1. What is a leakage protector?
A: The leakage protector (leakage protection switch) is an electrical safety device. The leakage protector is installed in the low-voltage circuit. When leakage and electric shock occur, and the operating current value defined by the protector is reached, the power supply is automatically disconnected for protection within a limited time.
2. What is the structural composition of the leakage protector?
Answer: The leakage protector is mainly composed of three parts: the detection component, the intermediate amplification section, and the operation actuator. 1 detection component. It consists of a zero-sequence transformer that detects the leakage current and sends a signal. 2 zoom in. The weak leakage signal is amplified, and the electromagnetic protector phase electronic protector is formed according to different devices (the amplifying component can be mechanical or electronic). 3 executive agency. After receiving the signal, the main switch is switched from the closed position to the open position, thereby cutting off the power supply, which is the tripping component of the protected circuit off the grid.
3. What is the working principle of the leakage protector?
Answer: 1 When electrical equipment leaks, two abnormal phenomena occur:
First, the balance of the three-phase current is destroyed, and zero-sequence current occurs;
Second, the normal-time uncharged metal casing appears to the ground voltage (normally, the metal casing and the earth are both zero potential).
2 Zero-sequence current transformer function The leakage protector obtains an abnormal signal through the current transformer detection, and transmits and transmits through the intermediate mechanism to make the actuator act and disconnect the power through the switch device. The current transformer is similar in construction to a transformer and consists of two coils that are insulated from each other around the same core. When the primary coil has residual current, the secondary coil induces a current.
3 leakage protector working principle The leakage protector is installed in the line, the primary coil is connected with the line of the power grid, and the secondary coil is connected with the trip unit in the leakage protector. When the electrical equipment is operating normally, the current in the line is in equilibrium, and the sum of the current vectors in the transformer is zero (the current is a vector with direction, such as "+" in the direction of the outflow and "-" in the return direction. The currents flowing back and forth in the transformer are equal in magnitude and opposite in direction, and the positive and negative offsets each other). Since there is no residual current in the primary coil, the secondary coil is not induced, and the switching device of the leakage protector is in a closed state. When the equipment casing leaks and someone touches it in time, the shunt is generated at the fault point. The leakage current flows through the earth ground of the human body and returns to the neutral point of the transformer (without the current transformer), causing the transformer to flow in and out. The current is unbalanced (the sum of the current vectors is not zero) and the primary coil is applied to generate residual current. Therefore, the secondary coil is sensed. When the current value reaches the operating current value defined by the leakage protector, the automatic switch trips to cut off the power.
4. What are the main technical parameters of the leakage protector?
Answer: The main operational performance parameters are: rated leakage current, rated leakage operation time, rated leakage non-operating current. Other parameters are: power frequency, rated voltage, rated current, etc. 1Rated leakage current The current value of the leakage protector under specified conditions. For example, a 30 mA protector, when the current value reaches 30 mA, the protector is disconnected from the power supply. 2 The rated leakage operation time is the time from the sudden application of the rated leakage current to the time when the protection circuit is cut off. For example, a protector of 30 mA x 0.1 s does not exceed 0.1 s from the current value of 30 mA until the main contact is separated. 3 rated leakage current does not work under the specified conditions, the current value of the leakage protector does not work, generally should choose one-half of the leakage current value. For example, a leakage protector with a leakage current of 30 mA should not operate when the current value reaches 15 mA or less. Otherwise, the sensitivity is too high and it is easy to malfunction, which affects the normal operation of the electrical equipment. 4 Other parameters such as: power frequency, rated voltage, rated current, etc., when using the leakage protector, should be compatible with the line and electrical equipment used. The working voltage of the leakage protector should be adapted to the rated voltage of the normal fluctuation range of the power grid. If the fluctuation is too large, it will affect the normal operation of the protector, especially for electronic products. When the power supply voltage is lower than the rated working voltage of the protector, it will refuse to operate. The rated working current of the leakage protector should also be consistent with the actual current in the circuit. If the actual working current is greater than the rated current of the protector, it will cause overload and malfunction of the protector.
5. What is the main protective effect of the leakage protector?
Answer: The leakage protector mainly provides indirect contact protection. Under certain conditions, it can also be used as supplementary protection for direct contact to protect against potentially fatal electric shock accidents.
