A critical alarm system that should have warned workers of danger aboard the Deepwater Horizon drilling rig had been disabled before the rig erupted into flames on April 20, the vessel's chief electrician testified Friday at a federal hearing into the accident.

Michael Williams, an employee of Transocean Ltd., the company that owned the rig, said the general alarm system aboard the Deepwater Horizon had been "inhibited." It was intended to automatically sound an alarm warning workers to move immediately out of harm's way....