The protective plug provides reliable personal protection against dangerous electric shocks. Residual currents are detected within fractions of a second and the power supply is interrupted. It includes a PRCD plug and safety coupling with protective cap. A function display and test circuit including an electronic measuring device are also included. The rated residual current is 30 mA and the temperature range is from -25°C to +40°C. Safety is tested according to VDE 0661-10 (HD639S1), VDE 0620-1, VDE 0285-525-21.