Rule 6 of 40 · Chapter I — Before You Touch Anything
Verify at the exact point of work
Why this rule exists
Voltage verified at the panel does not prove the conductor in your hand is dead. Wiring errors, unknown junctions, and multiple feeds mean the state upstream is not always the state downstream. A shared neutral can carry current from an adjacent live circuit even when yours is off. The only reading that protects your hands is the one taken where your hands will be. Distance introduces assumptions, and assumptions are what hurt people. Test the actual conductors you will contact, at the location you will contact them.
In practice
After de-energizing and locking out, bring your tester to the workpoint. Test line-to-line, line-to-neutral, and line-to-ground on every conductor in the box or enclosure, not just the one you think is hot. Include neutrals and the equipment ground in your checks; a floating neutral can sit at a dangerous potential. If you open the enclosure and find more conductors than expected, stop and identify them before proceeding. Re-verify if you leave and return.
When it doesn't apply
On large or complex equipment, qualified workers may apply safety grounds after verifying dead, which both proves and maintains the de-energized state. For simple branch-circuit work the tester at the workpoint is the standard. If the conductor arrangement is beyond what you can safely identify and test, stop and bring in someone qualified.