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ARC-FLASH PROTECTION - UNDERSTANDING NFPA 70E
By John
C. Klingler, P.E.
Lewellyn
Technology
Note: John will be presenting a short course on this
subsject at IMC-2005 December 6-9, 2005 in Tampa
You have decided to conform to the requirements of NFPA
70E, the standard for “Electrical Safety in the
Workplace”. You already have an electrical program for
preventing shock; here is an explanation of how to
address the 70E requirements for arc-flash.
Flash Hazard Analysis
On the subject of arc-flash, 70E requires a flash hazard
analysis. Although 70E does not explain how to conduct
an analysis it does say the analysis shall determine a
“flash protection boundary” and the personal protective
equipment (PPE) requirements when working within that
boundary.
Arc-Flash Boundary
Arc-flash boundaries are required around electrical
equipment such as switchboards, panelboards, industrial
control panels, motor control centers, and similar
equipment when an individual works on or in the
proximity of “exposed energized” (energized and not
enclosed, shielded, covered, or otherwise protected from
contact) components. This includes conducting
activities such as examination, adjustment, servicing,
maintenance or troubleshooting. Equipment energized
below 240V need not be considered unless fed by a
112.5KVA transformer or larger.
The arc-flash boundary is a distance at which a person
working any closer at the time of an arc-flash may
receive permanent injury (the onset of a second degree
burn or worse) if not properly protected by
flame-resistant (FR) clothing. Research has shown that
permanent injury results from an arc-flash that causes
an incident energy of 1.2 calories/centimeter2
(cal/cm2) or greater at the skin’s surface.
This distance (boundary) cannot be determined by casual
survey of electrical equipment. The only practical way
of determining this boundary is to calculate the
magnitude of the arc (a function of the available short
circuit current), estimate how long the arc will last (a
function of the interrupting time of the fuse or circuit
breaker), and then calculate how far away an individual
must be to avoid receiving an incident energy of 1.2
cal/cm2.
Small Facilities
In small facilities such as small businesses and offices
that only use 240 volts and less and have minor power
requirements (primarily lighting and receptacle loads),
it may not be practical or economical to calculate
arc-flash boundaries. It appears the authors of 70E
realized this, so they established a default flash
boundary that can be used without calculations. The
default boundary extends 4 feet from the energized
exposed components, and anytime an individual is inside
this boundary they must wear proper PPE to avoid a
permanent injury in the event of an arc-flash.
In most small facilities, the 4 foot boundary is likely
overly restrictive making it probable individuals will
attempt to avoid use of the PPE, potentially resulting
in enforcement issues.
In a few cases the opposite may be true; the 4 foot
boundary may be inadequate to avoid injury due to high
incident energy. 70E addresses this limitation in a
footnote, qualifying that the 4 foot boundary is only
applicable where the available short circuit current
does not exceed 50,000 amperes and the clearing time of
the fuse or circuit breaker does not exceed 0.1 seconds,
or any combination not exceeding 5,000 ampere seconds.
This footnote seems to place the small facilities back
into the position of collecting data and calculating
short circuit current and clearing times to justify
using the 4 foot boundary; however, in the vast majority
of small facilities, if the electrical system was
properly designed and if it has been properly maintained
by competent electricians (always installing properly
sized fuses and circuit breakers), the 4 foot boundary
should be more than adequate to avoid any permanent
injury from an arc-flash.
Other Facilities
For other facilities, especially those having employees,
contractors, or service personnel that perform functions
exposing them to energized components, the 4 foot
default boundary is probably not practical or
appropriate. The experience of this author indicates
that a substantial percentage of the equipment operating
at 480 volts and less in most facilities will have an
arc-flash boundary of less than 12 inches, which means
FR clothing for the face/chest area is not required when
working or near that equipment. However, experience has
also shown that practically every large facility has
some equipment where even the 4 foot default boundary is
not adequate to avoid permanent injury in the event of
an arc-flash. Consequently 70E provides an alternative:
a formula (based on IEEE Std 1584) to be used under
engineering supervision when the limitation of 5,000
ampere seconds is exceeded or when realistic flash
boundaries are desired. To use the formula requires
knowledge of available short circuit current and
corresponding clearing time. (See Sidebar 1)
Available Short Circuit Current
Determination of short circuit current starts with the
electric utility providing information about its
delivery capability at the service entrance/meter
point. Receiving this data from the utility can be as
easy as a phone call or as difficult as pulling teeth.
Proceeding from the service entrance to the equipment to
be worked on, the length, size and type of every
conductor and the nameplate information of every
transformer in that path must be recorded. With this
recorded data and the right software, a reasonable
estimate of the available short circuit current can be
calculated for use in the flash boundary formula.
Clearing Time
Determination of the arc-flash clearing time at
equipment requires collection of data on every fuse and
circuit breaker in the circuit between the utility
service and the equipment where the flash boundary is to
be determined. Time versus current interrupting
information is then acquired from the protective device
manufacturer based on the data collected.
Utilizing the short circuit current previously
determined and time-current data from the protective
device manufacturer, a reasonable estimate of the time
required to interrupt the arc-flash can be determined
for use in calculating arc-flash boundary.
Formula – Software - Consultant
For facilities having only a few circuits to be
evaluated, utilizing the 70E formula to determine the
arc-flash boundary may be feasible. However, if many
circuits are involved, commercially available software
or a consultant should be considered. Some commercially
available software performs all the calculations
required including determination of available short
circuit current, fault clearing time, and arc-flash
boundary. The price on this software can exceed
$10,000, and should be used under engineering
supervision. Keep in mind the data collection is still
required for input into the software program.
