Continuous search for safety enhancements, reported events from the past and changes in the power system motivate studies on the impact of power systems in the safety of operations of nuclear power plants (NPP). The on- and off-site systems of an NPP have interactions with each other, but these are usually simulated with separate tools and with varying levels of detail. This paper builds on previous work in developing a co-simulation platform that connects the simulation of the thermomechanical and automation systems of an NPP with the simulation of the related electrical power systems, composed of the on-site grid and the off-site grid (transmission system). This co-simulation platform enables the integration of largescale NPP models and performance of fault-based simulations to evaluate the impacts of selected grid fault scenarios on NPP operations. This paper presents an overview of the simulation models, describes the tested fault scenarios that affect both on- and off-site electrical grids, and offers a comparison between simulation results obtained with the co-simulation platform and simulations comprising only the electrical systems, which treat the NPP as a black-box. The presented results show the disturbance response at various system levels, allow comparison with alarms and protection function limits and assess where the co-simulation approach may be of benefit.