By the movement towards formation of standard international adjustment and the viewpoint of lightning protection，buildings adopting TN grounding systems are increasing. However there are many difference between TN grounding system in the West and TT grounding system in Japan. One example is grounding fault current. In the case of TN grounding system，grounding fault current flows mostly as much as short circuit current. However，the adequate operation of MCCBs is unknown，because　JIS does not define the capability of MCCBs to cut off one-line ground fault current over 5kA. Therefore，the authors built a high-voltage power receiving equipment to a test site，and conducted operating tests of MCCBs for one-1ine grounding fault on an TN grounding system. This paper discusses the result of them.

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