Rule 8 of 40 · Chapter II — The Body and the Current
Respect current, not just voltage
Why this rule exists
Voltage is the pressure; current is what does the damage. It is milliamps through the body, not volts across it, that stop muscles and hearts. Ohm's law ties them together, current equals voltage over resistance, so a low voltage across low body resistance can still push a lethal current. Around 10 milliamps you may not be able to let go; 50 to 100 milliamps across the chest can trigger ventricular fibrillation. So do not relax around 120 volts because it is not high voltage. At the body's resistance, it is plenty.
In practice
Treat every ordinary branch circuit as capable of killing you, because it is. Assume worst-case body resistance, wet skin, a good ground, a hand-to-hand path, rather than the reassuring dry-hand numbers. Keep current from finding a path through you: insulate yourself, use one hand, stand on a dry insulating surface. Where current could reach you despite precautions, rely on GFCI protection to cut it before it accumulates. Never gauge danger by voltage rating alone; gauge it by the current a fault could drive through your body.
When it doesn't apply
Very low voltages, nominal 24 volts and below in dry conditions, rarely drive dangerous current through intact skin, which is why control and doorbell circuits are treated differently. But wet or broken skin lowers resistance enough that even these deserve respect. Do not stretch low voltage is safe to cover the fault energy such circuits can still release.