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; parameters electrical circuit.
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 injury 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 irritations in the human body (a slight increase in temperature in the area of contact with the 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 the electrical circuit (Fig. 1).
Rice. 1. Simplified electrical circuit for equivalent resistance of the human body
Ra is the active (ohmic) component; Rc - capacitive component
The presence of a 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, equipment maintenance platform, 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, all 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 more high 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.
1. Electrical resistance of the human body.
2. The value of the potential difference in the electrical circuit.
3. Duration of exposure.
4. The path of current through the human body.
5. Rod and frequency of electric current.
6.Individual properties of a person.
7. Environmental conditions.
1. Electrical resistance of the human body.
The greatest resistance to electric current is provided by the skin, therefore the resistance of the human body is determined mainly by the resistance of the skin. The electrical resistance of the human body with dry, clean and intact skin, measured at 20 V, ranges from 3-100 kOhm, and the resistance of the inner layers is 300-500 Ohm. The electrical resistance of the human body is a complex quantity, it consists of active and capacitive, but, as a rule, capacitive is neglected. The skin of the face, neck, hands in the area above the palm has the least resistance, especially in the areas facing the torso. With an increase in exposure time, the resistance of the human body decreases, as this increases the local heating of the skin, which leads to vasodilation and an increase in the supply of blood to this area, and, accordingly, an increase in sweating.
Sensible current- an electric current that causes tangible irritations when passing through the body. For alternating current it is 0.6-1.5 mA, for direct current 5-7 mA.
Continuous current- an electric current that causes irresistible convulsive contractions when passing through the body. For alternating current it is 10-15 mA, for direct current 50-60 mA.
Fibrillation current is an electrical current that can cause asynchronous contractions of the heart muscle. The threshold current for AC is 100 mA, for DC it is 300 mA. With a duration of exposure of 1-2 seconds along the path of the arm - arm or arm - legs, the fibrillation current can reach 5 A. More than 5 A does not cause heart fibrillation - instant cardiac arrest occurs.
5. Type and frequency of electric current.
Direct current is approximately 4-5 times safer than alternating current. The significantly lower danger of direct current is confirmed by the practice of operating electrical installations. This provision is valid only for voltages of 250-300 V. But the greatest danger is alternating current with a frequency of 50-1000 Hz, with a further increase in frequency, the danger of damage decreases and completely disappears at a frequency of 45-50 kHz.
6. Individual properties of a person.
It has been established that physically healthy and strong people endure electric shocks more easily. Increased susceptibility to electric current have people suffering from diseases of the cardiovascular system, skin, organs of internal secretion.
7. Conditions of the external environment.
Dampness, conductive dust, caustic vapors and gases have a destructive effect on the insulation of electrical equipment. The impact of current on a person is also exacerbated by conductive floors and metal and grounded structures close to electrical equipment.
Premises according to the danger of electric shock are divided into:
1) premises without increased danger;
2) premises with increased danger, which are characterized by the presence of one of the following conditions:
Dampness or conductive dust;
conductive floors;
High room temperature (more than 35 C);
Possibility of simultaneous contact of a person with grounded metal structures on the one hand and metal cases of electrical equipment on the other.
3) especially dangerous premises - are characterized by the presence of one of the following conditions:
Particular dampness (relative humidity approx. 100%);
The presence of a chemically active or organic environment;
The presence of two or more high-risk conditions at the same time.
These are pronounced local (local) damage to body tissues caused by exposure to electric current or an electric arc. Local damage most often affects the surface of human skin, but in some cases, muscle tissue, as well as ligaments and bones, are also affected. Usually, local electrical injuries are cured and a person's working capacity is fully or partially restored. However, in some cases, local electrical injuries lead to the death of a person. Local electrical injuries include:
electric burns,
electrical signs (current tags),
skin electroplating,
· mechanical damage,
· electrophthalmia.
