I have an interesting question/situation and just polling the Electricians and EE's here on the board to get some insight.
Situation;
Six 100 gallon plastic stock tanks. Closest tank is 125' from the nearest electrical power source, the barn.
The Barn is subpaneled off the House main. 431' underground run of Triplex URD. Pre 2005 NEC.
Each stock tank has it's own dedicated 20 amp GFCI, which in turn are each on seperate independent 20 amp breakers.
125' "extension" cords are 10 AWG "SO" cord. Connections are environmentally wrapped. No exposed plugs, anywhere. Heaters are 1500 watts each, with auto shutoff for temp. They are also on timers. All cords and connections are two conductor with ground pins. Ground is carried throughout all equipment. Four of the heaters are brand new, imersion type. Two are of the floating type.
Problem;
I can measure ~1vac between the water in the troughs and stake driven into the ground near the troughs. Can't measure the Frequency, but It has to be within the tolerance of the Fluke I'm using. The 1vac may be inhibiting the horses from using the water troughs.
Can the solution be as simple as driving a ground rod in at each tank, or creating an equipotential plane under the tank, and dropping a ground wire into each tank, or will all of the tanks need to be grounded to the same potential? In other words, run #6 or #4 ground wire the length of the pastures and connect them together thus putting all of the tanks at the same reference.
My theory is that the tanks are far enough away from the ground rod for the main service panel, that what I'm seeing is a difference in ground potential between the ground at each tank, with the water heater in it, and the ground rod at the Main Service Panel.
I'm worried about creating a different grounding plane(s) and either faulting the GFCI's, or possibly worse, not having them protect the horses. Another concern is developing a situation of circulating ground currents, between the tanks, the barn, and the house Main Service panel ground.
My gut feeling is that a ground rod will need to be driven at each tank and a common ground wire connect these ground rods. Off of each rod would be a ground wire immersed in each tank.
Thoughts, Ideas?