase in frequency, the danger of damage decreases and completely disappears at a frequency of 45-50 kHz. These currents maintain a risk of burns. The reduction in the danger of current becomes practically noticeable at a frequency of 1-2 kHz.
Individual properties of a person. Physically healthy and strong people tolerate electric shocks more easily. People suffering from diseases of the skin, cardiovascular system, lungs, nervous diseases and others are distinguished by increased susceptibility to electric current. Such people are not allowed to work on electrical installations.
Conditions of the external environment.
The “Electrical Installation Rules” divide all premises 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, earthen, reinforced concrete, brick, etc.); e) the possibility of a person simultaneously touching the metal structures of the building that are connected to the ground, technological devices, mechanisms, etc., 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 close to 100%): the ceiling, shadows, floors and objects in the room are covered with moisture0; b) chemically active or organic environment (destroying insulation and current-carrying parts of electrical equipment); c) two or more conditions of increased danger simultaneously.
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 regulations for the operation of power plants, power networks
In order to prevent cases of electrical injury, it is prohibited:
carry out any kind of work on the power line under voltage;
perform installation and repair work on power lines, at radio stations without protective equipment (dielectric mats, gloves, aprons, galoshes), even if the current collectors are disconnected from the mains;
allow persons who do not have special training and permission to operate and carry out work on electrical networks, sources of electricity and electrical equipment;
turn on and off the power of power lines laid in target areas without the order of the flight director or the head of the aviation range.
Persons working with current should regularly conduct safety classes, which explain the inadmissibility of careless and careless handling of power sources, electrical networks and electrical equipment.
With persons working on machine tools, sawmills, circular saws and other equipment, organizes safety classes and systematic testing of their knowledge of safety rules.
In the workshop, in garages, at power plants, at radar stations and other facilities, there must be instructions approved by the head of the aviation range for observing labor protection rules.
It is 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 of any quantity that characterizes it.
According to the generation mechanism, vibrations with power, kinematic and parametric excitation are distinguished.
Power excitation is the excitation of system vibration by driving forces and (or) moments.
Kinematic excitation - excitation of the vibration of the system by communicating to any of its points of given movements that do not depend on the state of the system.
Parametric excitation is the excitation of system vibration by a change in time of one or more of its parameters (mass, moment of inertia, stiffness and resistance coefficients) independent of 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 = 2, 4, 8, 16, 31.5; 63 Hz.
2. Local, which is transmitted through the hands at frequencies f = 8, 16, 31.5; 63, 125, 250, 500, 1000 Hz.
According to the source of occurrence, vibration is divided into three categories:
1. Transport (mobile vehicles on the ground).
2. Transport and technological
(cranes, loaders).
3. Technological (jobs).
According to the time of action, vibration is divided into the following categories.
1. Permanent. Here, the value of the controlled parameter during the observation time changes by no more than two times;
2. Fickle. Here, the value of the controlled parameter changes by more than 2 times during the observation time of at least 10 min when measured with a time constant of 1 s.
Intermittent vibration can be oscillating, intermittent and impulsive.
For personnel working with electrical installations, the priority is to eliminate injuries. A feature of electric shock is the inability of people to remotely, visually, by smell or other signs to determine the threat. The use of special devices allows you to do this effectively, but not in all cases. Some dangers cannot be foreseen even by experienced specialists. To prevent injuries, special labor protection rules have been developed, under which the likelihood of injuries is significantly reduced.
The results of thermal and mechanical damage
Causes of injury from electric current
- Accidental contact, due to inattention to the exposed current-carrying elements of electrical installations under voltage. These can be bare wires, in the process of repairing contacts of household or industrial equipment, on knife switches or lampholders for lighting lamps.
- During operation, as a result of mechanical damage, parts of electrical installations can damage the insulating layer of current-carrying wires and be under dangerous voltage.
- Often the cause of electric shock is approaching along wet ground to a broken and fallen high-voltage power line wire.