An electric shock is the effect of an electric current on a human or animal body, as a result of which a convulsive contraction of the muscles of the body begins. Depending on the magnitude of the current strength and the exposure time, the biological object may or may not be conscious, but with the independent functioning of the respiratory organs and the cardiovascular system. In the most severe conditions, after an electric shock, not only loss of consciousness is observed, but also problems in the functioning of the cardiovascular system, and even death.
23. The sequence and content of first aid measures. Ways to free the victim from the effects of electric current, personal safety measures. Features of defeat by atmospheric electricity (lightning) during lightning discharges, first aid.
First aid in case of electric shock consists in immediately turning off the electrical installation or interrupting the circuit of exposure to electric current on a person in absolutely any way possible, then, depending on the degree of damage, will begin to perform a closed heart massage and organize artificial respiration if the victim has a cardiac arrest, as well as processing and bandaging the affected parts of the body.
In case of electric shock, it is necessary to free the victim from its effects as soon as possible, since the severity of the electrical injury depends on the duration of this action.
If the victim holds the wire with his hands, his fingers are strongly compressed and it is impossible to release the wire, then the first action of the person providing assistance is to turn off the electrical installation that the victim touches. Disconnection is carried out using switches, a knife switch or other disconnecting device, as well as by removing or unscrewing the fuses (plugs), the plug connector.
With the shutdown of the electrical installation, artificial lighting can go out at the same time, so you need to take care of this by turning on emergency lighting, etc.
In this case, it is necessary to take into account the explosion and fire hazard of the room. When assisting the victim, one should not touch him without taking precautions, as this is life-threatening; care must be taken not to be in contact with the current-carrying part and under step voltage.
To separate the victim from current-carrying parts or wires with voltage up to 1,000 V, it is necessary to use a rope, stick, board or other dry object that does not conduct electric current.
If the clothes are dry and lag behind the body, then you can pull the victim away from live parts, avoiding contact (touching) with parts of the body, by the floors of a coat or jacket, jacket, by the collar.
To isolate hands, the assisting person must wear dielectric gloves or wrap a hand with a scarf, put a cloth cap on his hand, or pull the sleeve of a jacket or coat over his hand. You can isolate yourself by using a rubber mat, a dry board, or other improvised items that do not conduct electricity (bedding, a bundle of clothes).
If an electric current passes into the ground through the victim, and he convulsively squeezes the wire in his hand, then you can separate the person from the ground by slipping a dry board under him or pulling him by his clothes. You can also cut the wire with an ax with a dry wooden handle or other tools with insulated handles (pliers, etc.).
After the release of the victim from the action of electric current, it is necessary to assess his condition.
If the victim has no consciousness, breathing, pulse, the skin is cyanotic, and the pupils are wide (0.5 cm in diameter), then he is in a state of clinical death and you should immediately begin to revive the body using artificial respiration using the mouth-to-mouth method. or "mouth to nose" and external heart massage.
Having begun to revive the victim, it is necessary to take care of calling for medical help.
The same pathological changes are noted in the body of victims of atmospheric electricity damage as in case of electric shock. The victim loses consciousness, falls, convulsions may occur, breathing and heartbeat often stop. On the body, you can usually find "current marks", the points of entry and exit of electricity. In the event of a fatal outcome, the cause of the cessation of basic vital functions is a sudden cessation of breathing and heartbeat, from the direct action of lightning on the respiratory and vasomotor centers of the medulla oblongata. The victim of a lightning strike needs hospitalization, as he is at risk of disorders of the electrical activity of the heart.
In case of a lightning strike, the same assistance is provided as in case of electric shock.
A person struck by lightning is immediately given artificial respiration, in case of cardiac arrest - his closed massage and warm the body. Inside give caffeine, analgin. If possible, anti-shock agents are administered subcutaneously: promedol, caffeine, ephedrine. After restoring breathing, the victim should be given hot tea to drink, burns should be treated and transported to the hospital.
