First, the harm of current to human body and its influencing factors
When the human body contacts a charged body, the current passes through the human body, causing part or the whole body to be stimulated and hurt by electricity, resulting in electric injury and electric shock. Electrical injury refers to the external harm of electricity to human body, such as arc burn and electrocautery. When the human body is close to high-voltage equipment and the distance is less than or equal to the discharge distance, arc discharge will occur between the human body and charged high-voltage equipment, and serious or even fatal arc burns will occur under the thermal and chemical effects of arc temperature and current as high as 3000℃ or even higher. Electric shock refers to the damage to the internal organs of human body, such as electric current acting on the nerve center of human body, destroying the normal work of the heart and respiratory system, causing convulsions, unconsciousness and other phenomena, and may also stop or greatly weaken the activities of respiratory organs and blood circulation organs, thus forming a so-called suspended animation state. At this time, if medical means such as artificial respiration are not used in time, people will not be resurrected.
The degree of injury when people get an electric shock is related to the voltage acting on the human body, the resistance of the human body, the current value passing through the human body, the frequency of the current, the time when the current passes through the human body, the way in which the current circulates in the human body and the physical condition of the human body. The current value is a direct factor that hurts the human body.
Second, safe current and safe voltage.
1, safety current
In order to ensure personal safety, it is generally based on the premise that there is no harmful physiological influence on the human body after electric shock. Therefore, the current passing through the human body generally does not have harmful physiological effects, which is called safe current. Safety current can be divided into allowable safety current and continuous safety current. When the human body gets an electric shock, the current value passing through the human body is not greater than the current value of the escape current, which is called the allowable safe current. 50 ~ 60 Hz AC regulation is 10mA (6mA for mining and other operations), and DC regulation is 50mA. When people get an electric shock, the current passing through the human body is greater than the escape current and the current value corresponding to the continuous power-on time is called continuous safe current. The relationship between AC continuous safe current value and continuous power-on time is as follows:
Iac= 10+ 10/t(0.03 seconds
T- continuous power-on time seconds
2. safe voltage
Under various environmental conditions, after the human body comes into contact with a charged body with a certain voltage, all its parts will not be damaged. This voltage second is called safe voltage.
Safe voltage is expressed by the product of the current allowed by human body and the resistance of human body. Generally speaking, the ground voltage below 40V can be regarded as safe voltage. The limit value of contact voltage stipulated by the International Electrotechnical Commission is 50 V. When it is lower than 25V, the safety measures against electric shock can be ignored. There are five grades of safe voltage stipulated in China: 42V, 36V, 24V, 12V, 6V and rating. At present, 36V and 12V are widely used in safe voltage. Generally, the running lamp used in power plant production sites and substations is 36V, and the voltage of running lamp should not exceed 12V in dangerous places or narrow workplaces, places with large area grounding bodies around them, and places with hot and humid environment, such as cable trenches and coal hopper oil tanks.
It should be pointed out that these voltages cannot be considered as absolutely safe, and if the human body contacts the power supply for a short time under the condition of sweating and broken skin, electric shock may also occur.
Third, the way the human body gets an electric shock
The basic ways of human electric shock are single-phase electric shock, two-phase electric shock, step voltage electric shock and contact voltage electric shock. In addition, there are people close to high-voltage electricity and lightning strikes.
1, single-phase electric shock
Refers to the electric shock caused by the human body standing on the ground or other grounding bodies and a part of the human body touching the charged body. The degree of danger is related to the voltage level, whether the neutral point of the power grid is grounded or not, and the unit relative capacitance. It is a common electric shock accident.
In daily work and life (three-phase four-wire system), if the insulation of switches, plugs and lamp holders of low-voltage electrical equipment, motors, electric irons, washing machines and other household appliances is damaged and the live parts are exposed, the shell and skin will be charged. When the human body touches these devices, a single-phase electric shock will occur. If the human body stands on the insulating board or wears insulating shoes at this time, the resistance between the human body and the earth will be great, and the current passing through the human body will be small, so there is no danger of electric shock.
2, two-phase electric shock
Refers to the electric shock when the human body has two contacts with any two-phase power supply at the same time. When two phases get an electric shock, the current flows from one wire to another through the human body, and the voltage acting on the human body is equal to the line voltage. If the line voltage is 380V and the current flowing through the human body is as high as 268mA, such a large current may cause the death of the electrocuted person as long as it flows through 0. 186s, so two-phase electric shock is more dangerous than single-phase electric shock.
