Infrared Windows and Arc Ratings


Written by:
Tim Rohrer, IRISS, Inc.

Pumps & Systems, February 2009

A dangerous misconception exists regarding the "Arc Rating" of infrared (IR) windows or viewing panes. Many reliability and maintenance professionals are under the impression that an IR window will protect them in the event of an arc blast. Others believe that installing IR windows will turn non-arc-rated switchgear or electrical equipment into "arc-rated" cabinets. Neither is true. Both misconceptions need to be corrected because they present safety concerns.

All three major brands of IR windows are available as standard options on certain brands of switchgear, MCC buckets and other electrical equipment. All three brands have undergone extensive evaluation and testing as parts of arc-rated systems. Yet none of these companies should ever lead the public to believe that any of these tests or system certifications have any broad-based or generic rating across all varieties of switchgear or electrical equipment. The simple fact is that there is no such "component rating" for generic "arc-resistance." Any claims to the contrary are dangerous and negligent.

IR windows are not intended to protect a user from an arc flash; they are intended to eliminate additional triggers of an arc flash during an inspection and replace a high-risk activity with a risk reduction/elimination strategy during inspection. IR windows and closed-panel inspection help companies to comply with the OSHA and NFPA mandates to eliminate risk wherever possible; conversely, a protection strategy is acceptable only after other methods of risk elimination or reduction have been exhausted.

The Anatomy of an Arc Flash

An arc flash occurs when a phase-to-phase or phase-to-ground fault causes a short circuit through the air. The core of the arc flash can reach temperatures of up to 38,000-deg F (21,093-deg C); at this high temperature copper turns to a plasma state instantaneously and expands 67,000 times its original volume in a fraction of a second. The heat and resulting expansion cause a pressure wave that carries thousands of pounds of force, a blinding flash of light and molten shrapnel.

Differences in the volume (cubic feet) of the switchgear, MCC bucket or electrical cabinet will affect the amount of force that impacts the cabinet panel/IR window. This concept is analogous to the way that a small explosive placed in the center of an empty train car would have very different consequences than if it were placed in a mailbox. Similarly, a cabinet with large amounts of copper available for expansion would be capable of producing an explosion with much more force than the same cabinet with less copper cable or bus-bar. Other differences, such as the use of current limiting fuses or distance and position of the arc flash relative to the panel door/IR window, will have a major impact on the force that impacts the panel/window.

As a result, arc ratings are given to systems that are able to withstand a blast in a specific model of switchgear with a standard size and configuration.

Arc FlashMCC arc flash

Arc-Ratings

Because of the near-infinite variety in size, content, and position of connections, a one-size-fits-all rating for arc-resistance is impossible. As a result, arc fault tests are performed on systems, not on the individual components that comprise a system. Therefore, the resulting arc ratings are given to the system and not to the individual components that happened to be in place during the test. Consequently, any changes in switchgear design require re-testing to verify the new design's rating, even though it might be similar to the previous arc-resistant version.

Arc rated switchgear and MCCs enlist a variety of safety mechanisms, such as additional barriers and pressure relief mechanisms, that redirect the forces and heat of an arc flash away from the panel doors and up through a series of plenums that systematically reduce the blast's force and minimize any damage that may have occurred had the blast escaped the system's confines. Any IR window, visual viewing pane or panel meter that happened to be in place during this test would not have been responsible for the system's arc-resistance, but would merely be shown not to interfere with any safety mechanisms in place to redirect the blast. For instance, the bolts holding the panel in place are not universally arc-resistant; they were simply the proper strength to hold the panel in place on that specific model of switchgear.

Using an infrared window.Using an infrared window.

Consider this analogy: A luxury car manufacturer is preparing its new sedan for crash testing. They include the high-end stereo option manufactured by Uber-Audio, complete with voice-activated MP3 catalog features. As expected, the crumple zones in the sedan's frame absorb huge amounts of force on impact; safety belts keep test-dummies properly placed for maximum protection and airbags deploy to cushion the occupants. The stereo, as expected, does not interfere with the proper functioning of any of the safety mechanisms; it stays on its mount and does not impale the test-dummies. The car receives a five-star crash rating. Would you expect to see the Uber-Audio stereo company claim that its stereo system received a five-star crash rating? Would you expect to see advertisements leading consumers to believe that this stereo could actually protect passengers in the event of a crash? Of course not.

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