At our previous barn the BO installed 7 heated Nelsons outside, accessible to all of the horses when turned out. My horse was on pasture board. I like the fact that water was the right temperature and always available. No one had to go out and pound ice when it was below zero. No need to deal with extensions cords and heaters. No need to keep the hose someplace warm and drag it around to the spigots. It was close to the gate so I checked it whenever I turned my horse out. The watering system is on a separate well and electricity from the house, barn and outbuildings. There is a hookup for a generator. They keep replacement parts there so they don’t have to be ordered.
We had a problem with stray current once in about 15 years. It was where my horse was turned out. If it’s not working they all gather near it and watch. If no one gets out there according to their timetable, they kick the unit. The stainless steel on the Nelson shows no signs of abuse. If a tub is out there they check the waterer first, then go to the tub.
If you test and get a very small reading it doesn’t mean the horses don’t get shocked, or it’s too small to make a difference. They got a tub immediately. I called Nelson and they were very helpful. Their rep gave me a lot of information on finding the source. He said the state of Wisconsin has a lot of information about grounding online. Apparently if a cow gets a tiny zap milk production stops and doesn’t come back. He also said to call the electric company to check all of their equipment. They came the morning after I called and did a thorough check. They said to make sure the electrician check all panels. We checked all of the electric fencing.
No one could figure out where it came from. The problem isn’t necessarily whatever is closest to the waterer. A grounding rod near the unit fixed the problem. I’ve always wondered how well the entire system was grounded when it was installed. It wasn’t done by an electrician. It’s really important, in my opinion, to be absolutely certain to focus on grounding during installation.
We moved to a new barn a couple of months ago. He’s 26 now and needed a change of environment. The stalls have heated Nelsons that were installed when the barn was built. My horse is now in a 12x12 stall with a “gossip window” in the stall grill so he can observe the aisleway, and a back door out to a large run. It functions like pasture board with a shed and he loves it. The barn is always left open, except for horrendous weather. The fields where they are turned out have tubs which are kept open.
My horse was the last one to use it when they first were installed. He didn’t like the swirling water. Throwing in a few apple slices worked. I haven’t seen a new horse outside have a problem; they probably learn from each other. There is one new horse at our new barn that is having a problem in the stall.
We are in southern Maine where the frost line is down about three feet. I used to do chores at the previous barn before the waterers were installed. It sucks pretty bad trying to keep water available when the high temperature for the day is below zero. Pounding ice is just the beginning. Whenever I heard someone complaining about chores I was tempted to turn off waterers and steal the Gator for a couple of days. 
In some areas of North America, the frost line is less than 12". Where we are, the frost line is 36" or 3 feet.