Safe electricity

(A) the basic knowledge of safe use of electricity

1. The importance of safe use of electricity

The so-called safe use of electricity refers to the correct use of electricity and the adoption of scientific measures and means on the premise of ensuring the safety of people and equipment.

In modern production and life, electric energy plays a very important role and brings great benefits to mankind. But if you can't use electricity safely, you will get an electric shock, causing death and equipment damage. Therefore, the safe use of electricity, as a professional knowledge, should be publicized to all power users and mastered by all power users.

2. Causes of electric shock in human body

The phenomenon that the human body is subjected to excessive voltage due to contact with a charged body, resulting in death or local injury is called electric shock.

Electric shock can be divided into electric shock and electric injury according to the degree of injury. Electric shock refers to the phenomenon that the internal organs are injured due to the current passing through the human body, which is the most dangerous electric shock accident. When the power frequency current passing through human body exceeds 10mA or the direct current exceeds 50mA, it will lead to dyspnea, muscle spasm and central nervous system injury, so that the heart stops beating and even dies. Electrical injury refers to the burning phenomenon caused by metal powder splashed outside the human body when the arc or fuse is blown.

The main cause of electric shock injury is current. The magnitude of current depends on the voltage acting on the human body and the resistance value of the human body. The resistance of different parts of the human body ranges from 2 K to 20 MΩ. Generally speaking, the resistance of the skin is large; However, the moisture on the skin surface will reduce its resistance. It can be seen that voltage and electric shock are important factors to determine the degree of electric shock injury.

There are many cases of electric shock, but the forms of electric shock can be summarized as single-line electric shock and double-line electric shock. As shown in Figure 4-29a, the human body touches two terminal wires at the same time, resulting in double-line electric shock. At this time, it is most dangerous for the human body to bear the line voltage. As shown in Figure 4-29b, it is neutral grounding single-phase electric shock. At this time, it is also very dangerous for the human body to bear the effect of phase voltage. As shown in Figure 4-29c, it is a single-phase electric shock with the neutral wire ungrounded. When the insulation of the charged body is poor and the capacitance to the ground is large, it is also dangerous for the human body to touch.

(2) Methods and safety measures to prevent electric shock

1. Protective earthing and protective earthing.

In the process of power generation, power supply and electricity consumption, electrical equipment is charged due to insulation aging, overvoltage breakdown or wear, resulting in parts that should not be charged originally (such as metal base, shell, etc. ), or the parts with low-voltage electricity are electrified, and the electrical equipment damage and personal injury accidents caused by these unexpected abnormal electrification often occur. In order to avoid this kind of electrical accident, the most commonly used protective measures are grounding and zero connection.

Figure 4-29 Various electric shock situations

(1) grounding

A good electrical connection between any part of electrical equipment and the earth is called grounding. Safe grounding is the most important measure to ensure the normal operation of electrical equipment and the personal safety of workers. A metal body or a group of metal bodies in direct contact with the earth is called grounding body or electrode. The metal wire used to connect the grounding body with electrical equipment is called grounding wire, which is divided into grounding trunk line and grounding branch line. Grounding body and grounding wire are collectively referred to as grounding device.

Grounding voltage refers to the rhyme potential difference between the grounding part of electrical equipment and the ground fog potential. Generally speaking, the earth more than 20 meters away from the grounding body can be considered as zero potential point. In addition, grounding resistance refers to the sum of grounding resistance and grounding resistance of grounding body, which is also equal to the ratio of grounding voltage to current flowing into the earth through grounding body.

Grounding can be divided into protective grounding and working grounding. Protective grounding is a measure taken to ensure personal safety, such as grounding the metal shell of electrical equipment; Working grounding, this is the grounding required for normal operation, such as the grounding of neutral point of power transformer and the grounding of overvoltage protection.

The protective grounding of low-voltage IT system and TT system are introduced as follows:

1)IT system protection grounding. In a three-phase, three-wire low-voltage IT system (Figure 4-30) where the neutral point of the power supply is not grounded, all electrical equipment enclosures are electrically connected to the earth, and protective grounding is implemented. In the IT system, when a person touches a single-phase collision device, the current Ib passing through the human body is only a part of the grounding current IE, that is, Ib=IERE/(RE+Rb). If a steel pipe with a length of 2.5m and a diameter of 50mm is used to drive sandy clay, and its soil resistivity is100Ω m, then its grounding resistance RE≈ resistivity/length =100Ω m/2.5m = 40Ω, and the human body resistance Rb =1000Ω is calculated according to the worst environment.

