The mathematical reliability model of system with complex whole reduced redundancy for failure cause analysis is proposed. Failure cause analysisof system is carry out based on minimal cut set probability characteristics. The analyzed system is composed of four components: two generators and two converters. The components are made up the main and the reserve subsystems. If the main subsystem is in up state, then reserveone is under reduced load. Time to failure for all components is distributed by Weibull and repairing time is distributed by exponentially. The reliability of the system is formalized by dynamic fault tree. Probability characteristics are calculated by split homogeneous Markov model. The main advantage of proposed mathematical reliability model for system consists in treating of load-sharing impact on minimal cut set probability characteristics.