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This paper presents a practical case for the safety assessment of ground grid design at 161/69-kV indoor-type and out- door-type substations in the system of Taiwan Power Company (TPC). For a substation grounding system to be safe, the touch and step voltages should be limited below the minimum tolerable voltages. Since the mesh voltage is usually the worst possible touch voltage inside an ac substation (excluding transferred potentials), the mesh voltage is used as the criteria for a safe ground grid design. In other words, safety assessment of a ground grid design at 161/69-kV indoor-type and outdoor-type substations in the TPC system is referred to as a procedure by which the mesh and step voltages at substations are computed and compared to the minimum touch and step voltages. Computation results of safety assessment for humans with 50- and 70-kg body weight are quantified in this paper. [ABSTRACT FROM AUTHOR]