The heart is -essentially- electrically controlled, in a complex pattern. Timing is everything.
There's also a world of difference between lab-bench experimentation and the real world, where the body, through mechanisms still not completely understood, provides the heart with a great deal of protection.
To further muddy the waters, I was present on one job, where the heart was shut down as the result of a shock to the brain. Again, the exact biological mechanisms are not nearly as well understood as we would like.
Information presented in electrician classes usually refer to 70mA as a level of shock that can cause heart problems and death. However, one needs to pay a great deal of attention to the 'fine print' that accompanies that statement. There are plenty of instances where far lower amounts have caused serious injury.
I used to have to teach this chart and I hated the whole concept. It implies there is some safe or acceptable level of shock and that is just courting disaster. Everyone is different but it is safe to say 100ma (a tenth of an amp) can kill the healthiest person and that kind of current is easily available on just about any fault condition on a line voltage circuit. "Freeze" current, when you can't let go happens right around the high end of a GFCI trip level. Your heart is in danger right then.
We receive quarterly formal safety training. Below is a copy of a handout on the effect of electricity on the body. The best protection against electrocution is to deenergize the equipment you are working on, but if that is not possible then the use of rubber gloves becomes very important.
How Electrical Current Affects the Human Body Three primary factors affect the severity of the shock a person receives when he or she is a part of an electrical circuit: • Amount of current flowing through the body (measured in amperes). • Path of the current through the body. • Length of time the body is in the circuit. Other factors that may affect the severity of the shock are: • The voltage of the current. • The presence of moisture in the environment. • The phase of the heart cycle when the shock occurs. • The general health of the person prior to the shock. Effects can range from a barely perceptible tingle to severe burns and immediate cardiac arrest. Although it is not known the exact injuries that result from any given amperage, the following table demonstrates this general relationship for a 60-cycle, hand-to-foot shock of one second's duration:
Current level (in milliamperes) Probable effect on human body
1 mA Perception level. Slight tingling sensation. Still dangerous under certain conditions.
5 mA Slight shock felt; not painful but disturbing. Average individual can let go. However, strong involuntary reactions to shocks in this range may lead to injuries.
6-30 mA Painful shock, muscular control is lost. This is called the freezing current or "let-go" range.
50-150 mA Extreme pain, respiratory arrest, severe muscular contractions. Individual cannot let go. Death is possible.
1000-4300 mA Ventricular fibrillation (the rhythmic pumping action of the heart ceases.) Muscular contraction and nerve damage occur. Death is most likely.
10,000 mA Cardiac arrest, severe burns and probable death.
I've seen other charts that further break this down into men and women, and give other ranges; I think body size has a lot to do with it. After a point, it becomes academic; when you're talking safety and prevention, it doesn't really matter whether 50ma will kill you or it takes 55ma. GFCI will protect from fatal shocks, but typical ground fault protection will not.
On the IBM chart it tops out at 500ma and says that may cause tissue damage, long after you are dead. I assume they err on the safe side. The cited list above is the OSHA chart. Just the fact that they are not the same points out this is all conjecture. I used to point out to the guys I was pitching safety to, that these studies are usually performed using college students who had to pass a physical before they could participate. This is a projection of what it takes to kill a healthy 20 year old. That ain't anyone here. I hate the idea of anyone walking away thinking there is a safe shock when you are dealing with electricity. A 10ma "freeze" could quickly rise to a hundred ma when the victim starts to sweat.