Which is a closed fire line?

In the context of fire protection systems, a closed fire line refers specifically to a system that can deliver water to fight fires but does not allow for any water to flow unless specifically activated. Option B describes an unmetered connection for a fire protection system, which indicates a dedicated line for supplying water to firefighting equipment such as sprinklers or standpipes without the need for metering. Because it is an unmetered connection, it is typically considered a "closed" system, meaning that it is not used for regular distribution purposes and is kept ready for emergencies.

The ability to keep this line closed from regular distribution helps ensure that the water supply for fire protection is available without interference from other system demands, making it essential to maintain the pressure and availability of the water source strictly for firefighting purposes. This is vital for ensuring that adequate water availability exists permanently for emergencies while preventing regular consumption that could compromise that water supply.

The incorrect answers pertain to different contexts or functionalities of fire protection systems. For instance, a closed valve to a fire hydrant or sprinkler system, while related to the overall fire protection setup, does not itself imply a dedicated, unmetered supply line that is closed off from everyday usage but could be opened when needed.