3, step voltage to get an electric shock
When an electrical device has a grounding fault or a wire is broken on the line, and the wire falls to the ground, the fault current will flow into the earth from the landing point of the grounding body or wire and be dispersed to the earth in a hemispherical shape. The closer to the current entry point, the higher the potential, and the farther away from the current entry point, the lower the potential. At a distance of 20M from the entry point, the ground potential is about zero. If someone enters this area at this time, the potential difference between the legs is the step voltage. The electric shock caused by step voltage is called step voltage electric shock. When the human body bears the step voltage, the current generally flows along the lower body of the human body, that is, from the feet to the crotch and then to the feet, forming a path with the earth. Current rarely passes through the important organs of the human heart, which seems harmless. When the stride voltage is high, people will fall to the ground because of leg cramps, which will not only increase the voltage acting on the body, but also change the path of current through the human body and through important organs of the human body, thus greatly increasing the risk of electric shock.
Therefore, electrical workers must be extra careful when working or walking at ordinary times. When it is found that the equipment has a grounding fault or the conductor is broken and falls to the ground, stay away from the broken area; Once you accidentally step into the broken line area and feel a step voltage, you should quickly put your feet together or jump with one leg to leave the broken line area; Wear insulating boots when you need to enter the disconnection area to save people or troubleshoot.
4, contact voltage to get an electric shock
Contact voltage refers to the voltage that people bear between their hands and feet when they stand next to the equipment that is short-circuited to the ground and touch the casing of the leakage equipment. The electric shock caused by contact voltage is called contact voltage electric shock.
In power plants and substations, the shell and base of general electrical equipment should be grounded. Usually, the housings and pedestals of these devices are free of charge. However, when the insulation of the equipment breaks down, the grounding part is destroyed, and there is a potential difference between the equipment and the earth, if the human body touches these equipment, it will bear the contact voltage between the hands and feet and get an electric shock. In order to prevent contact voltage from getting an electric shock, it is often necessary to bury all the equipment in a workshop and a substation respectively, and each motor uses a separate protective grounding.
5, arc discharge electric shock
Due to carelessness or failure to take safety measures, it will get close to exposed high-voltage live equipment and cause serious discharge and electric shock accidents.
6. Sudden call to power-off equipment leads to electric shock.
If reliable safety measures are not taken, such as temporary grounding and necessary signs are not installed, the equipment being repaired will be sent by mistake, causing the maintenance personnel to get an electric shock.
Fourth, technical measures to prevent personal electric shock
When the shell of electrical equipment is charged due to insulation damage, there is no sign of charging, so people will not have any premonition about the danger of electric shock, and electric shock accidents are often prone to occur. But many electric shock accidents can be avoided as long as we master the law of electricity and take corresponding measures.
1, protective grounding
Protective grounding is a good connection between the metal shell or frame of electrical equipment and the grounding body, so as to prevent the human body from being shocked when the insulation of electrical equipment is damaged. Protective grounding is suitable for small power grids with ungrounded neutral points. Using protective grounding can only reduce the risk of electric shock, but it can't completely guarantee personal safety.
2, protection by zero
In order to prevent people from getting an electric shock due to insulation damage of electrical equipment, the metal shell of electrical equipment is connected to the neutral line (transformer neutral point) of the power grid, which is called protective zero connection. Protective zero connection is suitable for low-voltage power system with three-phase four-wire neutral point directly grounded.
Requirements for protective neutral system:
(1) Fuses and circuit breakers cannot be installed on the zero line to prevent electric shock accidents caused by the zero line phase voltage when the zero line circuit is disconnected;
(2) In the same low-voltage power grid, part of electrical equipment shall not be connected with protective grounding, while the other part of electrical equipment shall be connected with protective grounding to zero;
(3) When connecting a three-eye socket, it is not allowed to connect the hole of the power zero line on the socket with the hole of the ground wire in series. The correct connection is that the hole connected to the zero line of the power supply and the hole connected to the ground are connected to the zero line with wires respectively;
(4) In addition to the neutral point must be well grounded, the zero line must also be repeatedly grounded.
3. working grounding
A point in the power system is connected to the ground directly through metal or through special equipment, which is called working grounding. Working grounding can reduce the contact voltage of human body, quickly cut off the power supply, reduce the insulation level of electrical equipment and transmission lines, and meet the special needs of electrical equipment operation.
4. Leakage protector
Its function is to prevent electric shock accidents caused by electrical equipment and line leakage, to prevent fire accidents caused by equipment leakage, to monitor or cut off grounding faults, and to automatically cut off power supply when equipment leakage and dangerous grounding voltage appear in the shell.