24. Flame burns, the sequence and content of first aid measures. Frostbite, first aid measures (by stages).
Burn – tissue damage caused by exposure to high temperature, chemicals, electric current, ionizing radiation. Depending on the cause of occurrence, thermal, chemical, electrical, radiation burns are distinguished. Sunburns are possible. Thermal burns are the most common.
Thermal burns. During fires, several damaging factors act on the human body. The most dangerous of them is the high temperature in the combustion zone, leading to heat stroke, burns to the skin and upper respiratory tract. Flame burns are much more severe than boiling liquid burns.. Among thermal burns of various localization, the most dangerous are burns of the face, they are accompanied by burns of the upper respiratory tract hot air.
The severity of the victim's condition depends on the degree of burn, its area and localization. The degree of burn is determined by the depth of damage to the skin and underlying tissues. Allocate IV degree burns.
1st degree burn manifested by redness of the skin, swelling, pain.
Second degree burn characterized by the formation of blisters filled with a clear yellowish liquid, a sharp reddening of the skin, burning pain.
Third degree burn accompanied by necrosis of all layers of the skin. The surface of the burn is covered with a scab - a dense gray-brown crust. In connection with the defeat of the nerve endings, the pain is insignificant or absent. Dead tissue suppurates and is torn away. Healing is slow. A scar forms at the site of the burn.
IV degree burn characterized by charring of the skin, subcutaneous fat, muscles and even bones. Pain sensitivity is lost. Skin grafting is needed to heal deep burns.
Determining the area of the burn
The area of the burn is determined by the "rule of nines". The surface of the head and neck is 9% of the body surface of an adult, one upper limb - 9%, one lower limb - 18% (thigh - 9%, lower leg and foot - 9%). The back surface of the human torso is 18% of the body surface, the front surface (chest, abdomen) - 18%, the perineum and external genitalia - 1%.
The area of the burn can also be determined by the “rule of the palm”. The area of the palm of the victim is 1% of the surface of his body. The palm is projected over the affected area, without touching the burned area of the body.
Burns over 15% of the body surface in adults are accompanied by burn shock. In children, burn shock develops when the burn area is 5–10% or more. There are 2 phases of burn shock: the first is the excitation phase, the second is inhibition. The first phase is short. The victims are excited, restless due to the continuous flow of pain impulses from burn wounds. The second phase is characterized by a pronounced inhibition of the activity of the nervous system, heart, lungs, kidneys and other organs. The indifferent look of the victims is noteworthy. Danger to life arises even with second-degree burns, occupying ⅓ of the body surface.
With extensive burns, toxic substances are formed in the affected areas. Penetrating into the blood, they are carried throughout the body and cause intoxication. Microorganisms get on the burned areas of the skin, burn wounds begin to suppurate. Burn disease develops. The deeper the damage to the skin and underlying tissues and the larger the area of the burn, the more severe the condition of the victim and the worse the prognosis.
Frostbite is damage to body tissues caused by cold. Frostbite is more susceptible to fingers and toes, nose, ears and face. The severity of frostbite depends on the duration of exposure to cold, as well as on the state of the body.
With alcohol intoxication, the thermoregulation of the body is disturbed, and the likelihood of frostbite increases! Sign: a sharp blanching of the skin and loss of its sensitivity. The main task of first aid is to stop exposure to cold and restore the normal temperature of the cooled tissues as quickly as possible. For this you need:
immerse frostbitten parts of the body in water with a temperature of 37 ° C to 40 ° C, but not higher due to the risk of burns;
make a light rubbing of frostbitten skin.
It is forbidden to rub frostbitten areas with snow or immerse them in cold water, as this causes further hypothermia!
Sterile dressings are applied to frostbitten skin to prevent infection. If pain, swelling of tissues, blisters appear, you should seek medical help.
The nature and consequences of exposure to an electric current depends on the following factors:
Electrical resistance of the human body;
Voltage and current values;
The duration of the electric current;
Current paths through the human body;
Type and frequency of electric current;
Individual properties of a person;
The conditions of the external environment.
