Which of the following duties is not allowed for a holder of a Fourth Class Engineer's Certificate of Competency?

A Fourth Class Engineer's Certificate of Competency restricts the holder from managing or taking charge of operations in larger power plants. Specifically, individuals holding this certification are permitted to operate, manage, or supervise power plants or systems of certain sizes, typically under 10,000 kW. The restriction mentioned in the correct answer reflects the regulatory framework that governs the responsibilities and limitations placed on operators of this certification level.

In the case of taking charge of a shift in a power plant exceeding 10,000 kW, this duty exceeds the qualification threshold established for a Fourth Class Engineer. Those holding higher-level certificates would be required for such responsibilities, ensuring that the complexity and scale of operation are managed by individuals with more advanced training and qualifications.

The other options presented indicate permissible activities under the Fourth Class certification level, such as managing a smaller power plant or specific types of boilers, as these do not exceed the capacity that the certification allows. Each of those scenarios aligns with the regulatory permissions given to holders of the Fourth Class certificate, thereby clarifying why they are valid and permissible roles.