Two major principles for engineers when facing leakage protection switches

The leakage protection device is a grounding protection device used to prevent accidents caused by electric shock and leakage. When the leakage current of the circuit or electrical equipment is greater than the setting value of the device, or when people or animals are in danger of electric shock, it can act quickly to cut off the accident power supply, avoid the expansion of the accident, and ensure the safety of people and equipment. Therefore, the correct selection and maintenance management of leakage protection switches are the main technical and management measures for safe electricity use in rural areas. 1. Correct selection of leakage protection devices The selection of leakage protection devices should be determined based on factors such as the system's protection mode, purpose of use, installation location, voltage level, leakage current of the controlled circuit, and grounding resistance value of electrical equipment. 1. Select according to the purpose of use The leakage protection device used to prevent personal electric shock accidents is generally selected based on two different requirements: direct contact protection and indirect contact protection, and there should also be a difference in the selection of action characteristics. (1) Direct contact protection is to prevent electric shock casualties caused by direct human contact with live conductors of electrical equipment. When the human body is in direct contact with live conductors, before the leakage protection device is activated to cut off the power supply, the electric shock current passing through the human body has nothing to do with the selection of the action current of the leakage protection device. It is completely determined by the voltage of the human body and the human body resistance. The leakage protection device cannot limit the electric shock current passing through the human body. Therefore, the leakage protection device used for direct contact protection must have a fast action performance of less than 0.1S, or have the inverse time characteristic specified in the IEC leakage protection device standard. (2) Indirect contact protection is to prevent the electrical equipment from presenting dangerous contact voltage on exposed metal parts such as metal casings when insulation damage occurs. The selection of the action current I△n of the leakage protection switch should be considered in conjunction with the grounding resistance R of the electrical equipment and the allowable contact voltage U. The contact voltage U on the electrical equipment should be less than the specified value. Selection of the action current I△n of the leakage protector: I△n≤U/R Where: U——permissible contact voltage R——contact resistance of the equipment Generally, for fixed electrical equipment with a rated voltage of 220V or 380V, such as water pumps, mills and other electrical equipment that are easily in contact with the human body, when the grounding resistance of the metal shell of the electrical equipment is below 500Ω, a leakage protection device with a 30-50mA action within 0.1s can be selected; when the grounding resistance of the metal shell of the electrical equipment is below 100Ω, a leakage protection device with a 200-500mA action can be selected; for more important electrical equipment, in order to reduce instantaneous power outages, a delayed protection device with an action current of 0.2s can also be selected. Household electrical equipment often has plugs that are frequently plugged in and out, and at the same time, some residential buildings do not consider grounding protection facilities. When insulation faults such as leakage and shell contact occur in electrical equipment, the housing of the equipment may present a dangerous voltage equal to the working voltage, which is very likely to cause electric shock casualties. Therefore, the electrical equipment installation regulations stipulate that a high-sensitivity leakage protection switch with an operating current of 30mA and an action within 0.1S must be installed behind the electric energy meter of the household incoming line. 2. Choose according to the place of use. Generally, in a 380/220V low-voltage line, if the metal shell and other metal parts of the electrical equipment are easily touched by people, and at the same time, these electrical equipment cannot be grounded to less than 4Ω or 10Ω according to the requirements of China's electricity regulations, it is advisable to install leakage protection devices in the power supply circuit of the electrical equipment in accordance with the requirements of indirect contact protection. At the same time, leakage switches with different operating currents should be reasonably selected according to different places of use. For example: In a humid workplace, the human body is more likely to sweat or get wet, which reduces the insulation performance of the skin and significantly reduces the resistance of the human body. When an electric shock accident occurs, the current passing through the human body will inevitably be greater than that in a dry place, which is highly dangerous. Therefore, it is appropriate to install a leakage protection device with a current of 15 to 30 mA and that can operate within 0.1S. 3. Select according to the normal leakage current of the circuit and electrical equipment (1) The lower the operating current of the leakage protection device is selected, the higher the sensitivity of the switch can be. However, the insulation resistance of any power supply circuit and electrical equipment cannot be infinite, and there will always be a certain amount of leakage current. Therefore, in terms of ensuring the stable operation of the circuit and providing uninterrupted power supply, the selection of the operating current of the leakage protection device is subject to the constraints of the normal leakage current of the circuit. (2) Since the leakage current of the measuring current must be measured by a more complex test method or by the use of special test equipment, for the convenience of selection, you can refer to the following empirical formula: For lighting circuits and single-phase circuits for residential electricity: I△n ≥ IH/2000 For three-phase three-wire or three-phase four-wire power lines and mixed power and lighting lines: I△n ≥ IH/1000 Where: I△n - the operating current of the leakage protection switch device IH - the actual maximum power supply current of the circuit For general household power supply circuits, if users use 3A electricity meters, under normal circumstances, the leakage current of each household is about 1mA. In principle, when the leakage current in a household single-phase circuit exceeds 1/3000 of the circuit's maximum power supply current, the circuit should be repaired.