Electrical resistance of the human body. The strength of the current Ih passing through any part of the human body depends on the applied 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 that touch 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 located under this layer, as it were, form the plates of a capacitor with a capacitance FROM with resistance r n (Fig. 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 Rn and the capacitance, the electrical resistance of which
, where Wpf - angular frequency, Hz; f - current frequency, Hz,
Rice. 7.1. Wiring diagram replacing the resistance of the outer layer of the skin
a – electrode contact diagram; b - electric equivalent circuit; 1 - electrode; 2 - the outer layer of the skin; 3 - the inner region of the skin.
Then the total resistance of the outer layer of the skin for alternating current:
(7.2)
Resistance r n and capacitance C depends on the area of the electrodes (contact area). With an increase in the contact area, rn decreases, and the capacitance C increases. Therefore, an increase in the contact area leads to a decrease in the total resistance 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 current path "hand - hand", the total electrical resistance of the body can be represented by the equivalent circuit shown in Figure 7.2.
Rice. 7.2. Electrical circuit for substituting the resistance of the human body: 1 - electrode; 2 - the outer layer of the skin; r vr, r vk- internal resistance of hands 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) practically becomes equal to the internal resistance rв. The dependence of the resistance of the human body on frequency is shown in fig. 7.3.
There is a non-linear 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 non-linearity of the electrical resistance of the human body. So, at a voltage on the electrodes of 40 ... 45 V, significant electric field strengths arise in the outer layer of the skin, at which the breakdown of the outer layer occurs completely or partially, which reduces the impedance of the human body (Fig. 7.4.) At a voltage of 127 ... 220 V, it is practically drops 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 dimension of the section of the body through which the current passes.
As a calculated value for alternating current of industrial frequency, the active resistance of the human body is taken equal to 1000 0m.
Under actual conditions, the resistance of the human body is not a constant value. It depends on a number of factors, including the condition of the skin, the state of the environment, the parameters of the electrical circuit, etc.
Damage to the stratum corneum (cuts, scratches, abrasions, etc.) reduces the resistance of the body to 500 ... 700 ohms, which increases the risk of electric shock to a person.
Moisturizing the skin with water or sweat has the same effect. 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.) leads to a decrease in its resistance.
The resistance of the body is influenced by the area of contacts, as well as the place of contact, since the same person has different skin resistance in different parts of the body. The skin of the face, neck, hands in the area above the palms and especially on the side facing the body, armpits, back of the hand, etc. has the least resistance. 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, as this increases the local heating of the skin, which leads to the expansion of blood vessels, to an increase in the supply of this area with blood 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: pain (blows, injections), light and sound.
The magnitude of voltage and current. The main factor determining the outcome of an 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 lesion, 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; relaxes the muscles slightly. Not felt
5 … 7
Pain intensifies throughout the hand; convulsions; mild pain in the whole arm up to the forearm The beginning of sensations; weak heating of the skin under the electrodes
8 … 10
Violent pains and cramps in the whole arm, including the forearm. It is difficult to take your hands off the electrodes. Enhancement of feeling.
10 … 15
Barely bearable pain in the whole arm. Hands cannot be torn off from the electrodes. With an increase in the duration of the flow of the current, the pain intensifies. Significant heating under the electrodes and in the adjacent area of the skin.
20 … 25
Strong pains. Hands are paralyzed instantly, it is impossible to tear them off the electrodes. Breathing is difficult. Feeling of internal heating, slight contraction of the muscles of the hands.
25 … 50
Very severe pain in arms and chest. Breathing is extremely difficult. With prolonged exposure, respiratory arrest or weakening of cardiac activity with loss of consciousness may occur. Strong heat, pain and cramps in the hands. When the hands are separated from the electrodes, severe pain occurs.