PPE Selection
NFPA 70E requires the employer provide and the employee
wear appropriate FR clothing and other PPE when within
the arc-flash boundary. Selection of FR clothing is
based on the level of incident energy the individual
will be exposed to in the event of an arc-flash. The
level of incident energy is a function of the distance
the individual is from the arc-flash (incident energy
increases rapidly as the individual moves closer to the
arc-flash). Generally 18 inches is assumed to be the
distance between a worker’s face/chest and the
arc-flash. Using the same information as was used to
determine the arc-flash boundary the engineer can
calculate the incident energy in cal/cm2 at
18 inches. Since FR clothing is rated in cal/cm2,
this allows selection of appropriate clothing to protect
against the incident energy of exposure.
It is not uncommon for calculated results at 18 inches
to show an arc-flash incident energy of less than 1.2
cal/cm2, resulting in no FR clothing
requirement for the face/chest area, only clothing that
will not melt such as cotton. However, additional PPE
may be required for parts of the body that are closer
than the 18 inch basis. It is also not uncommon to find
at least one location in facilities where the calculated
incident energy at 18 inches exceeds 40 cal/cm2,
the highest level that 70E recognizes as being practical
to protect (some clothing manufacturers offer clothing
with higher ratings).
Small facilities that choose to use the 4 foot default
boundary in lieu of using the formula will not have the
incident energy results necessary to select the proper
level of PPE for the arc-flash hazard. For these
facilities 70E provides two tables to use in selecting
PPE. The first table matches a “Hazard/Risk Category”
to a specific task by voltage level and type of
equipment. (See Sidebar 2) The second table describes
the FR clothing and corresponding incident energy for
each of five Hazard/Risk Categories. (See Sidebar 3)
Limitations of the NFPA 70E Tables
Use of the 70E tables to select PPE has limitations.
The first table matching the category to the task is
limited to electrical systems that do not exceed
specified levels of available short circuit current and
fault clearing times as described in the table
footnotes. Additionally, 70E states that for tasks not
included in the table and for electrical systems that
exceed the footnote limitations, the tables cannot be
used and the incident energy must be calculated for PPE
selection. Using the tables when the electrical system
exceeds the levels described may expose individuals to
hazardous energies beyond the protection of their FR
clothing, potentially resulting in serious injury or
death. On the other hand, when the footnotes are met,
the level of protection can be overly conservative,
which may increase hazards to the individual by limiting
vision, mobility, and dexterity. In other words, it is
always better to select the proper PPE based on the
calculated incident energy of exposure. Selecting PPE
based on incident energy may also result in substantial
savings over the cost of selecting PPE based on the
tables.
Labeling
Although not required by 70E, labeling of equipment is
an essential part of the Flash Hazard Analysis;
establishing an arc-flash boundary and determining the
appropriate PPE is useless if the information is not
communicated to the individuals working on or in near
the equipment with the hazard. The label should be
placed in a conspicuous location that will be seen by
individuals before opening the equipment. Since 2002
the National Electrical Code® (NEC) has
required labeling of equipment to warn of potential
flash hazards. Although the current NEC requirement
does not specify the information to be provided on the
warning label, it is likely that future editions will.
This author recommends that at a minimum the following
information should be included on the label:
Maximum Voltage in the Equipment
Arc-Flash Boundary
Required PPE (Hazard/Risk Category or cal/cm2)
Advantages of a Qualified Consultant
An arc-flash analysis by a qualified consultant should
provide more than just the results of the analysis. The
consultant should review each location that has an
arc-flash hazard requiring Category 1 FR clothing or
greater to determine if any changes can be made to
reduce the hazard. The consultant should evaluate
changing fuse types, breaker settings and other
opportunities to reduce or eliminate the need for FR
clothing. The result of these recommendations can be
substantial economic savings in FR clothing, as well as
reduction or elimination of arc-flash hazards.
The consultant should provide one-line drawings of the
electrical system evaluated and labels for all equipment
having the potential of a hazardous arc-flash.
As part of the short circuit analysis the consultant
should identify any problems in the interrupting
capacity of protective devices. Inadequate interrupting
capacity can result in the protective device exploding
during a major fault, potentially causing injury to
personnel and/or costly downtime.
The consultant should also make recommendations to
improve any overcurrent coordination problems. The
objective is for the interrupting device closest to the
fault to open first. This minimizes the equipment
effected in the event of a fault, improving operations
and safety by limiting exposure to electrical hazards
when troubleshooting.
Summary
Before purchasing FR clothing and requiring individuals
to wear clothing that they may or may not need, complete
an arc-flash hazard analysis. Identify the equipment
that has the potential to cause permanent injury or
death from arc-flash and then evaluate opportunities to
eliminate or reduce the hazard in lieu of using PPE.
After taking advantage of every feasible/realistic
opportunity to reduce or eliminate arc-flash hazards,
purchase or arrange through a uniform service to provide
the appropriate PPE. Label equipment with the
information necessary for individuals to know the hazard
and the required PPE. (This information is also
essential for contractors and service personnel that
work on or near exposed energized components.) Train
qualified and affected personnel on how to recognize and
avoid electrical hazards (shock and arc-flash), and
train them on the results of your Arc-Flash Hazard
Analysis!
John C. Klingler, P.E. is Vice President of
Lewellyn Technology. John also provides
training on the NEC®, NFPA 70E, Electrical Safe Work
Practices and other subjects for Lewellyn
Technology.
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