Final answer:
An excess flow valve acts as protection by restricting propane flow if a propane line ahead of a regulator is broken or opened, preventing dangerous leaks.
Step-by-step explanation:
The device that protects a propane system by severely restricting the flow of propane when a line ahead of a regulator is broken or opened is known as an excess flow valve. This safety feature is crucial as it helps to prevent dangerous gas leaks that can lead to fires or explosions. It automatically restricts the flow of propane to a lower level that would be insufficient to maintain a fire, thus providing a critical safety measure.