An Excess Flow Valve (EFV) is a safety device that can be installed on your natural gas service line (the underground line that runs from the main line to the meter).  Excess flow valves are designed to shut off or significantly reduce the flow of natural gas if a service line becomes damaged, and that damage causes a sudden and significant leak or pressure drop.  When activated, the excess flow valve may prevent the buildup of natural gas and may lessen the possibility of a natural gas fire, explosion, personal injury, and/or property damage.  However, they do not protect against leaks beyond the meter assembly (house piping).  A service line may be damaged by natural forces (such as seismic activity), excavation damage, or a vehicle hitting and severing a gas meter.