What type of current does an RCD measure to detect earth leakage?

An RCD (Residual Current Device) measures alternating current (AC) to detect earth leakage. It is designed to protect against electric shock and electrical fires by monitoring the balance of current flowing through the live (phase) and neutral wires in an electrical circuit. Under normal operating conditions, the current flowing into an appliance through the live wire should equal the current returning through the neutral wire.

If there is a fault, such as a person touching a live part of the circuit or insulation failure, some of the current will flow through the ground instead of returning through the neutral wire. The RCD detects this imbalance and trips the circuit, cutting off the power supply to prevent harm.

This functionality primarily applies to AC systems, as RCDs are typically used in residential and commercial environments where AC power is standard. While there are specialized devices to detect DC leakage current, conventional RCDs are not designed to function with DC currents. Therefore, focusing specifically on AC currents is critical for understanding how RCDs operate in real-world applications.