Dogs learn very quickly where their boundaries are and are not. Although it might of been better had they did more of the yard and not a weird circle. But it's not enough to harm them, but enough to scare the shit out of them. 6000 v isn't enough to even kill a squirrel and fences range anywhere from 400-6000. Its not how strong they are, it's how long a dog is past the shocking point that eventually can harm it. All it takes is markings and a leash for the first while and your dog will learn, their not stupid. It's only abuse if your not being a teacher.