Grant couldn't fit through. Lex, maybe. I think Tim could have though.
Also, Lex hanging on to Grant's arm probably prevented him from getting electrocuted too, and if that had happened, they both probably would've died then and there and then the raptors would've gotten Lex
I seem to remember a metal mesh (almost like a chain link fence pattern) between voltage lines on the perimeter fence, preventing anything from just slipping through the lines. So they had to climb all the way to the top and over to get past the fence.
The T-Rex Paddock fence earlier in the film didn't seem to have that mesh.