Researchers this month will begin testing a high-voltage circuit breaker that can quench an arc and clear a fault with supercritical carbon dioxide fluid. The first-of-its-kind device could replace conventional high-voltage breakers, which use the potent greenhouse gas sulfur hexafluoride, or SF6. Such equipment is scattered widely throughout power grids as a way to stop the flow of electrical current in an emergency.“SF6 is a fantastic insulator, but it’s very bad for the environment—probably t...