6. What is direct contact and indirect contact protection?
Answer: When the human body comes into contact with a charged body and has a current through the human body, it is called an electric shock. According to the cause of human body electric shock, it can be divided into direct contact electricity and electric contact electricity. Direct electric shock refers to the electric shock caused by the direct contact of the human body and the charged body (such as touching the phase line). Inter-contact electricity refers to the electric shock caused by the human body touching the metal conductor that is not charged under normal conditions and charged under the fault condition (such as the outer casing of the leakage device). Depending on the cause of the electric shock, the anti-shock measures taken against the electric shock are also divided into: direct contact protection phase indirect contact protection. Direct contact protection generally adopts measures such as insulation, protective cover, fence, and safety distance; indirect contact protection generally adopts measures such as protective grounding (zero connection), protection cutoff, and leakage protector.
7. What is the danger of human body shock?
Answer: When the human body is exposed to electricity, the greater the current flowing into the human body, the longer the phase current lasts and the more dangerous it is. The degree of danger can be roughly divided into three stages: perception - get rid of - ventricular fibrillation. 1 perception stage. Since the current is small, the human body can feel (generally greater than 0.5 mA), which does not pose a hazard to humans; Refers to the maximum current value that people can get rid of when the electrode is electrically touched (generally greater than 10 mA). Although this current is dangerous, it can be rid of itself, so it is basically not fatal. When the current is increased to a certain extent, the electric shock will be contracted by the muscles, causing the sputum to cause the gripping of the charged body, and cannot get rid of it by itself. 3 ventricular fibrillation stage. Increasing the current and increasing the time of the electric shock (generally greater than 50 mA and ls) will cause ventricular fibrillation, which will result in death if the power is not immediately disconnected. It can be seen that ventricular fibrillation is the most important cause of death from human electrocution. Therefore, the protection of humans is not commonly used to cause ventricular fibrillation, as a basis for determining the characteristics of electric shock protection.
8. What is the safety of "30mA·s"?
A: Through a large number of animal experiments and studies, it has been shown that ventricular fibrillation is related not only to the current (I) through the human body, but also to the duration (t) of the current in the human body, that is, the safety power through the human body Q=I× t to determine, generally 50mA·s. That is to say, when the current is not more than 50 mA and the current duration is within ls, ventricular fibrillation generally does not occur. However, if controlled according to 50 mA·s, there is still a risk of ventricular fibrillation when the energization time is short and the current is large (for example, 500 mA × 0.1 s). Although less than 50 mA·s will not cause the consequences of electric shock, but it will also cause the electric shock to lose consciousness or secondary injury. Practice has proved that using 30 mA·s as the operating characteristic of the electric shock protection device is suitable from the safety and manufacturing aspects of use, and has a safety ratio of 1.67 times compared with 50 mA·s (K=50/30). =1.67). From the safety limit of "30mA·s", it can be seen that even if the current reaches 100mA, as long as the leakage protector operates within 0.3s and cuts off the power supply, the human body will not cause a fatal danger. Therefore, the limit of 30 mA·s has also become the basis for selection of leakage protector products.
9. Which electrical equipment needs to install a leakage protector?
Answer: The "Safety Technical Specifications for Temporary Electricity Use at Construction Sites" stipulates that "all electrical equipment on the construction site, except for protection and zero connection, must be provided with leakage protection devices at the head end of the equipment load line." Aspects: 1 All electrical equipment on the construction site must be equipped with a leakage protector. Because of the open construction of the building, the humid environment, the changing personnel, and the weak management of the equipment, the danger of electricity consumption is high, requiring all power equipment including power and lighting equipment, mobile and stationary equipment. Of course, equipment that is powered by a safe voltage supply and an isolation transformer is not included. 2 The original protection zero (grounding) measures according to the regulations are still unchanged as required. This is the most basic technical measure for safe use of electricity and cannot be removed. 3 The leakage protector is installed at the head end of the load line of the electrical equipment. The purpose of this is to protect the load line while protecting the electrical equipment to prevent electric shock accidents caused by line insulation damage.
10. Why is the leakage protector added after the protection is connected to zero (ground)?
A: Regardless of whether the protection is connected to zero or grounded, the scope of protection is limited. For example, “protection to zero†means connecting the metal casing of the electrical equipment to the neutral line of the power grid and installing a fuse on the power supply side. When a fault occurs in the electrical equipment (a phase touches the outer casing), a single-phase short circuit of the relatively neutral line is formed. Since the short-circuit current is large, the fuse is quickly blown and the power supply is disconnected for protection. The working principle is to change the "cracking fault" to "single-phase short-circuit fault", so as to obtain a large short-circuit current cut-off insurance. However, electrical faults on the construction site are not frequent. Frequently, leakage faults, such as equipment moisture, excessive load, long lines, and aging insulation, are small. These leakage currents are small and cannot be quickly cut off. Therefore, the fault does not automatically disappear and exists for a long time. However, this leakage current poses a serious threat to personal safety. Therefore, it is necessary to install a more sensitive leakage protector for supplementary protection.
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