How does an RCD work?

• A. When the incoming current equals the outgoing current
• B. When the current in does not equal the current out
• C. It never trips
• D. It trips on overvoltage

Answer: (B)

An RCD protects by keeping a watch on the balance between the current leaving on the live conductor and the current returning on the neutral conductor. When things are normal, those two currents are the same. If some current leaks away, typically through a person or damaged insulation to earth, the returning current is less than the leaving current, creating an imbalance. The RCD senses this difference and quickly trips, disconnecting the circuit to prevent electric shock. It reacts to leakage levels designed for safety (often around 30 mA in domestic settings) and is not triggered by overvoltage. So the correct idea is that it trips when the current in does not equal the current out.