Electrical safety mainly includes personal safety and equipment safety. Personal safety refers to the safety of personnel engaged in work and the operation and use of electrical equipment; Equipment safety refers to the safety of electrical equipment and other related equipment and buildings.
Electrical accidents are often not caused by a single cause. To do a good job in electrical safety, measures including technology and organization management must be taken. With the progress of science and technology, all countries are actively studying and introducing advanced electrical safety technology, perfecting and revising electrical safety technical standards and regulations, which is very important for protecting the safety and health of workers and the safety of electrical equipment.
What is an electric shock? What is the harm to human body?
When the human body touches the charged body, or when the charged body flashes and discharges with the human body, or when the arc hits the human body, the current enters the earth or other conductors through the human body, forming a conductive loop. This situation is called electric shock. When people get an electric shock, they will be hurt to some extent. According to its opening, it can be divided into electric shock and electric injury.
Electric shock refers to the current flowing through the human body, causing pain, numbness, muscle twitching and severe convulsions. Cardiac fibrillation or respiratory arrest, even death due to fatal injury to human heart, respiratory system and nervous system. Most electrocution deaths are caused by electric shock. Electrical injury refers to the arc burn of the parts where the human body is in poor contact with the charged body, or the electrocautery of the parts where the human body is in contact with the charged body, and the skin metallization caused by the invasion of metal particles melted and evaporated by current into the human skin. This kind of injury will leave scars on the human body, and may even lead to death in severe cases. Electrical injury is usually caused by thermal effect, chemical effect or mechanical effect of current.
Electric shock and electric injury may also occur at the same time, which is common in high-voltage electric shock accidents.
How do common electric shock accidents happen?
There are many kinds of electric shock accidents, most of which are caused by direct contact with charged bodies, or equipment failure, or rest bodies being too close to charged bodies.
The human body is in direct contact with the charged body. When the human body is on the ground or other grounding conductors and a part of the human body touches any phase of the three-phase conductor, the electric shock accident is called single-phase electric shock. The harm of single-phase electric shock to human body is related to the voltage level of power grid and neutral grounding mode. The number of electric shocks in human body accounts for more than 95% of the total number of electric shocks. In addition to single-phase electric shock, there are two-phase electric shocks. Refers to the electric shock accident caused by the simultaneous contact of two charged bodies with different phases. Two-phase electric shock accident belongs to this kind of indirect electric shock accident.
Human contact with faulty electrical equipment. In the case of political parties, the shell of electrical equipment is not taken. However, when the line fails or the insulation is damaged, electric shock will occur when it comes into contact with the leakage or charged equipment shell. Electric shock is the same as direct contact with a charged body. Most electric shock accidents belong to this kind of indirect electric shock accident.
The distance from the charged body is too small. When the distance between the human body and the charged body is too small, although it is not in contact with the charged body, because the insulation strength of the air is less than the electric field strength, the air breaks down and an electric shock accident may occur. Therefore, in the Electrical Safety Regulations, the minimum allowable safety distance is specified for electrical equipment with different voltage levels.
Step voltage gets an electric shock. Due to the destruction of external forces (such as thunder and lightning, strong wind), there will be a large amount of diffusion current flowing into the earth near the grounding point of electrical equipment and lightning rod, or near the disconnection point, so that different potentials are distributed on the surrounding ground. When people step on two points with different potentials on the ground at the same time between their feet, they will get an electric shock due to the step voltage.
Why are high and low voltage electric shocks dangerous?
There is no doubt that high voltage electricity will kill people. When the human body comes into contact with a high voltage conductor of about 0 1 kV, it will be electrocuted and die. Therefore, there are words "high voltage danger, please don't go near" near the equipment of high voltage power station. But it can't be concluded that high pressure will kill people and low pressure won't kill people.
