I am a dealer and installer of invisible fence systems (http://www.dogfencenw.co.uk) and in answer to your question are shock collars dangerous? no they are not! Granted in the wrong hands the dogs could be open to abuse which i disagree with as it could affect the dog mentally! A dogs skin is 25% more resistant than a humans so the correctional stimulous (shock) is just uncomfortable opposed to dangerous! I have felt the shock myself an its no worse than us touching a farmers cattle fence!

I hope this has answered your question.

Dog training is the art of teaching your dog skills, The electric collar can't be cruel, it is an inanimate object. By "wait!" you say, "surely it is cruel to use the device on a dog. You are an awful and mean person for even thinking the electric collar might be OK.