Leakage protector rated leakage current selection - Database & Sql Blog Articles

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Correctly selecting the rated leakage current of a residual current device (RCD) is essential for both safety and system reliability. On one hand, the RCD should respond quickly when an electric shock or excessive leakage current occurs, ensuring protection. On the other hand, it must not trip under normal operating conditions to avoid unnecessary power outages and disruptions.

The rated leakage current of an RCD must meet three key criteria:

1. To ensure personal safety, the rated leakage current should not exceed the safe current threshold for the human body. Internationally, 30 mA is generally considered the maximum safe current level that can be tolerated without causing harm.
2. To guarantee reliable operation of the electrical network, the RCD’s rated leakage current should be higher than the normal leakage current in the low-voltage grid. This prevents false tripping due to minor, harmless currents.
3. To achieve selective protection across multiple levels, the rated leakage current of the downstream RCD should be less than that of the upstream one. A step difference of 1.5 to 2 times between stages is typically recommended to ensure proper coordination.

The first-level RCD is usually installed at the output of the low-voltage side of a distribution transformer. It protects long lines with higher leakage currents. In systems without a multi-level protection setup, its rated leakage current should not exceed 100 mA. In well-designed multi-stage systems, it may be set lower—75 mA during dry seasons and up to 200 mA during rainy periods. For grids with larger leakage currents, it might be set at 100 mA in dry weather and 300 mA in wet weather.

The second-level RCD is placed at the exit of branch circuits. These lines are shorter and have smaller loads, resulting in lower leakage currents. The rated leakage current here should be between the upper and lower stages, typically ranging from 30 to 75 mA.

The third-level RCD is used to protect individual or multiple devices directly, offering the highest level of personal protection. Since the load is small and the leakage current is minimal, it usually does not exceed 10 mA. It is recommended to use an RCD with a rated operating current of 30 mA and a response time of less than 0.1 seconds for optimal safety.