It is amazing the number of times I've been asked by a horse owner to look at a setup to evaluate why it is not keeping Neddy in. What is is about horsey people having difficulty in establishing where a problem is?
My first question is to establish what the fence voltage is running at and seven out of ten I cannot be given an answer. Very few have a tester and I'm afraid that is the only way to be able to RELIABLY establish what your fence is doing. The only other way is to grab the fence yourself and most are not too keen on that idea.
The next principle involves the earth post. Place ground rods in permanently damp ground and make sure you have sufficient soil/metal contact. Bear in mind that a length of mild steel will not be effective for long. Mild steel rapidly develops rust and this layer of corrosion is a poor electrical conductor. About 95 percent of all electric fence problems are due to poor grounding.
Please be very clear that wrapping the end of your fence around a tree is not going create a good fence. If your wires touch anything except a plastic insulator you WILL be losing power. If your fence line is running in close proximity to another wire fence line it will create a current in that fence - this is called an inducted current created by a magnetic field that develops around the wires due to the pulsing of the electric fence system. This can result in a significant power reduction. Keep wires at least 10cm (4 inches) apart.
Old and poor wires usually have breaks and joins in them. Poor connections will result in a reduced current being transferred down the line. Check all those little wire filaments as best you are able. If you must join a tape the bare the wire filaments and twist them together outside the knot.
Get the maximum contact possible on your joins
Using electric fencing is not rocket science, follow just a few simple guidelines and any horse will be looked after. The important thing is to know what current is in your fence.