The design status on the ITER insulation and grounding scheme is described with emphasis on some new design features, which are not common with the current generation of tokamaks. These modifications were found necessary because of the much larger stored magnetic energy, and need for additional conservatism caused by the presence of nuclear components.

All coils have "soft" ground connections. Each coil (or coil section) terminal is connected to a neutral busbar through the terminal resistor, and this neutral busbar is connected to the machine ground through the main grounding resistor (~1 kW) to limit fault current and related damage.

All Toroidal Field Coils and Poloidal Field Coils (except Central Solenoid) and Coil Feeders are designed in accordance with the double insulation concept. Two layers of full grade solid insulation are used instead of one. These layers are separated by an internal metallic screen and also covered externally by metallic screen. Each screen is grounded via resistors. Internal screens are made using a metal wrapping, conduit, or the function is provided by a structural element such as the TF radial plates. External screens are typically provided by TF coil cases, PF coil module separators, clamps and other structural elements. The main functions of internal screens are in initial fault detection and limitation of fault currents. External screens restrict fault voltages, e.g. voltages applied to vacuum interspaces, and also allow fault detection. These features combine to enhance the magnet protection and make many possible accidents within the magnet system irrelevant to nuclear safety. Required level of nuclear safety is ensured by a better protection of the magnets themselves.

Insulation and grounding schemes for structures located close to the TF and PF coils were developed in accordance with the enhanced magnets protection. As an example, the Vacuum Vessel Thermal Shield has 40 toroidal and 2 poloidal electric insulating breaks, plus a few redundant insulating barriers. This design allows initial fault monitoring and provides multiple insulating barriers, thus preventing short circuits from TF cases on the Vacuum Vessel, or short circuits between two TF cases.

Arrangement of electric connections and insulation barriers around the machine satisfies the single tree grounding concept. Its central point (Machine Ground) is represented by the Cryostat Vessel and Vacuum Vessel, which are interconnected through many ports.

The ITER site utilizes the unified ground grid concept. Overall ground grid connects all building ground systems and the two switchyards (pulsed and steady state power supply systems).

Work is provided under the ITER EDA agreement