Currently, low-voltage residual current protectors commonly used in China are categorized based on the characteristics of electrical signals that trigger their operation. These can be classified into voltage-operated types, current-operated types, and AC pulse types. In this article, we will focus on the AC pulse type protector, which is often used as a primary protection switchboard, and analyze its common external faults. The "four diagnosis" method—ask, smell, see, and test—is an effective way to identify and troubleshoot issues with these devices. 1. Ask: Begin by asking the electrician about the cause, process, and current status of the fault. This helps narrow down the possible scope or device affected by the issue. 2. Smell: Check for any signs of overheating by smelling the inside of the protector or around the contactor coil. A burnt odor may indicate a problem. 3. See: Examine the installation of the protector, the wiring, and whether there are visible damages. Ensure that the neutral point of the distribution transformer is properly connected. 4. Test: Use a multimeter to measure the input and output voltage of the protector. Check if the zero-sequence transformer is disconnected and measure the grounding resistance of the neutral point. 2. External Fault Analysis 2.1 Determine if the fault is internal or external using the four diagnostic methods. First, isolate all lines and electrical equipment after the protector and test it separately. After power is restored, press the test button several times to check if the indicator light functions correctly and if the relay makes a “click†sound. If everything works as expected, the fault is likely external. 2.2 Common human-related and installation errors: 2.2.1 Incorrect wiring of the three-phase load: The correct wiring should pass both the phase and neutral lines through the zero-sequence current transformer. Improper wiring can lead to malfunctions such as no grounding of the transformer’s neutral point, incorrect routing of the neutral line, or interference from nearby electromagnetic sources like contactors. 2.2.2 Wiring on the neutral grounding wire: Proper wiring ensures the neutral line passes through the zero-sequence transformer. However, if the neutral line has poor insulation or repeated grounding, it may cause false tripping or prevent the protector from operating correctly. 3. Troubleshooting Line and Electrical Equipment Issues 3.1 Common line and equipment failures include poor insulation leading to excessive leakage current, unbalanced phase insulation causing sensitivity issues, and poor neutral insulation or grounding affecting the protector's performance. 3.2 To address these issues, use a 500V megger to test the insulation of the low-voltage lines. If the insulation is low or damaged, locate and repair weak points, balance the leakage current across phases, and trim branches to ensure proper spacing between wires. Regular maintenance is essential to maintain safe and reliable operation of the protector.
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