The paper presents the application of IntervalMathematics as a new method to, rigorously, address uncertainties associated with designing substation earthing grid system. While several methods exist to determine the important factors for personnel safety in and around substations such as earth resistance, touch, step and mesh potentials of substation earthing grid, these methods usually require data which may be uncertain in nature. To account for such uncertainties the interval mathematics is developed with the integration of input parameters’ uncertainties, in interval format, into the governing standard expressions mentioned on IEEE STD 80- 2000. The effects of uncertain inputs within the proposedmodel are examined for various assumed levels of overall uncertainties. To assess the relative contribution of each uncertain input, an interval sensitivity analysis is carried out. While catering for uncertainties, the method also offers utilities with alternatives for selecting the standard conductor size to be used. Successful implementation of the proposed method is described for the design and configuration arrangement of a 115/13 kV substation earthing rectangular grid with ground rods system.