How does this work?

[House Doc]

This is making me nuts...Can anyone explain to me how a pipe nipple that comes with the water heater can be "dielectric"? Isn't it a solid steel nipple with just a plastic insert? How does it keep current from flowing from piping to the heater?

 

[Rickyman]

I think it is zinc

 

[GReynolds929]

My understanding is that the plastic lining of the nipple acts as an insulator preventing the water moving through it from creating an electrical current.

 

[House Doc]

I can see how an actual dielectric union would prevent current. There is the plastic "washer/sleeve" between the nut and brass part, and rubber washer to keep the brass part from the steel bottom fitting. There is no metal connection between copper and steel. How does that nipple do that?
What is it that I cannot see? I'm sorry but I am a logical person and I thought that the idea was to stop a current between the copper and steel.

 

[breplum]

You are right! It is not actually dielectric but serves as adequate:
The tank is steel and the nipple is steel, so compatible metals.
The nipple liner lip has contact with the nut on a WH flex, so a copper system would be isolated w/o electrolysis.
However, if a copper f.i.p. adapter were used, there would be no dielectric protection.