When Working Near An Energized Switchgear Do Not Wear Nylon, Polyester, Polypropylene And Acetate
Upcoming SlideShare
Loading in...5
×

Like this? Share it with your network

Share
  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
    Be the first to comment
    Be the first to like this
No Downloads

Views

Total Views
1,843
On Slideshare
1,843
From Embeds
0
Number of Embeds
0

Actions

Shares
Downloads
46
Comments
0
Likes
0

Embeds 0

No embeds

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
    No notes for slide

Transcript

  • 1. Arc Flash Exposure NESC Rules related to Arc Flash NFPA 70E Rules and other Standards
  • 2. Overview
    • Basic discussion about the ARC FLASH
    • Hazard Risk
      • Questions
    • Standards and Regulations
      • NESC,NFPA 70E, IEEE Standard 1584
      • Questions
    • Arc Flash Study
      • Calculations
      • Relay Coordination
      • Questions
  • 3. Megaway, Inc. Consulting Engineers
  • 4.  
  • 5.  
  • 6. Arc Flash
    • What is it?
    • What is the Source?
    • Hazard Risk Categories
  • 7. Arc Flash
    • Arc flash is an explosion with extremely high temperature, toxic and vaporized metals
    • The worst case is when a bolted fault occurs.
    • Hazard risk exists at all times, when working near the exposed and energized equipment
  • 8. ARC FLASH
    • When the normal flow of current in a conductor is disrupted, it takes a short cut to release its energy
    • The short path could be between the conductors, at the equipment or to ground, etc.
    • The released energy flashes an arc at the point of contact which is extremely hot and toxic
  • 9. Arcing Fault
    • Electrical shorts occurs due to:
      • Accidents
      • Touching the energized bus or conductors
      • Insulation breakdown of the equipment, bus conductors, etc.
      • Once the accident happens more events will follow:
        • Arc blast due to expanding gases
        • Arc plasma or Heat
  • 10. Arc Flash Characteristics
    • Amount of heat from an arc depends on the Voltage across it and the arc current
    • Power in arc ( V arc x I arc) which is radiated out and multiplied by time as incident Energy
    • The arc produces expanding gases by heat transfer such as radiant and convection
      • Gasses heat up the contact surfaces
      • Burn, melt and vaporizes the contacts
  • 11. Arc Resistance
    • Arc resistance is highly variable and changes non-linearly with the arc current duration each cycle.
    • Arc plasma at higher impedance conducts current.
  • 12. Electromagnetic Radiation
    • Produced by arc flash, is highly variable
    • The bandwidth Includes:
      • Radio waves
      • Microwave
      • Infrared( IR )
      • Visible, UV and X-Ray
  • 13. Arc Energy
    • Measured in Calories
      • 1 cal. = 4.1868 Joules
      • 1Joule = 1 watt-second
      • 1Kilowat-hour= 3600000 Joules
      • Exposure > 2Cal/cm-squ require PPE, NESC
    • Arc energy ( I squared) x t, calculated based on the fault current magnitude and its duration in cycles
  • 14. Hazard Risk
    • Risk exists during the fault
    • When it takes a longer time to clear the fault
    • When the prohibited areas are violated
      • Boundaries
      • Insulations defects
      • Old equipment
      • Equipment designed for lower SC withstand capability than the actual system faults
  • 15. Approach Boundaries
    • NFPA 70E annex C Sketch
  • 16. FLASH PROTECTION BOUNDARY LIMITED APPROACH BOUNDARY LIMITED SPACE RESTRICTED APPROACH BOUNDARY RESTRICTED SPACE PROHIBITED APPROACH BOUNDARY PROHIBITED SPACE EXPOSED CONDUCTOR
  • 17. Phase to phase voltage Exposed Movable Conductor Exposed fixed circuit part Restricted approach boundary Prohibited approach boundary <50 Not specified Not specified Not specified Not specified 50-300 10’ 3’ - 6” Avoid contact Avoid contact 301-750 10’ 3’ - 6” 1’ 1” 751-15kV 10’ 5’ 2’ - 2” 7” 15.1-36kV 10’ 6’ 2’ - 7” 10” 36.1- 46kV 10’ 8’ 2’ - 9” 1’ - 5” 46.1- 72.5kV 10’ 8’ 3’ - 2” 2’ - 1” 72.6-121kV 10’ – 8” 8’ 3’ - 3” 2’ - 8” 138 – 145kV 11’ 10’ 3’ - 7” 3’ - 1” 161 – 169kV 11’ – 8” 11’ – 8” 4’ 3’ - 6” 230 – 242kV 13’ 13’ 5’ - 3” 4’ - 9” Table 130.2(C) Limited, restricted, and prohibited approach boundary