- When approaching current-carrying elements under voltage above 1000V, electric shock can be caused by a breakdown of the air space.
- The cause of the defeat is the damp walls of buildings, structures, inside which there are wires with unreliable insulation and grounded elements of metal structures.
- There are cases of injuries as a result of poor organization of labor protection measures, with unauthorized connection to the power supply, when maintenance personnel are working. Work without first checking that safety measures are taken, as well as the absence of voltage, the presence of interlocks, warning posters and other elements that prevent injury.
Affecting factors
The duration of the impact of electric current on the body, the magnitude of the lesion current, the area of \u200b\u200bcontact and many other factors determine the nature of the injury and the degree of impact:
- mechanical actions - delamination, tissue breakage;
- thermal - burns, destruction of the blood vessel structure (Fig. above);
- electrolytic influence - the breakdown of organic matter in human flesh, including blood;
- biological influence - disruption of the work of natural biocurrents, which causes involuntary, convulsive contraction of individual muscles.
Types of injuries
There are the following main types of electric shock.
electrical injury
It is characterized by damage to individual organs, tissue fragments. These may be signs left by an electric discharge, metallization of the skin. An electric arc causes swelling of the surface of the eyeball, evaporation of the mucous membrane on it. Possible mechanical impact, leading to damage such as bruises, fractures.
- Electrical burn - this is the destruction of individual organs, sections of the skin as a result of the influence of current or electric arc on tissues. Burns resulting from the action of electric current can be of different types:
- Electric burn upon contact of a wet (sweaty) body with current-carrying elements, heating and boiling of liquids occurs on the surface and inside the tissues. This process depends on the resistance of the affected area and the current strength. The released heat energy causes burns. Such injuries occur on electrical installations with a power of up to 2 kW, causing burns of the first or second degree.
- Electric arc burn is obtained on a part of the human body under the influence of high thermal energy, which the arc possesses (temperature up to 350 ̊С). Burns of the third and fourth degree occur in electrical installations with a voltage of 6-10 kW.
- of the skin cover is obtained with a short circuit (short circuit) or an arc discharge, when an electrical circuit is opened with a large load. As a result of the melting of the metal at high temperatures, it splashes, gets on the surface of the skin.
Molten metal splashes during short circuit
Small metal fragments from conductive contacts (copper, aluminum or steel) stick to the skin and penetrate the tissue, piercing and burning the skin. Such lesions take on a rough metal cover. Subsequently, on the affected area, the skin exfoliates along with foreign bodies, the wounds heal.
An example of electrical metallization of the skin
- - the result of direct contact with current-carrying elements. The contours of their outlines reflect the surface of the elements with which there was contact, usually a circle or ellipse, from terminals and wires. Print sizes up to 10 mm, the material of the conductive parts determines the color of the signs, they can turn out yellow from copper, brass, gray from steel and white from aluminum. The result determines the chemical, mechanical effect of the current. The tumor under these signs does not have inflammation and heals quickly. With large areas of damage, there are numbness, loss of sensitivity.
Such marks can leave an electrical discharge
- Mechanical damage - the result of instant muscle contraction, the elements of the blood supply system, blood vessels, and skin are torn. There are fractures of the limbs, damage to the joints.
- Electrophthalmia - the effect of high power ultraviolet radiation on the eyeballs. The resulting arc has a wide spectrum of light rays, including infrared, visible colors and ultraviolet. The latter causes burns on the surface of the eyes.
electric shock
The human nervous system instantly reacts to a strong external stimulus. There may be high blood pressure, disruption of the functioning of the blood supply, respiratory organs. There are several phases following an electric shock:
- exciting phase;
- exhaustion and lethargy of the nervous system sets in, the victim remains conscious, but complete indifference to what is happening around him appears. Breathing weakens, the pulse rate increases, this can last up to 20 hours, then the heart stops, and the person dies.
electric shock
Passing through human tissues, electricity causes convulsive, involuntary muscle contraction. The degree of injury depends on the strength of the current and the duration of contact with the conductive surface. Small currents cause slight itching and tingling, at 10-15 mA, uncontrolled convulsions occur.
