In relation to boiler operation, what does the term 'gagged' refer to?

The term 'gagged' in relation to boiler operation specifically refers to securing the boiler to prevent any operation. This action is taken to ensure safety during maintenance or repairs and to eliminate any risk of the boiler inadvertently being brought online or operated while personnel are working on it. When a boiler is gagged, it typically involves implementing physical locks, tags, or other mechanisms that render the boiler inoperable. This practice is an essential part of safety protocols in facilities that operate boilers, ensuring that all necessary precautions are taken to protect workers.

In contrast, the other choices do not accurately capture the meaning of 'gagged' in this context. Being blocked for maintenance suggests a temporary state of unavailability without the specific focus on safety measures, while being prepared for safety tests implies readiness for operational checks rather than a prohibition of operation. Lastly, modifying a boiler for efficiency pertains to changes made to improve performance, which is unrelated to the concept of securing a boiler to prevent its use.