Construction of the JT-60 grounding system

JT-60 is a tokamak-type nuclear fusion apparatus for critical plasma tesing, which was constructed in April 1985 by Japan Atomic Energy Research Institute. Major features of the JT-60 grounding system are: 1) a combination of ground mat and ground rod is laid for each individual building (experiment building, power source building, control building), 2) the grounding mats are connected with each other by low-impedance grounding conductor, 3) the ground system is designed so as to reduce the potential differences over the large ground mats, which is achieved by grounding electrodes serving to decrease the grounding resistance. Testing is made on basic characteristics of components making up the JT-60 ground system. Duct-shaped inter-building grounding conductors are adopted to decrease the impedance effectively. The DC circuit breaker for the ohmic heating coil power supply is installed on a separate ground mat so that surge and noise will not affect other facilities. Special equipment for monitoring grounding resistance, which incorporates two current transformers for detection, is developed and installed to ensure that JT-60 will not operated by mistake with a loop circuit left in the ground system. The grounding resistance is constantly monitored by this equipment, leading to an increase in the reliability of the facilities. (Nogami, K.)