A large current paralyzes the nervous system, the victim cannot independently free himself from contact with current conductors, which prolongs the time of exposure to damaging factors. Currents of 20-25 mA / 50 Hz knock down the rhythm of the heartbeat, paralysis of the respiratory organs leads to death.
A current of 50-80 mA creates fibrillation of the muscle tissue of the heart, the heart and blood flow stop. Currents greater than 100 mA definitely kill a person in 2-3 seconds of exposure to the body. It is noted that voltage up to 100V is not as dangerous with direct current as with alternating current, it is especially destructive with a frequency of 50 Hz, close to the heart rate, so its effect instantly causes arrhythmia.
Currents at 20-100 Hz are the most dangerous. The probability of damage to internal tissues is less when the frequency increases.
Currents with a frequency of hundreds of kHz do not destroy internal organs; they can only cause burns on the surface of the body. Alternating and direct currents with a voltage of 500V have equally dangerous damaging factors. A voltage of 600V with direct current becomes more destructive for a person than with alternating current.
Electric shocks are divided according to severity:
- I - convulsive muscle contractions, while the person is fully conscious;
- II - the victim is unconscious, the heart and respiratory organs are functioning;
- III - the victim is unconscious, there are violations of the rhythm of the heart and failures in the functioning of the respiratory system;
- IV - breathing and blood circulation flows stop, death occurs (clinical).
Clinical death - no breathing, heartbeats are not heard, a person does not feel painful stimuli, wide pupils that do not respond to changes in light intensity. The transition to death is accompanied by a lack of oxygen supply to the structures of the brain.
The duration of the absence of oxygen in the brain is allowed for 4 minutes, a maximum of 8 minutes, after which irreversible destructive consequences occur.
Cardiac arrest is caused by a sharp contraction of the muscles in the affected area, in the areas of passage, including the heart. Reflex contraction of the heart, when the current flows bypassing the heart muscles, creates conditions for fibrillation and cardiac arrest. In these cases, the currents are called fibrillation, they prevent breathing when flowing through the muscles of the chest, which are involved in the respiratory process.
With a short-term touch to conductive elements in a healthy person, cardiac arrest does not occur, the muscle contracts, with the loss of current, it relaxes, and the heart continues to function. When paralysis of the heart or breathing occurs, both cases are possible simultaneously, the working capacity of the organs is not spontaneously restored, the heart needs an urgent forced massage in conjunction with artificial respiration.
Paths of electrical current through the body
The route largely determines the severity of the lesion, different organs have heterogeneous structures, the resistances of which differ.
Currents travel along paths with less resistance, higher conductivity. The main conductors are large elements of the circulatory system. There is a lot of fluid in these vessels, blood has good conductor properties.
Most likely routes:
- hand - through the chest area - the second hand;
- left or right hand - through the body - legs;
- head - through the neck - hands;
- head - through the body - legs;
- leg - through the inguinal region of the body - the second leg.
An example of the route of electric current through the human body right arm - through the torso - legs
The most dangerous routes are:
- hand - through the heart - leg;
- on the head;
- along the spinal cord.
Cases of death of the victim are not excluded, when the currents pass from one leg to the other or through the arm to the other arm.
The path of electric current through the human body from one hand to the other
The main dangerous case is considered to be the passage of current from the left hand to the legs, but according to injury statistics, the largest percentage of deaths occurs when the route is through the right arm to the legs.
It is possible that the right hand is used more often during work, therefore it is more often injured. The value of the current between the points where it flows depends on the voltage and tissues in its path with different resistance:
In =U\ Rt, where
- Ip - damage currents;
- U - voltage between the contact of the victim with the conductor and the current exit point;
- Rt is tissue resistance.
