This paper presents the EMTP modelling of mechanical DC CB for transmission applications. Modelling structure of a mechanical DC CB is firstly presented with all the main components included. A theoretical background for dimensioning main components is provided. The control logic for each subunit and the interlocks between different subunits are modelled. A self-protection is also modeled to trip the DC CB in case of malfunction of upper layer protection. A wide range of user-specified parameters that enable modelling of DC CBs made by different manufacturers are provided. In particular, several different internal delays are provided since speed of operation is very important for grid-level protection design. The DC CB model is tested for wide range of operating conditions including: DC fault clearing, reclosing operation, failure of grid-level protection leading of self-protection activation and reclosing into a DC fault. EMTP responses show expected results which are in agreement with those papered by manufacturers.