In recent years, Hydro-Quebec has undertaken a major program to upgrade the reliability of its transmission system. Much effort has focused on increasing the system's ability to withstand extreme contingencies, usually caused by multiple incidents or the successive tripping of transmission lines. In order to counteract such events on its system, Hydro-Quebec has adopted a number of special defensive measures, globally called the "defence plan against extreme contingencies". The goal of this plan, which will be fully operational by 1998, is to detect and confine incidents that exceed the system's strength. This paper summarizes Hydro-Quebec's current orientation for safeguarding its transmission system after extreme contingencies and describes the various automatic measures adopted and, in particular, the RPTC system, one of the main components of the defence plan. The RPTC system will detect multiple line losses directly from fifteen 735 kV substations, include four generation rejection sites and a centralized remote load shedding system.