In summary, metal-enclosed electrical installations are designed to provide protection and safety to unqualified persons, and should be marked with appropriate caution signs. Additionally, openings in equipment should be designed to deflect foreign objects away from energized parts to further increase safety.
NEC Article 110.31(B)
(1) In Places Accessible to Unqualified Persons.
Metal-enclosed electrical installations are designed to provide protection from direct contact with energized components and are a critical part of any electrical system accessible to unqualified persons. This includes switchgear, unit substations, transformers, pull boxes, connection boxes, and other similar associated equipment.
These metal-enclosed components are designed to be robust and durable, and to provide a high degree of safety to unqualified persons. Additionally, these components are often marked with appropriate caution signs that indicate the potential dangers of interacting with the electrical components and should be followed carefully.
To further increase safety, openings in ventilated dry-type transformers or similar openings in other equipment shall be designed to deflect foreign objects inserted through these openings away from energized parts. This can help to prevent accidental contact with live components and reduce the risk of injury.
(2) In Places Accessible to Qualified Persons Only.
Indoor electrical installations that are considered accessible only to qualified persons in accordance with the relevant safety regulations shall adhere to the requirements established by NEC 110.34, 110.36, and 490.24. These regulations provide guidance on the design and installation of electrical systems, and are intended to ensure the safety of qualified personnel who may be interacting with these systems.
NEC Article 110.31(C) Outdoor Installations.
(1) In Places Accessible to Unqualified Persons.
Outdoor electrical installations that are open to unqualified persons must comply with the safety regulations established by Parts I, II, and III of Article 225.
(2) In Places Accessible to Qualified Persons Only.
For outdoor installations with exposed live parts, these installations should be accessible only to qualified personnel in accordance with the safety regulations established by NEC 110.34, 110.36, and 490.24. To ensure the safety of qualified personnel, openings in equipment should be designed to deflect foreign objects from energized parts and nonmetallic or metal-enclosed equipment should be designed so that exposed nuts or bolts cannot be removed. Where nonmetallic or metal-enclosed equipment is accessible to the public and the bottom of the enclosure is less than 8 feet above the floor or grade level, the enclosure door or hinged cover must be locked. Underground box covers that weigh over 100 pounds must also be locked, bolted, or screwed on.
NEC Article 110.31(D) Enclosed Equipment Accessible to Unqualified Persons.
Indoor electrical installations that are accessible only to qualified personnel must comply with the safety regulations established by NEC 110.34, 110.36, and 490.24. To ensure the safety of qualified personnel, openings in equipment should be designed to deflect foreign objects from energized parts and nonmetallic or metal-enclosed equipment should be designed so that exposed nuts or bolts cannot be removed. Where nonmetallic or metal-enclosed equipment is accessible to the public and the bottom of the enclosure is less than 8 feet above the floor or grade level, the enclosure door or hinged cover must be locked. Underground box covers that weigh over 100 pounds must also be locked, bolted, or screwed on.