Final answer:
A normally closed solenoid valve in a propane system closes when there is no electrical current flowing through it. It is a safety feature to prevent the flow of gas when the system is not in use or when there is a malfunction.
Step-by-step explanation:
In a propane system, a normally closed solenoid valve means that the valve is in the closed position when there is no electrical current flowing through it. This type of valve requires power to keep it open and will automatically close when the power is removed. It is a safety feature to prevent the flow of propane gas when the system is not in use or when there is a malfunction.