In a power supply and distribution system for supplying critical and non-critical outputs to a user system:
- first primary power generator means and second emergency standby power generating means, first and second power buses, said first power bus connected to the output of said first power generator means, a first switching means connecting the output of said second bus to the output of said first power generator means when said first power generator means is operative, and connecting the output of said second bus to the output of said second power generating means when said first power generating means is inoperative, sensing monitor means comparing a sample input signal proportional in magnitude to the voltage on said first power bus with a reference signal proportional to the level on said second power bus and generating a control signal output when said sample and reference inputs thereto exhibit a predetermined discrepancy therebetween, further switching means through which at least a portion of power supply output lines supplied from said second bus are disconnected from said user system in response to said control signal output from said sensing monitor means,
- said first bus connected to a first power conversion means to provide said sample input to said sensing monitor means, said second bus connected to a second power conversion means and a power regulator circuitry from which critical power output is supplied said user system and a reference input is supplied to said sensing monitor means, the output of said second power conversion means being connected as a power input to said sensing monitoring means and to said second switching means, and with non-critical power output from said second power conversion means and said power regulator circuitry being connected through said second switching means to said user system.