Electrical safety mainly includes personal safety and equipment safety in two aspects. Personal safety is engaged in the work and the use of electrical equipment operation process of personnel safety; equipment safety refers to electrical equipment and other equipment, building safety.
Electrical accidents are often not the sound of a single cause, in order to improve electrical safety, must be taken, including technical and organizational management and other aspects of the measures. With the progress of science and technology, countries are actively researching and continue to introduce advanced electrical safety technology, improve and revise the technical standards and regulations for electrical safety, which is very important for the protection of the safety and health of workers, the protection of electrical equipment safety.
What is meant by electric shock? What harm does it do to the human body?
When the human body touches the electrified body, or the electrified full and the human body between the flash discharge, or arc and the human body, the current through the human body into the earth or other conductors, the formation of conductive circuits, this case, it is called electrocution, electrocution, the human body will be subjected to a certain degree of injury, according to its open can be divided into two kinds of electric shock and electric injuries.
Electrocution is the flow of electric current through the human body, causing pain and numbness, muscle twitching, severe will cause strong spasm. Cardiac fibrillation or respiratory arrest, and even death due to fatal injuries due to the human heart, respiratory system and nervous system. The vast majority of electrocution fatalities are caused by electric shock. Electrical injury refers to the electric shock, the human body and the charged body contact poor part of the arc burns, or the human body and the charged body contact part of the electric branding, yarrow due to the electric current melting and evaporation of metal particles and other invasions of the human skin caused by the skin metallization. This injury can leave scars on the body and can be fatal in severe cases. Electrical injuries are usually caused by the thermal, chemical or mechanical effects of electric current.
Shocks and injuries can also occur at the same time, which is common in high-voltage electrocution accidents.
How do common electrocution accidents occur?
Electrocution accidents are varied, and most are caused by the human body coming into direct contact with an electrically charged body, or by equipment malfunctioning, or by Hugh being too close to an electrically charged body.
Direct human contact with the charged body. When the human body on the ground or other grounded conductor, and a part of the human body touches the three-phase conductor of any one phase of the electric shock caused by the accident is called single-phase electric shock. Single-phase electrocution on the human body and the voltage level, the grid neutral grounding method and so on. The number of human electrocutions accounted for more than 95% of the total number of electrocutions, in addition to single-phase electrocution, there are two-phase electrocution. It refers to the human body in two places at the same time contact with different phases of the charged body and the electric shock caused by the accident. The two-phase electrocution accident belongs to this category of indirect electrocution.
The human body in contact with faulty electrical equipment. In the case of political parties, the shell of the electrical equipment is not charged. But when the line failure or insulation breakdown, contact this this leakage or charged equipment shell, the risk of electric shock will occur. Electrocution is the same as direct contact with a live body. Most electrocution accidents fall into this category of indirect electrocution.
The distance from the charged body is too small. When the distance between the human body and the electrified body is too small into, although not in contact with the electrified body, but due to the insulation strength of the air is less than the electric field strength, air breakdown, electric shock may occur. Therefore, the electrical safety regulations, the different voltage levels of electrical equipment, are specified in the minimum allowable safety distance.
Stepping voltage electrocution. As a result of external forces (such as lightning, wind) damage and other reasons, electrical equipment, lightning rod grounding, or broken down near the location of the wire severed, there will be a large number of diffusion of the current to the earth into the ground, and the ground around the cloth with a different potential. When a person's feet and feet at the same time stepped on the surface of the ground at different potentials at two points, will lead to Chi step voltage electrocution.
Why are high and low voltage electrocutions dangerous?
There is no doubt that high voltage can kill. When the human body comes into contact with a high voltage wire of about 01 kilovolt, it can be electrocuted to death. That's why there's a "High Voltage Danger, Do Not Approach" sign near high-voltage power plants. But we can't conclude that high voltage will kill you, while low voltage will not.
In fact, the value of the voltage to which the electrocuted person is exposed is only one of the determining factors in the degree of injury sustained. It is the current through the body, not the voltage, that is fatal to electrocution. Granted, the higher the voltage of the electrically charged body touched by the human body, the higher the voltage of the electrically charged body, the greater the current through the human body when the human body's electrical yin is unchanged, and therefore, the greater the danger. But according to statistics, the vast majority of electrocution accidents occur in 1 kilovolt below the low-voltage called on, and even as low as 24 volts under the AC voltage has occurred in the unfortunate accident. This shows that the human body card unfavorable environment (such as standing in the water or wet), there is no voltage can be regarded as safe.
