POWER August 2015 - 58

SAFETY
Table 2. Arc-rated head and face protection. This table is from 1910.269 Occupational
Safety and Health Standards Appendix E. Source: OSHA
Minimum head and face protection
Exposure
Single-phase, open air
Three-phase
None *
2-8 cal/cm2
2-4 cal/cm2
Arc-rated faceshield with a
minimum rating of 8 cal/cm2*
9-12 cal/cm2
5-8 cal/cm2
Arc-rated hood or
faceshield with balaclava
13 cal/cm2 or higher †
9 cal/cm2 or higher ‡
Notes:
* These ranges assume that employees are wearing hardhats meeting the specifications in § 1910.135 or §
1926.100(b)(2), as applicable.
† The arc rating must be a minimum of 4 cal/cm2 less than the estimated incident energy. Note that § 1910.269(l)(8)
(v)(E) permits this type of head and face protection, with a minimum arc rating of 4 cal/cm2 less than the estimated
incident energy, at any incident energy level.
‡ Note that § 1910.269(l)(8)(v) permits this type of head and face protection at any incident energy level.
proximately 8 cal/cm2
results in a third-degree
burn (all at six cycles or 0.10 second).
OSHA notes in Appendix E that it expects
cotton clothing to reduce the heat received to
the body to 1.2 cal/cm2
energy is 2 cal/cm2
.
Conversely, the 70E committee rejected
using cotton clothing as a form of arc-protective
clothing, as cotton will ignite and burn
vigorously at some value of incident energy.
Given the fact that arcs in field conditions are
somewhat unpredictable, we felt it was the
proper thing to do. (I served on this panel.)
Even though the 2.0 cal/cm2
or less when incident
that was estimated.
threshold seems
reasonable at first glance, the end result of an
arc flash event may not necessarily be the 2.0
cal/cm2
All calculations of this type, regardless
of methodology or software package,
are estimates based on fixed values. Most
people associated with the electric power
industry know that the power system is
not static, and there are a host of variables
that cannot always be accounted for. At
least in industrial facilities being fed by
a power transformer, its impedance will
help dampen surges and transients somewhat,
along with the capacitor banks that
are often installed.
I advise workers covered by 1910.269
and 1926 Subpart V to consider the consequences
of receiving a second-degree burn
versus the onset of a second-degree burn. It
may seem like a small difference, but with
the onset of a second-degree burn, the skin
is not uniformly blistered, and chances of
infection are relatively small. As incident
energy increases above the onset of seconddegree
burn threshold, the blistered area
will be larger. Blistering from burns greatly
increase the risk of infection, and with antibiotic-resistant
germs cropping up in some
medical facilities, survival chances are not
nearly as good.
Protecting Extremities
A worker's hands must be protected from an
electrical arc, and OSHA notes that wearing
rubber insulating gloves and leather protectors
is adequate protection. If the worker is
wearing heavy-duty leather gloves, they are
considered adequate up to about 14-cal/cm2
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