When i had Bishop (a Dogue de Bordeaux) in puppy training class back in 2002, there was a German Shepard in the class named Kase. Great dog and was very playful, friendly and attentive to its owners. Over the next year of classes the dogs grew up and the owners would always talk about our experiences with the dogs at home. The owners of Kase had the underground perimeter system installed when they felt Kase was old enough to be let outside by himself. They ended up installing a wood fence after a few months of training because Kase would go right over the wire and not be bothered by the zapping even on its highest setting. They said his neck would be convulsing due to the electrical current and it was as if Kase didn't feel it.
My neighbors have one and it works like a charm on their black lab.