Low-voltage electrocution accidents occur with high frequency also because: 1. contact with low-voltage electrical equipment, and these people than the lack of knowledge of electrical safety; 2. people do not pay enough attention to the dangers of low-voltage electricity, including defective equipment, operation is unreasonable, the protection device is not perfect, etc.; 3. low-voltage electricity is not enough to pay enough attention to the dangers of low-voltage electricity, including the easy to be "stuck", unable to get rid of the power supply. "and can not get rid of the power supply, and high-voltage electrocution of the human body to get rid of the power supply may be autonomous (commonly known as popping back).
Secondary factors affecting the degree of injury from electrocution include the frequency of the current, the age of the electrocuted person, size, health, etc.. AC is more dangerous than DC at the same voltage, and AC at 25-300 weeks is more dangerous than high-frequency currents, inrush currents and electrostatic charges. Generally women and children are more dangerous than adult men when electrocuted. Where patients suffering from heart disease, neurological diseases, tuberculosis, the degree of injury suffered when electrocuted than healthy people.
What are the factors that determine the degree of injury sustained by an electrocuted person?
The degree of injury sustained by a person electrocuted depends on the following major factors:
1. The current flowing through the body, measured in milliamperes. It is determined by the applied voltage and the total human impedance between the points where the current enters and exits the body. The higher the current flowing through the body, the stronger the physiological reaction of the human body, the greater the danger to life. 20-25 milliamps or more of industrial frequency current are prone to produce severe consequences. When the current is less than a few milliamps, the current mainly causes ventricular fibrillation sustenance, hundreds of milliamps or more of the current, in addition to cause unconsciousness, the heart stops beating immediately, breathing stops, but also leave a fatal electrical injury.
2, the way the current flows through the body. The heart, lungs, central nervous system and spinal cord are easy to hurt the human organs, therefore, the current flow through the body of the pathway to the chest to the hand, the hand to the feet of the most dangerous, the buttocks or the back to the hand, the hand to the hand is also very dangerous, the feet to the feet of the less dangerous. In addition, the passage of electric current through the brain is also quite dangerous and can cause immediate unconsciousness.
3, the duration of the current through the body, measured in milliseconds. The more time the human body is energized and,*** body resistance to engage in due to sweating and other decline, resulting in an increase in current, the consequences are serious. On the other hand, a human heart beat cycle (about 750 milliliters), there is a 100 milliseconds of susceptibility to injury period, this period of time to the electrical injury period phase heavy full and cause great danger.
How much current is allowed in the human body?
The human body is generally insensitive to industrial frequency currents below 0.5 milliamperes. Experimental data show that for different people to cause the feeling of the smallest current is not the same, the average adult male about 1.01 mA, adult women about 0.7 mA, this value is called the perception of the current. This value is called the perceived current. At this time, the human body feels a slight tingling due to the stimulation of the nerves. Similarly, stage different people can independently get rid of power after the maximum current is not the same, the average adult male is 16 mA, adult female is 10.5 mA, this value is called to get rid of the current. In general, 8-10 milliamps below the frequency current, 50 milliamps below the direct current can be used as the human body allows the safety of the current, but these currents for a long time through the human body is also dangerous. In equipped with protection against electric shock, the human body allows the frequency current of about 30 milliamps, in the air, not the surface of the occasions that may cause serious secondary accidents, the human body allows the frequency current should be considered according to the 5 milliamps does not lead to a strong spasm.
How large is the human body resistance?
When the human body comes into contact with a charged body, the human body is connected to the circuit as a circuit element. The human body impedance usually consists of the external negative impedance (related to the clothes, shoes and socks worn at the time of electrocution, as well as the body's moisture, ranging from a few thousand ohms - tens of megohms) and internal impedance (related to the electrocuted person's skin impedance and the internal impedance). The human body impedance is not a pure resistance and is mainly determined by the body resistance. Human body resistance is also not a fixed value.
