In a TT system, why is RCD protection often required?

• A. Because fault currents return through the local earth electrode and are detected only by the supply's main protective device.
• B. Because TT systems include an automatic disconnect on every fault regardless of earth path.
• C. Because TT systems do not require any protective devices if the circuit is new.
• D. Because the fault current path via the local earth electrode may not cause timely tripping without RCD protection.

Answer: (D)

In a TT system the consumer’s earthing is a separate local earth electrode, not the supply’s earth. That means any fault current to earth must travel through this local electrode and back to the source through the earth. The impedance of that path is usually high, so the fault current can be small and may not be enough to trip the main protective device quickly enough. An RCD detects any imbalance between live and neutral currents, which occurs when current leaks to earth (for example via a fault or a person), and it trips fast regardless of the earth fault current magnitude. This fast disconnection greatly reduces the risk of electric shock in TT systems.