Arc Flash Energy Computations and NESC Section 41

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Arc Flash Energy ComputationsandNESC Section 41

• Robert J. Rusch
• Stanley Consultants, Inc.
• September 11, 2008

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Goals Objectives

• What Is Arc Flash?
• Historical Context
• Rules Regulations
• NFPA 70E and NESC 41
• Arc Physics Computation Approaches

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What Is It?

• Need Basic Definition for Common Understanding
• Need to Understand Context of Issues
• Need to See What It Is

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Why of Concern?
Electrical Safety Triangle
Safety Triangle
i.e. electrical work is less forgiving
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Electrical Hazards

• Fire
• First hazard to be recognized due to economic
  losses
• Formed basis of first NEC in 1897
• Shock/Electrocution Second hazard to be
  generally recognized and addressed
• Arc Flash/Blast
• Last hazard to be recognized
• First addressed in NFPA 70E-1995

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Potential Consequences of an Arc-Flash Incident

• Injury
• Death (in the case of burn injuries, often a slow
  and painful death)
• Monetary Cost to Employer
• Damage to Equipment, Down Time
• Personnel Injury 25,000,000/person
• Death of Personnel 15,000,000/person
• 2 of workplace injuries are electrical in
  nature, but they account for 28 of the costs of
  injuries

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What Is An Arc Flash?

• Defined as a dangerous condition associated with
  the release of energy caused by an electric arc
• Created by such things as
• Equipment failure (misaligned contacts,
  ferroresonance, insulation failure)
• Human error (dropping a tool, body part getting
  too close to energized parts, stupidity)

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Classic Arc Flash
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What Is It?
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Staged Test
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When Do Arcs Occur?

• Primarily from physical movement
• Switch or Circuit Breaker is opened/closed
• Contactors/starters pick up or drop out
• Door or Cover is opened/closed
• Test Equip. and Safety Grounds are installed
• Equipment is inserted/withdrawn from energized
  bus

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Electric Arc History

• Arcs have been studied since Aristotle did
  experiments with sheeps wool
• First studies were static electricity and
  lightning
• Mid-1800s began to experiment and understand
  electric arcs
• Discharges studied by physicists as well as
  engineers

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Arc History

• Partial discharges along with arcs studied in
  early 1900s Peeks Gradient, Paschens Law, Etc
• Arc physics still under study as mathematical
  models still have limits

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Anatomy of An Electrical Arc Flash
Molten Metal
35,000 F
Pressure Waves
Sound Waves
Shrapnel
Copper Vapor Solid to Vapor Expands by 67,000
times
Hot Air-Rapid Expansion
Intense Light
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Components of Arc Flashes

• Two basic components of arc flashes
• Heat Radiation (quantified as Incident Energy
  Level)
• Pressure Wave (also known as Arc Blast)

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Heat

• Electric arc temperatures are considered to be
  approximately 4 times hotter than the suns
  surface.
• Temperature at arc terminals can reach 35,000F
  (for reference, the surface temperature of the
  sun is 9,000F, and the temperature of a wood
  fire is 900F)

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Heat Radiation

• Heat radiation exposure is a function of
• Distance to arc
• Available fault current
• Fault clearing time
• Equipment type
• Gap between conductors (determined by equipment)
• Vaporizes metals
• Ignites clothing
• 2030F - skin not curable (cell death)
• Possible to become fatally burned or seriously
  injured when working at a distance of 10 feet or
  more from an electrical arc.

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Personnel Reaction to Heat

• Some potential effects on personnel due to Heat
  Radiation
• External burns, potentially very severe
• Internal burns, such as to the lungs due to
  ingestion of vaporized metal and superheated air
• Health effects due to inhalation of toxic gases
  and heavy smoke due to the burning of paint,
  insulators, and other components
• Partial or total loss of sight
• Disability
• Death

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Pressure Wave

• Electrical blast (or explosion) is the result
  of the rapid expansion of air caused by an
  electric arc.

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Pressure Wave

• Peaks in the first ½ cycle of fault ( 9msec)
• Pressure levels of 2,160 pounds per square foot
  (psf) in the immediate vicinity of the blast have
  been detected
• Caused by superheating of air and vaporizing of
  conductors (air expands to roughly 1670 times and
  copper expands to roughly 67,000 times its volume
  i.e. 1 in3 becomes 1.4 yd3)
• Function of arc fault gap and available fault
  current

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Pressure Wave

• During an arc blast metal droplets travel 10 ft
  or more and faster than the speed of sound!
• Expansion produces explosion that results in
• Molten metal
• Fragmented metal
• High temperatures
• Pressure on the body

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Pressure Wave (cont.)

• Some potential effects on personnel due to
  Pressure Wave
• Injury due to blast
• Collapsed eardrums leading to partial or
  near-total loss of hearing and possibly tinnitus
  Sound levels of 141.5 decibels at 2 feet from
  the blast have been detected
• Collapsed lungs
• Injuries due to shrapnel being ejected from
  equipment
• One positive benefit can lessen effects of Heat
  Radiation due to personnel being thrown away from
  equipment, but this can also lead to other
  injuries

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Arc Study

• Studied in high voltage labs
• Voltage
• Steady State
• Dynamic
• Transient
• Use Gaps
• Spheres
• Rods
• Planes

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Applicable Testing Standards

• International Electrotechnical Committee (IEC)
• Canadian Standards Association (CSA)
• IEEE - IEEE 4
• European Union Standards BS, DIN,etc
• Japanese Standards (JIS)

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Arc Physics

• Multiple Parameters
• Research is Ongoing

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Rules Regulations

• OSHA
• NFPA
• NESC
• RUS

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Which Applies?

• Type of employer
• Risk Manager and Legal Counsel

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OSHA

• Federal
• Standard 1910.269 Employers must supply
  appropriate clothing
• General Duty Clause Employer must identify and
  quantify risks
• OSHA May 15, 2008 Ruling Employer must provide
  PPE at no cost
• State of Colorado See Regulations

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NFPA 70 and 70E

• National Electrical Code - NFPA 70
• NFPA 70E Standard for Electrical Safety in the
  Workplace
• Original 2000, Updated 2004 to compute flah
  energy
• Flash boundaries
• Flash identification with stickers
• Select PPE based on energy

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Flash Protection Boundaries
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Arc-Flash Hazard Warning Label
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NESC 2007

• Section 41
• 410 (A)
• Effective January 1, 2009
• Employer must do assessment to determine
  potential exposure to arc flash
• Clothing provided for gt 2 cal/cm2

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NESC 41

• Computations must be performed
• Include
• Available fault current
• Arc duration
• Distance from arc to employee
• Table 410-1

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RUS

• 7 CFR Ch. XVII Subpart E Electric System
  Design, 1724.40 Compliance with National
  Electrical Safety Code (a)
• Borrowers shall ensure that its electric
  systemis designed, constructed, operated, and
  maintained in accordance with all applicable
  provisions of the mist current and accepted
  criteria of the National Electrical Safety Code
  and requirements of State and local governmental
  entity.
• RUS has initiated a task force to review its
  position on arc flash

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NFPA 70E vs NESC 41

• Regardless of legal situation, computations must
  be performed
• Arc energy estimated
• Appropriate PPE selected
• NFPA 70E contains guidance on computations
• NESC 41 provides no guidance except that listed
  above

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Electric Arcs Are Functions Of

• Weather Temperature and Humidity
• Atmospheric Pressure Altitude
• Gap Size Type
• Terminal Metals / Characteristics
• Voltage Determines voltage gradient (V/cm) and
  flashover level
• Fault Current Power follow current magnitude
• Determines plasma temperature
• Heat Flux gt Heat Energy cal/cm2
• Time Fault duration

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Arc Effects Function Of .

