ARCFLASHSAFETY BY JIM PHILLIPS
So far, this series has demonstrated
how to calculate the arcing short-circuit
current, incident-energy and arc flash
boundaries using a series of examples
and worksheets. However, once the calculations have been completed, what do
you do with all of the results?
The primary use for the results is
to comply with the various arc-flash
risk-assessment requirements in NFPA
70E, Standard for Electrical Safety in
the Workplace. Incident energy calculations are one of the methods listed in
NFPA 70E that can be used for selecting
arc-rated personal protective equipment (PPE) and clothing. The arc flash
boundary is the distance where the
incident energy is 1. 2 calories per centimeter squared (cal/cm2) and must also
be known as part of the risk assessment.
Both of these values can be used to comply with the arc-flash label requirements.
There is more to the calculations
than just boundaries, PPE and labeling.
Results can be used to analyze the effectiveness of using a risk control hierarchy.
This hierarchy was first introduced in
the 2015 NFPA 70E and includes six
safety controls for the risk associated
with electrical hazards such as arc flash.
Controls are ranked in order from the
most to least effective (see my May 2015
article, “Every Line of Defense”).
1. Elimination: The most effective risk
control is to eliminate the hazard. This
means establishing and verifying an electrically safe working condition as defined
in NFPA 70E.
However, if the arc flash hazard is
eliminated, why would the calculations
matter? They matter because, as part
of the many steps in the process, the
absence of voltage must be verified. Until
all steps have been completed, the circuit
is assumed to be energized, and the arc
flash hazard is assumed to be present.
Knowing the incident energy enables a
person to select the appropriate arc flash
PPE to be used.
2. Substitution: Using substitution as a
safety control may be an effective method
to reduce the risk to a more acceptable
level. The incident-energy calculations
can be used to verify the extent to which
the hazard and risk are reduced.
3. Engineering controls: There are several methods that can be used to reduce
the risk associated with the hazard by
using engineering controls. These often
involve reducing the arc flash duration
with an alternate or additional protection scheme. A few of these schemes
may include differential protection, zone
selective interlocking or arc flash relays,
or alternate temporary relay settings.
Regardless of the methods selected,
the incident-energy calculations can be
used to evaluate the hazard before the
controls are implemented. The same calculation methods can be used with new
data that represents the system after the
engineering controls would be implemented to see how the hazard and risk
could be been reduced.
4. Awareness: NFPA 70E 130.5(D) has
specific requirements for warning labels
on equipment. As part of these require-
ments, the arc flash boundary must be
included. NFPA 70E provides several
options that can be used on the label
for selecting the PPE, such as listing the
calculated incident energy and work-
ing distance, minimum arc rating of the
protection and site-specific PPE. Each
of these requires knowing the calculated
prospective incident energy.
5. Administrative controls: NFPA 70E
Chapter 1 provides quite a bit of information about administrative controls. These
controls could include procedures, training, risk-assessment job briefings and
more. For many of these controls, the
incident energy and arc flash boundary
must be known.
6. PPE: This is often the first thing that
comes to mind when reviewing the incident-energy calculations. However, it is
considered the least effective in the risk
control hierarchy. When all else fails,
PPE is the last resort.
PPE and clothing have an arc rating in
terms of cal/cm2. The PPE and clothing
should be selected with a rating sufficient
for the calculated incident energy. Keep
in mind, the calculations are for a specific
working distance, and, if a worker gets
closer for whatever reason, the incident
energy could increase dramatically, rendering the selected protection insufficient.
Do you think you are done? Sorry, this
is only a temporary pause. NFPA 70E
130.5( 2) states that the arc flash risk
assessment must be updated when a
major modification or renovation takes
place and reviewed periodically at intervals not exceeding five years. So, the clock
Beyond Labels and PPE
Arc flash calculations, part 5
THIS ARTICLE IS THE FIFTH AND FINAL PART in a series that provides a
step-by-step approach for performing arc flash hazard calculations. The previous
parts appeared in the January, March, May and July 2016 issues of ELECTRICAL
CONTRACTOR and are on
PHILLIPS, P.E., founder of
training programs globally and is the author of “Complete Guide to Arc Flash Hazard
Calculation Studies.” He is Secretary of the IEEE 1584 Arc Flash Working Group and International
Chair of IEC TC78 Live Working. He can be reached at
firstname.lastname@example.org or 800-874-8883. IST