Dry skin is generally considered to have a fairly high resistance of about 100,000 ohms at low voltages. When the voltage is 500-1000 volts, this resistance drops to 1000 ohms. The epidermis has this high resistance because it has no capillaries. The skin of a certain part of the finger also has a stratum corneum, which has a higher resistance, while the resistance of the skin of the infrequently rubbed area is the smallest. Skin resistance is also related to the area of contact between the body and the body and pressure.
When the epidermis is damaged to expose the dermis, the body is full of blood vessels transporting salt solution and a very low resistance. It is generally recognized that the resistance of an arm or leg in contact with the dermis is about 500 ohms. Thus, the path from one arm to the other or from one leg to the other is equivalent to a resistance of 1000 ohms. Assuming that a person with both hands tightly a charged body, feet standing in a puddle and the formation of a conductive circuit, then the body resistance is basically the body resistance of about 500 ohms.
In general, the human body resistance can be considered as 1000-2000 ohms.
What is the safe voltage standard?
Safety voltage is a series of voltages adopted by a specific power supply in order to prevent electric shock accidents. Its power supply requires the implementation of output and input circuit isolation, and isolation from other electrical systems. The upper limit of this voltage series, in normal and fault conditions, any two conductors between any conductor and ground shall not exceed the AC (50-500 Hz) RMS value of 50 volts.
People can be based on the characteristics of the place, the use of China's safe voltage standards for the safety of alternating current voltage level: 1, 42 volts (no-load upper limit is less than or equal to 50 volts) can be available for use in places where there is a risk of electrocution, such as the use of hand-held power tools and other occasions; 2, 36 volts (no-load upper limit is less than or equal to 43 volts), can be in the mines, multi-conductive dust and other places such as the use of running lights and other occasions; 3, 24 volts, 12 volts, 6 volts and other occasions; 3 24 volts, 12 volts, 6 volts (no-load limit were less than or equal to 29 volts, 15 volts, 8 volts) three gears can be used in some of the human body may accidentally touch the electrically charged body of the equipment. Work in large boilers, metal containers or work in the generator, in order to ensure personal safety must use 12-volt or 6-volt low-voltage traveling lights. When electrical equipment using more than 24-volt safety voltage, must be taken to prevent direct contact with the charged body. Its circuit must be insulated from the earth.
The safe voltage is determined on the basis of the product of the permissible current in the human body and the resistance of the human body.
What measures should be taken after an electric shock?
The main point of first aid for electrocution is to grab quickly, save the law, not to be alarmed, helpless, people electrocuted, may be due to spasms or loss of consciousness, etc. and can not get rid of the power supply, this time to quickly make the electrocuted person from the power supply is the first step of first aid, and after that, it should be quickly to make a simple diagnosis of their injuries; observe the presence of a touch of the carotid or groin at the aorta has no pulsation, take a look at whether the pupil is dilated, generally can be dealt with according to the following situation: 1, the patient's state of mind awake, but there is weakness, dizziness, panic, cold sweat, nausea, vomiting and other such symptoms, the patient should be made to rest quietly on the spot, the symptoms are serious, carefully escorted to the hospital for examination and treatment, 2, the patient's heartbeat is still alive, but the consciousness of the coma, the patient should be kept, keep the surrounding air circulation, pay attention to warmth, and do a good job of artificial respiration and heart Squeeze the preparation work, and immediately notify the medical department or with a stretcher to send the patient to the hospital to rob, 3, if the patient is in the "false death" state, should be immediately symptomatic implementation of artificial respiration scribe heart squeeze method or both methods at the same time for the resuscitation, and quickly ask the doctor to diagnose and treat or to send to the hospital, special attention should be paid to the urgent need to be carried out as soon as possible, can not wait for the arrival of the doctor, and in the hospital to send to the hospital. The doctor's arrival, on the way to the hospital, can not stop the first aid work.
How to perform mouth-to-mouth artificial respiration?
Mouth-to-mouth artificial respiration is one of the most effective artificial respiration, before the implementation of the electrocution should be quickly impeded breathing on the body of the collar, blouse, skirt, etc., and electrocuted in the oral cavity of the dislodged dentures, blood clots, vomit, etc., so that the airway is open. Then make the electrocuted person lying on his back, head fully tilted back, so that the nose also face up.
