This paper analyzes the behaviour of a modular multilevel converter (MMC) based HVDC system under DC cable fault conditions. The behaviour of the HVDC system during a permanent line-to-line fault is analyzed in details at each stage of the fault timeline. Operation of the proposed system under a specific earthing configuration i.e. converter unearthed /AC transformer secondary side and DC cable solidly earthed, is also analyzed. Simulation studies are provided to assist the analytical analysis. It is observed that both faults leads to substantial AC and DC over-current and result in DC side oscillations.