MEETING NFPA 70E REQUIREMENTS FOR
On January 7, 1976 a new electrical standards development
committee was formed to assist the Occupational Safety and
Health Administration (OSHA) in preparing electrical safety standards.
This committee was needed for a number of reasons.
First, when adopted by Congress, OSHA incorporated the
1971 National Electrical Code®. OSHA refrained from adopting
later editions due to the danger that the NEC® requirements would
be significantly changed due to the required public comment.
Second, the NEC is an installation manual, while OSHA
addresses employee safety in the workplace.
Third, not all sections in the NEC are safety related.
Fourth, many safety related work and maintenance practices
are not covered, or not adequately covered, in the NEC.
As a result, the idea of a new standard was conceived. It consisted
of four parts;
Part I Installation Safety Requirements
Part II Safety-Related Work Practices
Part III Safety-Related Maintenance Requirements
Part IV Safety Requirements for Special Equipment
Since each part was independent of the others, it was decided
that they would be published as they were completed. The new
standard (NFPA 70E, Standard for Electrical Safety Requirements
for Employee Workplaces) was first published in 1979 and consisted
of only Part I.
The second edition, published in 1981, consisted of the original
Part I and a new Part II. The third edition was published in
1988. It included a revision of the original Parts I and II and a new
Part III. The fourth edition (1995) is a major rewrite of existing text.
Part IV will be developed at a later date.
The following will explore some of the new requirements in
Part II as they pertain to the protection of workers against burns
caused by electric arcs.
I. ARC BASICS
When a maintenance worker, that is “working a panel hot,”
goes to ground or phase to phase with a screwdriver, an arc is
often formed. The temperature at the ends of an arc can reach
approximate 35,000°F, or about four times as hot as the surface of
the sun. These temperatures can easily cause serious or fatal
burns to exposed skin and/or ignite clothing.
II. SAFE WORKING DISTANCE FORMULA
Because employees were being seriously burned by electric
arcs, NFPA 70E adopted formulas to define the safe working distance
from a potential arc. The formulas for this calculation are
based upon the work and a technical paper by Ralph Lee, “The
Other Electrical Hazard: Electrical Arc Blast Burns,” IEEE
Transactions on Industrial Applications, Volume IA-18. No.3,
Lee’s work showed, for example, that skin temperature above
96°C for .1 sec. resulted in total destruction of the tissue (incurable
burn) and that skin temperature below 80°C for .1 sec. allowed for
skin which could be cured (just curable burn). At a distance of 3
feet, the arc energy required to produce these temperatures was
determined to be 23MW and l7MW respectively. He also found that
the maximum arc energy occurred when it represented 50% of the
available three phase bolted fault. Therefore, the arc from a
46MVA available source for .1 second could cause an “incurable
burn” at a distance of 3 feet. And, the arc from a 34 MVA available
fault for .1 seconds at 3 feet would result in a “just curable” burn.
Following are the formulas developed by Mr. Lee and incorporated
into NFPA 70E.
Dc = (2.65 ≈ MVAbf x t)1/2
Df = (1.96 ≈ MVAbf x t)1/2*
Dc = distance in feet for a “just curable” burn
Df = distance in feet for an “incurable burn”*
MVAbf = bolted three phase MVA at point of short-circuit
= 1.73 ≈ VOLTAGEL-L ≈ AVAILABLE SHORT-CIRCUIT
CURRENT ≈ l0-6
t = time of exposure in seconds
*Not included in NFPA 70E.