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Lightning risk assessment of a 170 kV GIS substation connected to the Hellenic transmission system through underground cables has been performed. Fast-front overvoltages impinging on the substation due to shielding failure and backflashover of the incoming overhead transmission lines have been computed with the aid of ATPEMTP simulations and evaluated with respect to the insulation level of the substation equipment. The mean time between failures of the substation is greatly reduced for higher tower grounding resistance and varies significantly among lightning attachment models employed for the evaluation of the lightning performance of the incoming overhead transmission lines. Implementation of the IEEE Std 1243:1997 in lightning risk assessment of substations imposes high requirements on protection against incoming fast-front overvoltage surges. (5 pages)