  • 18. Key Concepts
    • Open Arc
      • Non Enclosed areas, energy is radiated equally
      • Overhead lines
    • Directed Arc
      • Arc in a box, energy is radiated and reflected
      • Switchgear enclosure with the door open
    • Radiant Arc
      • Light released by an arc ( radiant Energy)
    • Blast Energy
      • Energy of moving objects due to the arcing faults, 100s of MPH speed , penetrate human body
  • 19. RISK ANALYSIS GOAL
    • To determine the Incident Energy for Electrical path to the equipment
    • To examine boundaries and distances
    • To determine level of Personal Protective Equipment
    • To provide Labeling, training & guidelines
    • To minimize injury and death
    • To minimize insurance cost & legal issues
  • 20. Hazard Risk Categories FR=Fire resistance fabric Flash Hazard Risk Category Range of Calculated Incident Energy Min. PPE Rating Clothing Required 0 0-1.2 cal/cm 2 N/A 4.5-14 oz/yd 2 untreated cotton 1 1.2- 4 cal/cm 2 4 cal/cm 2 FR shirt and pants 2 4- 8 cal/cm 2 8 cal/cm 2 Cotton underclothing plus FR shirt and pants 3 8- 25 cal/cm 2 25 cal/cm 2 Cotton underclothing plus FR shirt, pants, overalls or equivalent 4 25- 40 cal/cm 2 40 cal/cm 2 Cotton underclothing plus FR shirt, pants, plus double layer switching coat and pants or equiv
  • 21. Standards & Regulations
    • Five separate industry standards focus on preventing arc flash incidents:
      • NESC (National Electric Safety Code)
      • NFPA70-2005
      • NFPA70E-2004
      • IEEE Standard 1584
      • OSHA 29 code of Federal Regulations
  • 22. NESC (National Electric Safety Code)
    • Section 41 Supply & Communications systems
    • Rule 410 A3, employees to wear clothing with a correct arc rating
    • Tables 410-1 and 410-2 to be used for quick reference to use PPE
    • Arc hazard Analysis must be performed
    • When working near an energized switchgear do not wear Nylon, Polyester, polypropylene and Acetate
  • 23. NESC Section 41 Supply & Communication Rules for Employers
    • General duties of Employers
      • Inform employees about all communication equipments or electrical supply equipment
      • Training has to be provided to all employees who work on or near the energized facilities.
      • Ensure that potential exposure has been assessed to an electric arc
      • (Effective as of Jan. 1, 2009)
          • Exposure > 2 cal/cm 2
          • PPE required .
  • 24. NESC Section 41 Supply & Communication Rules for Employers
    • Rule410A, general requirements
      • Employer shall perform an assessment on exposure to electric arc
    • Rule 410B Emergency procedures and first aid rules
    • Rule 410c responsibility
      • A designated person shall be in charge
  • 25. NESC RULE 410 A3
      • Applicable work rules and engineering controls for secondary systems below 1000 V is required to limit exposure .
      • Clothing system with Min. effective arc rating of 4 cal/cm 2 can be used instead of performing of an arc hazard analysis.
      • Using multiple layers clothing instead of one layer can be used. The outer layer should have flame resistant material and inner layer of normal wear.
  • 26. Table 410-1 NESC
    • Calculations for Energy levels based on Fault currents and Max. clearing time for voltages 1-46KV
      • Based on open air phase–ground arcs
      • Based on 15” separation distance from the arc
    • Arc Gaps used for this Table calculations:
      • 2” for 1-15KV, 4” for 15.1- 25KV
      • 6” for 25.1-36KV, 9” for 36.1-46KV
  • 27. Clothing Systems Table 410-1 Phase- to Phase voltage (kV) Fault current (KA) 4-cal system Maximum clearing time (cycles) 8-cal system Maximum clearing time (cycles) 12- cal system Maximum Clearing time (cycles) 1 to 15 5 46.5 93.0 139.5 10 18.0 36.1 54.1 15 10.0 20.1 30.1 20 6.5 13.0 19.5 15.1 to 25 5 27.6 55.2 82.8 10 11.4 22.7 34.1 15 6.6 13.2 19.8 20 4.4 8.8 13.2 25.1 to 36 5 20.9 41.7 62.6
