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 NFPA70E-2004 exacerbates this problem by classifying most equipment as "hazard class 0" (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. This is actually not the case. Scientific studies have demonstrated that metal enclosures can only contain electrical arc blasts of limited intensity and duration (Gammon & Matthews, 2003). Several common work practices and equipment failures can precipitate arc blasts that can exceed the structural limits of metal-clad enclosures and still cause injury to nearby workers. Additional hazards are present any time the metalclad enclosures have any intentionally installed openings, such as cooling vents. Research has revealed that no regulatory or manufacturing requirements mandate that the doors of metal-clad enclosures be of equal strength to the sides of the cabinets (Gammon & Matthews, 2003). This means that relying on closed and latched doors on enclosures to protect workers from arc blast hazards is sometimes inadequate and that wearing FR clothing even when working on locked enclosures is often a reasonable work practice.

The Dynamics of Electrical Arcs

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) and molten metal which are created by the initial fault that precipitated the arc. The oxygen (O2) component of the air 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 (Ferraz Shawmut, 2005). However, when an 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 deliberately 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 Lee (1982) and others reveal that the heat released in an electrical arc can rise to values of more than 35,000 ºF. This is approximately four times hotter than the surface of the sun. The metal in electrical panels will melt when heated to approximately 2,500 ºF (1,984 ºF for copper) (Kross, 2007).

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