Methods to Reduce Arc-Flash Hazards

117 Pages·2015·7.9 MB·English

This presentation reviews the typical protective relay methods used to reduce arc-flash hazards.
Background information on arc flash is presented along with a review of the standards impacting
arc-flash calculations. An exercise is included on how to calculate arc-flash energy.
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Part one of this presentation discusses the topics listed on this slide.
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Part two discusses arc-flash optical fiber installation examples, events with arc-flash detection
(AFD), special applications of AFD, and examples of how to calculate incident energy and settings
for an instantaneous trip for maintenance mode. A comparison of ARCPRO calculation results to
Lee Equation results is also shown.
Ulteig provided the ARCPRO calculations and results used in the examples discussed in this
presentation. Minnesota Power provided the real-life system examples and verified the example
results with third-party software.
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An arc-flash hazard is a dangerous condition associated with the release of energy caused by an
electric arc.
Arcs produce some of the highest temperatures on earth, up to 35,000°F(19,000°C) ormore. For
comparison, arc furnaces are approximately as hot as 3,000°to 5,000°F(1,600°to 3,000°C).
An arc flash produces the following:
• Dangerous levels of radiated energy—thermal and ultraviolet (UV).
• Flying shrapnel.
• Pressure waves.
• Sound waves.
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Reducing arc-flash hazards is important. There is an average of ten Occupational Safety and Health
Administration-reportable (OSHA-reportable) arc-flash incidents every day in the United States,
resulting in at least one worker fatality.
The statistic on this slide is from “The Myths and Realities of Arc Flash Protection,” an article by
Thomas E. Neal and Randell B. Hirschmann (available at http://www.electricenergyonline.com).
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The main picture on this slide shows an arc-flash incident scene. The portable light was not in
place during the incident.
This incident was caused by a mounting cover plate on the door of a 480 V circuit breaker cubicle.
Two substation secondaries were tied together, so the available fault energy level was higher than
normal. Circumstances suggest that the cubicle door was closed before the cover plate was
secured. An estimated exposure of over 60 cal/cm2resulted in four workers being seriously
injured.
The inset picture shows the damaged cubicle. Notice the following*:
• The top cover plate stud is intact.
• The bottom cover plate stud has melted away.
• Scorch marks indicating heat are visible from the top edge of the opening down.
• The chain mechanism is missing.
The fault started as a single-phase fault between the right-most phase and ground, and then it
proceeded into a three-phase fault.
*This information is courtesy of the case history “Two Similar Incidents—Two Different Outcomes” by D. Doan and H.
L. Floyd, which was presented at the 2006 IEEE IAS Electrical Safety Workshop.
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The picture on this slide shows completely melted medium-voltage switchgear at a taconite mine.
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Arcing faults produce the following severe worker hazards, which can have long-term effects:
• Infrared (IR) and UV radiation. IR, visible, and UV light levels are bright enough to cause
corneal and retinal eye damage.
• Shrapnel. Arcs spray molten metal in a lethal mist at high-speed pressure. The high-momentum
shrapnel can easily penetrate a body.
• Pressure. Blast pressure waves have thrown workers across rooms and knocked them from
ladders, resulting in concussions and severe brain damage. Pressure on the chest can be higher
than 2,000 lbs per square foot (100 kPa), which is sufficient to collapse a lung.
• Sound. Hearing loss can occur from a sound blast. The arc expands air rapidly, similar to
thunder caused by lightning. Researchers have recorded magnitudes as great as
140 dB at a distance of 2 ft from the arc.
• Heat. Fatal burns can occur less than 3 ft from the arc. Serious burns are common at a distance
of 10 ft. Staged tests show temperatures greater than 392°F (200°C) on the neck and hands for a
person standing 2 ft from the arc. Clothing can be ignited from several feet away and can cause
more severe burns than if the skin were exposed.
• Smoke. Toxic gases are formed by a chemical reaction between the molten copper dust and the
atmosphere. Breathing these gases can result in thermal injury to the upper airway. Chemical
injury to the lungs can cause narrowing of the airways and pneumonia.
• Psychological effects. Many workers report apprehension when they return to the work
environment. Some require desensitization psychotherapy or reassignment to different jobs.
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OSHA requires businesses to have a safety program, calculate arc-flash hazards, communicate the
hazards, train workers on the hazards, and provide the personal protective equipment (PPE) and
tools needed to work safely on energized equipment. The aim of OSHA compliance is to provide a
“workplace free from recognized hazards.”
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This slide lists some standards that address the issue of arc-flash protection and provide
calculations and tables for determining incident energy.
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