The basic condition of the ZUS type power supply system is the state in which SRK devices are powered from the 3 × 400V primary power network. In the case of a power outage in the primary network, the back-up network is automatically switched on. If within 4 min. from the return of the main network voltage there will not be another loss of power, switching to the supply from the main network takes place. In the case of incorrect parameters of the primary network and the reserve network, the generator is automatically switched on.