Conventionally，the earth of a washing machine was typical of the earth system for a house. However，recently，the number of home appliances，which need earth，is increasing in a variation of lifestyles :for example，water heaters，air-conditioners，and personal computers have spread. The purpose of grounding these appliances is various : preventing electric shock，maintaining the function of electronic circuits，or protecting themselves from surges，and so on. The SPD (surge protective device) for the switchboard using an earth is also spreading after an amendment of the regulation on indoor wires in 2005. One-report shows that about the half of the appliances，which need earth，are not grounded in almost all houses，under the limitation of the number of the earths. Some studies have tried to substitute the concrete foundation of a house for the earth in order for home appliances to be grounded at any place in the house. In this study，experiments were conducted with a miniature-scale system to clarify the influence of the lightning-surge current on the concrete foundation. This paper discusses the results of them.

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