  • 28. Clothing Systems Table 410-1 Phase- to Phase voltage (kV) Fault current (KA) 4-cal system Maximum clearing time (cycles) 8-cal system Maximum clearing time (cycles) 12- cal system Maximum Clearing time (cycles) 10 8.8 17.6 26.5 15 5.2 10.4 15.7 20 3.5 7.1 10.6 36.1 to 46 5 16.2 32.4 48.6 10 7.0 13.9 20.9 15 4.3 8.5 12.8 20 3.0 6.1 0.1
  • 29. Table 410-2 NESC Live –Line Tool Work Clothing
    • Calculations for Energy levels based on Fault currents and Max. clearing time for voltages 46.1-up to 800KV
      • Arc gap is calculated by using Phase – Ground Voltage of system divided by 10
      • Dielectric of air is taken at 10 KV per inch
      • Distance from arc is calculated based on minimum approach distance in table 441-2
    • Refer to table 441-2
  • 30. Live – Line Tool Work Clothing Table 410-2 Phase- to Phase voltage (kV) Fault current (KA) 4-cal system Maximum clearing time (cycles) 8-cal system Maximum clearing time (cycles) 12- cal system Maximum Clearing time (cycles) 46.1 to 72.5 20 8.5 17.0 25.5 30 5.3 10.5 15.8 40 3.7 7.3 11.0 50 2.8 5.5 8.3 72.6 to 121 20 8.2 16.5 24.7 30 4.7 9.4 14.1 40 3.1 6.2 9.3
  • 31. Table 410-2 Live – Line Tool Work Clothing Phase- to Phase voltage (kV) Fault current (KA) 4-cal system Maximum clearing time (cycles) 8-cal system Maximum clearing time (cycles) 12- cal system Maximum Clearing time (cycles) 50 2.6 5.3 7.9 161 to 169 20 9.3 18.6 27.9 30 5.7 11.5 17.2 40 4.0 8.0 12.0 50 3.0 6.0 9.0 230 to 242 20 10.4 20.9 31.3 30 6.4 12.9 19.3
  • 32. Table 410-2 Live – Line Tool Work Clothing Phase- to Phase voltage (kV) Fault current (KA) 4-cal system Maximum clearing time (cycles) 8-cal system Maximum clearing time (cycles) 12- cal system Maximum Clearing time (cycles) 30 6.4 12.9 19.3 40 4.5 9.0 13.5 50 3.4 6.8 10.1 345 to 362 20 22.6 45.3 67.9 30 14.0 28.1 42.1 40 9.8 19.6 29.4 50 7.4 14.7 22.1
  • 33. Table 410-2 Live – Line Tool Work Clothing Phase- to Phase voltage (kV) Fault current (KA) 4-cal system Maximum clearing time (cycles) 8-cal system Maximum clearing time (cycles) 12- cal system Maximum Clearing time (cycles) 500 to 550 20 18.9 37.8 56.7 30 11.7 23.3 35.0 40 8.1 16.3 24.4 50 6.1 12.2 18.3 765 t0 800 20 43.6 87.3 130.9 30 27.0 53.9 80.9 40 18.9 37.8 57.7 50 14.2 28.4 42.6
  • 34.  
  • 35.  
  • 36. Protective Methods and Devices
    • Rule411A methods :
      • Access for authorized personnel only
      • Diagrams and documents
      • Instructions and guide lines for employees
  • 37. NESC Rules continued
    • Rule411B Devices & equipment
      • Adequate supply
      • First aid
      • All protective devices shall conform with applicable standards
      • Grounding materials & devices
  • 38.
      • Devices and equipment
        • Voltage detection devices/meters & more
      • Signs and tags for employee safety
        • With the provisions of ANSI Z535.1-2006 through ANSI Z535.5-2006, the safety signs and tags are required.
    Protective Methods and Devices
  • 39. NESC Rules continued
    • Rule411C Inspection and testing of protective devices
      • All PPE and tools etc
      • shall be tested, inspected regularly prior to any usage
    • Rule 411F Fall Protection
      • Employers shall maintain an effective program for training
  • 40. NESC SECTION 42 General Rules for Employees
    • Rule420B qualification of Employees
    • Rule420C Safeguarding oneself & others
    • Rule420I Clothing ,suitable for the assigned work
    • Rule420Q Communication Antennas
      • Range 3KHZ-300GHZ
      • Workers shall not be exposed to radiation levels set by regulatory authority Jurisdiction.
      • Refer to FCC Bulletin #65, IEEE C95.1, OSHA29
  • 41. NESC SECTION 43 Additional Rules for Communications Employees
    • Approach to energized conductors or parts:
      • conductive object should not be brought within any distance of an energized part (see table 431-1)