Specific operational steps are as follows:
1, one hand to defend the nostrils of the electrocuted person, the other hand will be pulled forward and downward jaw (or hold the back of the neck), the rescuer inhaled a deep breath of air and then pressed against the electrocuted person's mouth to the inward blowing, the same into the observation of the chest is rumbling to ensure that the gas is effective, for about 2 seconds.
2, blowing finished, immediately leave the electrocuted person's mouth, and relax the pinched nose, let him automatically exhale, pay attention to the recovery of the chest, for about 3 seconds.
Follow the above steps continuously until the electrocuted person starts to breathe.
The electrocuted person, such as children, can only small mouth blowing (or do not pinch the nose, let the natural leakage of air, so as to avoid alveolar rupture; such as found that the electrocuted person's stomach inflated and swollen, can be gently pressurized with a hand on one side of the upper abdomen, and one side to continue to blowing and ventilation, such as the inability to make the electrocuted person's mouth open, you can change the mouth-to-nose artificial respiration.
How to give first aid with chest compressions?
Thoracic cardiac compression is a first aid method after the electrocution of the heart stops beating, its purpose is to force the heart to resume beating on its own, thoracic cardiac compression, the electrocution should be made in a relatively solid, flat, solid place, to keep the airway open (the specific requirements of the same mouth-to-mouth respiration), the rescuer enthusiastically in the patient's waist.
The action is as follows:
One hand with the tip of the middle finger against the lower edge of the patient's neck concave tao, the palm of the hand pressed on the chest, the other hand pressed on the back of that hand, the root of the palm forcefully downward, so that the lower part of the sternum and the connected ribs sunk 3-4 centimeters, pressing the heart to make the heart of the blood within the heart to beat.
Suddenly relax after squeezing, the root of the palm does not have to leave the chest, relying on the elasticity of the thorax, so that the sternum reset, this into, the heart diastole, the blood of the large veins back to the heart.
According to the above steps, continuously and rhythmically, once every second, until the electrocuted person's lips and the color of the skin on the body turned red, as well as feel the arterial pulsation until.
When performing chest cardiac compression, rely on the rescuer's weight and shoulder muscles to exert moderate force, with some impact force rather than slow force, but not too much force. If the electrocuted person is a child, you can use one hand to squeeze, have a lighter, so as not to damage the sternum, and it is appropriate to squeeze about 100 times per minute.
What is a broken circuit and a short circuit?
In a closed circuit part of the phenomenon of disconnection occurs, so that the current can not be the state is called disconnection (or disconnected state). After the occurrence of disconnection, electrical equipment will not work, the operation of the equipment will stop working or cause anomalies, or even cause failure.
The power supply to the electrical equipment (also known as the load) of the wire if not through the load (or load is zero) and direct each other's state, known as a short circuit (or short-circuit state). At this time the current in the circuit (short-circuit current) depends mainly on the supply of high-voltage circuit lines and transformers in the internal negative resistance, it will be far more than the conductor and the equipment allowed by the current limit. This will inevitably favor the overheating or burning of electrical equipment, or even cause a fire. At the same time, the short-circuit current Hoon will produce a great deal of electromotive force, which may lead to serious accidents such as power supply equipment. Therefore, electrical equipment must take appropriate protective measures, including the addition of appropriate protective devices to prevent the occurrence of short-circuit or limit the damage caused by short-circuit.
What is meant by overvoltage?
In the power system, various voltage levels of electrical equipment, in normal operating conditions only serve to withstand its rated voltage. But in abnormal circumstances, may be due to external lightning lightning induction suddenly added to the system, or the system operation, faults, and other reasons for the operation of the system caused by the internal electromagnetic energy oscillation, accumulation and propagation, resulting in a dangerous voltage rise on the insulation of electrical equipment, a phenomenon known as overvoltage. Over-voltage phenomenon although the duration of a very short period of time (generally from a few microseconds to tens of milliseconds), but the voltage rise in the energy is very large, in the absence of over-voltage protection measures seven equipment itself when the level of insulation is low, may occur in the transmission and distribution lines and electrical equipment, insulation breakdown, so that the operation of political parties of the electric power system suffered damage. In order to ensure the safe operation of the power system, must effectively take a variety of safety and technical measures, overvoltage protection, in order to minimize the harm caused by overvoltage.
