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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Electrical safety equipment in everyday life. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electrical safety, fire safety Passing through the human body, a current exceeding 0,015 A destroys it, and the assessment of electric shock is reduced to determining the strength of the current flowing through the human body. Small currents may not be harmful, while large ones can be fatal. With illiterate and inattentive handling, electricity can cause accidents. Injuries caused by electric current and fires in electrical installations occur primarily due to ignorance of the basic safety rules and only then - due to non-compliance with these rules. The voltage of electrical networks for domestic needs is mainly 220 V. Some often call this network "low voltage" and mistakenly consider it safe. electrical safety in the conditions of country life and in gardening partnerships differs from electrical safety in urban conditions. Statistics show that the greatest number of injuries occurs in the summer months, when adults and children spend most of the day outdoors. Approximately 90% of injuries occur in men and boys, 10% in women and girls. Often, developers and owners of summer cottages do some work on the installation and repair of electrical wiring and appliances on their own, connect various machines and devices to the network. For the most part, they are not specialists and carry out electrical work illiterately, which, of course, often leads to an accident or fire. Typical installation defects are incorrect assembly of electrical circuits. Instead of a phase wire, a zero wire is brought to the switch, they do not pay attention to connecting the phase wire in the lamp sockets, they lay unprotected wires (APV, APPVS, APPV, etc.) in accessible places - along the ground, attics, baseboards, over roofs. In order to avoid electric shock, it is necessary to clearly understand the danger of electric current, firmly know and steadily follow the basic rules for using electrical energy. External electrical networks located on the territory of the site, in relation to electric shock, are equated to especially dangerous by the Rules for the installation of electrical installations. As a result of unsatisfactory operation of external electrical networks, untimely and poor quality of their repair, lack of control over the condition of the routes, the wires of power lines sag or break. In contact with a broken or sagging wire, a person is injured. It is mortally dangerous not only to touch, but also to come closer than 8-10 m to a broken wire of an overhead line lying on the ground, since there is a so-called "step" voltage in the current spreading zone. You can leave the current spreading zone on legs closed together or by jumping on one leg. The action of an electric current outside of 8-10 m from a wire that has fallen to the ground is practically not felt. Especially often, children, mostly boys, are injured due to touching broken or sagging wires. It is forbidden to climb on the roofs of houses where electric wires pass nearby, on the supports of external power networks, play under an overhead line, fly a kite there, open the doors of transformer substations, etc. If broken or sagging wires of a high-voltage line are found, it is necessary to protect the place of damage, warn people about the danger that has arisen, and immediately report the accident to an electrician or to the district electrical networks. The telephone number of regional electrical networks must be written on the cabinet doors of a transformer substation installed on the territory with a village or a gardening partnership. The danger of electric shock can also arise when the external electrical networks are working, but the distance from the person to the wire is reduced. For example, work is carried out under the wires with long tools or objects, TV and radio antennas are not installed correctly near the branches and inputs to the building, fruits are removed from the trees in the garden using long metal objects, the crowns of trees growing under the wires are pruned, etc. In all of these cases, a person stands on the ground (a conductor of electric current), and his approach to bare current-carrying parts or to sections of wire with damaged insulation leads to an accident. So, the danger of electric shock depends on a number of factors: the mains voltage, the quality of the ground, the "connection circuit" of a person in the electrical network. Grounded neutral networks are used where it is impossible to provide good insulation or quickly find and repair insulation damage. These networks include distribution networks in rural areas, where citizens build cottages, develop garden plots Technical methods of electrical protection are: 1) zeroing; 2) use of safe voltage (12-42V); 3) use of consumers with double insulation. The main measure of protection against electric shock in case of touching the housings of electrical equipment that has become energized due to damage to the insulation is nulling, i.e., a deliberate electrical connection with a zero protective conductor of metal non-current-carrying parts that may be energized. Such a connection turns any short circuit of current-carrying parts to earth or to the body into a single-phase short circuit. Suppose a person is on a wooden floor in dry rubber shoes that do not conduct electricity. According to Ohm's law, the current strength is directly proportional to the voltage and inversely proportional to the resistance of the circuit section: I = V/R, where V is voltage 220 V; R - resistance equal to the sum of series resistances: human body - 1000 Ohm; human shoes - 45 ohms; floor - 000 ohms. Zeroing resistance, which is less than 1% of the sum of the reduced resistances (not taken into account) Total 146 ohms So, the strength of the current passing through a person is 220/146 = 000 A. Such a current is not dangerous to a person. And if the tile is not connected to the grounding system (position B), the person is standing on damp conductive floors, the resistance of which can conditionally be taken equal to 0, the person is in conductive shoes, the resistance of which is also conditionally taken to be 0. In this case, if the insulation of the phase wire is broken, the voltage gets on the body of the tile, passes through a person, conductive shoes, conductive floors and to the ground. The current passing through a person will have a deadly value: 220 / 1000 \u0,22d XNUMX A. An electric shock often occurs when a person is touched simultaneously with the body of an electrical device, on which, due to a malfunction, there is a phase voltage, and with a natural grounding conductor, which in domestic conditions turns out to be a heating pipe, water supply, sewerage, etc. In this embodiment, the current comes from case of a faulty device through the human body to a natural grounding conductor. Let's consider another example. The body of the washing machine is connected to a protective conductor. The machine is filled with water through a hose from the water supply. When the phase wire is shorted to the case, a short circuit occurs, which disables the phase wire fuse, and the power supply to the washing machine is turned off. The person serving this machine will get off with a slight fright. The result of a violation of the insulation of a phase wire is an electric shock. Cases of electrical machines, transformers, shield boxes, electrical