Figure 4-30 Information Technology System

As far as the equipment is concerned, IE is only the sum of two-phase leakage current (capacitive current) when there is no fault, and its value is very small, which will not endanger the safety of the equipment and cause malignant accidents such as fire, so it is not necessary to cut off the power supply, and the faulty equipment can continue to operate within the specified time. In view of the characteristics of many flammable and explosive gases in petrochemical, mining, shipbuilding and other industrial production, which need continuous power supply, more IT systems can not only improve the reliability of power supply, but also play a role in preventing electric shock. In order to find and eliminate faults in time and prevent two-phase grounding faults from happening at the same time, insulation monitoring devices must be installed in the IT system, and alarm signals should be given when single-phase grounding occurs in the system and handled in time.

2) 2) Protective grounding of TT system. In a three-phase four-wire TT system in which the neutral point of power supply is directly grounded, the casings of all electrical equipment are grounded through their own PE wires, which constitutes a TT system, also known as protective grounding (Figure 4-3 1). In TT system, when the equipment collides with the enclosure, its single-phase grounding current IE=220/(4+40)=5A (assuming that the neutral grounding resistance R0 of power supply is 4 Ω and the grounding resistance Re of equipment enclosure is 40 Ω), and the grounding voltage UE of equipment enclosure is 200 V. TT system is characterized by small IE and large UE. This single-phase grounding short-circuit fault current generally does not make the over-current protection device of the line act, so the equipment shell may be charged for a long time, which is dangerous to people. Therefore, the TT system must be equipped with leakage protection. The action current of leakage protection is generally set to 30mA (safe current value), so that the grounding voltage of the equipment shell does not exceed 50V (safe voltage value) in case of single-phase grounding fault. Therefore, the protective grounding resistance of the equipment shell is Re = 50V/30mA =1.667Ω. It is easy to take RE≤ 100ω in practical work. Therefore, protective grounding, as a safety measure, has been widely used in three-phase four-wire system with direct neutral grounding, especially in the case of wide power supply range, unbalanced load and high neutral voltage, it is reasonable to adopt TT system. This system is widely used abroad and has been popularized in China. "GB 50096-20 1 1 Residential Design Code" stipulates that the residential power supply system "should adopt TT, TN CS or TN S grounding mode".

Figure 4-3 1 TT system

(2) Zero connection

Zero connection refers to connecting the metal shell or frame of the electrical equipment insulated from the live part with the zero line (i.e. pen line or male ***PE line) in the neutral point direct grounding system, so as to avoid the danger of electric shock to human body. In TN system where the neutral point is directly grounded and the PEN line or PE line is led out, protection and zero connection measures must be taken to ensure the reliable operation of the equipment and prevent the danger of electric shock. According to different connection modes, TN system can be divided into TN-C system, TN-S system and TN-C-S system.

1)TN-C system. N line and PE line in TN-C system are merged into a PEN line, and the exposed conductive parts of all equipment are connected to the PEN line (Figure 4-32). Current can flow through the pen line, so it can cause electromagnetic interference to some equipment through the pen line. If the pen line is disconnected, the exposed conductive parts (such as the shell) of the equipment connected with the pen line will be charged, which may cause electric shock to people. Therefore, the system is not suitable for places with high electromagnetic interference and safety requirements. However, due to the integration of N wire and PE wire, non-ferrous metals (wires) and investment can be saved. In the past, the system was widely used in low-voltage distribution system in China, but now it is not allowed to be used in places with high safety requirements, including residential buildings, office buildings and places that need electromagnetic interference prevention.

Figure 4-32 TN-C system

2)TN-S system. In TN-S system, the N line is completely separated from the PE line, and the exposed conductive parts of all equipment are connected to the PE line (Figure 4-33). There is no current flowing in the PE line, so the equipment butting the PE line will not generate electromagnetic interference. If the PE wire is disconnected, the exposed conductive part of the equipment connected with the PE wire is not charged under normal circumstances; However, when some equipment has inter-shell faults, the exposed conductive parts of other equipment connected with PE line will be charged, which makes people have the danger of electric shock. Because the N line is separated from the PE line, compared with the above TN-C system, the consumption and investment of non-ferrous metals have increased. The system is widely used in important office places, experimental places, residential buildings and other places with high safety requirements and high anti-electromagnetic interference requirements.