In fact, the voltage received by the electric shock is only one of the decisive factors of the degree of injury. The fatal factor of electric shock is the current passing through the human body, not the voltage. Of course, the higher the voltage of the charged body contacted by the human body, the greater the current passing through the human body, so the greater the danger. However, according to statistics, the vast majority of electric shock accidents occurred at low voltage below 1 kV, and even at AC voltage as low as 24 volts, unfortunate accidents occurred. This shows that there is no voltage that can be regarded as safe in the unfavorable environment of human body card (such as standing in water or soaking wet).
The high frequency of low-voltage electric shock accidents is also due to: 1. Many people are exposed to low-voltage electrical equipment, and these people lack electrical safety knowledge; 2. People don't pay enough attention to the hazards of low-voltage electricity, including defective equipment, unreasonable operation and imperfect protection devices. 3. The harm of low-voltage electricity has not been paid enough attention, including being easily "stuck" and unable to get rid of the power supply, while the human body shocked by high-voltage electricity may get rid of the power supply independently (commonly known as rebound).
The secondary factors affecting the degree of electric shock injury include the frequency of current, the age, body shape and health status of the person who gets an electric shock. At the same voltage, alternating current is more dangerous than direct current, and alternating current for 25-300 cycles is more dangerous than high frequency current, impulse current and static charge. Generally speaking, women and children are more dangerous than adult men when getting an electric shock. Patients with heart disease and nervous system disease tuberculosis are more seriously injured by electric shock than healthy people.
What are the factors that determine the degree of injury of electric shock victims?
The degree of injury caused by electric shock depends on the following main factors:
1, the current flowing through the body, in mA. It depends on the total impedance between the applied voltage and the current in and out of the body. The greater the current flowing through the body, the stronger the physiological response of the human body and the greater the danger to life. Power frequency current above 20-25mA is liable to be severely punished. When the current is less than a few milliamps, the current mainly causes ventricular fibrillation and suffocation. When the current exceeds several hundred milliamps, it will not only cause coma, cardiac arrest and respiratory arrest, but also leave a fatal electric injury.
2. The way the current flows through the body. The heart, lungs, central nervous system and spinal cord are all easily damaged human organs. Therefore, the way the current flows through the body is from the chest to the hand, from the hand to the foot, which is the most dangerous. It is also very dangerous to carry the hip or back to the hand, and the hand to the hand is less dangerous. In addition, it is quite dangerous for electricity to pass through the brain, which will make people unconscious immediately.
3. The duration of current passing through human body is measured in milliseconds. The longer the human body is electrified, the lower the body resistance due to sweating, which leads to the increase of current and serious consequences. On the other hand, a human heart cycle (about 750 ml) has a vulnerable period of 100 milliseconds, which is very dangerous for the electric injury period.
How big is the current allowed by human body?
The human body is generally insensitive to the power frequency current below 0.5 mA. The experimental data show that the minimum current that causes sensation to different people is different. The average current of adult males is about 1.0 1 mA, and that of adult females is about 0.7 mA, which is called the perceived current. At this point, the human body will feel a slight tingling due to nerve stimulation. Similarly, the maximum current that different people can get rid of electricity independently is different. The average adult male is 16 mA, and the adult female is 10.5 mA, which is called desynchronization. In general, the power frequency current below 8- 10 mA and the direct current below 50 mA can be used as the safe current allowed by the human body every year, but it is also dangerous for these currents to pass through the human body for a long time. Under the condition of installing electric shock protection device, the allowable power frequency current of human body is about 30 mA. In the air, in places that may cause serious secondary accidents, the allowable power frequency current of human body should be considered as 5 mA, without strong convulsions.
How big is the human resistance?
When the human body touches the charged body, the human body is connected to the circuit as a circuit element. Human body impedance usually includes external impedance (related to the clothes, shoes and socks worn at that time and the humidity of the body, ranging from several thousand ohms to several tens of megohms) and internal impedance (related to the skin impedance and internal impedance of the electric shock victim). Human body impedance is not a simple resistance, but is mainly determined by human body resistance. Human body resistance is not a fixed value.
