"Electricity can kill"

Yes, I'm sure that it can.

"It's something you own goes over the fence, call and we will get it for you"

That's nice of you. Do you actually see any fence here? This is some kind of hinged opening on the side of a solid wall of a building. I really don't see the likelihood of a tennis ball being able to make it through the louvers and somehow find itself amongst high-voltage equipment. I do sometimes wonder if the people who put up safety signs actually stop and look around for a moment before they do so and contemplate with the sign has any particular relevance to the environment that their placing it in.