What is the only component that can be connected to a low water cut-out?

The only component that can be connected to a low water cut-out is a drain. A low water cut-out is a safety device designed to shut down the boiler in case the water level drops below a certain threshold, which can prevent overheating and potential damage to the boiler.

Connecting a drain to a low water cut-out allows for the removal of excess water from the system safely, ensuring that the water level does not drop dangerously low when maintenance or other issues arise. This feature is crucial for maintaining the integrity and safety of the boiler operation.

In contrast, a pressure relief valve, an expansion tank, and a steam trap do not have roles that align with the safety objectives of a low water cut-out. A pressure relief valve is designed to relieve excess pressure, an expansion tank accommodates thermal expansion in the system, and a steam trap removes condensate without losing steam. None of these components directly relates to the detection and management of water levels within the boiler, which is the primary function of the low water cut-out connected to a drain.