Risk of electric shock
As you know, the human body is a conductor of electric current. Therefore, in the case of direct contact of a person with bare live parts of an electrical installation or power line, there is a risk of electric shock.
In most cases, touch occurs when a person is standing on the ground or on a conductive base (floor, platform). In this case, an electrical circuit arises, one of the sections of which will be the human body.
The degree of electric shock injury is determined by the amount of current flowing through the human body.
It has been found that a current of 0.1 A is in most cases fatal to a person, and currents of 0.03 — 0.09 A, although not fatal, still cause serious damage to the human body.
The amount of current flowing through the human body depends on the voltage of the electrical installation, as well as the resistance of all elements of the circuit through which the current flows, including the resistance of the human body.
Human electrical resistance
Electrical resistance varies from person to person. Even for the same person, it can be different depending on a number of factors.So factors such as the condition of the skin, the degree of fatigue, the state of the nervous system, etc., have a great influence on the value of the electrical resistance.
Dry, rough, wrinkled skin, lack of fatigue and a normal state of the nervous system sharply increase the electrical resistance of the human body, and, conversely, moist skin, overwork, an excited state of the nervous system, as well as other factors, significantly reduce it.
The humidity and temperature of the room, the condition of clothes, shoes, etc., have a great influence on the resistance of the human body when passing an electric current.
What determines the severity of an electric shock for a person
The severity of an electric shock on the human body depends on the strength and frequency of the current, the path and duration of its action, as well as the resistance of the human body at the moment of contact with live parts.
The most dangerous is the path of the current through the heart, brain, lungs, and the most vulnerable parts of the body at the moment of touching the living part are the cheek, neck, lower leg, shoulder and the back of the hand.
An equally important factor is the area of contact of the human body with live parts of the electrical installation.
The larger the area of contact of the human body with the conductor and the longer the impact of electric current on the human body, the lower its resistance and therefore the greater the risk of electric shock.
Therefore, the risk of electric shock increases sharply in such types of work as welding in wells, tanks, reservoirs, inside pressure vessels (kftla, cylinders, pipelines), where there is a high probability of contact of the worker with metal structures.
Rooms with conductive floors (earth, concrete, metal, etc.) in which the relative humidity exceeds 75% are dangerous for electric shock.
Particularly dangerous are rooms in which the relative humidity reaches 100% (ceiling, walls, floor and objects in the room are covered with moisture), as well as rooms with a chemically active environment that has a destructive effect on the insulation and live parts of the electrical network equipment and others …
For normal operating conditions in dry rooms, a voltage that does not exceed 36 V is considered safe, and in particularly unfavorable conditions, a fatal electric shock is possible even at a voltage of 12 V. As the frequency of the current increases, the risk of injury decreases.
Currents with a frequency of 40 — 60 Hz are the greatest danger. At frequencies above 100 Hz, the risk of injury decreases sharply.
The amount of current in a person is also determined by the applied voltage at the time of touching live parts.
If a person closes with his body two phase conductors of a working installation, for example by holding them with his hands, he places his body under total mains voltage.
When a person touches a live wire of a three-phase network, he is placed under a voltage acting between that wire and the ground.
In this case, the insulation resistance (to the ground) of shoes, the floor, wires from other phases that the person does not touch, is usually included in the electrical circuit through which the current passes through the human body.
See also:
How environmental factors affect the outcome of electrical injuries
What is step voltage
It is called the voltage that arises in the earth fault current circuit between its two points at the moment when a person touches them touch voltage.
Electric shock can also occur under the action of a step voltage, which occurs under the action of a current that spreads to the ground when live parts are shorted to the frame of the equipment or directly to the ground.
Step voltage equal to the potential difference between two points on the earth's surface at a distance of one step (approximately 0.8 m). It increases when approaching the point of connection of live parts to ground and can be equal to the touch voltage.
Therefore, when detecting a connection to the ground of any current-carrying part of the installation, it is forbidden to approach the place of damage at a distance of less than 4 — 5 m in closed switchgears and 8 — 10 m in open ones.
The impact of alternating electromagnetic field on a person
Long-term exposure to a variable electromagnetic field on the human body also causes some disturbances in its normal activity — a person quickly gets tired, the accuracy of movements during work decreases, headaches and pain in the heart area appear, and sometimes blood pressure rises.
The industrial frequency electric field, in addition to the biological effect on the human body, causes it to become electrified as a conductor. Therefore, a person isolated from the ground and located in an electric field finds himself under a significant potential (several kilovolts).
If a person touches grounded parts of electrical equipment, an electric discharge occurs. Discharge current causes painful sensations.
The choice of means of protection against the harmful effects of electromagnetic fields depends on the frequency of oscillations of the electromagnetic field. In industrial frequency installations with a voltage of 330 kV and more, a protective suit made of a special metallized fabric is used as a protective device.
