This document provides an overview of arc flash analysis and mitigation methods. It discusses the general steps for performing arc flash analysis according to IEEE 1584 and NFPA 70E standards. It then describes various analysis considerations and how to analyze arc flash results. The document outlines several methods that can be used to mitigate incident energy, including reducing fault clearing time, increasing working distance, reducing short-circuit current, and reducing energy exposure. It provides examples and drawbacks for each mitigation method.

1. Arc Flash Analysis & Mitigation
Methods
By Albert Marroquin
Operation Technology, Inc.

2. IEEE 1584 2004a “Guide for Performing Arc Flash
Hazard Calculations”
NFPA 70E 2004 “Standard for Electrical Safety
Requirements for Employee Workplaces”
Analysis Methods for Arc Flash Hazards

3. • AF Concepts and Analysis
• What is new in NFPA 70 2009
• NFPA 70E & IEEE 1584 Methodology – Arc Flash exercise 5
• Arc Flash study case & display options - Arc Flash exercise 1
• AF Analysis for LV - Arc Flash exercise 4
• AF Report Analyzer- Arc Flash exercise 2
• AF differential protection – Arc Flash exercise 6
• AF for 1-Phase applications – Arc Flash exercise 7
• AF Options (preferences) – Arc Flash exercise 3
• Questions and Answers session
AGENDA

4. General Steps for Performing
Arc Flash Analysis
• Collect system information required for the arc flash
calculation
• Determine the system operating configuration
• Calculate 3-Phase bolted fault currents
• Calculate arcing fault current (IEEE only)
• Determine arc fault clearing time (arc duration) - TCC

5. • Calculate incident energy
• Determine flash protection boundary
• Determine Hazard/Risk Category based on NFPA 70E
requirements
• Select appropriate protective equipment (PPE Matrix)
General Steps for Performing
Arc Flash Analysis

6. AF Analysis Considerations
• Possible Arc Fault Locations
Line side arc faults
Load side arc faults
• Arc Flash Analysis Worst Case Scenarios
Maximum bolted short-circuit fault current
Minimum bolted short-circuit fault current
• Arcing Current Variation
Incident Energy at 100% of arcing current
Incident Energy at 85% of arcing current

7. Analysis of AF Results
• Arc Flash Analysis Scope
100s or 1000s of Buses
High/Medium/Low Voltage Systems
Multiple Operating Configurations
Dozens of Multiple Scenarios to be considered
• Sorting the Results According to NFPA 70E
Categories
Categories 0 - 4
Locations with Arc Incident Energy > cat 4 limits

8. Analysis of AF Results
• Determine Which Protective Device Clears the Arc Fault
Is it the first upstream device in all cases?
• Determine the Locations with Special Analysis
Conditions
Ibf is less than 700 or higher than 106,000 Amps
The bus nominal kV less than 0.208 kV
The feeder source has capacity less than 125 kVA (may
not have enough energy to generate the arc)

9. Analysis of AF Results
• Arc Flash Analysis should include:
Labeling of equipment and PPE requirements
Recommendations on mitigation methods
Engineering required to reduce the arc flash hazard

10. Methods to Mitigate the
Incident Energy
• Methods to Reduce the Fault Clearing Time
Improving coordination settings of OC PDs.
Type 50 protective devices (Instantaneous)
Arc Flash light sensors
Maintenance mode (switch)
Differential protection
Zone selective interlocking protection (ZSIP)
• Methods to Increase the Working Distance
Remote racking of breakers/Remote switching
Use of Hot Sticks

11. • Methods to Reduce the Short-Circuit Current
Current limiting fuses and circuit breakers
Current limiting reactors, Isolating Transformers
High resistance grounding
• Methods to Reduce the Energy Exposure
Arc resistant switchgear
Arc shields
Infrared scanning, Partial Discharge and or Corona
Cameras
Methods to Mitigate the
Incident Energy

12. 600 Volt Arc in Closed Box Incident energy Exposure @ 18 in.
0
5
10
15
20
0 10 20
Fault clearing time (Cycles)
Calorie/cm^2
NFPA 70E-2000
IEEE 1584-2002
Incident energy exposure at a working distance of 18”
for a 19.5 kA Arc @ 600 Volts (enclosed equipment)

13. Improving Over-Current Device
Coordination Settings
• Purpose is to isolate the fault with the nearest
upstream over-current protective device
• Arc flash results are extremely dependent on
coordination settings
• Unnecessarily high time dial settings for type 51
over-current devices
• Selection of fuses with faster total clearing time
characteristic curves can reduce the energy significantly

14. Coordination
t
I
C B A
C
D
D B
A

15. Arcing current
through A
50/51-1
Incident Energy
released is greater
than 27 cal/cm²
Category 4
Fault Clearing Time
is 37 cycles with
current time dial
settings