50 … 80
Breathing is paralyzed after a few seconds, the work of the heart is disturbed. Prolonged exposure may cause cardiac fibrillation Very strong surface and internal heating. Severe pain in the arm and chest. Hands cannot be torn off the electrodes due to severe pain.
80 … 100
Fibrillation of the heart after 2 ... 3 s .; after a few seconds, breathing stops. The same action expressed more strongly. With prolonged action, respiratory arrest.
The same action in less time. Fibrillation of the heart after 2 ... 3 s .; after a few seconds, breathing stops.
From the table below, the following threshold current values can be distinguished:
O u t and m y current- an electric current that causes perceptible irritations when passing through the body. Perceptible irritations are caused by an alternating current with a power of 0.6 ... 1.5 mA and a constant one with a power of 5 ... 7 mA. The indicated values are threshold sensible currents; the region of perceptible currents begins with them.
N o t o r e c u r c u r t- 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 10 ... 15 mA AC and 50 ... 60 mA DC. With such a current, a person can no longer independently unclench the hand in which the current-carrying part is clamped, and turns out to be, as it were, chained to it.
F ibrillation current- an electric current that causes fibrillation of the heart when passing through the body. The threshold fibrillation current is 100 mA AC and 300 mA DC for a duration of 1 ... 2 s along the “hand-hand” or “hand-foot” path. Fibrillation current can reach 5 A. Current more than 5 A does not cause cardiac fibrillation. With such currents, instantaneous cardiac arrest occurs.
Threshold (lowest) values of perceptible, non-letting and fibrillation currents are random variables, the normalized values of which are determined by the distribution law and its parameters. The numerical values of the currents correspond to a certain probability of the occurrence of a given biological reaction.
Permissible currents for humans are evaluated according to three electrical safety criteria.
First criterion- perceptible current. As the first criterion for an alternating current with a frequency of 50 Hz, the current I = 0.6 mA was taken, which does not cause disturbances in the body's activity. The permissible duration of the flow of such a current through a person is not more than 10 minutes.
Second criterion- releasing current. As the second criterion of electrical safety, the current I = 6 mA was adopted, when it flows through a person, the probability of release 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, with a long exposure of 1 ... 3 s, does not cause heart fibrillation in a person weighing 50 kg, with a certain margin, it is taken equal to 50 mA.
Thus, the magnitude of the current has a significant impact on the degree of human injury. 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-release (6 ... 24 mA) and heart fibrillation (more than 50 mA).
The duration of the electric current. The duration of the passage of current through the human body has a significant impact on the outcome of the lesion. Prolonged exposure to current leads to severe and sometimes fatal injuries.
With a short exposure (0.1 ... 0.5 s), a current of about 100 mA does not cause heart fibrillation. If you increase the duration of exposure to 1 s, then the same current can lead to death. With a decrease in the duration of exposure, the values \u200b\u200bof the currents permissible for a person increase significantly. So, when the exposure time changes from 1 to 0.1 s, the allowable current will increase by about 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.
Diagram of an electrocardiogram
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 (the QRS peak) 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 fill with blood. Phase P corresponds to atrial contraction. It has been established 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 current exposure to 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.
In the event of a mismatch between the time of passage of current through a person with phase T, currents significantly exceeding the threshold values will not cause heart 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 = 50/ t (7.3)
where I h is the current passing through the human body, mA; t is the duration of the passage of current, s.
This formula is valid within 0.1 ... 1.0 s. It is used to determine the maximum allowable currents passing through a person along the path "arm - legs", necessary for the calculation of protective devices.
Current paths through the human body. The path of the current in the human body depends on which parts of the body the victim touches the current-carrying 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 way "hand - hand" 3.3% of the total current passes through the heart, "left hand - legs" - 3.7%, "right hand - 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 "arm - 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 pass along the shortest path, which is explained by the large difference in the resistivity of various tissues (bone, muscle, fat, etc.).