The set of a protective suit includes a coverall or jacket with trousers, a hat (helmet, cap) and leather boots with electrically conductive soles that ensure good electrical contact with the surface on which the person stands.
All parts of the suit are interconnected with special flexible wires. For protection, special grounded screens in the form of shields made of metal mesh are also used. Their protective effect is based on the effect of weakening the electric field near a grounded metal object. Screens can be permanent and portable in the form of canopies, canopies, partitions or tents.
See here for more details: How electromagnetic fields from overhead power lines affect people, animals and plants
Static electricity hazard
It is also a danger to people static electricity… Static electricity is formed as a result of complex processes associated with the redistribution of electrons or ions when two different materials come into contact. Static electricity sparks can cause ignition of flammable substances and explosions, cause deterioration or destruction of materials, and adversely affect the human body.
The accumulation of static electricity discharges in stationary and mobile installations becomes:
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when filling electrifying liquids (ethyl ether, carbon disulfide, benzene, gasoline, toluene, ethyl and methyl alcohol) in ungrounded tanks, tanks and other containers;
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during the flow of liquids through pipes insulated from the ground or through rubber hoses,
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when liquefied or compressed gases exit the nozzles, especially when they contain finely atomized liquid, suspension or dust;
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during the transportation of liquids in ungrounded tanks and barrels;
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when filtering liquids through porous partitions or nets;
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when the dust-air mixture moves in ungrounded pipes and devices (pneumatic conveying, grinding, sieving, air drying);
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in the processes of mixing substances in mixers;
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for mechanical processing of plastics (dielectrics) on metal-cutting machines and manually;
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when the transmission belts (rubberized and leather dielectrics) rub against the pulleys.
The build-up of static electricity in humans becomes:
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when using shoes with non-conductive soles;
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clothing and linen of wool, silk and man-made fibres;
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when moving on floors that do not conduct electric current, when performing manual operations with dielectric substances.
Prolonged exposure to static electricity (for example during manual operations) has a detrimental effect on the health of workers.
Grounding devices are used to remove static electricity built up on installations, devices and equipment.
Mixers, gas and air lines, air and gas compressors, pneumatic dryers, exhaust ventilation air lines and pneumatic conveying systems, especially in the removal of synthetic materials, unloading devices, tanks, containers, apparatus and other devices in which dangerous electrical potentials arise, must to be grounded in at least two places.
All movable containers temporarily located under the filling or discharge of liquefied combustible gases and flammable liquids must be connected to the earth electrode during filling.
To avoid ignition and explosion of dust-air mixtures, it is necessary:
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preventing the formation of mixtures within the limits of explosiveness;
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beware of the formation of fine dust;
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increase in relative air humidity;
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to ground process and transport equipment, especially discharge nozzles, to sew filters made of textiles and other non-conductive materials with copper wires and then to ground them;
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prevents dust from accumulating in the room, falling or throwing it from a great height, as well as its swirling.
Conductive shoes are used to drain static electricity - boots with leather soles, conductive rubber soles or rivets (brass) pierced by conductive and non-distorting rivets (brass) during friction and impact, grounded door handles, ladders, tool handles and others.
Protection against static electricity:
How to protect yourself from static electricity at home and at work
Lightning hazard
An electric shock may occur and by lightning... The lightning current can reach 100-200 kA. By producing thermal, electromagnetic and mechanical effects on the objects through which it passes, the current can cause the destruction of buildings and structures, fires and explosions and pose a great danger to people.
The destructive and damaging effect of lightning can be caused by a direct (direct) strike to an object introduced with a high potential (on the wires of overhead lines or pipelines struck by lightning during a lightning discharge), induced voltages under the action of electrostatic and electromagnetic induction (secondary lightning effects), as well as step voltage and touch voltage in the lightning current propagation zone (when discharged into the ground, tree, building, lightning protection device, etc.).
To obtain an electrical discharge of lightning (lightning current), devices are used - lightning rods, consisting of a supporting part (for example, a support), an air terminal (a metal rod, cable or network), a down conductor and a ground electrode.
Each lightning rod, depending on its design and height, has a certain protective zone inside which objects are not subject to direct lightning strikes.
In order to protect against electromagnetic induction between pipelines and other elongated metal objects in the places of their mutual approximation by 10 cm or less, steel jumpers are welded every 20 m so that there are no open circuits (sparking is possible in places of interruptions and therefore, the danger is not excluded explosion and fire).
Electrical injury statistics
Statistics show that about 9.5% of all cases of electrical injuries occur in electrical lighting systems, and more than half of them are cases of electric shock when changing lamps when touching a base or an incorrectly filled cartridge. To avoid the risk of electric shock when replacing an electric lamp, it is necessary to turn off the power before replacing.
Other materials with electrical injury statistics: