Rule 1 of 40 · Chapter I — Before You Touch Anything
Assume every conductor is live
Why this rule exists
A wire tells you nothing by looking at it. Insulation color, a flipped switch, a labeled breaker, none of these prove the absence of voltage. Backfed circuits, mislabeled panels, shared neutrals, and induced voltage from parallel runs all put potential where you did not expect it. NFPA 70E builds its entire electrically-safe-work-condition process on this assumption for a reason: the cost of being wrong is measured in burns and cardiac arrest, not inconvenience. Treat every conductor as energized until your own test says otherwise.
In practice
Before contact, identify the circuit and its source. Open the disconnect, then verify at the conductor with a tester you trust. Keep your hands off any conductor, terminal, or bus you have not personally tested dead. When you leave and return, re-verify, because someone may have re-energized it. Treat the load side of a switch as live even when the switch is off, because switches fail and get bypassed. Build the habit so it runs without thinking, on the day you are distracted.
When it doesn't apply
There is no exception to assuming live. What changes is the verification method: on high-voltage or utility-owned equipment, the tester, PPE, and procedures differ, and that work belongs to qualified crews. If you cannot establish and verify a de-energized state, stop and call someone who can.