The Rt of each person is different, this determines the skin, which can be wet, damaged, in these cases it will be less. The current will increase accordingly, the defeat will be more severe. The cornea of the skin has the greatest resistance. When the surface is dry, on intact skin, the resistance can be between 10 and 100 kOhm. Wet skin has a resistance of 1000 ohms, on damaged skin, with cuts, abrasions 500–800 ohms.
On internal tissues in the range of 300-500 ohms, practice shows that a voltage of 50-200V already breaks through the stratum corneum. The breakdown voltage difference is determined by certain conditions:
- thickness of the stratum corneum;
- density of distribution and filling of vessels with blood;
- completeness of filling and distribution over the surface of the sweat glands.
Based on these conditions, the resistance in different areas is different.
The severity of the injury is influenced by environmental conditions, moist air. High temperatures increase conductivity.
Factors that determine the degree of electric shock
Which is dangerous. Video
You can learn about the consequences of electric shock in the video below.
It is important to take into account the condition of the injured person, age, psychological characteristics. People with a sick heart sweat faster and more profusely during physical exertion, alcohol reduces the resistance of body tissues. You need to know all this in order to take timely measures to prevent injuries, make work safer, and, if necessary, correctly determine the degree of damage and provide first aid.
The nature and consequences of human exposure to electric current depend on the following factors:
Electrical resistance of the human body;
The magnitude of the voltage and current acting on a person;
Duration of exposure to electric current;
Type and frequency of electric current;
Current paths through a person;
Environmental conditions and factors of the labor process.
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 provided by the skin, therefore the resistance of the human body is determined mainly by the condition of the skin.
The skin is made up of two main layers: the outer layer, the epidermis, and the inner layer, the dermis. The epidermis also has a layered structure, in which the topmost layer is called the stratum corneum. The stratum corneum in a dry and uncontaminated state can be considered as a dielectric - its electrical resistivity reaches 10 5 ... 10 6 Ohm m, i.e. thousands of times greater than the resistance of other layers of the skin and internal tissues of the body. The resistance of the inner layer of the skin (dermis) is insignificant; it is many times less than the resistance of the stratum corneum. The resistance of the human body with dry, clean and intact skin ranges from 3 to 100 kOhm or more, and the resistance of internal organs is only 300 ... 500 Ohm.
As a calculated value under the action of alternating current of industrial frequency (50 Hz), the active resistance of the human body is equal to 1000 ohms. Under actual conditions, the resistance of the human body is not a constant value. It depends on a number of factors, including: the condition of the skin and the environment; electrical circuit parameters.
Damage to the stratum corneum of the skin (cuts, scratches, abrasions, etc.) reduces the resistance of the body to 500 ... 700 Ohm, which increases the risk of electric shock. The same effect is exerted by: moisturizing the skin (for example, sweat); pollution with harmful substances (eg dust, scale, etc. substances).
The resistance of the human body is influenced by the area of contact with the current source, the larger it is, the less resistance. Up to tens and even units of ohms, the resistance of the skin can decrease at the locations of acupuncture points on the human body.
The magnitude of the current and voltage. The main factor determining the outcome of an electric shock is the strength of the current passing through the human body. The voltage applied to the human body also affects the outcome of the lesion, but only insofar as it determines the amount of current passing through the person.
In the practice of electrical injury, it is customary to distinguish the following thresholds for the action of electric current:
- threshold electric current - the magnitude of the current that causes barely perceptible irritation in the human body (a slight increase in temperature in the contact zone with a source of electricity, irrepressible trembling of the fingers, increased sweating, etc. factors). These sensations are caused by the current strength: 0.6 ... 1.5 mA (for alternating current with a frequency of 50 Hz); 5…7 mA (for direct current);
- non-letting current, - the magnitude of the electric current, causing irresistible convulsive contractions of the muscles of the hands in which the conductor is clamped. The value of the non-releasing current with an action time of 1 ... 3 s is 10 ... 15 mA for AC and 50 ... 60 mA for direct currents. With such a current strength, a person can no longer unclench his hands on his own, in which the current-carrying parts of electrical equipment are clamped;
- fibrillation (lethal) current - the magnitude of the electric current that causes fibrillation of the heart (simultaneous and scattered contraction of individual fibers of the heart muscle, unable to support its independent work). With an action duration of 1 ... 3 s along the hand-hand, hand-foot path, the value of this current is ~ 100 mA for AC and ~ 500 mA for DC. At the same time, a current of 5 A or more does not cause fibrillation of the heart muscle - an instantaneous cardiac arrest and paralysis of the chest muscles occur.
