What documentation is essential prior to extending the internal inspection?

The essential documentation prior to extending the internal inspection of a boiler involves water treatment records. These records provide critical information regarding the chemical composition and treatment of the boiler water, which is vital for ensuring the system's efficiency and longevity. Proper water treatment helps prevent issues such as scaling and corrosion, which can compromise the integrity of the boiler.

Without accurate water treatment records, there may be undetected problems that could lead to major failures or safety hazards. This documentation serves as a history of how the water has been managed over time, allowing inspectors to assess whether the water conditions have been optimal for the boiler's operation and maintenance.

In contrast, although the other types of documentation, such as employee training certificates, insurance policies, and historical maintenance reports may hold their own importance in different contexts, they do not provide the specific insights needed for evaluating the current state of boiler water quality prior to an internal inspection. Thus, water treatment records are the key focus here for this aspect of boiler management and safety assurance.