Understand your shock path

Why this rule exists

Current only hurts you if it completes a path through your body. It enters at one contact point and must exit at another, hand to hand, hand to foot, hand to elbow on steel. No path, no current, no injury. That is why standing on a dry insulator with one hand behind your back matters, and why a bird on a wire is unharmed. Severity depends on the path (does it cross the heart?), the current, and the duration. Think in entry and exit points before you reach toward anything live.

In practice

Before you approach live parts, ask where current would enter and where it would leave. Break the likely path: insulate your feet, keep one hand clear, avoid contact between a working hand and any grounded metal. Remove rings, watches, and metal bands that create low-resistance contact points. Keep tools and your body from bridging line-to-ground or line-to-line. If de-energizing is possible, do it, since that removes the source and makes the path question moot. Otherwise, engineer the path so it cannot run through your chest.

Example

Dry skin, hand-to-hand: ~100,000 ohms
I = 120 / 100000 = 1.2 mA (felt, survivable)
Wet skin, hand-to-hand: ~1,000 ohms
I = 120 / 1000 = 120 mA
50-100 mA across the chest can stop the heart
Same voltage, wet skin: a lethal path

When it doesn't apply

Some paths you cannot fully control, a fall onto a bus, a tool slipping, which is exactly why GFCI protection and de-energizing exist as backstops. High enough energy can also injure by arc and blast without any conductive path through you. The path model explains shock; it does not cover arc flash, which needs distance and PPE instead.