What is the purpose of a residual current device and what is the typical domestic rating?

• A. To detect earth leakage by current imbalance and trip the circuit; commonly 30 mA
• B. To limit short-circuit current to protect cables; 300 mA
• C. To provide surge protection for mains; 1 kA
• D. To monitor voltage levels and warn of imbalance; 1000 mV

Answer: (A)

The key idea is protection from electric shock. A residual current device (RCD) keeps an eye on the current in the live and neutral conductors and assumes everything is balanced. If some current leaks to earth (for example through a person who touches a faulty appliance), the balance is upset. The RCD detects that imbalance and quickly disconnects the circuit, reducing the risk of serious shock. In homes, the usual protective rating is about 30 milliamps—the leakage level at which it trips—because this is low enough to protect people but high enough to avoid constant nuisance trips from normal appliance use. Other devices handle different faults: short-circuit protection with fuses or MCBs, surge protection devices guard against voltage spikes, and voltage-monitoring devices alert you to abnormal supply levels.