The strength of threshold currents is considered to be a long-term safe value for humans.
There are no safe voltages among those values that are used in human practice, since the current strength at any small of the indicated voltages can exceed the threshold current strength at abnormally low resistances of the human body. For example, the contact of the poles of a galvanic cell (U = 1.5 V) with acupuncture points of a person (R ~ 10 Ohm) can cause a constant electric current to flow between them with a force of 1.5 A, which even with a short-term action exceeds the lethal value by 3 times.
Duration of exposure to electric current. With an increase in the time of current flow through a person, the probability of its passage through the heart increases at the moment of the most vulnerable phase T for the entire cardiocycle (the end of the contraction of the ventricles and their transition to a relaxed state ~ 0.2 s). In addition, with an increase in the time for the flow of electric current through a person, all negative phenomena, both local and general, are aggravated.
Type of current and frequency of alternating electric current. Direct current is about 4 ... 5 times safer than alternating industrial frequency (50 Hz). This fact can be explained by the complex structure of the resistance of the human body. The resistance of the human body includes active (ohmic) and capacitive components, and the latter occurs when a person is included in electrical circuit(Fig. 1).
Rice. 1. Simplified circuit diagram replacement resistance of the human body
Ra is the active (ohmic) component; Rc - capacitive component
The presence of the capacitive component is due to the fact that between the electrode touching the human body (housing of electrical equipment, electrical wires, etc.) and the ground (floor, platform for equipment maintenance, etc.) on which the person stands, there is a stratum corneum skin cover is practically a dielectric, which forms a capacitor system (electric capacitance). If a direct current flows through a person, then it only affects the active component of the total resistance (Ra), since the electric capacitance for direct current is an open circuit. Alternating current flows through both the active and capacitive components of the total human resistance (Ra and Rc), which, other things being equal, leads to a greater negative effect on the body.
With an increase in the frequency of alternating current (relative to 50 Hz), its general negative effect decreases, comparing at a frequency of ~ 1000 Hz with the effect of direct current. At a frequency of ~ 50 Hz and above, the alternating current has practically no general effect on a person. This phenomenon can be explained by the fact that the highest density of charges (ions, electrons) in the plane of the cross section of the conductor during the flow of high-frequency alternating current is observed on the periphery of this section; if we consider a person as a conductor, then on the periphery of the cross section of the trunk and limbs we will see a skin covering that has a resistance close to that of dielectrics. The local effect of high frequency alternating current is preserved.
This provision is valid only up to voltages of 250 ... 300 V. At higher voltages, direct current is more dangerous than alternating current with a frequency of 50 Hz.
The current path through the human body plays a significant role in the outcome of the lesion, because. electric current can pass through vital organs: the heart, lungs, brain, etc. The influence of the current path on the outcome of the lesion is also determined by the resistance of the human skin in various parts of his body.
The number of possible current paths through the human body, called current loops, is quite large. Most often, the current flows through the loops: hand-hand; arm-leg; leg-leg; head-hands; head-legs. The most dangerous are the loops: head-arms and head-legs, but they occur relatively rarely.
Environmental conditions and factors of the labor process have a significant impact on the resistance of the skin and the human body as a whole. So, for example, elevated temperature (~ 30 ° C and above) and relative humidity (~ 70% and above) contribute to increased sweating, and, consequently, a sharp decrease in the active resistance of the human body. Intensive physical work leads to a similar result.