Five, get an electric shock scene first aid
Electric shock accidents often occur in an instant, and the situation is critical. There can be no hesitation. Time is life.
After electric shock, some people may die or die clinically, even though their heartbeat and breathing have stopped. If the rescue is correct and timely, it is generally possible to save lives. Whether you can save the life of an electric shock depends on whether you can quickly get rid of the power supply and carry out correct emergency rescue.
1, power off
When people get an electric shock, the first thing to do is to disconnect the electric shock victim from the power supply, which is the key to first aid the electric shock victim. However, before the person who gets an electric shock leaves the power supply, the emergency personnel are not allowed to directly pull the person who gets an electric shock by hand to prevent the emergency personnel from getting an electric shock. In order to get the electric shock person out of the power supply, emergency personnel should take appropriate methods and measures decisively according to the on-site situation. There are generally the following methods and measures to get rid of power supply.
(1) Low voltage electric shock is out of power supply.
(1) There is a power switch or plug near the low-voltage electric shock. Unplug the switch or plug immediately and cut off the power supply.
(2) If the power switch is far away from the electric shock site, the wires can be cut off with insulating tools, but the wires on the power side must be cut off, and the cut wires should be prevented from touching others by mistake.
(3) When the charged low-voltage conductor falls on the person who gets an electric shock, the conductor can be removed with insulation to isolate the person who gets an electric shock from the power supply. However, it is not allowed to use any metal bars or wet objects to move the wires to prevent emergency personnel from getting an electric shock.
(4) If the clothes of the person getting an electric shock are dry, emergency personnel can strictly wrap their hands with dry clothes and scarves, and then use the wrapped hands to hold the clothes of the person getting an electric shock, or tie them with the dry clothes of emergency personnel to drag the person getting an electric shock away from the power supply.
⑤ If the electric shock victim is far from the ground, cut off the power supply to prevent the electric shock victim from falling from a height and causing trauma.
(2) The person who gets an electric shock at high voltage is out of power supply.
In case of high voltage electric shock, cut off the power switch quickly. If you can't cut off the power switch, you should use insulation tools suitable for this voltage level to keep the electric shock away from the power supply. During rescue, the first responder should protect the voltage level within a certain safe distance to ensure the personal safety of the first responder.
(3) The overhead line is shocked by the power supply.
When someone gets an electric shock on the overhead line, you should quickly pull down the switch or call the local power supply department to inform the power failure. If the power supply cannot be cut off immediately, the power supply side switch can be tripped by short circuit. When throwing short routes, you should prevent arc burns or disconnection from endangering personal safety. After the person who gets an electric shock on the pole disconnects the power supply, use a rope to send the person who gets an electric shock to the ground.
2. On-site first aid treatment
When the electric shock person cuts off the power supply, the emergency personnel should carry out on-site first aid treatment according to the different physiological reactions of the electric shock person.
(1) The electric shock victim is conscious, but feels weak, flustered, breathless and pale. At this point, the person who gets an electric shock should lie flat and rest quietly. Don't let the electrocuted person walk around to reduce the burden on the heart, and closely observe the changes of breathing and pulse. If you find that the pulse of the electric shock person is too fast or too slow, please ask the medical staff to check and treat it immediately.
(2) When the person who gets an electric shock is delirious and has a heartbeat, but his breathing stops or is very weak, he should hold his chin up with his head in time to open the airway and carry out mouth-to-mouth artificial respiration. If artificial respiration is not carried out in time, the heart will stop beating because of lack of oxygen for too long.
(3) When the electric shock victim is delirious, his heart stops beating, but his breathing is weak, he should be given CPR first aid immediately. We can't think that there is only chest compression when there is still weak breathing, because this weak breathing can't play the role of gas exchange.
(4) When the electric shock person's heartbeat and breathing stop, first aid should be given to cardiopulmonary resuscitation immediately, and first aid should still be given according to the regulations of cardiopulmonary resuscitation on the way to the hospital.
(5) When the electric shock person's heartbeat and breathing all stop, accompanied by other injuries, first aid of cardiopulmonary resuscitation should be carried out quickly, and then the trauma should be treated. For those who get an electric shock due to cervical fracture, when opening the airway, the head should not lean back to avoid high paraplegia, and the jaw-supporting method should be applied.
(6) When a person is struck by lightning, because the lightning current will depolarize the heart, the brain will have a short metabolic rest and central apnea. Therefore, when the victim's heartbeat and breathing stop, he should be given cardiopulmonary resuscitation first aid, otherwise he will die of anoxia. You can't insist on resuscitation just because the pupil of the victim has been dilated.