Which of the following is not a method for keeping sprinkler valves open?

The appropriate method for keeping sprinkler valves open should be effective and reliable under various conditions. Manual operation involves human intervention to open and maintain the valve position, which can be prone to error or oversight. This means that relying solely on manual operation does not provide the necessary assurance for ensuring that sprinkler systems remain operational during emergencies.

In contrast, options like electronic detection devices, locked chains, and other mechanisms are designed to provide consistent and fail-safe methods for keeping the sprinkler valves open. Electronic detection devices can automatically activate and ensure the valves remain open based on certain conditions, like the detection of fire or heat. Locked chains can physically secure the valves in the open position to prevent accidental closure. Other mechanisms might refer to automated systems that keep the valve in the correct state without direct human operation.

Thus, the reason manual operation is not considered an ideal method for keeping sprinkler valves open is due to its reliance on human action, which can lead to inconsistencies, making it less reliable compared to other options.