Symmetrical monopolar HVDC links, based on Modular Multilevel Converters (MMCs) represent the state of the art for submarine and underground cable interconnections over long distances. MMCs exhibit significant advantages if compared to other converter topologies, such as lower losses, footprint and harmonic distortion. However, due to the use of a symmetrical monopolar arrangement, severe temporary overvoltages on the dc side (dc-TOV) can arise, due to single pole to ground fault, endangering cable insulation. The paper presents a possible countermeasure for dc-TOV, using an innovative grounding device, which does not hinder the use of 3rd harmonic over modulation. The proposed method allow to reduce the dc-TOV below 1.4 p.u., thus allowing a safe operation of HVDC cables.