What is an RCD?

An RCD (residual current device) constantly compares the current flowing out through the live conductor with the current returning through the neutral. In a healthy circuit the two are equal. If some current is missing — because it is leaking through damaged insulation, a faulty appliance or a human body — the RCD disconnects the circuit within milliseconds.

What it protects against

Electric shock: a 30 mA RCD trips far below the current level that stops a heart.
Fire: long-term leakage through damaged insulation heats up and can ignite.

What it does NOT do

An RCD does not protect against overload or short circuit — that is the job of a circuit breaker (MCB ratings and curves). This is why panels pair RCDs with MCBs, or use combined devices (RCBOs).