What is overload? What is low voltage?
There are many forms and specifications of electrical devices, the selection must be based on the actual load to determine its specifications. For transformers, oil circuit breakers, disconnect switches, timing, etc., and load-related data is the rated current or capacity; power capacitors, the allowable load by its rated voltage, while the conductor to the safety of the load capacity for the selection of allowable load basis. If the rated current of the selected electrical device is less than the actual load, overloading will occur.
The impact of overload phenomenon on electrical equipment is mainly easy to cause equipment, line higher temperature rise as well as accelerate the aging of insulation, shorten the service life. Electrical equipment for a long time overload operation will be due to severe overheating and damage to electrical equipment, and even cause fires, explosions and other major accidents.
Voltage good or bad directly affects the safety and economic operation. Power supply line is old, power supply line layout is unreasonable, power transformer overload power supply, power factor of the grid load is too low, etc. will cause voltage quality decline. When the voltage of the line is lower than the rated voltage of the equipment, low voltage phenomenon will occur. Prolonged low voltage operation of electrical equipment will bring great difficulties and losses to users. It not only makes the power supply line power loss increase, dim lighting, television, radio, communication quality decline, motor output and efficiency, and often due to overcurrent caused by motors, communication equipment, televisions and other electrical appliances overheating or even burned. State regulations allow the voltage load deviation: 35 kV or more for -7%, 10 kV and low voltage power for -5%; low voltage lighting for -10%.
What is the safety current of wires and cables?
Safety current is also known as safety flow or allowable continuous current.
When current passes through a wire or cable, the presence of impedance causes a drain on electrical energy, causing the wire seven cable to heat up and increase in temperature. The higher the current through the wire or cable, the higher the temperature of the wire or cable. When the temperature rises to a certain value, may make the wire seven cable insulation damage, connected to the outside of the oxidation intensified, thus causing leakage, disconnection, etc., severe punishment may cause fire and other accidents.
In order to ensure the safe operation of electrical lines, all lines of wire and cable cross-section must meet the thermal conditions, that is, in any ambient temperature, when the wire and cable through the maximum load current continuously, the line temperature is not greater than the maximum permissible temperature (usually about 700C), when the load current is called the safety current.
The safety current of wires and cables is determined by their type, specification, ambient temperature and laying method, there are special tables can be checked.
What are the neutral point operation methods in electrical systems?
China's three-phase alternating current power system, the neutral point of the generator and transformer have three kinds of operation
Neutral ungrounded system, when the system has a single-phase fault grounding, three-phase power equipment can run as usual, generally allowed to continue for 2 hours, such as 2 hours can not be ruled out the fault, only to excise faulty lines, the system of communication lines interfere with a small, single-phase The system has little interference with communication lines and little danger of single-phase electric shock, but the system has higher requirements for ground fault detection and protection and line insulation. Often in the 3-10 kV system and 01 kV below the working conditions of special, high security requirements of the place used.
Neutral point directly grounded system, the line insulation of the system as long as the phase voltage considerations, can inhibit the high voltage into the lightning strike, pull and close the gate operation and induction of the ground voltage rise, it can also provide a stable phase voltage to facilitate the power supply, but the system of single-phase electrical danger, grounding device is easy to affect each other, often in the 01 kilovolts and above, ultra-high voltage system and 01 kilovolts below the three-phase four-wire system used, 3, neutral fault detection and protection requirements and high safety requirements of special working conditions below the 3-10 kilovolt system and 01 kilovolts. The neutral point is grounded by canceling coil, which can eliminate the intermittent arc of the grounding point and avoid over-voltage, and at the same time, it maintains the advantages of neutral ungrounded system that allows the operation of one-phase grounding fault for a short period of time. It is 3-60 kV system when the single-phase grounding current is greater than a certain value, the grounding method commonly used.
Different grounding methods for power system operation, especially when a single-phase grounding has a significant impact. Each has its own advantages and disadvantages.