Abstract: A vital power supply system that utilizes a two out of three determination scheme to control the tolerance of the output voltage and also to shut down the output voltage. The power supply system includes monitoring circuits that determine over-voltage and under-voltage conditions and when a redundant supply is switched-in in the event the main power supply fails. One of the monitoring circuits includes switches that shut down the supplied power when at least two out of three processing units determine that such action is needed. Another monitoring circuit includes different switches that are selectively enabled to control a level of a reference voltage which indicates the voltage level being supplied by the supply. By changing the reference voltage up or down the tolerance for voltage monitor is controlled. Preferably, the system complies with CENELEC standards.