Why Arc Flash Studies Are Important
By Julian R. Burns CPQ
What Is An Arc Flash/Blast?
An Arc Flash/Blast is the event that occurs
when a large amount of electrical energy is
released during an electrical fault. A fault is
caused during a Phase-to-Phase or a Phase-to
-Ground short, which can result from a tool
being inserted or dropped across energized
parts. The resulting Arc Flash is an explosion
which creates extreme heat, UV light, molten metal, flying shrapnel along with a tremendous pressure wave. The size of the Arc
Flash/Blast (explosion) is based on the electrical system operating voltage, available
short circuit current and the operating time
for an upstream overcurrent device to open
(clearing time). The Arc Flash event can be
as small as a firecracker or as large as a grenade.
OSHA & NFPA 70E
The Occupational Safety and Health Administration 29 CFR 1903.1 states;”that
every employer must furnish to his employees a place of employment which are free
from recognized hazards that are causing or
likely to cause death or serious physical
harm to his employees.”
Working on or near electrical equipment
that is energized is not considered a safe act,
a hazard exists. Thus an employer is responsible to train it’s employees how to safely
work on and around this equipment and furnish the appropriate safety equipment. The
appropriate document to assist an employer
with these tasks is NFPA 70E “Electrical
Safety in the Workplace”, which is a consensus standard developed by the National Fire
Protection Association (NFPA) at the request of OSHA. NFPA 70E sets the ground
work for what training is required for a person (employee) needs to be a qualified person and what types of personal protective
equipment (PPE) are required to work on or
near energized electrical equipment. In order
to properly select the type and level of PPE
an Arc Flash Study must be performed for
the electrical distribution system of the facility.
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Appropriate Steps for a
Successful Arc Flash Study.
I. Field Verification & Audit
II. Update one-line drawings
III. Short Circuit analysis
IV. Protective device coordination
study
V. Software modeling
VI. Arc Flash Study/Analysis
VII. Labeling
VIII. Training
I. Field Verification & Audit:
This step is very critical and impacts all
other steps in the process. Prior to this
step the facility engineering department/maintenance department should
furnish, to the firm performing the Arc
Flash Study, a drawing indicating layout of all electrical distribution equipment throughout the facility and a current one-line drawing. If these drawings
are not available it will greatly impact
the required time to complete the study
and also will increase the cost. During
this step the provider’s personnel will
verify the one-line drawing for accuracy and collect data on; Transformers,
Switchgear, Motor Control Centers,
Distribution Panels, Circuit Breakers,
Relays, Disconnects, Conductor Type
and lengths where obtainable
II. Update One-line Drawing(s):
Create a new one-line drawing to include modifications to the electrical
distribution system that were discovered during the onsite Field Verification & Audit step. This updated drawing should include transformer ratings,
system voltages, conductor sizes and
various lengths. This is basically a
working drawing and helps preserve the
data that was collected.
III. Short Circuit Analysis:
This step evaluates the maximum
fault levels at each point or node
throughout the electrical distribution
system and identifies any problem
areas. A problem exists if at any point
in the electrical system the available
short circuit current is greater than the
rating of the panel or overcurrent device. Example: A circuit breaker has a
short circuit rating of 10,000AIC and
the available short circuit current is
14,000AIC. This can cause the circuit
breaker to explode during a downstream fault. This is a critical step in
keeping both the electrical system and
an employee safe.
IV. Protective Device Coordination
Study:
A coordination study is an added
value to an arc flash study. The coordination study is to help ensure that
the an overcurrent device (fuse/circuit
breaker) nearest a fault/short opens
clearing the fault condition before an
upstream overcurrent device opens.
By having selective coordination of
overcurrent devices minimizes the
area or portion of the facility that
looses power when a fault/short occurs.
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V. Software Modeling:
The modeling process utilizes the data
acquired during the field verification/
audit process to perform calculations
and systematically begins the process of
the Arc Flash study and red flags any
required missing data. During the modeling process an accurate one-line drawing is created and within this process
changes can made easily and quickly to
create different scenarios that may show
how the changes in breaker settings or
fuse type will make the electrical system
operate safer and more reliable.
VI. Arc Flash Study/Analysis:
Utilizing all of the information in steps
I. through V. calculations are made to
determine the available incident energy
at each point/node throughout the electrical system. Then the calculated incident energy level is used to determine
the Flash Boundary, Limited Approach Boundary, Restricted Approach Boundary, Prohibited Approach Boundary and the Hazard/
Risk Category. Some if not all of these
values should be included on the final
one-line drawing for reference.
VII. Labeling:
After the Arc Flash study/analysis has
been completed and all calculations
have been validated and the final oneline drawing is complete then a durable
arc flash warning label is printed for
each node in the electrical distribution
system. The information included on
each label should include, but not limited to; Available incident energy, Arc
Flash Hazard Boundary, Limited Approach Boundary, Restricted Approach
Boundary, Prohibited Approach Boundary, Shock Hazard Voltage, Hazard/Risk
Category,
Category, Equipment Designation and
Upstream Overcurrent Protective Device.
VII. Training:
After the completion of the Arc Flash
study and the labels have been affixed to
all of the electrical distribution equipment a training class is held for all of the
facility’s electrical and maintenance personnel, who will be involved in electrical maintenance and trouble-shooting.
This training class should indoctrinate
attendees as to the safety procedures of
working on or near exposed energized
electrical equipment, how to select, test
and maintain proper personal protective
equipment (PPE) and tools for various
hazard/risk categories, the purpose of an
energized work authorization permit,
how to read and understand an Arc
Flash warning label along with the
safety requirements on OSHA and
NFPA 70E “Electrical Safety in the
Workplace” as they apply to electrical
safety.
About The Author:
Julian Burns is a committee member of
NFPA 70E “Electrical Safety in the
Workplace” and serves as Chairman of
NFPA 70 National Electrical Code
(NEC) Panel 8. He is a consultant with
over 40 years in the electrical industry
and conducts Arc Flash Studies throughout the country as well as related seminars