What is the function of an RCD and what is a typical domestic trip current?

• A. It detects imbalance between live and neutral and trips when leakage exceeds its rating; typical domestic trip current is 30 mA.
• B. It provides overcurrent protection for short circuits.
• C. It regulates voltage to appliances.
• D. It protects against voltage transients.

Answer: (A)

An RCD’s job is to monitor the current in the live and neutral conductors and trip the circuit if there is any leakage to earth. This imbalance means some current is escaping the intended path, which could flow through a person and cause an electric shock. When leakage exceeds the device’s rating, the RCD quickly disconnects the circuit to reduce the risk. A typical domestic RCD rating is around 30 mA, chosen to provide fast protection for people while avoiding nuisance trips from normal small leakage currents. The other functions listed—overcurrent protection for short circuits, voltage regulation, and protection against voltage transients—are handled by other devices (like fuses/MCBs, voltage regulators, or surge protectors), not by an RCD.