In order to exploit high wind conditions, wind turbine generator systems are often constructed in places where few tall structures exist; therefore, they are often struck by lightning. Much of the damage caused by lightning is from the resulting breakdown and malfunction of the electrical, communication, and control systems inside the wind turbine generator system; these breakdowns can be attributed to a rise in electric potential both within the system and in the surroundings due to lightning. Impulse tests were conducted on a wind turbine generator system at a disposal site where the conductivity of the ground was very low, like that found on the surface of the sea. The rise in ground potential of the system, and around its foundation, was measured. When a wind turbine generator system is constructed at a site where the grounding resistivity is very low, the potential rise at the wave front typically becomes larger than that of the steady state because of the inductivity of the grounding system. Therefore, it is very important that the transient characteristics of the grounding system are well understood. Copyright © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.