Water Shocking Horses?

We have a drop-in, sinking heater (this one.... Allied Precision Sinking De-icer, 1500 W - 2170712 | Tractor Supply Company ) Although you are not really supposed to use extension cords with them, we have no choice, so we have a heavy-duty, outdoor extension cord plugged in through an outlet in the chicken coop. The heater keeps the water unfrozen. However, this morning my mare, Goldie, went to the water (after a long gallop around the pasture with Spirit ), put her head in and threw it back up and jumped back, she did this a couple more times and than gave up. I believe she was getting shocked. Spirit, did not have this reaction, but I am unsure of rather he actually took a drink or not, as I was at the house watching them. We bought this heater last year but only put it in this December so I don't think it would be the heater itself doing this but probably the extension cord. Has anyone else ever had this experience? And what did you do? Also, could it have anything to do with the water not being full past the top of the heater (by past the top I mean past the red thing with the yellow sticker)? Also, is there any way for me to test if it's shocking the horses without actually getting shocked myself? I know water and electricity is a very, very dangerous mix! A few days ago when the water level was just slightly below the actual heater (not the cage, just the black part) there was a very thin sheet of ice in the tank that I got out and I did not get shocked, however, I believe I was wearing rubber-soled boots. Any help is greatly appreciated and I hope all this made sense! Thanks~ poundinghooves

Good thing it was volts, NOT watts or amps, or she'd be dead. I do NOT trust these heaters, and I'm not the only one. I've been discussing this topic for over 20 years now. I even posted in the Sufficient Self forum asking how to heat the tank without exposing my horses to any wiring. Right now in the winter I pull my big tank and alternate between a 20 gallon and a 30 gallon rubber round waterers. I can break up the ice in those, when necessary. They double during the summer for my north pasture, when I have to do work in other pastures, so they are worth purchasing.
Take it back and try to get a refund, or at least, a refund card from the store.
The ONLY heated watering container I use is a dog heated bowl, for my chickens, bc they won't chew or peck on the cord.

The heater is supposed to be 1500 Watts!

I have a small tank, I'm not sure of how many gallons and I can not get it unfrozen during the winter. Obviously, during the summer there is no problem, but in the winter, when it's below freezing, I can't bust the ice for the life of me, unless it's only a thin layer. Also, we have to fill the trough with a hose so I can't just bust the ice, pick it up out of the tank and pitch it because I can't just refill it lickety split, I can only refill it if the hose isn't frozen, or with tons and tons of trips back and forth with a bucket, and I'm a small person so this doesn't come easy to me. I've got no idea how to fix this problem.

I'm not much help, I don't use electric tank heaters either because of the chance of them getting shocked, just the thought scares the snot out of me. Glad your mare is okay! I use gas tank heaters for the stock tanks, get the big propane tanks topped off in the summer when price is low and runs them all winter usually with a bit left over (it cost $36 to top off the propane in July).

Buy one bc you CAN break up the ice.

What do you use to break the ice?

I just throw it, step on it and bang it out. It only takes a few minutes and rubber is very forgiving.

This is why I wont use water heaters in the actual troughs or buckets too
The whole thing is supposed to be submersed but I don't think that's the problem
It might be the extension lead - especially if they warn to not use one but most likely a loose connection somewhere

20 or 30 gallons is much less than what my trough will hold (although I don't know my trough's exact capacity) so that would be much easier to break. How do you fill them back up once you break the ice? Right now I only have my (large) mini horses so it wouldn't be too bad to keep them in water, but by next winter I will have a barrel pony/horse and he or she will drink much more water but by then we will hopefully have a more sufficient way (besides the hose) to fill the water.

I have a couple of smaller lots with 70 gallon rubbermaid tanks that don't have heaters. For those I use a sledge hammer to bust the ice and a metal potato fork to scoop it out. I fill them back up with hoses, then walk the water out of the hoses so they don't freeze. Smaller containers outside when you don't have heaters are a pita.