There is no research on the quantitative risk assessment of schemes for equipping new devices for service by now. This paper introduced two schemes of equipping brand new primary devices for service. By researching the events of the two schemes, which may hazard the safety operation of electric grid, fault tree analysis method is used to construct the fault tree of the two schemes respectively. Based on probability analysis of all the events, this paper deduces the risk probability of each scheme. By comparing the result, some general suggestions are given to help decide which scheme is better according to practical conditions. Using the described method, we successfully complete the scheme design of a new sub-station equipping for service.