[en] The results of investigation of stationary and emergency operation modes of permanent current source (PCS) and the description of the electric guard system against three phase shorts are presented. PCS consists of silicon rectifier (SR), connected to the secondary winding of step-down transfomer (SDT) and is controlled with power saturation choke (SC), operating windings of which are switched in series to the primary (high-voltage) SDT windings. The electric guard circuit is based on the analysis of harmonic components of the SDT secondary coil voltage. The guard circuit has operating and brake control circuits. The harmonic analyser performance is based on the regularity that in three phase short circuit regime between the SC operating winding and the SDT primary winding, the SC does not practically halts valve current commutation. The electric guard system is being successfully used for some years for the PCS protection, which feeds magnetic elements of charged particle transport channels