The switch in the operating control of a steam boiler is closed by which condition?

The switch in the operating control of a steam boiler is closed by the condition of falling steam pressure because this scenario requires the boiler to respond and generate more steam to meet the system's demand. When the steam pressure drops, it indicates that the steam system is not adequately pressurized, which typically means that either there is a demand for steam that exceeds the current production capacity or that there is a problem in the system.

In response to a decrease in steam pressure, the operating control system activates the combustion process by closing the switch, thereby allowing more fuel to be burned to increase steam production. This automatic adjustment helps maintain the desired pressure levels within the boiler system and ensures safe and efficient operation.

Other conditions presented in the options involve scenarios that do not typically engage the operating control switch in the same way. For instance, rising water temperature and increasing fuel input could indicate that the boiler is already operating effectively, while decreasing water level would typically trigger a different safety response associated with low water conditions rather than the operating control for steam production.