The development of two computer algorithms to assess the performance of IDMT over-current protection for interconnected power systems is described. The first algorithm considers the case of three-phase short-circuit faults, and the second considers single-phase to earth shortcircuit faults. Both can include multiple bulk supply points, parallel lines and transformers, directional and nondirectional relays, including both on the same circuit, teed points, and instantaneous high-set elements. The algorithms currently incorporate relays with a standard inverse characteristic but any relay with a current/time characteristic can easily be included. Results are presented of the application of these algorithms to the assessment of the relay performance for several actual power distribution systems. For each selected fault position, the operating time of each relay is calculated and the sequence of relay operation shown until the fault is completely isolated. The results also illustrate, in particular, the ways in which the performance of the relay systems can be tabulated to greatest effect.