      • Supply lines should be de-energized and grounded when voltage is involved.
  • 42. NESC SECTION 43 Additional Rules for Communications Employees
    • Joint-use structures:
      • Certain safe distances should be taken into consideration according to table 431-1 while working on poles or structures.
      • This rule does not apply on voltages of 140 kV and below, when installing a rigid fixed barriers between the supply and communication facilities, when attached above electric supply conductors.
  • 43. NESC SECTION 43 Additional Rules for Communications Employees
    • Joint –use manhole, Metallic or semi conductive sheath should be used when working on buried cable or on cable in manholes.
    • While working in a manhole a member of the personnel should be on hand at the surface to render any assistance required.
  • 44. NESC SECTION 44 Additional Rules for Supply Employees
    • Minimum approach distances should be followed on chart 441-1 when voltages of 301 V to 72.5 kV is involved.
    • Rubber insulated gloves must be worn when personnel are within reach or minimum approach distances of the above voltages.
  • 45. NFPA 70E
    • All protective equipment must meet the requirements as shown in the latest edition of NFPA 70E.
    • Protective equipment, sufficient for protection against the potential electrical flash, required for every part of the body.
    • Selection of the required thermal rated PPE depends on the incident energy level at the point of work.
  • 46. NFPA 70E PPE
    • Employees should be trained in the use of proper protective equipment as well as using proper wearing cloths.
    • Employees must wear and be trained in the use of appropriate protective equipment for the possible electrical hazards they may face.
    • PPE include hard hat, face shield , flame resistant neck protection, ear protectors , insulated rubber gloves with leather protectors, and insulated leather footwear .
  • 47. NFPA70E Hazard Risk Categories FR=Fire resistance fabric Flash Hazard Risk Category Range of Calculated Incident Energy Min. PPE Rating Clothing Required 0 0-1.2 cal/cm 2 N/A 4.5-14 oz/yd 2 untreated cotton 1 1.2+ to 4 cal/cm 2 4 cal/cm 2 FR shirt and pants 2 4+ to 8 cal/cm 2 8 cal/cm 2 Cotton underclothing plus FR shirt and pants 3 8+ to 25 cal/cm 2 25 cal/cm 2 Cotton underclothing plus FR shirt, pants, overalls or equivalent 4 25+ to 40 cal/cm 2 40 cal/cm 2 Cotton underclothing plus FR shirt, pants, plus double layer switching coat and pants or equiv
  • 48. PPE Standards Head Protection ANSI Z89.1,1997 Eye and Face ANSI Z87.1,1989 Gloves ASTM D 120-95 Sleeves ASTM D 1051-95 Gloves and Sleeves ASTM F 496-97 Leather Protectors ASTM 696-97 Footwear ASTM 1117-93 Visual Inspection ASTM F 1236-96 Apparel ASTM F 1506-00 Rainwear ASTM F 1891-01
  • 49. Standards for Other Protective Equipment Ladders ANSI AA14.1, A14.3, A14.4, A14,5 Safety signs & tags ANSI Z535 Blankets ASTM D 1048 & F 479 Covers ASTM D 1049 Line Hoses & covers ASTM D 1050 & F478 Fiber Glass tools ASTM F711 Plastic Guards ASTM F712 Temporary Grounding ASTM F 855 Insulated Hand Tools ASTM F 1505
  • 50. Signs
  • 51. Safety Video
    • This is a graphic video. Viewer discretion is advised.
  • 52.  
  • 53. Break
  • 54. Arc Flash Study
    • Involves four Major Tasks
      • Existing System & Data Gathering
      • Engineering analysis of Data
      • Personal Protection Equipment procurement and labeling
      • Training program
  • 55. Existing System & Data Gathering
    • Final set of drawings
    • Field verify all drawings and system configurations
    • Check all equipment name plates
    • Prepare an accurate data ready for modeling and software inputs
  • 56. Engineering Analysis of Data
    • Perform Short Circuit Analysis and calculations for each bus and equipment
    • Calculate for maximum fault
    • Perform arc flash energy calculations using two approaches, IEEE 1584 & NFPA70E
      • Incident Energy
      • Flash Boundary
      • Determine Working Distance
