A cage tender was waiting in the cage for approximately 2 1/2 hours while electricians were troubleshooting the back-up generator during a planned power outage. The generator shut down at 9:15 PM and returned to service at 11:50 PM. The MSHA Helena field office was contacted immediately upon notification (9:35 PM), and again at 12:10 AM and 7:30 AM with follow up information.