Final answer:
The overspeed trip mechanism protects a turbine by closing the steam supply valve when excessive speeds are detected, preventing damage and maintaining safety.
Step-by-step explanation:
The mechanism that protects the turbine against rotating at excessive speeds by causing the steam supply valve to close is called the overspeed trip. This safety feature is crucial for preventing damage to the turbine and ensuring operational safety. When the turbine reaches a speed higher than its designed operating range, the overspeed trip activates and causes the valve that controls the steam supply to shut. This action reduces the steam flow to the turbine, preventing further acceleration and protecting the system from potential harm due to overspeeding.