Electrical Theory Question. Not shop related.

[Mike Cutler]

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?

[Phil Thien]

Full disclosure: I'm not an electrician or an EE.

But I'd sink one ground rod next to a test tank, clamp a wire to the rod and drop the wire into the water tank, and take another reading. You should reduce your voltage by an order of a magnitude. If that works as I expect, I'd duplicate the effort at the other tanks.

[Dan Hintz]

I can't quite picture your setup, but GFCI will trip around 5-6mA difference... if they're not tripping, you're below that amount. Also, I would question whether a mere 1V difference is causing the horses not to drink... they may be more sensitive to voltage variations, but I know I can't feel 1V, particularly at mA levels of current.

[Lee Schierer]

Since the heaters are on timers, set the on time so the circuit is only live after dark. See if the horses drink from the tanks......

Meanwhile check the ground of your sub panel and recheck your wires for breaks or cracks. Are your heater plugs three prong grounded plugs? If not your GFCI will never work.

[Chuck Wintle]

yes it sounds like you need to provide grounding at the stock tanks. I am not an electrician though. Remember that at 125 feet of run you caould have a significant voltage develop along the length of the wire.

[John Lanciani]

Are your heater plugs three prong grounded plugs? If not your GFCI will never work.

This is incorrect. A GFCI works by sensing an imbalance between the hot and neutral conductors, no ground connection is required.

[Mike Cutler]

Lee

All heaters are equipped with a ground lug, and all of them are intact. All 10AWG SO extension cords are also ground lug equipped.
The grounding scheme for the 1st barn sub panel predates the requirement for running a quadplex type cable. It's a three wire fed. Two hots and a neutral is all there is on Triplex. Sub Panel for the heaters carries the ground through the equipment ground bus back to the 1st Sub Panel in the barn. The heater sub panel is in the same building, barn , as the 1st sub panel off the main.
I went back tonight to check, and with one probe in the tank and the other just held in the air I was getting 1100-1800mVAC. Put the probe in the ground and I got a repeatable 3.5 VAC at all troughs. Disconnect the heaters and the reading goes to <5mVAC tank to ground.
I wonder if I don't have a second distractor involved. That being the Electric Fence Charger. It's an AC powered fence. I know the fence is a chopped DC with a variable pulse, and it's possible it could be bleeding and getting picked up by the heater cords on the ground. I'll have to check it's grounding when there is daylight. Tonlte I was out in a fog with cold rain, and it was dark.
I need to get another meter too. The Fluke I have is a DMM, and it may be that a Simpson 260, or a Triplett, with a D'arsonval movement would be better. The 1100-1800mVAC with one probe held in the air was a little unexpected.

This also could be much ado about nothing, other than one of the horses stuck it's nose on the float type heater when it was on and got in contact with the hot element in the water. We'll watch the water consumption and make sure they're using the outdoor troughs. I just want to have a plan with a few options in the event I have to come up with a grounding scheme for these troughs.

[Stephen Cherry]

First, I'll have to say that everything I say is just thinking out loud.

There have been many cases when I have used a fluke meter and have been faked out, looking at voltages across the very high resistance input of a meter. I would consider adding a resistor across the probes of the meter, maybe try a 1k ohm, 100 ohm, etc. If you measure 1 volt across a 100 ohm risistor, you have some real current going- about 10 ma.

Maybe even use the current measuring part of the meter and measure the current from the water to the stake- just to see what you get.

Does the voltage go away when you turn off the breaker to the heaters?

Is it possible to ground the water in the tank by running a copper wire between the water in the tank and a stake in the ground?

If there is a problem, I would look at the viability of using a 1-1 transformer at the heaters, and grounding the center tap of the transformer to the local ground at the tank. There are a lot of people out there watering anamals, this thing had to have been gone through before. Maybe use the world wide web, and see what has been done?

[Rick Christopherson]

Contrary to common belief, the Earth is not all at the same voltage. Both man made and natural voltage gradients exist throughout the Earth. It is actually livestock watering troughs where this becomes most noticeable due to the animals. The voltage of the ground at your barn is different than it is out in the field. However, the ground wire in the power cord is transferring that voltage reference artificially out to the troughs and creating another voltage gradient local to the troughs.

Installing 6 equipotential grids around the troughs that is large enough to cover the area covered by the horses will be difficult. It should be effective though.

On the other hand, because these are fed from extension cords from GFCI protected circuits, I would also consider removing the ground wire that's causing this in the first place, by changing one of the extension cords to a 2-wire cord. GFCI does not use the ground as any type of sensing, so the horses will still have full GFCI protection from electrocution, but you will remove the cause of the voltage gradient around the troughs. If you still have problems, then you could install the grid.

[Mike Cutler]

Stephen
Yeah, I kinda question the numbers I'm getting from the meter, but it is a consistent 3.5vac at each trough, and when the cords are disconnected with the heaters still in the tank, it drops to <5mVAC. So the potential I'm seeing is directly influenced by the installation. I'll try cycling a breaker tonight and leaving ground intact.

Believe it or not, this is a problem in the equine community, more so than beef or dairy due to the nature of livestock management techniques, and facilities involved.
The portable stock tank deicers are a compromise solution at best. the best being an installed auto system like a Nelson waterer, followed by the insulated heated outdoor waterers, which is basically a tank inside a tank with insulated foam and heat trace in between the two tanks. No element in the water.
I don't know if the problem exists with metal troughs, but I suspect not. There would be a lot of ground contact there. The problem is most apparent with the large plastic type stock tanks you see at a Tractor Supply.
There are a few solutions out there on the web, but some of them would have to be effected prior to initial trough placement. It's also hit or miss based on soil type, animal temperament and just luck of the draw.

Rick
I thought about cutting the ground pins last nite after I got cleaned up. It may be the easiest solution, as long as I have total protection. I'll probably just get some 3 to 2 pin plug adapters at Lowes.