Abstract: A device for protecting a polyphase electric power line against high transient overvoltages comprises at least two stages in series, the first stage being provided with a spark-gap and a decoupling inductor and being followed by a stage having a bridge limiter including two oppositely-biased power diodes each connected in series with a high-value capacitor, a time-delay contactor being placed at the terminals of each power-diode/capacitor assembly so as to permit charging of the capacitors through a resistor and auxiliary diodes and then to permit cut-off of the bridge limiter. The protection device has only one pair of high-value capacitors each connected in common in series with one of the power diodes of each phase, an auxiliary diode being associated with each capacitor.