How much voltage can a human being carry safely?

The voltage doesn't determine the danger. Your body gets charged up

to several thousand volts when you scuff across the carpet on a dry day ...

just before you come up behind your sister and zap her ... but if you know

what you're doing, you can kill yourself with a 12-volt car battery.

The danger depends on how much current flows through your body, and

on the path it takes. Something like 15 mA (0.015 Ampere) of current can

contract a muscle with enough force to break a bone, or can stop the heart.

The current, and the path it takes, depend on . . .

-- the voltage between two points on your body,

-- where the two points are, and

-- the resistance of the contacts with your body, i.e. how easily the

connection gets through your skin to the warm slithery juices inside.