Just three inches high, this robot could help keep the power grid humming by diagnosing faulty power lines in difficult-to-access tunnels and pipes. The robot hugs underground power cables, rolling along them on small plastic wheels; it carries a thermal sensor to locate hot spots, an acoustic sensor to listen for the crackle of sparks, and a dielectric sensor to detect moisture. The battery-powered robot also has a gyroscope to help maintain its balance and stabilizing arms to right it if it slides off track. The gadget is the fruit of a project led by Alexander Mamishev, professor of electrical engineering at the University of Washington, Seattle. The technology “looks very promising,” says Dave Hawkins, a project manager at the California Independent System Operator, a nonprofit organization that manages much of the state’s power grid. Though the robot can access only the 10 percent of underground cables that are found in pipes or tunnels (as opposed to those buried directly in the ground), those are often the ones that suffer damage from water and other causes. The robot was recently tested in New Orleans, where it was sent underground to search for Hurricane Katrina damage.