Inverter's incoming power supply can not use leakage protector - News - Global IC Trade Starts Here Free

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When using a frequency inverter, it's crucial to avoid installing a residual current device (RCD) or leakage protector. This is a fundamental principle that many engineers and technicians follow. The reason is simple: the inverter itself behaves like an adjustable RCD, and adding another one can cause conflicts and system failures. Some users mistakenly believe they need to install a specific RCD for their inverter. However, this often leads to unexpected tripping, where both the inverter and the RCD activate simultaneously, causing the entire system to shut down. Why does this happen? One key factor is the use of a four-wire system with the inverter. With four wires, it's easier for the RCD to detect imbalances. The basic principle of RCDs is that they monitor the difference between the incoming and outgoing currents. In a three-phase system, the currents are usually unbalanced. If the neutral wire (zero line) isn't connected to any equipment, the imbalance might not result in actual current flow through the neutral. However, the RCD may still trip due to this imbalance. The correct wiring setup involves ensuring the inverter’s metal casing and its terminal connections are properly grounded. If a four-wire RCD is used, the inverter’s housing should be insulated from the ground and connected to the RCD’s zero line output terminal. But this practice violates the safety principle of repeated grounding, which is essential for electrical safety. The main solution is to never install an RCD inside the inverter cabinet. Instead, if an RCD is necessary, it should be placed at the main power input, with a setting of over 300mA. This is because, after repeated grounding, the leakage current on the neutral line will be shunted, making the RCD less likely to trip. However, the inverter may still leak current through electromagnetic interference or parasitic capacitance, which can exceed normal levels. The core issue lies in the zero-line connection. Many people misunderstand how grounding works in inverter systems. Even with proper shielding, if grounding is missing, the inverter’s metal casing could develop a potential difference relative to the ground. Repeated grounding is the safest approach, but it also causes some leakage current to be reflected back to the RCD, leading to false trips. Another important point is that the inverter outputs a high-frequency PWM wave, which mimics AC but introduces high-order harmonics. These harmonics can create large and unstable leakage currents. If an RCD is used, its threshold must be set higher—typically 2–3 times the normal leakage current. Determining the exact value requires practical experience and testing, adjusting from a higher setting until the first trip occurs. Siemens systems, for example, strictly prohibit the installation of RCDs in the incoming distribution system. If a short circuit occurs, the system initiates self-protection. Their design emphasizes proper grounding of all equipment housings, ensuring equipotential bonding across all conductive parts. This ensures the overall safety and stability of the system.