Figure 4-33 TN-S system

3)TN-C-S system. In TN-C-S system, according to the load characteristics and environmental conditions, N line and PE line can be laid together or separately, and pen lines are not allowed to merge again (Figure 4-34). Its advantage lies in solving the problem that the neutral line voltage at the end of TN-C system is too high. It has the characteristics of the first two systems and is suitable for situations where the environmental conditions at the end of distribution system are harsh or there is data processing. It must be pointed out that in the same low-voltage distribution system, some can not take zero protection, and some take grounding protection. Otherwise, when the equipment with grounding protection does not touch the casing, all the equipment casings with zero protection may bring dangerous voltage to the ground.

Figure 4-34 TN-C-S system

2. Safe operation measures to prevent electric shock

(1) Power outage operation and safety measures

1) Disconnect the power supply. When repairing the electrical circuit, disconnect the low-voltage switch first, and then disconnect the high-voltage switch. For multi-circuit lines, it is necessary to prevent reverse power transmission from the low voltage side to the equipment to be repaired.

2) Check the power. Check the incoming and outgoing lines of the maintenance equipment with the voltage level electroscope, and work can be started only after it is confirmed that there is no electricity.

3) Install the grounding wire. Portable temporary grounding wires shall be installed for each phase of disconnected power output and each phase of maintenance lines. When installing the grounding wire, the grounding terminal should be connected first, and then the conductor terminal. When removing the grounding wire, the conductor end should be removed first, and then the grounding end should be removed. When installing and removing the grounding wire, you should wear insulating gloves and hold the insulating rod of the temporary grounding wire for operation. The human body shall not touch the grounding wire and accept supervision.

4) In case of gale, rainstorm and lightning above level 6, it is forbidden to climb poles and switch.

5) Before climbing the pole, check whether the pole root is firm. Don't climb until the new pole is completely stable.

6) When working on the pole, the ground should be supervised; Materials and tools should be transported by hanging rope; No one is allowed to stand within 2m below the pole, and the field staff should wear safety helmets.

7) Safety belts must be used when working on poles. Seat belts should be tied to poles and solid parts, not to cross arms. The safety belt should be prevented from coming off the top of the rod.

8) When using the ladder, someone should support it or have anti-skid measures.

(2) Anti-electric shock measures for live working

1) Live working must comply with relevant safety regulations, be conducted by trained and qualified electricians, and be supervised by experienced electrical professionals.

2) Use tools with good insulation, wear insulated shoes without damage and oil stain, and stand on dry insulation.

3) First of all, we should distinguish between phase line and zero line. When disconnecting the conductor, disconnect the phase line first and then the neutral line. You can't use insulating pliers to clamp off the phase line and the zero line at the same time to avoid short circuit. When the wires are overlapped, the zero line should be connected first, and then the phase line. Never let the human body touch two wires at the same time.

4) Insulation or isolation measures should be taken for disconnected phase lines and charged bodies.

5) When overhauling low-voltage lines erected on high-voltage poles, the distance between maintenance personnel and high-voltage lines shall meet the requirements in Table 4-9.

Table 4-9 Safe Distance

3. Description of airport safe electricity use

1) It is forbidden to install electrical appliances on the ground (earth) with wires (phase wires), and the live wire must enter the control switch.

2) Don't connect too many electrical appliances to a socket. Don't tie the power cord with wires.

3) Don't touch live equipment with wet hands, and don't wipe electrical appliances with wet cloth.

4) Don't wear clothes and sit on the floor on the motor and electrical equipment; Electrical equipment should be used in an environment far away from humidity and corrosion; Electrical equipment should have a certain insulation resistance, and insulation materials must be regularly checked for damage and aging during use; The load of airport electrical equipment should match the power supply capacity.

5) Bare wires shall not be used for the power cord introduced into the drilling airport; The telephone pole near the airport should be higher than 2.5m, and the iron foot of the insulator on the pole should be led into the ground wire; If cables are used, they should be intact, without connectors in the middle, and the cables in the computer room should be laid under the floor.

6) The power switch in the airport must be intact, with reliable insulation, and put in the box. Wear insulating gloves when pulling the switch; The working lamp voltage should be lower than 36V. When moving electrical appliances, the power supply must be cut off, and copper and aluminum wires are not allowed to replace fuses.

