In the UK, over 50V is generally thought of as being potentially
lethal to someone who's cut and has a dodgy heart, i.e. Anything
over 50V AC should be protected.  50V DC will also give a slight
jolt.  Saying that though, I never been harmed by 50V and I
consider it safe for me to work on live, however I'm aware of
the risks/precautions.  Play it safe and put perspex over it.