What are the three factors that cause electric shock accidents?

According to the statistics of accidents in recent years, there are three kinds of common electric shock accidents in water environment: electric shock accidents of electric water heaters in bathrooms; Electric leakage and electric shock accidents of electrical equipment in water; Accident of electric power line falling into water, leakage and electric shock.

1. Analysis of accident characteristics and causes

(1) Characteristics of electric shock accident in water environment

1. Because the accident occurred in water environment, it is not easy for people to escape after electric shock.

2. Generally, people are not in direct contact with electrical equipment and conductors, but get an electric shock through the conductive effect of water, which is difficult to detect and prevent beforehand.

3. People who come to the rescue afterwards often think that drowning, fainting and other accidents have occurred, but they go directly into the water for first aid, which is very likely to cause secondary electric shock accidents.

In fact, after electric shock, the degree of injury is not directly related to voltage, but depends on five aspects: the magnitude of current passing through human body, the duration of current, the way of current passing through human body, human body condition and the frequency of current.

studies have proved that if the current passing through the human body reaches or exceeds the free current (generally 16 mA for men and 1 mA for women), the human body will not be able to get rid of the charged body on its own, and it will feel extremely painful and unbearable. If the time is too long, it may be coma, suffocation and even death. The current of 3 mA is a dangerous current. When the current passing through the human body reaches or exceeds 3 mA, it will make people's heart beat irregularly, coma, blood pressure rise and severe spasm in a few seconds to several minutes, which may cause abnormal heart beat and cause death. 5 mA is the lower limit of ventricular fibrillation current (or lethal current), that is, the minimum current that causes ventricular fibrillation through human body. When the current of 5 mA passes through human body for 1s, ventricular fibrillation may occur and people may die.

according to ohm's law, it = u/Rb, under the same voltage condition, in the environment where the skin is immersed in water, the smaller the human body resistance, the greater the current passing through the human body. In addition, after research, the frequency of 3 ~ 3 Hz alternating current is the most harmful interval to human body, and the power frequency alternating current of 5 Hz is just in the interval, so the power frequency current of 5 Hz is very harmful to human body. At the same time, because the electric shock current in water often passes through the whole body and flows through the heart, the electric shock accident in the water environment is particularly serious.

(II) Analysis of the cause of the accident

1. Electric shock accident of the bathroom electric water heater

There are three reasons for the electric shock accident of the bathroom electric water heater:

First, it comes from the water heater itself. The electric heating tube in the water tank of the water heater causes fatigue on the metal surface, and when it reaches its limit, it will break, leading to the connection between the electric heating wire and water. The voltage of 22V directly passes through the water flow and flows through the human body to form a passage, and a large amount of current. Second, it comes from the water supply pipeline, especially the old building built in the early stage. The water supply pipeline is made of metal pipe. When the old building is rebuilt or the ground wire is pulled indiscriminately or the pipe network is not maintained properly, the water pipe is often electrified, which also causes electric shock accidents in the water of the electric water heater. Third, the ground wire itself is electrified, and most of the water heaters themselves have good ground wires, so the elimination of leakage faults depends on good grounding. However, when the ground wire is electrified, the water heaters suffer from it, which also causes electric shock.

2. Electric leakage, electric shock and drowning accident of electrical equipment in water

The electrical equipment in water that occurred in rural electric shock accidents mainly included submersible pumps, water pumps, fish pond oxygen pumps, oxygen machines, etc. The underwater lighting equipment in the city is mainly the park. Due to the quality problems or long-term use of these equipment, the equipment is corroded, the water seal condition of electrical parts is damaged and the insulation of electrical lines is damaged, and most of these electrical equipment are not equipped with leakage protection devices or the installation is not standardized, which eventually leads to electric shock accidents. At the same time, the probability of death is much higher than that of ordinary electric shock accidents because the human body can't act autonomously due to spasm after electric shock in water.

3. Electric shock accident of power line falling into water

The main causes of electric shock accident of power line falling into water are: first, the support of power line tower does not meet the requirements of line erection specifications, resulting in tower collapse and line falling into water in typhoon or other bad weather, which is more common in rural areas; Second, the line insulation is poor or damaged, which is mostly caused by private connection and random connection when electricity is used in construction sites or rural areas; Third, in sudden floods or when the water level in pits and basements exceeds the normal state, some normal electrical equipment, electrical lines and sockets are submerged, resulting in leakage.

II. Preventive measures

To prevent the above-mentioned electric shock accidents in water environment, the key is to do practical and effective preventive work in both technology and management.

