MCC-NH Gas Fitters Practice Exam

The correct answer is that plastic piping encased in non-pressure-carrying metal pipe requires a special transition fitting as defined in NFPA 54. This is because the use of different materials in gas piping systems generally necessitates specific fittings to ensure safe and effective transitions between them. The NFPA (National Fire Protection Association) standards, specifically NFPA 54, outline the requirements for gas installations, including stipulations for transition fittings to address issues such as potential leaks, structural integrity, and compatibility of materials.

When moving from plastic to metal, the transition needs to be carefully managed to avoid any vulnerabilities during operation. A special fitting serves to maintain the required strength and sealing capability at the joint, mitigating risks associated with gas leaks or failures in the piping system.

In contrast, metal piping does not require a transition fitting as it maintains consistent properties throughout, flexible rubber piping tends to be more suited to specific applications and doesn't typically interact with traditional metallic systems in a way that necessitates a special transition, and composite piping often has its own criteria and does not directly align with the same requirements as plastic encased in metal.