I'm new here but hardly new to electric fence. I have been working with one type or the other of electric fence since the mid 1950's. Over the years we have fenced beef cattle, dairy cattle, hogs, sheep and for several years now, horses with electric fences. Sheep are the toughest, too much insulation. Still with some planning and training (things like aluminum foil pie pans wired to the wire to be curiously sniffed with a wet nose) it works fine. BTW, strips of cheap raw bacon hung over the bottom line is a good way to train neighborhood dogs and critters like coyotes. The longer it hangs and spoils the more they want to get at it. ZAP!!!
The scheme I have currently settled on and works well for us is a strand of "Equi-rope" at 42" from the ground and a second conductor of white "poly-wire" 12 inches below that. The poly wire is "a whole lot" cheaper. Especially when you have as many fences as we have. For one pen I would just use all Equi-rope. The horses can eat just enough below that bottom line so that I don't have to mow under it much.
Use a good hot charger (but it does not have to be silly hot), right now I am using two 50 mile chargers. One would do the job especially a 100 mile one but I am fencing 5 horses on one side of the road and 11 (at the moment) across the road and it is just easier to use two units. I have 8 pasture lots so far ranging from 1 to 5 acres. The 11 horses across the road are all client horses and as each of them came to the farm I put them in a cattle panel pen or a round pen with a hot rope across one side for them to get acquainted with it. About 2 zaps is about all it ever takes to convince them on a permanent basis to give it a lot of respect. Actually it seems that almost all horses of any years at all have been exposed at some time in their lives to electric fence and it shows up quickly when you put them near one. I do like to walk them around the perimeter of a new pasture so they see just where the fence is. After that they just know. You can see them checking the perimeter as I walk them.
Yes, all 16 here will reach across the fence to be petted or to visit with horses on the other side. They will often reach over and nuzzle necks with each other. They NEVER try to challenge the fence even reaching over like that. They will often get within an inch of the rope with their necks but magically do not touch it. They know exactly where each fence is at and do not challenge it even if it is completely under drifted snow. I have had a fence knocked down by a deer a few times but still they don't walk out over the downed line. We did have one where a deer being chased by a dog ripped a spot completely away and a new horse wandered out and was grazing in a neighbors yard but her pasture mate stayed in the pen.
When I am working on a fence or moving one it not uncommon to have it off for a half day and they never challenge it. I do have an old (maybe 25) quarter horse mare that likes to know if the fence is hot or not. She frequently "checks" the fence by lightly touching it with a single whisker. Apparently she can feel it without actually getting zapped. It does not seem to matter to her if it is on or off since she never challenges it, she just "likes to know".
One thing that I have done that works well is that I use a section of cattle-panel about 4 to 8 feet long on each side of all of the gates. By doing that they can walk through the gateway with out becoming nervous about maybe being too close to the electric fence or any risk of them getting their butt over against it and bolting as they pass through.
One of my next projects is to put an extra perimeter white polywire around the entire perimeter of the horse areas. That line will stop a wandering horse if it happens to get out. Sometimes clients forget to fasten gates... I have 24 foot wide grass lanes around each field (total of a little over 4 miles) that we all use for riding. This farm is a little over 100 acres.
Pick a good charger and supplies. After that the most important thing you need to do is provide an excellent ground. Put down several ground rods as you will be advised by the maker of your charger in the instructions but don't stop there. I have one grounded to a complete but abandoned galvanized pipe water system. It is also grounded to an underground tank that has drain-back water from a frost-proof hydrant flowing near it daily so that even in the drought we have been having in Central Indiana it sits in wet soil. The other is not only hooked to ground rods but to a metal fence with steel post and a power pole soil anchor (Not the power company ground). If you have a steel culvert handy, ground to it. If your mother-in-law will stand still, run a wire to her... Get the picture?
Also use lightning protectors and a lightning choke coil (Google it) in the hot line feed near the charger.
Now I am probably going to tick some folks off here but I will tell you that what I am telling you is first hand experience, long experience... It is not something I read. it is not something I heard about from my wife's second cousin's car mechanic's granddaughter... It is not stuff I dreamed or imagined. It works.
You will also get all manner of silly dire warnings from those that are afraid of "everything". Read and research such warning but take most of them with a grain of salt and temper them with common sense. You do have to be careful. I often say that the horse is the only creature that can injure itself playing with a nerf-ball.
We have been lucky, the only injury on the farm over the last several years has been one smallish cut on a right front leg from a kick with a sharp hoof this summer. Then again we don't have pastures full of junk. I use a lot of steel "T" post but never without a top safety cap. I don't allow barbed wire on the farm.
My wife and I have traveled around the eastern half of the US a good bit over the last few years and we see an awful lot of those 3 errors. We started making sort of a game as we traveled of keeping track of what kind of fencing we saw and we were both amazed at how many horses we saw fenced with barbed wire.
Maybe 60% in some areas. I found a roll of new barbed wire in one barn left over from the old days and the scrap collector got that.