16. Arcing current
through A
50/51-1
Incident Energy
released is less than
8 cal/cm²
Category 2
Fault Clearing Time
= 10 cycles
with lower time dial
settings

17. Sample Fuse TCC Curves used in Arc Flash Analysis

18. Fuse Total Clearing Time based on 3.5 kAArc Fault

19. Incident Energy Released for Each Fuse

20. Type 50 Protective Device
• Relays with instantaneous settings
• Molded case circuit breakers
• Insulated case breakers
• Power circuit breakers with instantaneous direct acting
trip elements

21. Type 50 PD Advantages
• Fast acting to reduce the fault clearing time since it can
operate within 3 to 6 cycles
• Commonly available for most MV and LV applications
• Cost effective and do not require special installations
• Already installed in electrical system and may only
require adjustments to reduce the incident energy

22. Type 50
Protective
Devices

23. Equipment Specific Incident Energy
Equations for Molded Case CBs
• Equation based calculation of incident energy for molded
case CBs (Eaton Electrical MCCBs)
• Equations were developed based on extensive testing
• Equations typically yield smaller incident energy results
when compared to those obtained with the TCC curve
analysis methods.
• Maintenance and aging of breakers can change the
predicted incident energy release

24. Example of Eaton Molded Case CB
Equations
• Please note that equations should be used for values
higher than 15*Ir.

25. Type 50 PD Drawbacks
• To achieve coordination with downstream elements,
upstream source Protective Devices have longer time
delays (do not have instantaneous protection)
• The arcing current magnitude passing through the Type 50
protective device must be higher than the device’s
instantaneous pickup setting

26. Selective
Coordination
introduces
time delays
Type 50 PD Drawbacks

27. Maintenance Mode
• Very fast acting trip device reduces the Fault Clearing Time
(FCT)
• Are designed to pickup under very low arcing current
values (instantaneous pickup setting is very low)
• Does not require complicated installation and will
effectively protect locations downstream from the trip unit
with maintenance mode

29. Maintenance
Mode
Normal
Operating
Mode

30. Normal Operating
Mode

31. Normal Operating
Mode

32. Maintenance Mode

33. Maintenance Mode
ON

34. Maintenance Mode Drawbacks
• System will not have coordination during the maintenance
period because of reduced instantaneous pickup settings
• Does not increase equipment protection unless the
maintenance mode is ON
• May not protect certain zones where energized equipment
tasks may be performed

35. Zone Selective Interlocking
Protection (ZSIP)
• Reduced arc fault clearing times
• Zone selection is accomplished by means of hard wired
communication between trip units
• Only the trip unit closest to the fault will operate within
instantaneous since upstream units are restrained by the
unit closest to the fault
• Equipment and personnel arc fault protection

36. Normal
Coordination
Settings

37. Arc Faults at
different bus levels
without ZSIP

38. ZSIP hard-wired
communication for
restraining upstream
trip units

39. Arc Flash at different
bus levels using
ZSIP (observe the
reduced energy)

40. ZSIP Drawbacks
• May take a bit longer to operate than type 50 devices
because of the inherent time delay required for the ZSI
logic operation
• If system is not coordinated, ZSIP does not necessarily
force coordination and other upstream devices may
operate before the device closest to the fault
• Arcing current must still be above short time pickup

41. Arc Flash Light Sensors
• Detect the light emitted by the arc
• Very fast operation (5 to 10 ms) after the light is detected
• Provide comprehensive zone or individual cubicle arc flash
protection (doors open or closed) when correctly applied
• Light sensor protection can be worn at time of task being
performed for additional safety

42. Enclosures

43. Light Sensors

44. Kema-Laboratory Tests
50 kA - 500 ms Arc Fault Clearing Time

45. Arc Flash without Light Sensors

46. Kema-Laboratory Tests
50 kA - 500 ms Arc Fault

47. Kema-Laboratory Tests
50 kA Arc Fault with 50ms Fault Clearing Time

48. Kema-Laboratory Tests
50 kA Arc Fault with 50ms Fault Clearing Time

49. Arc Flash Light Sensor Drawbacks
• Nuisance trips caused by light emitted from sources other
than electrical arcs (can be remedied by using a more
robust approach by combining over-current and light
sensors)
• Positioning of the light sensors poses a possible problem if
they are obstructed or blocked and cannot see the light
emitted by the arc

50. Light Sensor and
Over-Current Relay
Combination

51. Differential Protection
• Short Arc Fault Clearing Times
Differential protection can operate (relay plus breaker)
within 4 to 6 cycles
Relay can operate within ½ to 3 cycles
• Maintain coordination between protective devices upstream
and downstream from the Differential Protection Zone
• Differential protection provides continuous equipment arc
flash protection

52. Types of Differential Relays
• Generator Differential Protection
• Transformer Differential Protection
• Bus Differential Protection
• Line Differential Protection

