Low speed of overcurrent relay forces application of distance relay for protection sub-transmission system. Hence, coordination of distance and overcurrent relay plays important role in protection of sub-transmission system. Before coordination it is essential to find the critical fault locations. These are critical fault locations at which the time margin between main distance relay and backup overcurrent relay is at minimum. The coordination is made based on the constraints derived from the values of for critical fault locations. Five different critical points as the fault position are taken into account for development of OCR and distance relay combined coordination scheme.