Current kills. Period. You have to have enough voltage to overcome your body's resistance, but once that condition is met it is the current that determines lethality.
That's an approximation of the truth. If the current is at a fatal level you could also argue the voltage is at a fatal level by re-arranging the equation.
It's better to break down how electricity can kill you by different means. I only know of a couple:
- Organ failure through communication interruption (eg fibrillating/stopping a heart, causing electrical issues in brain)
- Dissipating lots of power in your cells -> organs cooked or blood poisoning from dead cells (burns)
Communication disruption I don't know much about this, but it's why AC can be more dangerous than DC. It's also why certain 'paths' (eg hand to foot vs hand to hand) can be less or more safe/dangerous. Only tiny amounts of power/current are necessary for this.
Given that human R is decently high in most scenarios you need a reasonably high voltage for a effective amount of current to flow. Hence the 'fear voltage' saying, which is
slightly better than 'fear current' but still misses the complexity of some situations. It is better advice however to give to an average joe, as most power supplies you can kill yourself on are
constant-voltage of some variety.
Cooking Ie raw watts over time. In certain situations you are kept in the circuit because you lose control over eg your hands and they grip hard. In other situations you release immediately. Some current levels are actually safer than ones below them because they ensure you can't hang on to die
but this is heavily situation dependant.
Overall I prefer not to tell people to fear current, as I've encountered people that think car batteries ("100s of amps") are more dangerous than ("10A") wall powerpoints. If anything say voltage is a better way to judge situations simply because it's statistically more likely people will electrocute themselves on a constant-voltage (AC or DC) source than a constant current or more complex one.
Opinions on 'safe voltage' levels vary Depending on the country and guidelines you will find everything from above 40V is dangerous to above 100V is dangerous. It really depends on the situation -- if you're sweaty and barefoot on a metal or damp floor, then think a bit more about that 50V AC supply a bit more than normal. If you're in rubber boots and thinking about what you're doing with your limbs then over 100V constant-voltage supply is
probably fine.
When I'm dealing with peers getting into electronics I generally say "please don't play with anything that can deliver over 40V without talking to me". It's an overly safe threshold BUT I don't want them setting fire to their projects either