Concern about energy security on domestic Department of Defense installations has led to the possibility of using natural gas-fired electricity generators to provide power in the event of electric grid failures. The natural gas system in the United States is partly dependent on electricity for its ability to deliver natural gas from the well-head to the consumer, but it also uses natural gas from the system itself to fuel some of the drilling rigs, processing units, and pipeline compressors. The vulnerability of the system to a disruption in the national electricity supply network varies depending on the cause and breadth of the disruption and where in the country one is located relative to that disruption, as the interconnected nature of transmission pipelines, the penetration of electric motor-driven compressors and other equipment, and the availability of nearby gas production, import terminals, or storage varies. In general, the gas supply system is reliable for short-term, limited-area disruptions in the electricity supply, and firm delivery contracts for natural gas increase the likelihood of continued operation, but for disruptions that cover large sections of the electric grid encompassing areas from extraction wells to customers and which last longer than available gas in storage or transmission pipeline constraints from elsewhere, contractual force majeure limits will come into play rendering the firm delivery contracts void; operation of gas-fueled power generation systems that are not dual-fuel capable for longer than weeks to a few months (depending on time of year) will be unlikely. Several weather-related outages in recent years have provided limited case studies showing the system's resilience, but no long-term, widespread electricity grid failures have occurred.