  • 57. Relay Setting and Coordination
    • Perform relay settings and coordination to include all protective devices for different system configurations
      • Normal configuration settings
      • Alternate settings for temporary or emergency configurations
  • 58. Data Collection
    • Fault Current Limits
    • Information of Physical Structure
    • Bus Isolation
    • Grounding
    • Collect all System Impedance data
  • 59. Calculations
    • One line diagram of a switchgear
    • Use computer, typical software for fault current calc.
    • Most programs provide a complete and comprehensive calculations for all variable items ( SC, Arcing Current , Incident Energy, boundaries PPE materials etc.)
  • 60. System Fault Currents
  • 61. IEEE 1584
    • Incident Energy :
    • lg En = K1 + K2 + 1.081lgIa + 0.0011G    (1)
    • En = J/cm2
    • Distance from the arc = 610 mm.
    • Arcing time = 0.2 sec. K1 = -0.792 for open configurations, -0.555 for box configurations / enclosed
    • K2 = 0 for ungrounded and high resistance grounded systems, -0.113 for grounded G = gap between conductors in mm. Ia = Arcing current in kA.
  • 62. IEEE STANDARD 1584
    • Incident Energy Calculation
    System K 1 open K 1 box K 2 open K 2 box I a 1000 V or lover -0.792 -0.555 0 -0.113 Arcing current 1kV up 15 kV -0.153 -0.097 0 0 Arcing current
  • 63. IEEE 1584
    • For 1000V and lower systems: lgIa = K + 0.662lgIbf + 0.0966V + 0.000526 G + 0.5588VlgIbf - 0.00304GlgIbf    (2a)
    • lg - is logarithm base 10 (log10). Ia - arcing current in kA. En - normalized incident energy in J/cm2
    • K – is -0.153 for open arc, -0.097 for box configurations. Ibf - bolted fault current in kA symmetrical rms. V - system voltage in kV. G - gap between conductors in millimeters.
  • 64. IEEE 1584
    • voltage ranging from 1 up to 15kV:
    • lgIa = 0.00402 + 0.983lgIbf (2b)
    • E = 4.184CfEn(t / 0.2)(610x/Dx)    (3)
    • E - incident energy exposure in J/cm2.
    • Cf - calculation factor equal to 1.0 for voltages >1 kV, 1.5 for voltages <1 kV.
    • En - normalized incident energy in J/cm2 t - arcing time in seconds. D - distance from possible arcing point to workers in mm x - distance exponent .
  • 65. IEEE 1584
    • Flash Protection Boundary equation :
    • DB = [4.184 * Cf * En * (t / 0.2) * (610x/EB)]1/x (4)
    • DB - distance of the boundary from the arc point in mm. Cf - calculation factor equal to 1.0 for voltages >1 kV, 1.5 for voltages <1 kV. En - normalized incident energy in J/cm2
    • EB - incident energy in J/cm2 at the boundary distance. Ibf - bolted fault current for three phase faults in kA symmetrical rms. t - arcing time in seconds. x - Distance exponent .
    • EB is usually set at 5 J/cm2 (1.2 cal/cm2 ) for bare skin, or at the rating of proposed PPE.
  • 66. IEEE1584 Bus Gaps
    • Typical bus gaps are shown below:
      • Open Air 10 – 40 mm
      • Low-voltage switchgear 32mm
      • 15kV switchgear 152mm
      • 5kV switchgear 104mm
      • Low-voltage MCC,PNL 25 mm
      • Cable 13 mm
  • 67. Working Distance
    • Typical working distance is the sum of the distances between the worker standing in front of the equipment, and from the front of the equipment to the potential arc source inside the equipment.
  • 68. IEEE1584 Working Distances
    • Classes of equipment:
      • LV Switchgear 610 mm
      • 15KV/5KV switchgear 910 mm
      • LV MCC & PNL B 455 mm
      • Cable 455 mm
      • 1”=25.4mm
  • 69.  
  • 70. Personal Protective Equipment (PPE)
    • Use the calculations results to apply on the given tables to assign type of clothing for the given example
      • Gloves
      • Tools
      • Face Protection
      • Glasses
      • Clothing
  • 71. Show photos of PPE
  • 72. Grounding Type
    • Two grounding classes are applied in the IEEE 1584 procedure, as follows:
      • Ungrounded systems, which includes, high-resistance grounding.
      • Solidly grounded systems.
  • 73. Solidly Grounded System
    • X0/X1< 3 and R0/X1 <1
