In code class, we had a similar discussion concerning the hot water piping, with the isolation at the water heater, the use of plastic piping, and the use of non-metallic sinks. I have copper and cast iron drains in my house and rely heavily on the garbage disposal being grounded properly. Seems like the same principal of the proper grounding of the furnace circuit grounding the gas piping. Does this mean we're relying on the plumber and tinner to do our grounding? Are we supposed to check? Who pays? Stay tuned for the answers to these and other important questions.