It is generally believed that dry skin has a fairly high resistance at low voltage, about 65438+ million ohms. When the voltage is 500- 1000 volts, the resistance drops to 1000 ohms. The reason why the epidermis has such high resistance is that it has no capillaries. There is also a stratum corneum in the skin of a certain part of the finger, and the resistance value of the stratum corneum is higher, while the resistance value of the skin that is not rubbed frequently is the smallest. Skin resistance is also related to the contact area and pressure between human body and body.
When the epidermis is damaged and the dermis is exposed, the human body's resistance is low because there are blood vessels that transport saline solution. It is generally believed that when the arm or leg is in contact with the dermis, its resistance is about 500 ohms. Therefore, the path from one arm to another or from one leg to the other is equivalent to a resistance of 1000 ohm. Suppose a person holds a charged body tightly with both hands and stands in a puddle with both feet to form a conductive loop. At this time, the human body resistance is basically around 500 ohms.
Generally speaking, human body resistance can be considered as 1000-2000 ohms.
What is the safe voltage standard?
Safe voltage is to prevent electric shock accidents by using specific power supply voltage series. Its power supply needs to be isolated from the input circuit and other electrical systems. Under normal and fault conditions, the upper limit value of the voltage series between any two conductors and the ground shall not exceed the effective value of 50 volts AC (50-500 Hz).
According to the characteristics of the site, people can use the AC safe voltage grade (1, 42 volts (no-load upper limit is less than or equal to 50 volts) specified in safe voltage standards for hand-held electric tools used in places with electric shock danger; 2, 36 volts (no-load upper limit is less than or equal to 43 volts), can be used in mines, conductive dust and other places; 3. For some live equipment that the human body may accidentally touch, you can choose 24 volts, 12 volts and 6 volts (the upper limit of no-load is less than or equal to 29 volts, 15 volts and 8 volts respectively). When working in large boilers, metal containers or generators, 12V or 6V low-voltage running lights must be used to ensure personal safety. When the electrical equipment adopts safe voltage of more than 24 volts, measures must be taken to prevent direct contact with charged bodies. Its circuit must be insulated from the earth.
Safe voltage is determined according to the product of human allowable current and human resistance.
What measures should I take after getting an electric shock?
The key point of electric shock first aid is to grab quickly and rescue appropriately. Don't panic and be helpless. After electric shock, people may be unable to get rid of the power supply by themselves because of convulsions or unconsciousness. At this point, it is the first step to quickly disconnect the electric shock victim from the power supply, and then, a simple diagnosis should be made quickly. Observe whether it exists, touch the aorta in the neck or groin to see if the pupil is dilated. Generally, it can be treated as follows: 1, the patient is awake, but has symptoms such as fatigue, dizziness, palpitation, cold sweat, nausea and vomiting. Let the patient rest quietly on the spot. If the symptoms are serious, he should be carefully escorted to the hospital for examination and treatment. 2. The patient's heartbeat is still alive, but he is unconscious. Keep warm, prepare for artificial respiration and cardiac compression, and immediately notify the medical department or send the patient to the hospital on a stretcher. 3. If the patient is in a state of "suspended animation", artificial respiration or cardiac compression should be performed at the same time immediately, and a doctor should be asked for treatment or sent to the hospital as soon as possible. He should pay special attention to the urgency as soon as possible, and he can't wait for the arrival of the doctor, nor can he stop the first aid work on the way to the hospital.
How to carry out mouth-to-mouth artificial respiration?
Mouth-to-mouth artificial respiration is one of the most effective artificial respiration methods. Before its implementation, the collar, coat and nepotism that hinder breathing should be untied quickly, and the dentures, blood clots and vomit falling off the mouth of the electrocuted person should be electrocuted to make the respiratory tract unblocked. Then let the electrocuted person lie on his back, with his head completely backward and his nose up.
