NFPA
NFPA 70E
Standard for Electrical Safety in the Workplace
The working practice standard that sits on top of an arc flash study: the justification required to work on or near energised equipment, the energised work permit, approach boundaries, PPE category selection, and the establishment of an electrically safe work condition. Its central idea is that de-energising is the default and energised work is the exception requiring justification, which is the opposite of how most plants actually schedule instrument and switchgear work.
This is an explanation, not the standard. PipingInspector is not affiliated with NFPA and reproduces no part of the document. Always work to the current edition of the standard itself and to your project specification.