wiring pipes, metal structures on which electrical equipment is installed, cases of mobile and portable electrical receivers are subject to grounding. Outdoor lighting fixtures installed on reinforced concrete and metal poles must be zeroed, and fixtures installed on wooden poles do not require zeroing. Neutral wires (both working and protective) with a cross section of a phase wire up to 16 mm (for copper) must have cross sections equal to the phase wire. The insulation of zero protective and zero working wires must also be equivalent to phase ones. It is not allowed to install disconnecting devices and fuses in the circuits of neutral protective wires. It is allowed to use switches with simultaneous disconnection of all live wires. The connection of zero protective wires to each other must ensure reliable contact. Steel wires, if possible, should be connected by welding, steel and copper wires - with soldering of the connection, in other cases "under the bolt" with preliminary surface maintenance. In dry rooms, without aggressive environment, zero protective conductors are laid directly along the walls. In humid, damp and especially damp rooms and in rooms with an aggressive environment, zero protective conductors should be laid at a distance of at least 10 mm from the walls. Zero protective conductors in outdoor installations are allowed to be laid in the ground. In electrical installations with a grounded neutral, it is recommended to lay zero protective conductors together with phase ones. In the living quarters of a garden house and kitchens, grounding of metal cases of permanently installed lighting equipment, household machines, mechanisms and electrical appliances with a power of up to 1,3 kW is not required. The housings of water heaters, stationary electric stoves, appliances and machines with a power of more than 1,3 kW and metal pipes of electrical wiring must be zeroed. In one room there should not be electrical receivers working with and without grounding. In this case, the neutral wire must be re-grounded at the building inputs. An effective method that increases electrical safety is operating voltage reduction up to the limits from 42 to 12 V. When using a power supply with a voltage of not 220 V, but 12 V, the current passing through a person will be 12 / 1000 \u0,012d XNUMX A. Such a current does not pose a danger to humans. Transformers are used as a low voltage power source; using autotransformers for this purpose is strictly prohibited. One of the most effective ways to protect against electric shock is to double insulation. The fact is that the product, in addition to working, has additional insulation, equivalent to working and independent of it. This insulation will protect a person from electric shock if the working insulation is damaged. Simultaneous damage to the working and additional insulation is unlikely. Additional insulation is plastic housings, plastic push buttons, rubber-coated handles, etc. Recently, circular saws, pumps, electric drills and other household appliances and mechanisms have found wide application in personal households. Mostly these are portable and mobile mechanisms that are used outdoors, in damp rooms; work standing on earthen conductive floors. Mechanisms are connected with portable wires and cables. It is forbidden to use unprotected wires APPV, APV and others instead of cables KRPT or 1PRPS, specially designed for such work. Do not move equipment connected to the electrical network, as the supply cable may be pulled and the insulation may be broken. Do not clean, lubricate, adjust or repair equipment while the engines are running. To prevent accidents when working with mechanisms, it is necessary to follow all the instructions set out in the manufacturer's instructions. It is unacceptable to use equipment, wires and cables intended for indoor installation outdoors. Kettles, irons, table lamps, vacuum cleaners and other household electrical appliances may only be used indoors, in dry rooms. The lack of factory-made portable lamps, and sometimes exorbitant prices for them, force people to make them on their own, often with great violations. So, unprotected wires are used from separate pieces connected by twists, the lamps are not covered with nets, there are no plugs. The danger is aggravated during operation: devices are turned on in sheds, basements, other especially dangerous rooms with earthen floors, in the open air. Operation of welding installations without observing the necessary safety regulations, improper connection to the mains leads to electrical injuries, fires. It is strictly forbidden to connect welding transformers to the network before entering the house, since such connection most often does not create reliable contact and is associated with work at height. The main condition for the safe use of electricity inside cottages, in garden houses is the good condition of the insulation of wiring, electrical appliances, switches, sockets, lamp sockets, as well as cords with which the devices are connected to the network. It is not allowed to hang electrical wiring on nails, various objects, lay it behind pipes, twist the wires. Do not hang on the wires or lay household items for them, pull out the plug from the socket by the cord - such actions lead to damage to the insulation. During repair work inside the premises, it is strictly forbidden to whitewash and paint over wires and cables, as this violates the insulation, worsens the cooling conditions, and makes it difficult to visually inspect the surface of the wiring. Moreover, if the network is not de-energized, and the wire or cable has damaged insulation, then when painting, you can get under voltage. With hidden electrical wiring, it is forbidden to drive nails into the walls anywhere for hanging lamps, paintings and other objects. Doing so may damage the insulation and cause electric shock. Chandeliers and other lamps, electric bulbs must be cleaned standing on non-conductive dry stools, a rubber mat. It is forbidden to clean the lighting fittings from dust and dirt when the switch is on, as well as with a wet rag. With open wiring, care must be taken when moving and installing furniture, refrigerators, etc., as heavy objects of large size can damage the insulation of the wires. It is impossible to repair damaged switches, cartridges, socket outlets, to eliminate loose connections in junction boxes when they are energized. First, in order to avoid accidents, it is necessary to de-energize the network: turn off the circuit breakers or unscrew the plugs, warn others about the repair work, take measures to exclude the supply of voltage to the network. In the case of repair or inspection of non-switchable live parts, devices and tools with insulated handles should be used, rubber mats should be used. When using portable appliances and power tools, one should not forget that touching any parts of the human body with pipes and metal structures is dangerous. In this case, if the insulation is damaged, an electric current will pass through the human body. Particular care must be taken in basements, bathrooms, showers, toilets and other damp and especially damp areas. In rooms without increased danger, about 20% of all domestic injuries occur, in rooms with increased danger and especially dangerous - 80% of accidents. It is not allowed to install switches and sockets, use electric heaters, washing machines, portable lamps that do not meet safety requirements. Author: Bannikov E.A.
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