Which of the following is NOT a type of fitting allowed for gas installations in concealed areas?

In gas installations, particularly those in concealed areas, safety is of utmost importance. The use of fittings is strictly regulated to prevent leaks and ensure the integrity of the gas supply. Plastic fittings are not permitted for gas installations in concealed areas due to their susceptibility to degradation over time, particularly under environmental stresses such as heat, pressure, and exposure to chemicals. Unlike metal fittings, plastic can become brittle and may not maintain a reliable seal, which poses a significant risk of gas leaks.

On the other hand, threaded elbows, welded fittings, and brazed fittings are considered suitable for gas installations because they provide the necessary durability and can withstand the pressures associated with gas systems. Welded and brazed fittings create strong, permanent connections that are less prone to leakage, while threaded fittings, when properly installed with the appropriate thread sealants, can also provide effective seals, making them acceptable for use in these applications.