It is widely understood that clothing made from non-flame resistant synthetic
fabrics, such as polyester, nylon and polyester/cotton blends, are not appropriate
when working on or near electrically energized parts and equipment. If
these garments are exposed to an electric arc flash they can ignite, melt and drip,
which can lead to severe contact burns to the skin. In fact, the OSHA 1910.269 and
NFPA 70E standards prohibit this type of clothing. However, many people still consider 100% non-flame resistant cotton fabrics to be safe in an electric arc flash. The only thing safer about 100% cotton is that it does not contain a meltable component; it will ignite just as readily in an arc flash. In fact, cotton fabrics present major hazards in arcs; they burn hotter than polycotton, and are generally worn in slightly heavier weights, which means more fuel for the fire. Once ignition occurs, more fuel and a hotter fire combine to cause severe burn injury.

Heavier fabrics generally require higher incident energies to ignite, and published
ignition thresholds often are quite close to the oz/sq/yd weight of the test fabric. This
has led to policies which allow use of 100% non FR cotton (of >11opsy), typically
denim jeans, where hazard analysis has determined arc energies are well below
8 calories. While at first glance this may seem reasonable, the logic is predicated
on two conditions that rarely, if ever, exist outside the lab. One involves the fabric, and the second involves molten metal created by the arc. The lab test that
generates ignition thresholds uses new fabric (the ASTM 1958 “ignitability” std),
not used garments, and creates a very“clean” arc with minimal molten metal
and a relatively stable placement, while an actual equipment arc usually involves a
large volume of molten copper or aluminum and the arc can ‘wander.’
Things can be very different in the real world because as garments age in use,
three things typically happen which each reduce energy necessary to ignite: 100%
cotton garments lose mass through linting and abrasion (and remember, weight is directly related to ignition threshold); cotton garments typically develop thin spots, often at the knees, elbows, collar and above pockets; these areas can ignite at much lower energies than the same fabric when new, and support flame spread to the rest of the garment; and flammable contaminants, whether they be hydrocarbons from work or fabric softener, bug spray, etc from home, will all reduce the amount of energy necessary to ignite the fabric. While it is recommended to thoroughly clean or retire an FR garment that becomes soiled with a flammable contaminant or worn out, the primary (and significant) difference between non-FR 100% cotton fabrics vs. FR is, if the garment is not cleaned or retired, FR garments will self-extinguish after the source of ignition is removed while non-FR 100% cotton will continue to burn long after the fuel that initially cased ignition has been consumed. This continued burning can be the difference between life and death.

 

The lab arc uses a thin wire to fault, and electrodes with a 12” gap; this creates much less molten metal than most utility or industrial equipment. Molten copper spraying out at 1900°F can easily cause ignition of non-FR cotton regardless of weight. Many manikins exposed to arc flash at the KEMA lab in Chalfont, PA (while dressed in FRC) were covered with copper to well below the knees, even though the faulted disconnect or splice was at chest level. Another rationalization used to support wearing of non-FR cotton pants is that arcs are expected to occur at torso height. The lab uses a Faraday cage to contain the arc, but of course there’s no such restrictor in real gear. While the equipment which arcs may be at chest level, the arc can travel out in any direction, including down, and there’s plenty of high speed video analysis of arcs in both low and high voltage equipment which shows exactly this phenomenon. Some hot stick video even shows several cycles “stay home” while the last cycle rockets 8-10 feet out, directly at the maniken.

Another rationalization is that working in a bucket obviates the need for FR pants, because, the theory goes, the bucket will protect the legs. Again, video analysis of arcs and lab tests paint a very different picture. Most people would agree that when an electrician is in a bucket, the work is almost always above the top lip, usually by several feet. We’ve just discussed how arcs in real life wander; there is frightening video of an arc shooting directly into a bucket at KEMA. The bucket then becomes a confined space, and in effect intensifies the arc. The non-FR pants instantly ignite, and burn so fiercely that the instrumented maniken required extensive repairs. Test witness Ed Smith, Shop Steward for the UWUA 1-2, says “There’s a dangerous but common misperception that heavy cotton won’t burn. In fact, it ignited easily, burned vigorously, and consumed the clothing quickly. If you work around electricity, you need to be in FR clothing.”