We are having a debate with a contractor about when to use dielectric unions, when to use these new plastic-lined "dielectric" nipples, and if brass nipples are ever appropriate to adapt from galvanized steel to copper. Your comments would be appreciated.

Here is our current thinking:

It doesn't appear to us that brass nipples ever would work, as they don't break the electrical continuity between the copper and steel.

We don't understand how the plastic-lined steel "dielectric" nipples work as a dielectric connection, as the steel nipple threads directly to both the copper and steel piping and doesn't break the electrical continuity. I did see an explanation on a plumbers forum web site that supposedly the plastic lining provides a break in the "electrolytic continuity" of the water. Also, I found that Rheem's technical service department recommends the lined nipples over dielectric unions.

Dielectric unions appear to be the only thing that actually provides the electric continuity break and is the only way allowed by code. I can't find any other place that can explain what this "electrolytic continuity" concept advocated for the lined nipples.

Nonetheless, I saw quite a few posts on various plumber's forums that stated "from experience" that dielectric unions don't work, they corrode themselves, they form deposits, leak, etc. and that these nipples work better.

Any dielectric gurus out there?