• Electric Arc Parameters (Previous Slide)
• Working Distance Distance from arc source to
  person

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Arc Physics and Computation Approaches

• IEEE 1584
• Industry Research
• Computer Simulations

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Flash Boundary and Incident Energy Level
Calculation IEEE 1584

• Formulas based on testing are applicable for
  systems with
• Voltages from 208V to 15kV
• Bolted fault current from 700A to 106kA
• Gaps between conductors of 13 - 152 mm
• Faults involving three phases
• Formulas also based on statistical analysis to
  provide results that covers 95 of the cases
  (i.e. 5 of the arc flashes statistically will be
  worse than calculated)
• Also have theoretical formulas for systems where
  the testing-based formulas do not apply

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Arc Calculations Research

• All have limitations
• Assumptions for atmospheric conditions
• 3LG vs 1LG
• Arc behavior
• Choice of computation approach is based on what
  results are desired in what environment
• None Currently Address Pressure Wave

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Approach May Be Different Based on Type of
Equipment

• Metal Clad, Metal-Enclosed
• Padmounts
• Gas (SF6) Substations
• Air Bus Substations
• Underground Lines
• Overhead Lines

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Example Theoretical Computation230kV Air Bus /
Overhead Line

• Operating Voltage 230kV
• 1LG Fault 15kA Symmetrical rms
• Total Clearing Time 9 Cycles
• Distances (NESC 410.1 Does Not Specify)
• Gap 15.6 in
• Working Distance 30 in

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230kV Air Bus / Overhead Line
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230kV Air Bus / Overhead Line
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Example Theoretical Computation12.47kV Overhead
Line

• Operating Voltage 12.47kV
• 1LG Fault 600A Symmetrical rms
• Fault Location Midpoint of line
• Distances (NESC 410.1)
• Gap 2 in
• Working Distance 15 in
• Fault Interrupter 50-4H Recloser

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50-4H Recloser Sequence of Operation

• Fast Trip Setting
• Trip 0.047 sec or 3 cycles
• Open Time Delay 1 ½ sec delay
• Reclose
• Fault Still Exists
• Time Delay Trip 0.2257 sec or 14 cycles

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50-4H Recloser First Trip 0.047 sec ( 3
cycles)
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50-4H Recloser Trip 0.2257 sec ( 14 cycles)
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Example Theoretical Computation24.9kV Overhead
Line

• Operating Voltage 24.9kV
• 1LG Fault 600A Symmetrical rms
• Fault Location Midpoint of line
• Distances (NESC 410.1)
• Gap 4 in
• Working Distance 15 in
• Fault Interrupter 50-4H Recloser

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50-4H Recloser Sequence of Operation

• Fast Trip Setting
• Trip 0.047 sec or 3 cycles
• Open Time Delay 1 ½ sec delay
• Reclose
• Fault Still Exists
• Time Delay Trip 0.2257 sec or 14 cycles

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50-4H Recloser First Trip 0.047 sec ( 3
cycles)
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50-4H Recloser Trip 0.2257 sec ( 14 cycles)
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Theoretical Computation24.9kV Overhead Line
Example 2

• Operating Voltage 24.9kV
• 1LG Fault 300A Symmetrical rms
• Fault Location Midpoint of line
• Distances (NESC 410.1)
• Gap 4 in
• Working Distance 15 in
• Fault Interrupter 50-4H Recloser

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50-4H Recloser First Trip 0.047 sec ( 3
cycles)
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50-4H RecloserTrip 0.2257 sec ( 14 cycles)
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Means of Mitigating Arc Flash Hazards

• Work only on electrically safe (i.e.
  de-energized) equipment still exposure to
  hazard in making equipment electrically safe
• Increase distance to possible arc
• Remote mounting of control equipment or SCADA
• Remote racking of equipment move operators away
  from flash zone
• Hookstick operation
• Reduce fault current level
• Use current-limiting devices need to cover
  minimum arc current

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Means of Mitigating Arc Flash Hazards (cont.)

• Reduce fault clearing times (does not affect
  pressure wave component)
• Bus differential
• Temporarily change relay settings while work is
  being performed (e.g. 50 device with safety
  switch)
• Non-coordinated overcurrent protection
• Zone-selective interlocking
• Arc detection systems such as ABB Arc-Guard or
  similar system in metal clad or metal enclosed
  switchgear
• Fuses need to cover minimum arc current
• Use Arc-Resistant equipment
• Will likely be other means developed

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Summary

• Arc Flashes Components
• Heat Radiation Pressure Wave,
• Both pose serious dangers to personnel
• Regulations vary with industry
• Differing computations for differentr equipment
• Must mitigate
• Change working conditions
• PPE (No pressure defense)

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Thank You
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