The paper presents a general computational method and the relevant computer program which can be easily used to perform availability/unavailability analysis on power-system configurations. The mathematical availability formulation of the program is based primarily on: the tie sets (or cut sets) of the system configuration, the component failure rates and repair and maintenance times, some aspects of a Markov model of two states and the functional block concept. A real prerequisite for the successful utilisation of the program is the availability of realistic statistical data for the system components. For purposes of illustration, the computer program has been applied to evaluate the availability of two practical high-voltage/medium-voltage (HV/MV) substation configurations, from the standpoint of design, practical functionality and repair and maintenance procedures. The program is suitable in performing system availability/unavailability sensitivity analysis, based on value variations of system-component failure rates and repair and maintenance times.