I don't know about sheep (In fact I was suprized that electric fence was even used on sheep I would have thought the wool would insulate them) but horses,cattle, hogs, and even deer learn to avoid the fences unless scared or there is something on the other side of the fence that they really want (not used alone on intact males for example). When my neighbor used to turn his milk cows into a new pasture the fence would only be on for a couple weeks or if the cows had gotten out, the rest of the time he kept it off since he found it inconvient to himself.~gd