It's tempting to treat voltage as the danger scale - the higher the number, the scarier the equipment. But the human body doesn't respond to voltage. It responds to current, and how much current you get depends on voltage and resistance together, in a relationship Ohm's Law makes uncomfortably concrete.
What current actually does, by the milliamp
- ~1 mA - perceptible threshold. A slight tingle. Not dangerous on its own, but it means current is flowing through you.
- ~10-20 mA - the "let-go" threshold. Muscle contractions can prevent someone from voluntarily releasing an energized object - the "can't let go" grip that turns a brief shock into a sustained one.
- ~100-200 mA - ventricular fibrillation range. The heart's rhythm is disrupted and it quivers instead of pumping. This is the range that kills.
- Above ~1 A - severe burns, breathing paralysis, cardiac damage.
Why 120V easily reaches the lethal range
Body resistance isn't fixed. Dry skin might present 500-1,000Ω or more; wet skin or deep tissue contact can drop to 300Ω or less. Run the numbers on a fairly ordinary case: 120V across 500Ω of body resistance gives I = 120/500 = 240 mA - well into the fibrillation range, from a voltage most people consider harmless. Voltage alone never tells the whole story. Current path and contact resistance are doing most of the work.
Current path matters too. Hand-to-hand current crosses the chest and the heart directly. Hand-to-foot does the same. Foot-to-foot current - the kind you'd get from step potential during a ground fault - mostly affects the legs and is less likely to stop the heart, though it can still cause falls and burns.
Why GFCIs trip so early
A GFCI compares current on the hot and neutral conductors. In a healthy circuit they match exactly - all the current that leaves comes back. If current leaks somewhere else (through a person, damaged insulation, or a wet surface), that balance breaks, and the GFCI trips in about 1/40 of a second once the imbalance hits 4-6 mA. That's well below the 10-20 mA let-go threshold and far below the 100-200 mA fibrillation range - the trip point is a protective margin, not a "close call" number. A GFCI doesn't prevent shock; it limits how long and how severe it gets. And it has real limits of its own: it won't catch a line-to-neutral fault, and it won't help if someone contacts both hot legs simultaneously, since no current is leaking to ground in that case.
Grounding, bonding, and the rest of the shock-physiology material are covered in the Study Guide, with practice questions in the Exam Companion.