How long should an RCD take to trip?

RCDs are designed to trip extremely quickly — fast enough to prevent a fatal electric shock. The standard requires a 30mA RCD to cut power within 300 milliseconds (0.3 seconds) when the exact fault current it's rated for flows. At higher fault currents it trips even faster. In practice, most modern RCDs trip in 20–100ms. You can't measure this yourself with a stopwatch — it requires a special electrical test instrument. An electrician will test this during an installation inspection (EICR) or when commissioning a new installation. If your RCD ever takes more than a second to trip, or if pressing the Test button doesn't trip it instantly, have it checked immediately — a slow or failed RCD provides no protection.