The use of dielectric unions is prohibited on which of the following?

Dielectric unions are typically used to prevent galvanic corrosion that occurs when two dissimilar metals come into electrical contact with each other in a plumbing system. These fittings create an insulating barrier between the metals, which helps to prolong the life of the plumbing components.

The correct answer indicates that there are no prohibitions on the use of dielectric unions, meaning they can be used in all the specified situations: water mains, high purity water mains, and water lines 1" and smaller. By not restricting their use in these scenarios, the plumbing code allows for flexibility and can help in preventing issues related to corrosion, thereby ensuring the longevity and efficacy of the plumbing systems.

Water mains, particularly in larger systems, often include sections made from different materials. In high purity water mains, maintaining the integrity of the water quality is essential, and dielectric unions can provide a safeguard against corrosion that might contaminate the water supply. Additionally, smaller water lines (1" and under) are often subjected to conditions that make dielectric unions beneficial to avoid metal corrosion.

Therefore, the lack of prohibition on dielectric unions in these contexts supports good plumbing practices aligned with preventing corrosion and protecting the integrity of the plumbing system.