In a static converter with thyristor-rectifiers, a protection against auto-ignition includes an arrangement for sensing the polarity over the rectifier, and an And-gate. The sensing arrangement emits a first signal to the And-gate when a voltage after a commutation becomes negative and a second signal when a voltage after a commutation becomes positive. The And-gate is connected to the ignition system of the rectifiers to supply both signals thereto. The connecting means for the first signal includes a delay device which delays the signal by a time corresponding to the longest recovery time for the thyristors, so that, if the second signal is emitted during this time, an ignition signal is allowed to pass the And-gate.