DC grids offer highly efficient distribution of electric energy, eliminating components and optimizing the use of cables. Efficient generation, however, is still based on AC generators. Power-electronic devices link generators and grid and customize energy flow to the loads. Efficient distribution with low losses demands high voltage, e.g. in the range of 10 kV for ship-size grids. Such voltages challenge power electronics as well as protective devices: Harmonics, converter design and switch design are issues to be discussed. Fault scenarios, accepted recovery time, size and weight of components and redundancy as well as available technologies influence the selection of solutions. This paper analyses fault scenarios in DC grids with special regard to on-ship requirements. Existing power-electronic solutions are compared with options resulting from modular multilevel converters (MMC), analysing advantages and disadvantages of this relatively new technology for on-board use.