Which of the following is a critical safety procedure when testing low pressure gas lines?

Ensuring all connections are tight is a critical safety procedure when testing low pressure gas lines because loose connections can lead to gas leaks, which pose serious risks including fire and explosion. Tight connections help ensure that there is no escape of gas during the testing process, allowing for accurate testing results and promoting the overall safety of the installation. In addition, proper connection integrity is essential for maintaining the operational safety of the gas appliances connected to those lines.

Testing while appliances are in use is not advisable, as it can create unsafe conditions and risk exposure to gas leaks during the process. Removing all gas regulators is unnecessary for low pressure testing and could compromise the system's integrity. Allowing for high-pressure testing does not apply in this context since it can exceed the safe operating limits of low pressure systems and lead to potential failures or explosions. Hence, ensuring all connections are tight is the most appropriate and crucial procedure in maintaining safety during low pressure gas line testing.