The smallest current through the heart passes when the current passes through the lower leg-leg loop. However, one should not draw conclusions from this about the low danger of the lower loop (the action of step voltage). Usually, if the current is strong enough, it causes leg cramps and the person falls, after which the current already passes through the chest, i.e. through the respiratory muscles and heart.
Type and frequency of current. It has been established that alternating current is more dangerous than direct current. This also follows from Table. 7.1., since the same effects are caused by greater values of direct current than alternating current. However, this is typical for relatively low voltages (up to 250 ... 300 V). A voltage of 120 V DC under the same conditions is considered to be equivalent in danger to a voltage of 40 V AC of industrial frequency. At higher voltages, the danger of direct current increases.
In the voltage range of 400 ... 600 V, the danger of direct current is almost equal to the danger of alternating current with a frequency of 50 Hz, and at voltages over 600 V, direct current is more dangerous than alternating current. When exposed to direct voltage, especially sharp pain sensations occur at the moment of closing and opening the 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.
Rice. 7.6. The dependence of the non-release current on the frequency:
1 - for 0.5% of the subjects; 2 - for 99.5% of the subjects
With an increase in frequency (more than 50 Hz), the values of the non-letting current increase. A further increase in the frequency of the current is accompanied by a decrease in the danger of damage, 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. The 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 established that physically healthy and strong people tolerate electric shocks more easily.
Persons suffering from skin diseases, cardiovascular diseases, internal secretion organs, lungs, nervous diseases, etc., are distinguished by increased susceptibility to electric current.
The safety regulations for the operation of electrical installations provide for the selection of personnel for the maintenance of 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 maintenance of existing electrical installations.
Conditions of the external environment. Humidity and air temperature, the presence of grounded metal structures and floors, conductive dust have an additional impact on the conditions of electrical safety. The degree of electric shock largely depends on the density and area of contact of a person with current-carrying parts. In damp rooms with high temperatures or outdoor electrical installations, unfavorable conditions arise under which the area of contact of a person 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 with one pole (ground) of an electrical installation. In this case, any touch of a person to the current-carrying parts immediately leads to a bipolar inclusion in the electrical circuit. Conductive dust also creates conditions for electrical contact with both current-carrying parts and the ground.
Depending on the presence of the listed conditions that increase the danger of current exposure to a person, all premises are divided into the following classes according to the danger of electric shock to people: without increased danger, with increased danger, especially dangerous.
Premises without increased danger characterized by the absence of conditions that create an increased or special danger.
Premises with increased danger are characterized by the presence in them of one of the following conditions that create an increased danger:
Dampness (relative air humidity exceeds 75% for a long time) or conductive dust;
Conductive floors (metal, earthen, reinforced concrete, brick, etc.);
High temperature (above +35 0 C);
Possibilities of simultaneous contact of a person with the metal structures of buildings connected to the ground, technological apparatuses, mechanisms, etc., 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:
Special dampness (relative air humidity 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 current-carrying parts of electrical equipment);
Simultaneously two or more conditions of increased danger.
The main factors that determine the degree of electric shock are:
- current path through the human body. The most dangerous paths are - "head - legs" - options 11, 12, 14 and 15, "head - hands" - options 10, 12, and 13, and, "arm - leg" - options - 2, 3, 4, 5, 6, 7, 8. The current paths through the human body are shown in the figure;
Rice. 15. Characteristics of the current path in the human body
- - current strength (BUT). A person begins to feel an electric current at a strength of 0.6 - 1.5 mA (mA - milliamp = 0.001A). With a current strength of 20 - 25 mA, the functioning of the lungs and heart is disrupted. At a current strength of 100 mA, fibrillation occurs - a convulsive non-rhythmic contraction of the heart muscle. The magnitude of the strength of the electric current has a decisive role in the defeat of a person. An electric shock occurs when a closed electrical circuit is created, in which a person is also included. According to Ohm's law, the current strength I is equal to the electrical voltage U divided by the resistance of the electrical circuit R:
Therefore, the greater the voltage, the greater and more dangerous the electric current. The greater the electrical resistance of the circuit, the lower the current and the danger of human injury. The electrical resistance of the circuit is equal to the sum of the resistances of all sections that make up the circuit (conductors, floor, shoes, human body, etc.);
- electrical resistance human body. Clean, dry, undamaged human skin has a high resistance - up to several hundred thousand ohms. With damaged (wounds, scratches), as well as delicate and thin skin (in women and children), the resistance is less; with rough calloused skin of the hands (in men), the resistance is greater. Therefore, the degree of exposure to electric current is different for different people. In calculations for electrical safety, the resistance of the human body is usually taken = 1000 Ohm (1 kilo Ohm). The resistance of the internal organs of a person is small and therefore almost irrelevant.
