A Brief Discussion on the Application of Residual Current Operated Protective Devices in Rural Power Grids

Introduction

Correctly understanding and treating the significance and role of residual current operated protective devices (hereinafter referred to as protectors), scientifically selecting, verifying, configuring, installing and using protectors, and strengthening and improving their maintenance and management will all have a multiplier effect on enhancing the safe use of electricity in rural power grids and preventing electric shock casualties.

Problems existing in the installation, use and management of protectors

(1) Poor product quality, abnormal operation (breaking), and failure to provide protection. For instance, some products have defects such as high failure rates, abnormal operations, and inability to determine their quality by using the test jump button. However, users and many grassroots rural electricians are relatively lacking in the ability to distinguish right from wrong. This not only harms the interests of users but also poses hidden dangers to the safe production of power and the safety of people's lives and property.

(2) It is indeed damaged during operation but not updated. For the damaged protectors during operation, if they are not replaced in time, they should be directly withdrawn.

(3) Improper calibration and test jump methods or failure to verify and repair. In some places, there is a lack of dedicated personnel to conduct professional verification and testing. Only unified batch calibration and inspection are carried out before installation. When put into operation, no test trip is conducted or only the test trip button is used for test trip inspection, without actual grounding tests. After being put into operation, no regular or irregular maintenance checks will be carried out. Once installed, it will no longer be managed.

(4) The installation location and configuration are unreasonable. If only the first-level protection on the outgoing line side of the power transformer and the third-level protection on the user side are installed, while the second-level protection on the main line and branch line sides is ignored, when a leakage or electric shock fault occurs on a certain branch line, it will cause a complete power outage in the entire distribution transformer area. For single-phase circuits, only protectors without overvoltage and overcurrent protection functions are installed, and no separate switch devices are set up. In this way, when the neutral point is severely displaced and the neutral line becomes electrified, and the neutral line breaks or the neutral line fuse melts (the neutral line is originally prohibited from installing fuses), the protector will not act, and a safety hazard will arise.

(5) The type of protector used is inappropriate. In the selection of household protectors, regardless of the actual function of the protector and the size of the rated operating current, and even without considering the specific environment of the installation site, they are installed and used at will. In the selection and configuration of protectors for the three-level installation of distribution transformers, branch lines and users, in addition to ignoring the above-mentioned factors, the step difference of the operating current and the step difference coordination of the operating time of the protectors are also ignored, and they are installed at will.

(6) Poor management and maintenance of the protector. Insufficient attention has been paid to the professional management of protectors, and no professional centralized management has been implemented for protectors. The power supply station has not established a complete management system for the verification and maintenance of protectors, and does not conduct regular tests on protectors. Failing to pay attention to improving the professional technical quality and management level of employees; There is a slackness in the management of protectors in terms of ideology. There is a lack of publicity and education on safe electricity usage knowledge. When the behavior of not installing or removing protectors is discovered, it is not stopped and corrected in time.

2 Countermeasures

(1) The product quality is substandard. Once substandard products are found to be in operation on the power grid, they should be replaced in a timely manner. At the same time, when placing an order, it is necessary to pay attention to choosing products that comply with the regulations and have passed the inspection tests.

(2) The grounding protection method of the low-voltage power grid is wrongly matched with the wiring of the protector. Due to the fact that electrical equipment may experience insulation aging, wear, overvoltage breakdown, etc. during operation, causing originally non-electrified parts to become electrified, safety measures such as protective grounding and protective neutral line connection are adopted in low-voltage power grids. However, if the connection methods of the protective grounding and the protective neutral line are not distinguished when installing the protector, once the wiring is incorrect, it is very likely to cause the protector to malfunction. In this regard, when installing protectors, it is necessary to strictly follow relevant standards such as "Installation and Operation of Residual Current Operated Protective Devices" and "Technical Regulations for Rural Low-Voltage Power".

(3) The grounding wire was disconnected, causing the protector to fail to operate. When the neutral grounding wire of the distribution transformer is disconnected, the grounding current does not flow back through the grounding point when the electrical appliance is grounded, causing the protector to fail to operate. In this regard, during the construction of the transformer area, the grounding wire must be installed strictly in accordance with relevant norms and standards. During normal operation, strengthen inspection management, regularly check the integrity of the grounding wire, test the grounding resistance value, and strictly prevent the grounding resistance from abnormally increasing when the grounding wire is disconnected.

(4) Unbalanced three-phase load causes the protector to vibrate frequently. The start-up of a single-phase motor is very likely to cause the protector to malfunction. The more single-phase motors there are on a certain phase, the more likely the protector on that phase is to malfunction. In this regard, efforts should be made to ensure that single-phase loads, especially single-phase motor loads, are evenly distributed across all phases.

(5) Incomplete protection configuration leads to false operation. According to the regulations, low-voltage power grids and users should be protected at different levels, namely, three levels of main protection, branch line protection and terminal protection, or two levels of main protection and terminal protection. However, often due to the lack of secondary protection or the low operation rate of tertiary protection, the main protector trips during peak electricity consumption periods. In this regard, it is necessary to improve the installation and operation of secondary and final-stage protectors. Pay attention to improving the insulation level of the lines; Press the test button every month to test and check whether the protector operates normally. During thunderstorm seasons and peak electricity consumption periods, the number of tests should be increased. Regularly check whether all the terminal blocks are firmly connected to ensure the normal operation of the protector.

(6) Inadequate personnel training and publicity. The skill level of the low-voltage operation and maintenance personnel in the power supply station directly determines the operation level of the protector. For the abnormal operation of protectors caused by limited personnel skills and low operation and maintenance levels, it is necessary to strengthen personnel training and improve the quality of operation and maintenance personnel. Meanwhile, front-line personnel should take every opportunity to inform users about the function and significance of the terminal protectors, as well as how to conduct tests, under what circumstances they can be self-recessed and put into operation, and when maintenance personnel from power supply enterprises need to handle them.

Introduction to Acrel ASJ Series Products

The ASJ series residual current operated relays and multi-loop residual current monitors from Acrel can be combined with low-voltage circuit breakers or low-voltage contactors to form a combined residual current protection device. They are mainly suitable for AC 50Hz, rated voltage 400V and below TT and TN system distribution lines, and are used to provide ground fault protection for electrical circuits. It can prevent equipment damage and electrical fire accidents caused by ground fault current, and also provide indirect contact protection against the danger of electric shock to people.