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The simple answer to your question is that dielectric couplings need to be installed at ALL connections between ferrous (iron/steel) pipe and copper pipe. These couplings place an insulator (otherwise known as a “non-conductor” or “dielectric”) between the two types of pipe. Without this insulation between the two metals, an electrochemical reaction (called “Red-Ox” or “reduction-oxidation”) will occur, leading to corrosion of the pipes and leaks. Red-Ox reactions depend on a basic physical property of the metals, called “Red-Ox Potential.” This is the kind of stuff you learn in high-school chemistry, and I’ve forgotten most of it. Iron and Copper have enough Red-Ox potential difference to cause problems in a plumbing system.
Various commercial dielectric fittings are available, mostly unions and things like those flex fittings used to connect water heaters. A commonly accepted alternative to a true dielectric fitting is the use of a yellow brass nipple to connect the two types of metal, especially in the case where a brass valve is to be connected to galvanized iron pipe. Screwing a galvanized pipe directly into a brass valve leads to corrosion and leakage, but the use of a yellow brass nipple between the two seems to work quite well.