Recent demand of renewable energy generation has placed new requirements on the flexibility of transmission systems. These requirements together with technical advancements of high-voltage direct-current (HVDC) technology have resulted in the ambition to build large-scale multi-terminal DC grids (MTDC). These large-scale MTDC grids are expected to require fast DC breakers which allow for selective fault clearing. Selective fault clearing strategies avoid decreased availability during faults because only the faulted part can be isolated whereas the healthy part continues operation. For successful operation of a MTDC grid with multiple breakers, a protection system with selective fault detection is thus required such that the DC breakers achieve correct fault isolation in the event of a fault. Selective fault detection in MTDC grids is however not trivial because of low impedance characteristics and short required isolation times. This paper summarizes previously suggested selective fault detection methods for protection of a MTDC transmission system using DC breakers.