The specific operation steps are as follows:
1. Hold the nostril of the electrocuted person tightly with one hand, and pull the jaw forward with the other hand (or hold his back neck). After taking a deep breath, the rescuer will blow inward close to the mouth of the electrocuted person, and observe that the chest bulges together to ensure that the gas is effective for about 2 seconds.
2, after blowing, immediately leave the mouth of the electric shock, and relax the pinched nose, let him exhale automatically, pay attention to the recovery of the chest, for about 3 seconds.
Continue to operate according to the above steps until the electrocuted person begins to breathe.
If the person who gets an electric shock is a child, you can only blow through a small mouth (or let the nose leak naturally, don't pinch it to avoid alveolar rupture; If the stomach of the electric shock victim is found to be swollen, you can gently press the upper abdomen with your hands and continue to blow air for ventilation. If you can't open the mouth of the electrocuted person, you can change your mouth to artificial respiration.
How to use chest compressions for first aid?
Chest compressions are the first-aid methods for electric shock patients after their hearts stop beating, and their purpose is to force their hearts to return to spontaneous beating. When using chest compressions, the person who gets an electric shock should be placed in a solid, flat and stable place to keep the respiratory tract unblocked (mouth-to-mouth artificial respiration is especially required), and the rescuer should take the initiative in the patient's waist.
The operation is as follows:
One hand points the tip of the middle finger at the lower edge of the concave pottery in the patient's neck, presses the palm on the chest, and the other hand presses on the back of the hand. The palm root is pressed hard to make the lower sternum and the connected ribs sink 3-4 cm, and presses the heart to make the blood in the heart fight.
Suddenly relax after squeezing, and the palm roots don't have to leave the chest. Relying on the elasticity of the chest, the sternum is reset, the heart is relaxed, and the blood from the great vein returns to the heart.
Follow the above steps, continuously and rhythmically, once a second, until the lips and skin of the electrocuted person turn rosy and the arteries pulse.
When performing chest compressions, use moderate strength according to the rescuer's weight and shoulder muscles, and have a certain impact force. Don't use force slowly, but don't use force too hard. If the person who gets an electric shock is a child, you can squeeze it with one hand, which is lighter to avoid damaging the sternum. It is appropriate to squeeze it about 100 times per minute.
What are open circuit and short circuit?
A state in which a part of a closed circuit is broken, making the current unusable, is called an open circuit (or an open circuit state). After the circuit is disconnected, the electrical equipment will not work, and the running equipment will stop working or cause abnormal state, or even cause failure.
The state that the wires from the power supply to the electrical equipment (also called load) are directly connected to each other without passing through the load (or the load is zero) is called short circuit (or short circuit state). At this time, the current in the circuit (called short-circuit current) mainly depends on the internal reactance of the line and transformer in the high-voltage power supply circuit, which will far exceed the current limit allowed by wires and equipment. This is bound to be conducive to overheating or burning electrical equipment, and even cause fire. At the same time, short-circuit current will produce great electromotive force, which may lead to serious accidents such as power supply equipment. Therefore, electrical equipment must take corresponding protective measures, including installing corresponding protective devices to prevent the occurrence of short circuit or limit the damage caused by short circuit.
What is overpressure?
In the power system, electrical equipment of various voltage levels only bears its rated voltage during normal operation. However, under abnormal circumstances, due to the sudden addition of external lightning induction, or the operation and failure of the system, the electromagnetic energy in the system may oscillate, accumulate and spread, thus causing the voltage to rise, which is dangerous to the insulation of electrical equipment. This phenomenon is called overvoltage. Although the overvoltage phenomenon lasts for a short time (usually from a few microseconds to tens of milliseconds), the medium energy of voltage rise is very large. Without overvoltage protection measures, when the insulation level of the equipment itself is low, insulation breakdown of transmission and distribution lines and electrical equipment may occur, thus damaging the operation of political parties in the power system. In order to ensure the safe operation of power system, all kinds of safety technical measures must be taken for overvoltage protection to minimize the harm caused by overvoltage.
