Rule 12 of 40 · Chapter II — The Body and the Current
Fear the arc flash, not only shock
Why this rule exists
Shock needs contact; arc flash does not. A fault can ionize air into a conductive plasma reaching thousands of degrees, brighter than the sun, with a pressure blast that throws molten metal and shrapnel. You can be badly burned a foot away having touched nothing. NFPA 70E exists largely for this hazard, incident-energy analysis, boundaries, and arc-rated PPE. A dropped tool across a bus, a loose lug, or a wiring error can start it in an instant. Respect the energy in the equipment, not only the voltage on the wire.
In practice
The surest control is to de-energize and verify, since no energy means no arc. When work must be done near energized gear, respect the arc-flash boundary, wear arc-rated PPE matched to the incident energy, and remove watches and rings. Do not stand directly in front of equipment while operating a breaker under load; stand to the side. Use insulated, rated tools and keep them from bridging conductors. Close panels you are not working in. Treat every large service, motor, and industrial panel as capable of a serious arc.
When it doesn't apply
Arc-flash energy scales with available fault current and clearing time, so a small branch circuit poses far less than a large service, but less is not none. Determining incident energy and proper PPE is engineering work; without that analysis for the equipment in front of you, treat it as hazardous and keep clear, or bring in a qualified professional.