7) Grounding devices must be installed for electrical equipment in the airport. When starting the motor, you should issue a password to the airport personnel and pay attention to the running status of the motor and other equipment at any time. If smoke and sparks are found, the power supply should be cut off immediately for inspection.

8) In thunderstorm weather, airport staff should not casually move outside the airport to avoid being injured by lightning, and should not be close to telephone poles and lightning protection grounding wires to avoid electric shock caused by the surrounding step voltage when lightning strikes the ground.

9) Power transformation and distribution equipment (such as transformers) used in the airport should be far away from the airport, and effective protective measures should be taken to prevent ordinary people from approaching high-voltage equipment, and warning signs should be hung.

10) The telephone facilities in the airport should be equipped with lightning protection devices; Don't use the phone when it thunders, in case lightning is introduced to hurt people.

1 1) In case of external power failure, all power switches in the airport should be turned off to prevent sudden calls from causing accidents.

12) electrical equipment must be installed according to the specifications, and the live parts should have protective covers. When restarting after a long-term outage, the grounding resistance and insulation resistance should be measured again. The drilling rig shall not be too close to the high-voltage line.

(3) Electric shock first aid

1. Electric shock first aid

In the first aid of electric shock, we should follow the eight-character policy; Fast, accurate, on-site and lasting.

(1) Make the person who gets an electric shock leave the power supply quickly.

The sooner the better, because the longer the electric shock time, the heavier the injury, but pay attention to the correct operation.

1) disconnecting from the power supply is to disconnect the switch of the part of the charged equipment that the electric shock victim contacts or try to separate the electric shock victim from the charged equipment. Before the electric shock is turned off, the ambulance personnel shall not directly touch the electric shock with their hands. When disconnected from the power supply, the rescuer should not only save people, but also pay attention to protect himself from electric shock.

2) If the person who gets an electric shock touches the low-voltage live equipment, the ambulance personnel should try to cut off the power supply quickly. For example, turn on the power switch or unplug the power plug, or use non-conductive objects such as insulating tools and dry wooden sticks to get rid of electric shock. You can also grab the dry but uncomfortable clothes of the electrocuted person and drag them away. You can also wear insulating gloves or wrap your hands in dry clothes to get rid of electric shock. Rescuers can also stand on insulating mats or wooden boards for rescue. In order to get the electric shock person out of the charged body, it is best to rescue him with one hand.

3) If the person who gets an electric shock touches high-voltage live equipment, the ambulance personnel should quickly cut off the power supply and use insulating tools (wearing insulating gloves, insulating boots and insulating rods) suitable for the voltage level to rescue the person who gets an electric shock. Rescuers should pay attention to keeping a necessary safe distance from the surrounding live parts during rescue.

4) If the person who gets an electric shock is at a height, the person who gets an electric shock may fall from a height after disconnecting the power supply, so corresponding safety measures should be taken to prevent the person who gets an electric shock from falling or dying.

5) When cutting off the power supply to rescue those who get an electric shock, the emergency lighting needed for rescue should be considered in order to continue the first aid.

(2) On-site rescue for those who get an electric shock.

1) If the person who gets an electric shock is still conscious, he should lie flat on the spot and observe closely. Don't let him stand or walk for the time being.

2) If the electric shock victim has lost consciousness, he should lie flat, make sure the respiratory tract is unobstructed, and call the injured person or pat him on the shoulder for 5 seconds to judge whether he has lost consciousness. Don't shake the head of the wounded and don't call the wounded.

3) If the electric shock victim loses consciousness and stops breathing, but his heart beats slightly (use two fingers to test whether the carotid artery in the depression next to the Adam's apple is beating), mouth-to-mouth or mouth-to-mouth artificial respiration should be performed immediately after the respiratory tract is unobstructed.

4) If the electric shock victim is seriously injured, his heartbeat and breathing have stopped, and he is completely unconscious, mouth-to-mouth (nose) artificial respiration and artificial circulation chest compression should be performed immediately after the respiratory tract is clear. If there is only one person at the scene to rescue, artificial respiration and artificial circulation can be carried out alternately. First press the heart 4 ~ 8 times outside the chest, then blow the heart 2 ~ 3 times from mouth to nose, then press the heart 4 ~ 8 times, blow the heart 2 ~ 3 times from mouth to nose ... and so on.

