Existing data on fragility functions of the 110 kV and 220 kV equipment are summarized and expressed by double logarithmic model to reflect the significant discreteness between different studies. The median capacities, randomness, and uncertainties of major 110 kV and 220 kV equipment are determined. Based on these parameters, the seismic performance, in particularly, the system functionality, of a 220 kV substation is assessed. The double logarithmic model is capable of reflecting the discreteness of the seismic fragilities of equipment in different studies, especially the uncertainties caused by modelling and analysis methods. The substation is found to be more vulnerable to earthquakes compared with individual pieces of equipment, indicating that the redundancy considered in current design should be improved. The substation’s median capacities are positively related to component median capacities, negatively related to component randomness and uncertainties.