What is a residual current device (RCD/GFCI) designed to do?

• A. A device that increases current
• B. A device that detects current imbalance between live and neutral and trips to prevent electric shock
• C. A device that measures voltage drop
• D. A fuse

Answer: (B)

RCDs/GFCIs are protective devices that guard people from electric shock by sensing leakage of current and quickly disconnecting power. They continuously compare the current flowing in the live conductor with the current returning in the neutral conductor. Under normal use these currents are equal. If some current leaks to earth—such as through a person touching exposed conductors or through a fault—the two currents become unbalanced. When the imbalance reaches a small threshold (often around 30 milliamps), the device trips in milliseconds, isolating the circuit.

This function is what makes them effective at preventing electric shock and reducing the risk of fire from ground faults. They’re not simply measuring voltage or increasing current, and they aren’t a fuse that melts under overcurrent; their job is to detect leakage and cut power quickly.