This footnote seems to place the small facilities back into the position of collecting data and calculating short circuit current and clearing times to justify using the 4 foot boundary however, in the vast majority of small facilities, if the electrical system was properly designed and if it has been properly maintained by competent electricians (always installing properly sized fuses and circuit breakers), the 4 foot boundary should be more than adequate to avoid any permanent injury from an arc-flash. 70E addresses this limitation in a footnote, qualifying that the 4 foot boundary is only applicable where the available short circuit current does not exceed 50,000 amperes and the clearing time of the fuse or circuit breaker does not exceed 0.1 seconds, or any combination not exceeding 5,000 ampere seconds. In a few cases the opposite may be true the 4 foot boundary may be inadequate to avoid injury due to high incident energy.

In most small facilities, the 4 foot boundary is likely overly restrictive making it probable individuals will attempt to avoid use of the PPE, potentially resulting in enforcement issues. The default boundary extends 4 feet from the energized exposed components, and anytime an individual is inside this boundary they must wear proper PPE to avoid a permanent injury in the event of an arc-flash. It appears the authors of 70E realized this, so they established a default flash boundary that can be used without calculations. In small facilities such as small businesses and offices that only use 240 volts and less and have minor power requirements (primarily lighting and receptacle loads), it may not be practical or economical to calculate arc-flash boundaries. The only practical way of determining this boundary is to calculate the magnitude of the arc (a function of the available short circuit current), estimate how long the arc will last (a function of the interrupting time of the fuse or circuit breaker), and then calculate how far away an individual must be to avoid receiving an incident energy of 1.2 cal/cm2. This distance (boundary) cannot be determined by casual survey of electrical equipment. Research has shown that permanent injury results from an arc-flash that causes an incident energy of 1.2 calories/centimeter2 (cal/cm2) or greater at the skin's surface.

The arc-flash boundary is a distance at which a person working any closer at the time of an arc-flash may receive permanent injury (the onset of a second degree burn or worse) if not properly protected by flame-resistant (FR) clothing. Equipment energized below 240V need not be considered unless fed by a 112.5KVA transformer or larger. This includes conducting activities such as examination, adjustment, servicing, maintenance or troubleshooting. Arc-Flash BoundaryĪrc-flash boundaries are required around electrical equipment such as switchboards, panelboards, industrial control panels, motor control centers, and similar equipment when an individual works on or in the proximity of exposed energized (energized and not enclosed, shielded, covered, or otherwise protected from contact) components. Although 70E does not explain how to conduct an analysis it does say the analysis shall determine a flash protection boundary and the personal protective equipment (PPE) requirements when working within that boundary. On the subject of arc-flash, 70E requires a flash hazard analysis.

Arc flash protection boundary