There was an uproar
when Federal OSHA issued a rule that recommended training and careful selection
of clothing for electrical workers. Experts on this subject disagreed on
approaches, feasibility, costs and most strongly, the need. While tempers rose
and lawsuits were filed or threatened, some utilities took a "wait-and-see"
attitude. Compliance, which was required by May 1994, was slow while speculation
brewed that the rule would be withdrawn or changed dramatically. OSHA, however,
has remained steadfastly committed to the rule and has offered a number of
clarifications. As more electrical workers wear specialized clothing there is a
history of "saves" developing. This rule has reduced the severity of burn
injuries and has saved lives.
Confusion continues, however, about how
to comply with 1910.269 (1)(6)(iii). This part of the rule prohibits ...
"clothing that, when exposed to flames or electrical arcs, could increase the
extent of injury that would be sustained by the worker." Certain fabrics and
fibers are prohibited. Acetate, nylon, polyester and rayon used alone or in
blends are listed as unsuitable unless included in a flame-resistant fabric that
meets the requirements of ASTM F 1506-98. ASTM F 1506 is a specification for
fabrics used in apparel designed to protect electrical workers who may be
exposed to momentary electric arc hazards and related thermal hazards.
Performance standards in the specification identify flame-resistant fabrics that
are serviceable and durable to extended laundering. OSHA offered the
clarification that flame resistant clothing is acceptable under all conditions
if it is made of fabrics that meet ASTM F 1506.
OSHA issued additional
clarifications to assist compliance officers and others gain a better
understanding of the rule. One such clarification deals with untreated cotton
clothing. Eleven-ounce cotton was described in the preamble of the rule because
it was determined as being suitable for a specific arc hazard. Under certain
circumstances, non flame-resistant fabrics such as untreated cotton are
acceptable. To determine if they are suitable, an employer must first assess the
hazards present in the workplace and then evaluate the performance of the
selected fabrics: F195899 is the standard method designed to evaluate untreated
fabrics for ignition when subjected to an electric arc. The test method
describes the use of shirts of a standard design made of the fabric in question.
A series of at least 50 "shots" must be conducted to produce sufficient data.
Complex statistical methods are then used to predict the probability of
ignition. OSHA has discussed introducing an appendix to 1910.269 that would
pinpoint the appropriate level of probability of ignition to select when using
the method to select a suitable fabric. The draft appendix has been circulated
to "stakeholders" and is likely to be incorporated. It would serve as a
clarification. As written, the method allows one to choose the level of
probability of ignition. Clearly, textiles that would not ignite at a 50%
probability may ignite if a probability of 95% were required. The appendix will
also address the issue of what distance from the arc to use in calculations
because OSHA points out that accidents usually happen at less than the approved
approach distance.
Simply put, if an employer determines that workers are
potentially at risk of exposure to an accidental electric arc there are two
choices. They may select clothing made of flame resistant fabrics that meet the
requirements of the ASTM F 1506 specification. Alternatively, they may evaluate
their non flame-resistant clothing materials using ASTM F 1958-99. Proper
selection of either of these alternatives will yield a choice that will satisfy
the requirement of not..."increasing the extent of the injury" and will also
meet OSHA's original objectives in developing the rule.
Through years
of study, experiment and experience, a body of knowledge about the interaction
of electric arc and clothing materials is emerging. Although many think that low
voltage systems do not represent an arc hazard, this has been disapproved. In
addition to system voltage other important factors include severity of the
available fault current; duration or cycles; the gap or length of the arc and
proximity to the arc. Arcs generated in a confined space, also know as "arc in a
box," are intensified compared to those generated in an open situation such as
on a pole. Given all these considerations, one cannot assume that a low voltage
system does not present a significant arc hazard. With this information some
electric utilities have made engineering changes to reduce the hazard or used
appropriate clothing as a solution, or both.
One rule of thumb to
remember about flame-resistant clothing is that the weight of the fabric is
generally directly related to arc resistance. Fabric weight is expressed in
weight per area such as ounces per square yard (oz./yd. 2) or grams per square
meter (gm./m. 2). Heavyweight fabrics that are flame-resistant generally offer
more protection and thus have higher Arc Thermal Protective Values (ATPV)
ratings than lightweight fabrics of the same fabric or fiber type. Likewise,
heavyweight non flame-resistant fabrics, such as untreated cotton denim, are
generally less likely to ignite from an electric arc than are lightweight
fabrics of the same fabric or fiber type. ATPV is measured by standard test
method F1959-99 using fabric samples over sensors exposed to electric arc. The
following table shows that the fabrics listed in ascending order of weight are
also in ascending order of arc thermal protection value (ATPV). All fabrics
listed meet the F 1506 specification.
Another consistent rule of thumb is that two
layers of a flame-resistant fabric are more protective than one might guess from
the total weight. If you added the ratings of the single layers in a system, you
would expect much less protection than the layers provide. Examples of layered
combinations in use would be a two-layer flash jacket, a shirt tucked into pants
or with a coverall over shirt and pants. Thus, many clothing programs
incorporate several layers of flame resistant fabrics to achieve added
protection from electric arc hazard.
The same principals do not
necessarily apply when layering a combination of flame-resistant and non
flame-resistant fabrics. This is because the test method is designed to raise
the available arc energy until the values are close to the predicted onset of
second degree burn using the Stoll bum calculation. The energy to achieve the
Stoll curve may be sufficient to break open the outer fabric, exposing the under
layer to the possibility of flaming ignition. For that reason the break open
threshold, not ATPV, is reported and this would be at a lower level of energy.
At high energies most flame-resistant and non flame-resistant fabrics have a
propensity to break open in the impact of the arc. An example of this is a
flame-resistant shirt over a cotton undershirt. A much higher level of
protection would be achieved if a flame-resistant undershirt were used. Most
clothing programs advocate the use of cotton underwear because although it may
bum in an arc accident, it will not melt and drip and stick to the skin as a
synthetic would.
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As ASTM continues to test and redefine the specifications
and test methods and as customers gain more experience in selection of fabrics
for protective clothing programs, it is possible that rather complex clothing
solutions will be discussed. It is important to stay focused first on compliance
with the OSHA rule as written and clarified. While it is possible to use the
available tools to match the hazard with the clothing, remember that the use of
flame-resistant clothing that meets F1506 constitutes immediate compliance. The
reason for this is simple, regular lightweight clothing will readily ignite in
an electric arc accident and it will continue to burn, adding to the burn
injury. This includes clothing made of natural fibers such as wool and cotton.
Any fabric that ignites and continues to burn will "increase the extent of
injury."