    • Low impedance return path for Fault current during a L-G fault.
    • Protective devices will clear the fault faster
    • Stabilizes the line to ground voltage during the normal and abnormal (lightning) conditions
    • Higher ground faults, higher incident energy due to arc flash
  • 74. Ungrounded System
    • Incident Energy is usually higher
    • During the fault large over voltage occurs between the un-faulted phases and ground which can cause high transients
    • Increases L-L and L-G voltages.
  • 75. High Resistance Grounding Systems
    • Intentional resistance between neutral and ground.
    • R is usually 1/3 of coupling capacitive reactance between the phases and ground
    • Same advantages as ungrounded systems
  • 76. Training
    • According to Rule 410 A 2
      • Training shall include applicable work rules to section 41 NESC
      • Re-training until the employee demonstrated complete proficiency
  • 77. Relay Settings & Coordination
  • 78.
    • Protection Engineers have a main objective to reduce the effects of electrical disturbances
    • This obviously includes protecting the safety of personnel.
    • Lately there has been more emphasis on arc flash events.
      • The need to protect against fatal levels of heat, radiation, blasts producing shrapnel and deafening sound waves.
  • 79.
    • The energy produced is proportional to voltage, current and time. ( V*I*t )
      • To reduce energy then reduce one of V,I or t.
      • Voltage is fixed
      • Current can be reduced by the use of current limiting fuses and to some extent good grounding practices.
  • 80.
    • It would seem that time should be the easiest one to change by shortening the clearing time.
    • Looking at shortening the clearing time it also gives us less coordination time.
  • 81.
    • Let us look at the way we produce a coordination study.
  • 82.
    • Working along the system from the source, we add up all the impedances and produce the bolted short circuit currents.
    • This current is the highest expected.
    • Then the times are used to produce the coordination.
  • 83. SOURCE S RELAY Fault Level 13,000 A 1 RELAY Fault Level 2,300 A 2 RELAY Fault Level 1,100 A 3 Inst. Set at 3,000 A P.U. 350 Inst. Set at 1,400 A P.U. 40 Inst. Set at 500 A P.U. 20
  • 84.  
  • 85.
    • The arc fault will have some extra impedance from the arc so less current and therefore
  • 86. SOURCE S RELAY Fault Level Arcing 12,560 A 1 RELAY Fault Level Arcing 2,289 A 2 RELAY Fault Level Arcing 1,066 A 3 100% 85% 10,676 A 1,946 A 906 A
  • 87.
      • Longer times to trip if using an inverse time overcurrent relay or fuse.
      • This extra time increases the arc energy.
    • Now it is normal to do two studies using 100 % and 85 % of the arc fault current.
  • 88.  
  • 89.
    • Working down the system we can add new times for, the new currents, and from these new times and current (the breaker operating times need to be added to the TOC times.) Try to improve the coordination.
    • We can then document the incident energy from information already given.
    • Once this is accomplished a tabulation will help to show the flash boundaries due to these energy levels.
  • 90.
    • Other than cutting down the tripping times we can
      • use some form of differential scheme
      • use a communication scheme to tell quickly where the fault has occurred
      • put settings in relay with lower instantaneous times for maintenance
  • 91.  
  • 92.
    • Other than cutting down the tripping times we can
      • use some form of differential scheme
      • use a communication scheme to tell quickly where the fault has occurred
      • put settings in relay with lower instantaneous times for maintenance
  • 93.  
  • 94.
    • Other than cutting down the tripping times we can
      • use some form of differential scheme
      • use a communication scheme to tell quickly where the fault has occurred
      • put settings in relay with lower instantaneous times for maintenance