What is overload? What is low voltage?
There are many forms and specifications of electrical equipment, and the specifications must be determined according to the actual load when selecting. Used in transformers, oil circuit breakers, disconnectors, timers, etc. The data related to the load is rated current or capacity; For power capacitors, the allowable load is determined by their rated voltage, while the conductor is determined by the safe current carrying capacity. If the rated current of the selected electrical equipment is less than the actual load, overload will occur.
The influence of overload phenomenon on electrical equipment is mainly that it is easy to cause excessive temperature rise of equipment and lines, accelerate insulation aging and shorten service life. Long-term overload operation of electrical equipment will damage electrical equipment due to serious overheating, and even cause major accidents such as fire and explosion.
Voltage directly affects the safety and economic operation. Old power supply lines, unreasonable layout of power supply lines, overload power supply of power transformers and low power factor of power grid load will all cause voltage quality degradation. When the voltage of the line is lower than the rated voltage of the equipment, the phenomenon of low voltage will appear. Long-term low-voltage operation of electrical equipment will bring great difficulties and losses to users. It not only increases the power loss of power supply lines, darkens lighting, reduces the quality of television, broadcasting and communication, and reduces the output and efficiency of motors, but also often causes overheating or even burning of electrical appliances such as motors, communication equipment and televisions due to overcurrent. The allowable deviation of voltage load stipulated by the state is: -7% for voltages above 35 kV, 10 kV and-5% for low-voltage power; Low-voltage lighting is-10%.
What is the safe current of wires and cables?
Safe current is also called safe flow or allowable continuous current.
When the current passes through the conductor or cable, the existence of impedance will cause the consumption of electric energy, which will make the conductor 7 cable heat up and the temperature rise. The greater the current through the wire or cable, the higher the temperature of the wire or cable. When the temperature rises to a certain value, it may damage the insulation of wires and cables, and the external oxidation will intensify, resulting in leakage and disconnection, and even fire and other accidents.
In order to ensure the safe operation of electrical lines, the wire and cable sections of all lines must meet the heating conditions, that is, at any ambient temperature, when the wire and cable continuously pass through the maximum load current, the line temperature is not greater than the maximum allowable temperature (usually about 700C), and the load current at this time is called safe current.
The safe current of wires and cables is determined by their types, specifications, ambient temperature and laying methods, and there are special tables for reference.
What are the neutral point operation modes in the electrical system?
In China's three-phase AC power system, there are three kinds of neutral points of generators and transformers.
In ungrounded neutral system, when single-phase fault grounding occurs in the system, the work of three-phase electrical equipment can run as usual, which is generally allowed to last for 2 hours. If the fault cannot be eliminated within 2 hours, the faulty line will be cut off. The interference of the system to communication lines is small, and the risk of single-phase electric shock is also small, but the system requires high grounding fault detection and protection and line insulation. Commonly used in 3- 10 kV systems and places with special working conditions and high safety requirements below 0 1 kV.
Neutral direct grounding system, as long as the line insulation of the system is considered according to the phase voltage, can suppress the voltage rise to the ground caused by high voltage intrusion, lightning strike, switching operation and induction, and can also provide stable phase voltage for convenient power supply. However, the single-phase electricity of the system is dangerous and the grounding is easy to affect each other, so it is often used in ultra-high voltage systems above 0 1 kV and three-phase four-wire systems below 0 1 kV. 3. The neutral point is grounded through the coil, which can eliminate the intermittent arc at the grounding point and avoid overvoltage, while maintaining the advantage that the neutral point ungrounded system allows short-time one-to-one grounding fault operation. When the single-phase grounding current is greater than a certain value, it is a common grounding method for 3-60 kV systems.
Different grounding modes have obvious influence on the operation of power system, especially when single-phase grounding occurs. Each has its advantages and disadvantages.