Layer your FR clothing

January 11, 2010
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An arc flash incident such as this one (captured on video) can have deadly consequences.


You don’t need to be Ben Franklin with a kite, a key and a wicked thunderstorm to know that electricity is dangerous. If you’ve ever seen an arc flash in person or on video, you’re well aware of the frightening force that an errant electrical current can pack. As a result, you know how important it is to take appropriate measures to minimize injury and avoid potentially fatal situations for employees who work with or near energized equipment.

We all know that some types of clothing are flammable — cotton and polyester, to name two. What many may not realize, however, is that under some exposure conditions — such as an arc flash — these flammable clothing fibers can ignite even when worn beneath flame-resistant (FR) clothing.

That’s why it’s so important for safety managers to rethink the standards they maintain for their employees’ personal protective equipment (PPE) and to require workers to layer only FR clothing in order to provide maximum safety — and comfort — and to achieve the required arc rating.

Is your PPE adequate?
In brief, an arc flash is an electrical phenomenon that results when electricity flows through a medium that it shouldn’t. In most cases, that medium is air, which becomes like a piece of copper in the way it conducts electricity. With air, however, you can see the massive discharge of the electrons from the discharging element. This is the arc flash, and you can think of it as a lightning bolt on a smaller, yet more deadly, scale.

The most common causes of arc flashes are equipment failure, human error such as the improper placement of tools or improper use of equipment, or the conduction of electricity due to foreign particles in the air (often metal shavings). But whatever the cause, a worker’s greatest fear with an arc flash is that its force may be powerful enough to break open all the FR clothing outer layers and ignite a non-FR layer that’s being worn as a base underneath.

This set of circumstances can result in more severe burn injuries to an expanded area of the body due to a “chimney effect” in which cotton, for example, will burn underneath the FR layers onto areas of the worker’s body that were not exposed by the arc flash. Polyester garments will result in even worse injury — the material can melt and drip onto skin and into wounds.

Layer only with FR clothing
In 2009, the National Fire Protection Association (NFPA) 70E Standard for Electrical Safety in the Workplace was revised to address safety gaps and increase electrical worker protection. The new 70E Standard permits cotton clothing to be worn as a base layer beneath a worker’s PPE, but the cotton layer does not count toward the PPE’s required arc rating.

Because of the safety hazards posed by cotton and the fact that it isn’t considered protective layering by the NFPA, it makes sense for workers to eliminate cotton and other flammable materials altogether and layer only with FR clothing. In fact, Annex M of the NFPA 70E regulations states, “the use of all FR clothing layers will result in achieving the required arc rating with the lowest number of layers and the lowest clothing system weight.”

As Annex M also notes, to achieve, for example, a total system arc rating of 40 cal/cm2, an arc flash suit with an arc rating of 40 cal/cm2 could be worn over a cotton shirt and cotton pants. Alternatively, an arc flash suit with a 25 cal/cm2 arc rating could be worn over an FR shirt and FR pants with an arc rating of 8 cal/cm2 to achieve a total system arc rating of 40 cal/cm2. This second approach provides the required arc rating at a lower weight and with fewer total layers of fabric, providing the required protection with a higher level of worker comfort and safety.

It is important to understand that the total system and arc rating cannot be determined by simply adding the arc ratings of the individual layers. The only way to determine the total system arc rating is to conduct a multi-layer arc test on the combination of all the layers assembled just as they would be worn.

By understanding the risks involved with arc flashes along with the solutions available to you through FR layering, there’s no reason you cannot combine safety and comfort to help ensure that your workers head home safe and sound at the end of every shift.

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