A common misconception among Electrical Workers and Electrical Engineers is that Flame Resistant (FR) clothing is not necessary when working on equipment that is enclosed inside a metal cabinet ("metal-clad"). The "Tabular Approach" for selecting FR clothing in the NFPA 70E-2004 exacerbates this problem by classifying most equipment as "Hazard Class Zero" (lowest hazard) when the equipment is contained inside a locked metal enclosure. Hazard Class 0 equipment does not require FR clothing. This has contributed to the belief that metal-clad enclosures can always be trusted to protect workers from electrical arc-blasts when this is actually not the case.
Scientific studies have demonstrated that metal enclosures can only contain electrical arc-blasts of limited intensity and duration. There are a number of common work practices and equipment failures that can precipitate arc-blasts that can exceed the structural limits of metal-clad enclosures and still cause injury to nearby workers.
Further, additional hazards are present anytime the metal-clad enclosures have any intentionally installed openings in them, such as cooling vents. Our research has revealed that there are no regulatory or manufacturing requirements mandating that the doors of metal-clad enclosures be of equal strength to the sides of the cabinets(1). This means that relying on closed and latched doors on enclosures to protect workers from arc-blast hazards is sometimes inadequate and wearing FR clothing even when working on locked enclosures is often a reasonable work practice.
An electrical arc is actually Electrical Current (measured in Amperes) flowing through the air via a conductive path comprised of conductive gases or vapors. The "conductive gases" are "ionized gases and plasma" mostly comprised of Ozone (O3) which is created by the initial fault that precipitated the arc. The Oxygen (O2) component of the air we breathe is actually a very good "insulator" with respect to conducting Electrical Current. The insulating property of Oxygen is what allows energized electrical terminals to be located within only a few inches of each other in electrical equipment without "flashing-over" to each other. However, when even a small arc occurs near electrical conductors, the energy of the arc converts the Oxygen to Ozone which creates a very conductive atmosphere within the electrical equipment. This reaction has the same effect as if a person were to purposely "short-out" (touch two electrically energized conductors, or one energized and one neutral or grounded conductor together) electrical components (an "Electrical "Fault") by bridging them with a metal conductor. The electrical system supplying the panel will then supply every available Ampere to the fault which precipitates the arc-blast that is often depicted in Safety videos about FR clothing.
Studies conducted by H.R. Lee and others reveal that the heat released in an electrical arc can rise to values of >35,000°F (2). This is approximately 4 TIMES hotter than the surface of the Sun. The metal in electrical panels will melt when heated to approximately 2,500°F (3). (1,984°F for Copper).
