What 29 CFR 1926.404 requires (plain English)
29 CFR 1926.404 governs wiring design and protection for construction: the temporary electrical power that runs tools and equipment on a jobsite. Construction sites are especially prone to electrical shock: temporary wiring is exposed to weather, water, and physical damage; extension cords and portable tools are everywhere; and workers stand on damp ground and hold conductive tools. The standard's central protection against this is ground-fault protection for personnel.
The core requirement gives the employer two options. Under the first, all 120-volt, single-phase, 15- and 20-ampere receptacle outlets on the site that are not part of the permanent wiring and are in use by employees must have approved ground-fault circuit interrupters (GFCIs) for personnel protection. A GFCI monitors the current going out and coming back; if even a small amount (about 5 milliamps) leaks to ground (for example, through a worker's body) it trips in a fraction of a second, before the shock can be fatal. Under the second option, the employer may instead establish and implement a written Assured Equipment Grounding Conductor Program, with a competent person conducting daily visual inspections and periodic continuity and terminal tests of cords and equipment.
Beyond ground-fault protection, 1926.404 sets grounding and wiring-design requirements: equipment grounding conductors must be properly connected, the path to ground must be continuous and low-impedance, and outlet devices, cords, and receptacles must be suitable for their use. Together with the general construction electrical requirements (1926.416) and the equipment-use practices, it ensures that temporary construction power is designed and protected so a ground fault becomes a tripped GFCI rather than a fatal shock.