Which type of piping material can only be used outdoors and below ground in fuel gas distribution?

The use of plastic piping in fuel gas distribution is specifically allowed for underground installations and certain outdoor applications. This is due to plastic's resistance to corrosion, which makes it suitable for environments where it might come into contact with moist soil conditions that could deteriorate other materials over time. Furthermore, plastic materials are lightweight, easy to install, and often more cost-effective, contributing to their popularity in specific applications.

When considering the other materials listed, steel is typically favored for both indoor and outdoor above-ground uses due to its strength and durability. However, when buried, steel is prone to corrosion unless properly treated, making it less suitable for constant underground applications without additional protective measures. Copper is generally not used for underground fuel gas services because it is susceptible to corrosion in the presence of certain soil conditions. Aluminum is also not a common choice for underground installations due to its susceptibility to oxidation and the challenges it presents in joining and maintaining a gas-tight seal.

Thus, plastic stands out uniquely for its designated application in outdoor and underground settings for fuel gas distribution.