2020-07-01
164
Laboratory work 8
Electrical safety in the different types
of electrical networks
Objective: explore the dependence of the current flowing through the person touching the wire three-phase mains voltage up to 1 kV, from the power supply neutral system (transformer), wire insulation parameters of the network and resistance in the human body circuit.
Basic concepts and definitions
Electricity is used in all industries of the national economy and in everyday life. Practice shows that in all areas of use of electrical energy accidents happen where the victim is struck by electric shock. Electric shocks make up a small percentage of the total number of work-related injuries, but it occupy one of the top places on the number of injuries with severe and fatal consequences. The greatest number of electric traumas (60 … 70%) occurs during the work of electro installations up to 1000 V.
The actions of electric current on the human body are diverse. Passing through the body, the electric current causes thermal, electrolytic, and biological action.
The thermal effect of the current burns is shown in some of the individual parts of the body, heating of the blood vessels, nerves, blood, etc. The electrolytic action of the current is shown in the expansion of the blood and other organic body fluids, causes significant violations of their physical and chemical composition. The biological effect of the current manifests itself as anger and excitement of living tissues of the body, as well as a violation of the internal biological processes. The mechanical action results in the current bundle break the tissues as a result of electrodynamic effect, as well as the explosive instantaneous formation of steam from the tissue fluid and blood. This variety of electrical current actions can lead to two types of lesions – electrical injury or electric shock.
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Electrical injuries are distinct when local damage to tissues caused by exposure to an electric current or an electric arc where these following electrical injuries occur: electrical burns, electric signs, metallization, ophthalmia, mechanical damage.
Electrical shock is the excitement of living tissues of the body passing through electric shock, accompanied by involuntary spasmodic contractions of the muscles. Depending on the outcome of the current impact on the body electric shocks are divided into the following four degrees: I – twitching muscles without loss of consciousness, II – muscular twitching, unconsciousness, but the preservation of breathing and heartbeat, III – loss of consciousness and cardiac abnormalities or breathing (or both together), IV – clinical death, i.e., the absence of respiration and circulation.
The main causes of electric shock are:
– Violation of rules of technical operation of electrical installations;
– Contact with current carrying parts;
– Touching the metal not current carrying parts, become live due to faulty insulation or grounding devices.
If a person comes under stress, then through his body electric current flows. The effects of the electric current through humans depends on many factors: the current type (AC or DC), alternating current at - on its frequency; the magnitude of the current (or voltage); duration of current flow; from the current path through the human body; physical and mental state of a person.
The most dangerous for the human being is an alternating current with a frequency of 50... 500 Hz. The ability of self-liberation from the power of this frequency, most people only saved when its value is very small (up to 10 mA). The magnitude of current flowing through an affected person voltage depends on the voltage setting circuit and the resistance of all the elements on which the current flows.
It is exposed to the greatest danger of people, when current passes on vitals (heart, lungs) or cells of the central nervous system. However death is possible even at a small tension (12 … 36 V) as a result of contact of current carrying parts with the most vulnerable parts of a body – the palm back, a cheek, a neck, a shin, a brachium.
It is established that at the time of electric shock is very important physical and mental condition of the person. If a person is hungry, tired, drunk or ill, his body's resistance is reduced, i.e., the probability of severe injury increases. At observance of safety rules, i.e., with attentive and careful work, the possibility of electric shock is reduced. The extent of the current human exposure is given in Table. 8.1.
State of the environment (temperature, humidity, dust, acid vapors) affects the resistance of the human body and the insulation resistance, which ultimately determines the nature and consequences of electrical shock. From the standpoint of environmental production facilities may be dry, wet, damp, especially raw, hot, dusty with conductive and nonconductive dust, or with a reactive organic medium. All but dry, the body resistance is reduced.
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Table 8.1
The nature of the current impact on the human body
| Current | AC | DC |
| Up to 1 | Do not feel | |
| 1…8 | Sensations are painless. Muscle control is not lost. Perhaps the self-liberation from contact with live parts energized | Easy itching |
| 8…15 | Painful sensations. Muscles management has not been lost and perhaps self-liberation from the action of the current is possible | Feeling of warmth |
| 20…50 | Feelings from th current are very painful. Action from the current extends to thoracic muscles that lead to difficulty and even the termination of respiration. At long exposure for several minutes, death can occur when the lungs stop functioning | Light traction of arm muscles |
| 50…100 | Direct effect on the heart muscle. When the flow length of more than 0.5 seconds, can cause fibrillation or cardiac arrest, i.e. rapid and chaotic heart muscle fibers, in which the heart stops working as a pump, resulting in the body stops the blood-circulation and death occurs | Paralysis of respiratory |
| 100…200 | The occurrence of ventricular fibrillation | |
According to the Rules for Electrical Installation (PUE), all production facilities for electrical hazards are divided into three categories:
1. Premises with increased danger, characterized by the presence of one of the following factors (signs): moisture, when the relative humidity exceeds 75%; high temperature exceeding 35°C; conductive dust; conductive floors; the simultaneous touch of having a connection to the land steel structures of buildings, technological devices, mechanisms, etc., on the one hand, and the metal casing – on the other.