(3) The first aid process must be carried out persistently.

Before the future medical staff take over the rescue, they can't give up the on-site rescue, and they can't arbitrarily judge the death of the wounded and give up the rescue just because the wounded have no breathing and pulse. Only doctors have the right to diagnose the death of the wounded.

2. Cardiopulmonary resuscitation is used for electric shock first aid.

Here are two first-aid methods to revive people who get an electric shock:

(1) artificial respiration

The methods of artificial respiration include chest compression in supine position, back compression in prone position and mouth-to-mouth (nose) blowing. This paper only introduces the mouth-to-mouth (nose) blowing method, which is now recognized as a simple and effective method.

1) First of all, quickly untie the clothes and belts of the electrocuted person, and untie tights, bras, scarves, etc. This way, the chest can expand freely without obstructing breathing.

2) Let the person who gets an electric shock lie on his back, without a pillow, with his head tilted to one side first, and remove foreign bodies such as blood clots and dentures from the exit. If the base of the tongue sinks, pull out the tongue to make the airway unobstructed; If the electric shock victim's teeth are clenched, the rescuer should hold the back corner of the mandible with both hands, put his thumb on the edge of the mandibular corner, and slowly move the mandible forward with his hands to move the lower teeth to the front of the upper teeth; You can also use open pliers, small pieces of wood, metal pieces, etc. , and carefully extend from the mouth to the teeth to pry open the teeth and remove foreign bodies in the mouth. Then straighten your head so that it leans back as far as possible, with your nostrils facing the sky, so that the airway is smooth.

3) The rescuer is located beside the person who gets an electric shock, covering his nostrils with one hand to avoid air leakage; Pull your jaw forward with your other hand and open your mouth. You can cover your mouth with a layer of gauze and prepare to be blown.

4) After the rescuer takes a deep breath, stick it on the mouth of the person who got an electric shock and blow it (Figure 4-35a). If you can't open your mouth, you can also squeeze your mouth tightly and blow close to your nostrils. Let your chest expand when you blow.

Figure 4-35 mouth-to-mouth artificial respiration

5) When the rescuer takes a breath, he should immediately leave the mouth (or nostril) of the electrocuted person, relax the pinched nose (or mouth) and let it exhaust freely (Figure 3-35b).

Blow the electric shock repeatedly according to the above operation requirements, about 12 times per minute. When this method is applied to young children, don't pinch the nose too tightly, let it leak freely, and don't blow hard to prevent the lungs from bursting.

(2) chest compressions were performed on the heart by artificial circulation.

1) Like the requirements of artificial respiration mentioned above, you must first untie clothes, belts, bras, scarves, etc. And remove foreign bodies in the mouth to make the respiratory tract unblocked.

2) Make the electrocuted person lie on his back in the same posture as the above-mentioned mouth-to-mouth blowing method, but the ground behind him must be flat and solid, such as hard ground or wooden board.

3) The rescuer is located on the side of the electrocuted person, and it is best to kneel opposite the waist of the electrocuted person, with his hands folded (children can only use one hand), and the root of his palm is placed at a slightly higher place in the heart socket (such as under the sternum 1/3) (Figure 4-36).

4) After the rescuer finds the correct pressing point of the electrocuted person, he should press down from top to bottom, vertically and evenly, and squeeze out the blood in the heart (the force is less for children) (Figure 4-37a).

Figure 4-36 chest compression method

Figure 4-37 Artificial Heart Chest Compression

5) After pressing, the root of the palm is quickly relaxed (but the palm should not leave the chest), so that the chest of the electrocuted person will automatically recover, the heart will expand, and blood will return to the heart (Figure 4-37b). Press and relax the heart of the electrocuted person repeatedly according to the above operation requirements, about 60 times per minute. When pressing, the positioning should be accurate and the strength should be appropriate. In the process of artificial respiration and cardiac compression, the rescuer should closely observe the reaction of the electric shock. As long as the electrocuted person is found to have signs of awakening, such as eyelid blinking or lip movement, the operation should be stopped for a few seconds to let the electrocuted person breathe and heartbeat spontaneously. It's very tiring for the paramedics. In order to treat people who get an electric shock, they should persevere until the medical staff come to treat them. As long as the manual treatment is carried out correctly and timely, the possibility of being rescued by electrocution and suspended animation is very great.