Safety grounding approach for the National Ignition Facility power conditioning system

This paper describes a set of analyses and tests performed to evaluate approaches to provide a safe and robust grounding approach for the main Power Conditioning System (PCS) in the National Ignition Facility (NIF) facility presently under construction at the Lawrence Livermore National Laboratory (LLNL). The Power Conditioning System consists of up to 192 capacitor bank modules, each storing 2.2 MJ and capable of producing a peak current over 500 kA. The grounding system must minimize touch potentials associated with operation of the Power Conditioning System. In the event of severe faults, the system must assure that the energy delivered to a person through contact with "grounded" structures is very low. Based on computer modeling and low-voltage, low current tests, we have concluded that the most effective approach is a set of metal enclosures around the output cables (effectively heavy-wall closed cable trays) extending from the capacitor bank modules to their flashlamp loads. This paper will discuss the safety standards identified for this application, the approach to meeting the standards, and the predicted performance of the safety system.