I think that an actual connection to a metal rod pounded into the earth is
best whenever possible. Nowadays, plumbing can consist of PVC pipe which is
not metalic and won't work, so you have to be careful about that. I've read
of using a heavy duty mesh or screen of copper metal buried under the earth
can work also. I'm not sure you would get what you actually need by using
the grounding plug of an electrical outlet.