Arc Flash Analysis
Nicholas Abraitis, EIT
GRP Engineering, Inc.
Petoskey, MI
Arc Flash Analysis
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Understanding Arc Flash
Required System Data
Arc Flash Analysis Setting and Preferences
Delta Windings on Transformers
Windmil Arc Flash Results
Methods to Reduce Incident Energy in Arcs
Summary
Understanding Arc Flash
Causes of Arc Flash
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Dust
Dropping tools
Accidental contact
Faulty installation
Corrosion and equipment failure
Understanding Arc Flash
Factors Determining Incident Energy in Arcs
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Duration of the arc
Bolted fault current
Enclosed or in open air
Gap between conductors
Proximity of worker to the arc
Understanding Arc Flash
Hazards of Arc Flash
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Fire
Burns
Sound Blasts of 140dB
Heat upwards of 35,000 F
Flying objects and molten metals
Blast pressure upwards of 2000 lbs/ft2
2012 NESC Requirements
Rule 410A3
• Complete an assessment to determine the potential
exposure to an electric arc at all points of their
electrical system. Optionally, use NESC Tables 410-1,
410-2, 410-3 to determine the effective arc rating of
clothing or a clothing system to be worn by employees
working on or near energized lines, parts, or
equipment at voltages 50V to 800,000V.
• Include estimated arc-energy based on available fault
current, duration of the arc in cycles, and the distance
from the arc to the employee in the assessment.
• Require employees wear clothing or a clothing system
that has an effective arc rating not less than the
anticipated level of arc energy.
IEEE Standard 1584
Applies to the following system parameters
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Three phase faults
Frequency of 50Hz to 60Hz
Voltage range of 208V to 15,000V
Bolted fault current 700A to 106,000A
Enclosures of commonly available sizes
Gaps between conductors 13mm to 152mm
Required System Data
• Create the devices in Light Table
• Set the pick up curve, tap, time dial, and CT
ratios for each breaker or recloser
• Check the “Use LT” box in the EQDB for all
over-current devices
• Accurate connectivity and construction of
lines, protective devices, and equipment
Arc Flash Analysis Settings
• Model Subset – Selected Current Work Environment
with entire model active
• Selected an Explicit Working Distance of 18 inches,
can set different working distances for each element
using the Global Editor and setting the field named
“Arc Flash Working Distance”
• Use Typical Values Determined by Equipment Type,
configured in Arc Flash Preferences
• Use Max Arcing Time, calculated based on available
fault current and device operating time in Light Table
• Fault Current Analysis was previously run on the
reduced model, box can be unchecked
Arc Flash Preferences
• Sets the typical gap between
conductors (mm) and the
distance x factor
• Based on IEEE 1584 table
Delta Windings on Transformers
• Unbalance causes a neutral shift
• Windmil models a pseudo source at the delta
banks to represent the fault contributions for
the neutral shifts.
• Modeling the pseudo source is referred to as
the “looped” method
• Analysis is exponentially longer for each
element in the model
• Large delta systems can run several days
without finishing analysis
How Can We Make the Analysis Run
Faster?
• Run circuit reducer to reduce the number of
elements in the model
• Run looped method on selected elements vs.
all elements
• Remove the delta system from the model and
replace it using thevenin equivalent sources
Current Model
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138kV wye system
34.5kV delta system
13.2kV wye system
Generation assets on
the 138kV and 34.5kV
systems
• Model has
138kV:34.5kV wye to
delta transformers
and 34.5kV:13.2kV
delta to wye
transformers
Current Model Zoomed in
Model Reduction
• Select first element downline of the delta
transformers secondary and run fault current
analysis in looped method for the selected
element
• Record the positive and zero sequence
impedances of the element
• Create Zsm Impedance in the EQDB
• Insert a source as the parent of the element with
the newly created impedance code
• Disconnect and remove the delta system from
the model
Model Reduction – Central Substation
• Selected one OCR on
the secondary of each
34.5kV:13.2kV delta
to wye transformer
• Ran Fault Current
analysis
• Recorded the
thevenin equivalent
positive and zero
sequence impedances
Model Reduction
• Replace
34.5kV:13.2kV
transformers with
thevenin equivalent
sources
• Remove the upline
138kV and 34.5kV
transmission system
• Sources include
contributions from
all upline
generation
Model Reduction Completed
• Removed three
138kV:34.5kV
transformers and
seventeen
34.5kV:13.2kV
transformers
• All generation assets
removed from
model
• All 138kV and
34.5kV transmission
lines removed
Windmil Arc-Flash Analysis Results
• Analysis was
completed in
minutes and not in
days
• Arc Flash was
calculated for the
entire 13.2kV
distribution system
Relating the Incident Energy to
Required PPE
Table from NFPA 70E
Reducing the Available Fault Current
Arc Flash Incident Energy vs. Available Fault
Current
ARC FLASH INCIDENT ENERGY (CAL/CM2)
50.00
45.00
40.00
35.00
30.00
25.00
20.00
15.00
10.00
5.00
0.00
30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9
8
7
6
5
4
3
2
1
AVAILABLE FAULT CURRENT (KA)
Working distance (18in) and protective
device clearing time (.4s) held constant
Reducing the Incident Energy in Arcs
Reducing the available fault current
• Limit on-line generation
• Install current-limiting reactors
• Do not operate substation transformers in parallel
Reduce the Protective Device
Operating Time
Arc Flash Incident Energy vs Protective Device
Operating Time
ARC FLASH INCIDENT ENERGY (CAL/CM2)
60.00
50.00
40.00
30.00
20.00
10.00
0.04
0.05
0.045
0.055
0.06
0.065
0.07
0.08
0.075
0.085
0.1
0.09
0.15
0.2
0.25
0.3
0.4
0.35
0.45
0.5
0.55
0.6
0.7
0.65
0.8
0.75
0.85
0.9
0.95
1
0.00
PROTECTIVE DEVICE OPERATING TIME (S)
Working distance (18in) and available fault
current (15,000A) held constant
Reducing Incident Energy in Arcs
Reduce the protective device operating time
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Change the tap setting
Change the curve type
Change the time delay
Add zone protection
Add an instantaneous element
Increase the Working Distance
Arc Flash Incident Energy vs Working Distance
200.00
ARC FLASH INCIDENT ENERGY (CAL/CM2)
180.00
160.00
140.00
120.00
100.00
80.00
60.00
40.00
20.00
0.00
6
8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60
WORKING DISTANCE (INCH)
• Available fault current (15,000A) and protective
device operating time (.4s) held constant
Reducing Incident Energy in Arcs
Increase the working distance
• Use hook sticks to complete work when applicable
• Use remote breaker racking devices on indoor
switchgear
• Use infrared windows
Summary
• Having accurate connectivity in the model is crucial to
Arc Flash Analysis
• Need to have all protective devices modeled in Light
Table and linked to the Milsoft model
• Transformer banks with delta configured windings
cause analysis to run for prolonged periods of time
• Reducing the protective device clearing times or
increasing the working distances have the greatest
effect on the incident energy contained in an arc
• The safest way to avoid an arc flash injury is to
de-energize the equipment as part of the work
procedures
Contact Information
Nicholas Abraitis
Electrical Engineer
nabraitis@grp-engineering.com
Petoskey Office
GRP Engineering, Inc.
660 Cascade W. Pkwy Suite 65
459 Bay Street
Grand Rapids, MI 49546
Petoskey, MI 49770
616.942.7183
231.439.9683
616.285.6448 Fax
231.439.9698 Fax