Ultrafast disconnector (UFD) is a key component of hybrid DC circuit breakers and it is also studied as the main switch in some DC grid topologies. A UFD model suitable for DC grid studies and considering both normal operation and failure mode is presented. The dynamic motion of contacts is analysed in detail and it is concluded that Thomson coil inductances including parasitic parameters play important role and it is recommended to use finite element modelling. The arcing mode of UFD is repressed using a variable resistance in series with an ideal switch. The variable resistance is calculated analytically based on instantaneous position of contacts and the circuit conditions. Two different arc models are recommended: for the air insulated UFD and SF6 UFD, and in each case two operating regimes should be considered: high and low currents. The UFD model is verified for both normal operation and failure mode using measurements on a 5kV laboratory UFD and the results show very good matching. The 320kV SF6 UFD model is evaluated using limited reported results from manufacturers.