I think that when the pipes became copper, things change. You can cause current based copper erosion from galvanic action.
Practically, it should be a non-issue since ground shouldn’t have (significant) current flowing through it under normal operating conditions. Yeah, I know some timers and things like that can use it as a return path, but it’s rare.[/quote]It is actually quite significant. I know of more than one house where the copper plumbing had to be redone because of the interaction between galvanized steel and copper. It acts like a battery. Direct connection between the two can help create a closed path/short. The two metals adjacent to each other act like aluminum vs silver in an acid. Normally zinc acts as sacrificial in such a reaction, but the zinc is just a ‘coating’, which erodes quickly. With normal galvanized pipe, the zinc starts reacting to protect the steel. It will swell and close scratches. The problem is that there is a second path through the copper. This is why ‘mixing’ pipe types can have unexpected results. Generally when doing copper to galvanized connections, you want an insulator pipe segment in between. It is even in California plumbing code. http://homeguides.sfgate.com/connect-copper-galvanized-plumbing-20916.html
Remember, you are dealing with the galvanic action occurring over years.. not minutes, and that soil is used to create ‘neutral’ and ‘ground’ for AC supply.
