This is about corrosion and the "sacrificial anode" in the tank.

Consider the idea that a standard, American, 240V water heater is installed with plasic for the last 10 inches of both water pipes. All other pipes are copper. Of course, the water tank, the sheet metal case, and the outer shells of the heating elements are connected to "bond", which is the center tap of the secondary of the transformer owned by the power company. The mains water pipe is connected to the center tap with a substantial wire just before the pipe enters the house. The hot water delivery pipes are electrically connected to the cold water pipes at each faucet assembly. Every cold water delivery pipe is an unbroken connection to the center tap of the electric service.

Do the plastic pipes have any effect on corrosion? Is there any benefit to having that electrical path broken at the water heater? The fact that water heaters come with plastic circuit interrupters to be installed where the tank meets the water pipes tells me there is something going on that I do not understand. The number of "grounded" conductors and possible current loops exceedes my ability to analyse this.

 

Not sure about the electrical side of it but when going from galvanized to copper you must use a dielectric fitting or the galvanized will eat up the copper.

 

This video shows what happens when copper meets galvanized. http://www.youtube.com/watch?v=0igfll2tVrU

 

These photos show what happens when PVDC plastic meets a galvanized iron nipple on a water heater.

 

These photos show what happens when PVDC plastic meets a galvanized iron nipple on a water heater.

OR does that show what happens when you over torque a threaded connection exceeding the hydrostatic stress.
Looks like it was a "slow" leak at one time.

 

It's possible.

No reason to over-torque it. I used pipe dope on the iron and attached the plastic parts last, so there was no need to adjust the rotation of the iron parts. Still, I could have had a tiny leak.

My point is that it looks a lot like, "When Copper Meets Galv" so there must be more than one way to grow ugly stuff on a pipe joint.

Never did figure out whether I should leave the sections of plastic pipe as a dielectric or bridge the copper pipes with a wire jumper a couple of feet away from the water heater.