What 29 CFR 1910.303 requires (plain English)
29 CFR 1910.303 sets the general requirements for electrical installations in general industry: the design-and-installation foundation of OSHA's electrical standards (Subpart S). Electrical hazards cause a disproportionate share of workplace fatalities through shock, arc flash, arc blast, and fire, and 1910.303 exists to ensure that the conductors and equipment workers rely on are inherently safe: approved for the use, examined for safety, properly installed, marked, guarded, and accessible for safe operation and maintenance.
Several duties anchor the standard. Conductors and equipment are acceptable only if "approved": a defined term (1910.399) meaning acceptable to the authority having jurisdiction, typically demonstrated through listing and labeling by a nationally recognized testing laboratory. Equipment must be examined for safety considerations including suitability for the use, mechanical strength, insulation, heating and arcing effects, and proper classification by voltage and current. Listed or labeled equipment must be installed and used in accordance with its listing instructions, and equipment must be marked with the manufacturer and ratings needed to judge safe application.
The standard also protects the people who work around electrical equipment. Live parts of equipment operating at 50 volts or more must be guarded against accidental contact, by approved enclosures, location, or barriers. Sufficient access and working space must be provided and maintained around electrical equipment to permit ready and safe operation and maintenance (with minimum clearances that increase with voltage), at least one entrance must give access to the working space, and the space must be illuminated and never used for storage. These are the conditions that let a qualified worker service equipment without contacting an energized part.