What 29 CFR 1910.333 requires (plain English)
29 CFR 1910.333 governs the safety-related work practices for electrical work in general industry: how workers must interact with electrical circuits to avoid shock, arc flash, arc blast, and electrocution. Where 1910.303 and 1910.305 address how systems are installed, 1910.333 addresses how people work on and around them. Its foundational rule is straightforward: de-energize first. Working on live parts is the exception, not the default.
The core requirement is that live parts to which an employee may be exposed must be de-energized before the employee works on or near them, unless the employer can demonstrate that de-energizing introduces additional or increased hazards (for example, interrupting life-support equipment) or is infeasible due to equipment design or operational limitations. When a circuit is de-energized, it must be locked out and tagged out and, critically, tested to verify it is actually dead (the "live-dead-live" test: verify the tester works on a known live source, test the de-energized circuit, then re-verify the tester). Only after verification may work proceed.
The standard also draws a sharp line between qualified and unqualified persons. Only qualified persons (those trained and familiar with the special precautionary techniques, PPE, insulating tools, and shielding needed) may work on or near exposed energized parts when energized work is justified. Unqualified persons must keep their distance: they may not approach exposed energized overhead lines closer than the minimum approach distances (for example, at least 10 feet for lines rated 50 kV or below, increasing with voltage). This de-energize-verify-or-keep-qualified-and-clear structure is what prevents the majority of electrical injuries.