An alarm that should have alerted workers on the Deepwater Horizon oil rig to a deadly build-up of gas that sank the platform, sparking a huge oil spill, had been muted months before, a former rig worker said Friday.

The alarm system which uses lights and alarms to warn of fire or high-levels of toxic or explosive gases, had been "inhibited," Mike Williams, the chief electronics technician on the rig, told a hearing.

Williams, who survived the April 20 blast on the rig, which claimed the lives of 11 oil workers and turned the platform into a fireball, said the sensor was functioning, but was not set to ring an alarm in emergencies.

"Inhibited," he said, "means the sensor is active and sensing and... it will give the information to a computer but the computer will not trigger the alarm."

Senior managers on the rig, which was leased to BP, had asked that the alarms be inhibited because "they did not want people to be woken up at 3 o'clock in the morning due to false alarms," Williams said.

He said he had first noticed a year ago that the alarms had been set to not go off.

Williams was testifying at the third in a series of hearings to try to find out what caused the blast on the Deepwater Horizon rig.

The hearing on Friday was conducted jointly by the US Coast Guard and Interior Department.