Direct bonding is required for all gas-piping systems incorporating standard Corrugated Stainless Steel Tubing (CSST). CSST is a flexible yellow pipe used to provide natural gas and propane to many homes. If you live in a residential structure built between 1990 and 2006, or if gas line work was performed during that period, it’s best to work with a professional electrician to ensure safety and eliminate any fire hazards associated with CSST.