2. Particularly dangerous areas, characterized by the presence of one of three conditions: extreme humidity, when the relative humidity is close to 100%; reactive environment when contained pairs or form deposits are destructive to insulation and conductive parts of equipment; two or more attributes simultaneously, but inherent in the premises with increased risk.
3. Premises without increased danger, characterized by the absence of signs of increased and particular risk.
System safety standards (GOST 12.1.030-81 «Electro protective means. Protective earth, neutral earthing») is defined as the electrical system of organizational and technical measures and means to ensure the protection of people from harmful and dangerous effects of electric current, electric arc, electromagnetic fields and static electricity.
The severity of a person touching uninsulated live parts of electrical installations under voltage depends on the type of touch and type of the electrical network. Touches can be one-and two-phase to three-phase networks, as well as single- and double-pole single-phase networks.
The two-phase bipolar touches are very dangerous, because the person is under the nominal voltage of the electric power source. The current value flowing through a person,
I=U/R, (8.1)
where U – rated supply voltage (V), R – the human resistance, Ohm.
The electrical resistance of the human body (human resistance) consists of the resistance of the skin and internal organs. When calculating the resistance of the human body a thumb rule is to use 1000 Ohms.
The technical methods and means of protection include: the insulation of live parts with a continuous monitoring device; fences; electrical separation of networks; the use of low voltages; electro protective agents (lock); alarm and safety signs; protective earth; neutral earthing; safety shutdown; protection against the risk in the transition from a higher voltage to a lower side; compensation of earth fault currents.
Neutral earthing – a deliberate electrical connection with a zero protective wire of metal not current carrying parts which can appear alive. The physical essence of the neutral earthing is in the fact that it is intentionally performed by protective conductor metallic bonding equipment enclosures with earthed neutral power, any circuit on the case turns into a single-phase short circuit with an automatic disconnection emergency site from the network protection devices (fuses, circuit breakers and et al.).
Systems for protective shutdowns are the express electric devices intended for shutdown of electro installations in cases of danger of housing breaking down. As violation of isolation is the main reason for short circuit on a housing of current carrying parts of an inventory, systems of protective shutdown exercise constant control behind an insulation resistance or leakage currents between current carrying and not current carrying parts of an equipment.
One of the measures to ensure the electrical safety when working on electrical equipment is protective earth (protective grounding).
Protective grounding is called an intentional electrical connection with the ground or with an equivalent metal live element, which may be energized. Protection is achieved by reducing the voltage of touch by aligning the runoff potential in the electrical current from the ground in the breakdown phase in the installation package. The earthed current spreads uniformly in all directions on the surface and in the depths of the earth. As the distance from the earthing current density decreases, as it increases the cross-section of the layer of earth through which the current flows. It established by calculation that the potential of the ground surface decreases with increasing distance from the earthing by hyperbola law: the maximum value (for earthing) to zero at a distance of about 20 m.
In the current spreading areaa person can be under the potential difference, for example, a step away. Tension between the two points of the current circuit, located from one another at a distance of a step in which a man stands at the same time, called the step voltage.
The value of the step voltage depends on the step width and distance from the place of man in the earth circuit. Step voltage decreases with the distance from the place of circuit. Voltage step takes into account the shape of the potential curve.
Grounding is designed as a unit, consisting of the earthing and grounding conductors. Earthing can be natural and artificial. As a natural earthing metal elements are used that laid in the ground, such as: metal elements (valves) of reinforced concrete structures of buildings, water and other metal piping (except flammable gases, liquids and pipes, coated wire), metal sheaths of cables etc. When natural grounding resistance is absent or insufficient, and then arranged for artificial-earth lead.
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Depending on the location of the earth relative to grounded object artificial grounding devices are divided into contour and remote. Typically grounding electrodes are immersed vertically or horizontally in the ground. Group grounding device, consisting of vertical rods, interconnected strips or rounded steel, are most often used. To enhance the effect of the potential equalization grounding contour is built in the form of a grounding grid.
Artificial earth electrodes are made of steel of various profiles. For the purpose of mechanical, thermal and corrosion resistance are advised to take the following dimensions: diameter 40 – 80 mm and length 2 – 3 m.
Ground wires are usually made of steel of rectangular or circular cross-section. In networks with voltage up to 1000 V adopted conductivity grounding conductors less than 1/3 of the phase conductor. When laying the ground bus within the building smaller section rectangular tires should be 24 mm2, from the smallest circular diameter of 5 mm.
Requirements to the device of protective grounding and neutral earthing of electric equipment are defined in «Rules for Electrical Installation» according to which they have to settle at nominal tension of 380 V and above alternating current and also 440 V and above a direct current. Works in rooms with the increased danger and particularly dangerous areas have to be carried out in installations with power supply voltage more than 42 V variable and more than 119 V of a direct current. Metal parts of electro installations, exposed the person which can appear alive Uph as a result of isolation damage are subject to protective grounding and neutral earthing. In this case the current passing through the person
I 1 = U av /(R h +R IPM ), (8.2)
where Rh – the resistance of the human body; RIPM – resistance of individual protection means, in their absence RIPM = 0.
Protective earth is used to protect people against electric shock when touching the metal conductive equipment that can be energized as a result of damage to the insulation. Protective grounding is performed by an intentional connection (metal wires) live elements with electrical «ground» (Fig. 8.1) or its equivalent (GOST 12.1.030-81).
