Rule 11 of 40 · Chapter II — The Body and the Current
Know the let-go threshold
Why this rule exists
There is a current above which your own muscles betray you. Past roughly 5 to 10 milliamps through the hand, the flexor muscles clamp and you cannot release your grip, the let-go threshold. The longer you hold, the more the current dries and heats your skin, lowering resistance and raising the current further, a vicious circle. That is how a supposedly survivable shock turns fatal: the victim cannot let go, the path stays closed, and the heart is soon affected. Knowing this changes how you position your hands.
In practice
Approach potential contact so that if you are shocked, your hand is thrown clear rather than clamped on. Use the back of your hand to check for warmth or vibration near suspect equipment, so a clench pulls you away, not into it. Keep one hand out of the circuit entirely. Rely on GFCI protection, which trips in milliseconds at a few milliamps, well below the let-go level, as your margin. And de-energize, because you cannot be frozen to a wire that is not live.
When it doesn't apply
Let-go thresholds are lower for children and vary with body size, skin condition, and frequency; DC behaves differently than AC. Do not treat any specific milliamp figure as a safe ceiling, it is a danger floor. Above it you are in trouble; below it you can still be hurt. The only safe current through you is none.