[en] The DIII-D neutral beam injection systems (NBIS) at GA Technologies rely on a substantial array of control and instrumentation for routine operation. Functions include shot sequencing, power supply control, vacuum control, and valve control, long with interlocks for safequarding personnel and equipment. Failures resulting from improper operation of the neutral beamlines, their power supplies, and supporting services have the potential to cause extensive damage to personnel, equipment, and beamline components, as well as delay of the experimental schedule. This paper will address only equipment and component failures. Intelligence and control are distributed over dissimilar types of equipment in separate locations. An analysis has been made of the possible failure modes of the subsystems and the interlocking of systems and operations. This has helped us to improve the design of the safety interlock system and to minimize the risk to the NBIS project. The analysis technique will be presented and possible outcomes of typical failure modes explored. 5 refs., 2 figs