(1) Technical measures

In terms of electricity safety technology, insulation, safe spacing, leakage protection, safe voltage, fences and barriers are all protective measures to prevent direct electric shock; Protective grounding and protective zero connection are the most basic measures in indirect electric shock protection measures. The so-called indirect electric shock protection measures refer to the technical measures to prevent all parts of the human body from touching the metal parts of electrical appliances that are not charged under normal circumstances but become charged under fault conditions. Limited by space, this paper mainly explains the correct selection of leakage protection devices for electrical equipment in water environment.

functionally, the leakage protection (residual current protection) device is mainly used to prevent indirect contact electric shock and direct contact electric shock, as well as to prevent leakage fire and monitor a ground fault. Commonly used current leakage protection devices take leakage current or electric shock current as action signals. After processing, the action signal drives the actuator to act, which makes the line break quickly.

the operating current of current-type earth leakage protection device is divided into 15 grades, such as .6, .1, .15, O.3, .5, .75, .l, .2, .3, .5, 1, 3, 5, 1 and 2A, among which 3mA. Above 1mA is low sensitivity, which is used to prevent leakage fire and monitor a ground fault. In order to avoid misoperation, the rated non-action current of the protection device shall not be less than 1/2 of the rated action current.

according to the specification requirements, I-type mobile electrical equipment and hand-held power tools with metal casings, electrical equipment installed in harsh places such as damp or strong corrosion, construction electrical equipment in construction sites, temporary electrical equipment, plug-in seats in guest rooms of hotels, and underwater lighting equipment in civil buildings with high electric shock risk, swimming pools or baths. Power supply lines and electrical equipment installed in water shall be equipped with leakage protection devices.

many factors should be considered in the selection of leakage protection devices. In bathrooms, swimming pools, tunnels and other places with great electric shock risk, high-sensitivity leakage protection devices should be selected. According to IEC4.79 (the effect of current passing through human body), when the AC current passing through human body is less than 3mA, human body will not die due to ventricular fibrillation, which is not directly related to the humidity and contact voltage of human body. Therefore, the international electrotechnical standard stipulates that leakage protection devices with operating current not greater than 3mA are used in all provisions for preventing personal electric shock. Therefore, leakage protection devices with action current of 3mA can be installed in hospital operating rooms, bathrooms and other places with high risk of electric shock to prevent personal electric shock.

leakage protection devices with higher sensitivity (for example, 1mA) are used in some construction sites, families and rural areas. In fact, the leakage protection devices of 1mA and 3mA have the same effect in preventing electric shock, which can prevent people from dying of ventricular fibrillation. Because 1mA leakage protection device is expensive, it is not suitable to be widely used, and its rated non-operation current is only 5mA. Rural low-voltage power grid equipment is often in outdoor and humid places, so the normal leakage current is large, which is easy to cause misoperation. The consequence of frequent misoperation and power failure is often that the leakage protector is shorted or removed, which makes the line lose the protection of grounding fault and leads to dangerous consequences.

leakage protection devices in operation should be inspected and tested regularly. All parts of the protector shell, its upper parts and connecting terminals shall be kept clean and intact; Bakelite shell should not be deformed or discolored; There should be no cracks and burn marks; The name (or trademark), model, rated voltage, rated current and rated operating current of the manufacturer shall be clearly marked, and shall be consistent with the conditions and requirements of the running line. The protection grade of the protector shell should be suitable for the environmental conditions of the place of use. The terminal should not be loose; The connection parts shall not change color. There should be no obvious corrosion on the terminal. There should be no noise when the protector works. The operating handle of the leakage protection switch should be flexible and reliable, and its reliability should be tested regularly with the test button during use.

(II) Management measures

In addition to the corresponding technical measures, the management of water environment electricity consumption is also essential. According to the common types of electric shock, it is suggested to adopt the following management measures and master the necessary common sense of prevention.

1. government departments and electric power management departments should strengthen supervision, strengthen guidance on electricity use and regulate citizens' electricity use. In order to strengthen the safety management of rural power supply and distribution lines, electrical equipment near ponds and reservoirs should be equipped with qualified leakage protection devices and necessary insulation measures. Put an end to private connection and random connection of lines. Do a good job in the management of the "three-line crossing" line.

2. When professional electricians are working on all or part of the electrical equipment with power failure, they must complete the power failure, electricity inspection, installation of grounding wire, hanging signs and installation of barrier before they can start working.

3. For activities in places with water, such as lakes, outdoor swimming pools, park waters, etc., attention should be paid to whether there are power supply lines, production workshops and the distance between lines and electrical equipment and the water surface nearby, so as not to get into the water easily. Especially in thunderstorm weather, don't swim in the open air.

4. Don't touch the trees near the power line. In some places, the trees and wires are parallel. As the trees grow taller year by year, some tree crowns surround the wires. When encountering thunderstorms and strong winds, the trees and wires collide and rub against each other, which will lead to short circuit and discharge.

5. Don't take shelter from the rain under overhead transformers and power line towers.

6. After the rainstorm, water is likely to accumulate on the pavement in some places, so it is best not to wade in the water at this time. If you have to wade through the water, you must always observe whether there are any wires broken in the stagnant water near the section you pass. Do not handle it yourself when you find that the supply line falls into the water. Mark the surrounding area immediately to remind other pedestrians not to approach, and notify the power supply department for emergency treatment in time.

7. If you are a non-professional, you must be careful when buying second-hand electrical appliances, and fully consider their safety performance.

8. It is very necessary to strengthen the publicity of electricity safety knowledge and electric shock first aid knowledge. If you know electric shock first aid knowledge, many tragedies may not happen.