(3)我国农村低压电网的绝缘水平较低，泄漏电流较大。根据实测结果表明，泄漏电流的数值和配电变压器容量的大小关系不显著，但和低压电网中生活用电的居民户数有明显的关系，也就是不管变压器容量是多少，其中供给生活用电的户数越多，泄漏电流就越大。因此，农村电网中装置漏电开关时，应考虑到这点。一般而言，为了保护电网可靠运行，保证多级保护的选择性，下一级漏电保护动作电流应小于上一级漏电保护动作电流，各级漏电动作电流应有1.2～2.5倍的级差。第一级漏电保护装置应安装在配电变压器低压侧主干线出线端，该级保护的线路较长，叠加的泄漏电流较大。其漏电动作电流在未完善多级保护时，最大不得超过100mA,在完善多级保护时，其漏电动作电流最大不得超过300mA。第二级漏电保护装置应安装在各分支线的出线端，由于被保护线路较短，泄漏电流相对较小，其漏电动作电流应介于上、下级保护的漏电动作电流之间，一般取30～75mA。第三级漏电保护装置(又称末级保护)用于保护用电设备及人身安全，被保护线路短，泄漏电流小，一般不超过10mA，漏电动作电流应按人体触电摆脱电流值(10～20mA)选择，不应大于30mA，一般取15～30mA。

2. Management of leakage

protection switch after it is put into operation After the leakage protection switch is put into operation, effective management must be carried out to ensure that the leakage protection maintains a good operating state and truly plays a protective role. The management work mainly includes the following aspects:

1. After the leakage protection switch is put into operation, the operation record should be consciously established and the corresponding management system should be improved;

2. After the leakage protection switch is put into operation, the test button should be pressed once or twice a month in the power-on state to check whether the leakage protection switch is operating normally and reliably, especially in the thunderstorm season, the number of tests should be increased;

3. Regularly analyze the operation of the leakage protection switch and replace the faulty leakage protection switch in time;

4. The maintenance of the leakage protection switch should be carried out by professionals. If there are any abnormal phenomena during operation, electricians should be asked to deal with them to avoid expanding the scope of the accident;

5. After the leakage protection switch is activated by thunderstorms or other unknown reasons, an inspection and analysis should be carried out;

6. After the leakage protection switch is activated, if no cause of the accident is found after inspection, it is allowed to try to close the switch once. If it is activated again, the cause should be found out and the fault should be found out. If necessary, the action characteristics should be tested. Continuous forced power supply is not allowed. Unless it is confirmed that the leakage protection switch itself is faulty after inspection, it is strictly forbidden to remove the leakage protection switch privately and force power supply;

7. Before the leakage protection switch that has been out of operation is used again, the action characteristic test should be carried out according to the items specified by the relevant departments;

8. The action characteristics of the leakage protection switch are set by the manufacturer and used according to the product manual. It shall not be changed at will during use;

9. After an accidental electric shock casualty accident occurs within the protection range of the leakage protection switch, the action of the leakage protection switch should be checked, and the reasons for the failure to play a protective role should be analyzed. The site should be protected before investigation, and the leakage protection switch shall not be dismantled;

10. In order to check the action characteristics and changes of the leakage protection switch during operation, the action characteristic test should be carried out regularly. The characteristic test items include: testing the leakage action current value, testing the leakage non-action current value, and testing the breaking time; 11. When the leakage protection switch is tested for the action characteristics, a special test instrument qualified by the relevant national departments should be used. It is strictly forbidden to use the test method of directly touching the grounding device with the phase line; 12. In addition to regular tests according to the leakage protection characteristics, the circuit breaker part of the leakage protection switch should be regularly inspected and maintained according to the relevant requirements of low-voltage electrical appliances.