Measures and means of protection against electric shock
Electrical installations, which include almost all computer equipment, pose a great potential danger to humans, since during operation or maintenance work, a person can touch live parts. The specific danger of electrical installations is current-carrying conductors, computer cases and other equipment that is energized as a result of insulation damage (breakdown).
An important measure to prevent electrical injury is the correct service organization operating PCs, carrying out repair, installation and preventive work.
To ensure electrical safety, the following must be used alone or in combination with each other: technical methods and means of protection:
- insulation of current-carrying parts (working, additional, reinforced double). Correct insulation is the main condition for ensuring the safe operation of electrical installations. The main reasons for the violation of insulation and deterioration of its qualities are:
Heating, for example, by short-circuit currents, as well as heat from external sources;
Dynamic forces (displacement, abrasion, mechanical damage);
Exposure to pollution (oils, gasoline, moisture, chemicals).
The condition of the insulation is checked before putting the electrical installation into operation, after its repair, and also after its long stay in a non-working position;
- protection against contact with current-carrying parts is performed in the form protective devices . They are made of non-combustible or hardly combustible material in the form of casings, lids, boxes, nets and must have sufficient mechanical strength and be of such a design that their removal or opening is possible only with the help of special tools or keys and by employees who are entrusted with this.
- warning signal . To prevent accidents during the operation of electrical equipment, an important role belongs to the marking, inscriptions indicating the condition of the equipment, the name and purpose of the connections. In the absence of markings and inscriptions, maintenance personnel may, during repairs, inspections and operation of electrical equipment, confuse the purpose of wires, knife switches, switches, etc. All keys, buttons and control knobs must have inscriptions indicating the operation for which they are intended (“turn on” , "disable", "reduce").
- low voltage (42 volts and below). The use of such voltages sharply reduces the danger under all conditions of destruction;
- protective earth . This is a deliberate electrical connection to the ground of metal non-current-carrying parts of electrical equipment that may become energized by accident. For protective grounding, artificial and natural grounding devices are used: metal pipes, fittings, corners, building foundations, etc. Grounding devices should be located at a certain depth in the ground - deeper than the freezing level of the soil in winter (in Udmurtia - about 2 meters);
- protective shutdown equipment. This is a fast-acting automatic shutdown of an electrical installation in the event of a danger. There are several types of residual current devices. For example, a residual current device and a circuit breaker;
- individual protection means . They are divided into basic and additional. The main means of protection withstand long-term operating voltage in electrical installations. The main means of protection include insulating hoses, insulating handles of electrical measuring and electrical tools (screwdrivers, etc.), dielectric gloves, voltage indicators. Additional protective equipment does not withstand prolonged exposure to voltage. Additional means of protection include dielectric galoshes, rugs, coasters (wooden). All protective equipment must be marked with the voltage for which they are designed.
The following state regulations apply in the field of electrical safety:
- GOST R 50571.1-93 Electrical installations of buildings. Basic provisions.
GOST IEC 60536-04. Classification of electrical and electronic equipment according to the method of protection against electric shock.
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 striking 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 is in the zone of current spreading, falling under 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.