53. Generator
Differential Relay

54. Transformer
Differential Relay

55. Bus Differential
Relay

56. Actual System with
Differential
Protection

58. Arc Fault with Bus Diff Protection
With differential
protection the
incident energy is
only 5.5 cal/cm2

59. Fault I = 13.83 kA
OC Protection
FCT = 0.643 sec
Fault I = 51.2 kA
Diff Protection
FCT = 0.060 sec
Bus Diff Protection vs. OC Relay

60. Differential Protection Drawbacks
• Nuisance trips caused by transformer inrush currents which
are seen by relay as internal faults - the magnetizing
current has particularly high second order harmonic content
which can be used to restrain or desensitize the relay
during energizing
• Higher equipment and installation costs - relatively higher
costs when compared to traditional over-current protective
devices
• Limited zone of protection for differential ct nodes

61. Current Limiting Methods
• Current Limiting Fuses
• Current Limiting Circuit Breakers
• Current Limiting Reactors
• Isolating transformers
• High Resistance Grounding

62. Current Limiting Fuses
• Current limiting fuses can operate in less than ½ cycle
• Current limiting action is achieved as long as the
magnitude of the arcing current is within the current
limiting range
• Current limitation curves (peak let-through curves) are
needed in order to check if the fuse can limit the current
• Can be very effective at reducing the incident energy if
properly used

63. Current Limiting ActionCurrent(peakamps)
tm ta
Ip’
Ip
tc
ta = tc – tm
ta = Arcing Time
tm = Melting Time
tc = Clearing Time
Ip = Peak Current
Ip’ = Peak Let-thru Current
Time (cycles)

64. Current Limiting Action

65. Analysis of Current Limiting Action
Current Limiting
Action from this
point based on peak
let-through curves

66. Analysis of Current Limiting Action
Current Limiting
Range
Not in Current Limiting
Range (10 times higher)

67. Analysis using IEEE 1584 Equations
Current Limiting Equation for RK Fuses
2
2
/615.0
/57.2
)9321.00302.0(184.4
cmcalE
cmJE
IE bf




68. Current Limiting Fuse Drawbacks
• Current limiting action is achieved as long as the
magnitude of the arcing current is within the current
limiting range
• Can be thermally damaged and have altered characteristics
• Needs spares (which may be expensive) and there is not
indication of the type of fault.
• Energization on pre-existing fault = another blown fuse

69. Current Limiting Reactors
Isolating Transformers
• Current limiting reactors can help to reduce the available
fault current and thus reduce the available energy
• Isolating transformers help to reduce high kA short-circuit
levels (down to less than 10 kA).
• Isolating transformers add impedance between the main
switchboard and the smaller panels fed from it. The short-
circuit available at the switchboard may be considerably
higher

70. Increasing the Working Distance
• Hot Sticks
• Remote Racking
• Remote Switching

71. • Used to insulate the electrician from electric shock and to
increase the distance from arc flash/blast
• Should be inspected prior to each use for signs of cracks or
physical damage which may affect the insulating capability
• Need to wear additional PPE
Using Hot Sticks

72. Using Hot Sticks

73. • Are used to increase the personal space between the
potential source of the arc and the electrician
• Can be combined with high strength plastic shields to
reduce the effects of the arc flash/blast
Remote Racking/Remote
Switching

74. Remote Racking/Remote
Switching

75. Remote Racking/Remote
Switching

76. Mitigating/Avoiding the
Incident Energy
• Arc Resistant Switchgear
• Arc Flash Shields

77. Arc Resistant Switchgear
• Funneling or re-directing the incident energy away from
the personal space
• Special design and construction allows the front of the
equipment to experience low levels of energy
• Arc flash may still be very severe and equipment will
suffer considerable damage

79. Arc Resistant Switchgear

80. Infrared Scanning
• Infrared scanning can help detect loose connections by
detecting hot spots and thus avoiding an arc flash
• Infrared scanning only helps to detect equipment failure
which may cause an arc, but it does not reduce the risk of
arc flash incidents caused by human error

81. Infrared Scanning

82. Partial Discharge Measurements
• Measures partial discharge activity through analysis of high
frequency activity
• Identifies areas where insulation is breaking down
• Digital Oscilloscope with noise reduction software
• Connects to existing CT’s, PT’s or RTD circuits
• No shutdowns required

83. Partial Discharge Measurement

85. Results of Partial Discharges
<<< Corona
Corona (Close up) >>>

86. Results of Partial Discharges
<<Tracking 4,160 Volt
Surface Discharges >>>
and Void Type Defect

87. Best Solution to
Mitigate the AF Risk
• De-energize the equipment
The best strategy to protect against arc flash dangers
is to de-energize the equipment before working on it

88. Acknowledgements
• Eaton Cutler-Hammer
• ETAP Canada Ltd.
• QIP2
• VAMP
• Shaw

89. Thank You.