KINECTRICS INC.
AC Arc Flash
for
Transmission
and Distribution
Systems
GENERATING SUCCESS --- FOR 100 YEARS
life cycle management solutions
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Page 1
Kinectrics - Proprietary and Confidential
Agenda
1.
2.
3.
4.
About Kinectrics
Why should you be concerned about arc flash?
Basic information on arc flash
AC arc flash
Low voltage arc flash computations
Medium and high voltage arc flash computations
Arcpro
5.
6.
7.
8.
Comparison of arc flash assessment tools
Arc hazard mitigation
Conclusion
Q&A
life cycle management solutions
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Kinectrics has 100 years of advanced
technical expertise and experience
• An established independent
consulting engineering and testing
firm
• Formerly the R&D Division of
Ontario Hydro, one of North
America’s largest, most reliable
utilities
• Over 400 scientists, engineers and
professional staff
• Over 25 independent test facilities
and labs, complemented by field
inspection services
• Over 150 clients in North America
and worldwide
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Kinectrics owns one of the few High
Current Laboratories in North America
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Kinectrics was the pioneer and
continues to stand on the cutting edge
• Has been conducting arc hazard
analysis and testing since 1995
• Developed Arcpro ($500K arc
modeling project) – recommended by
OSHA
• Assigns majority of ATPV on thermal
protective PPE worldwide
• Provides technical support to groups
and committees: IEEE, NFPA, IEC,
ASTM, etc.
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Kinectrics teamed up with many to
strive for a safer work environment
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Kinectrics has world-renowned experts
in the arc hazard discipline
Kenneth Cheng, M. Eng, P. Eng, MBA
Ken.Cheng@Kinectrics.com
416-207-6000 X 6032
Senior Engineer of Distribution & Asset Management
PL for AC and DC arc testing and analysis
Sits on both the NFPA and IEEE 1584 WGs
Other Kinectrics Arc Experts:
Stephen Cress, Manager, Co-Developer of Arcpro
Carl Keyes, Senior Associate
Claude Maurice, HCL Manager
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Agenda
1.
2.
3.
4.
About Kinectrics
Why should you be concerned about arc flash?
Basic information on arc flash
AC arc flash
Low voltage arc flash computations
Medium and high voltage arc flash computations
Arcpro
5.
6.
7.
8.
Comparison of arc flash assessment tools
Arc hazard mitigation
Conclusion
Q&A
life cycle management solutions
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People are your most important asset.
• As much as 80% of all electrical injuries are
burns resulting from an arc-flash and ignition
of flammable clothing
• More than 5 workers a day were victims of
an arc flash
• 80% of electrically related accidents and
fatalities involving “Qualified Workers” are
caused by arc flash / arc blast
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It is the employer’s responsibility to
implement an arc flash program.
• Between 2007 and 2011, more than 2,880
fines were assessed for not meeting OSHA
regulation 1910.132(d) which averages out
to 1.5 fines a day
• The National Safety Council estimates workrelated injuries can cost businesses over
$30M in fines, medical costs, litigation, lost
business and equipment costs
• Employer’s responsibility to implement a
programs to select proper PPE for
employees to limit burns to levels that are
considered curable
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Workers exposed to arcs in open
spaces are often less severely injured.
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Workers exposed to arcs in confined
spaces are subject to worse injuries.
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Agenda
1.
2.
3.
4.
About Kinectrics
Why should you be concerned about arc flash?
Basic information on arc flash
AC arc flash
Low voltage arc flash computations
Medium and high voltage arc flash computations
Arcpro
5.
6.
7.
8.
Comparison of arc flash assessment tools
Arc hazard mitigation
Conclusion
Q&A
life cycle management solutions
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An arc is initiated when a passage of
current is established in ionized air.
• Arc temperature 15,000 to
35,000C
• High radiant heat – 95%
• Explosive (cu vapor expands
67,000 times solid cu
volume)
• Pressure – 2,000 lb/ft2
• Molten particle splatter
• Noise – 150 dB
• Light intensity from 108,000
lux to more than 249,000 lux
at 3 meters
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Arcs caused by bridging of phases, dielectric
breakdown, mechanical failure.
Hazards from electric arc include:
• Electrocution
• Burn hazard from high radiant heat
– No contact
– Severe if ignites, melts or breaks open
clothing
• Physical injury from pressure
• Injury from molten material
• Heating damage
• Vision damage
• Secondary fire or explosion
• Equipment damage
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Chance of Survival (%)
Percentage of body burn determines whether
an arc flash incident is fatal.
25% Body
50% Body
75% Body
100
90
80
70
60
50
40
30
20
10
0
20-29
30-39
40-49
50-59
Age Range (yr)
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Third degree burns are considered incurable.
Burn depth determines severity
• 1st degree: red skin no blister
• 2nd degree: skin blisters, epidermis must
regenerate, 100-micron depth
• 3rd degree: full thickness destroyed, skin
cannot regenerate, scar tissue, 1000micron
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Existing arc flash standards provide various
tools to conduct incident energy calculations.
Relevant standards:
• OSHA Standards 29 CFR, Parts 1910
and 1926
• IEEE 1584
• NFPA 70E / CSA Z462
• NFPA 70B (electrical maintenance)
• NESC 2007, 2009
• ASTM
• CAN / ULC-S801-10 (CEA Guide)
Circuits should always be de-energized unless deenergizing creates a bigger hazard.
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General but important information on arc
flash.
• Live parts should be de-energized unless
de-energizing increases hazard or is
infeasible due to operation limits
• PPE is the last line of defense
• Protection is to mitigate the impact of arc
flash, not prevent all injury – 2nd degree burn
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General but important information on arc
flash. (cont…)
• Balance between avoiding 2nd degree burn
protection and other hazards such as heat
stress, poor visibility, limited body movement
• Several hazard assessment methodologies
exist, each with limitations – use best tool
• Professional judgement must be used
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Arc flash terms that everyone should
understand:
• Arc Flash Hazard: A dangerous condition
associated with the release of energy
caused by an electric arc
– NFPA 70E says “exposed” live parts or
“interacting with equipment” can create arc flash
hazard
• Plasma: Superheated gas that is created by
the arc flash and expands rapidly from the
arc location
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Arc flash terms that everyone should
understand: (cont…)
• Incident energy: Amount of energy
impressed on a surface, a certain distance
from a source, generated during an electric
arc event
– Measured in calories/cm2 or joule/cm2
• Arc Thermal Performance Value (ATPV):
The mean incident arc energy on a textile
material that can cause the onset of a
second degree burn on the skin beneath
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Arc flash terms that everyone should
understand: (cont…)
• Calories/cm2: The total energy on a surface area. It
is the amount of energy required to raise the
temperature of one gram of water one degree
centigrade
Palm in the hottest part of
the flame for one second
Will receive about 1.2
calories per cm2 — the
onset of a second degree
burn (blister burn).
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Arc flash terms that everyone should
understand: (cont…)
• Arc Flash Protection Boundary: An approach limit
at a distance from exposed live parts within which a
person could receive a second degree burn if an
electrical arc flash were to occur
– The boundary is set at the point where a second
degree burn is probable (1.2 cal/cm2 or 5
joules/cm2)
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Here are the typical garments for different
Hazard Risk Categories (HRC)
•
•
•
•
•
Category 0:
Category 1:
Category 2:
Category 3:
Category 4:
Up to 1.2 cal/cm2
1.2 cal/cm2 to 4 cal/cm2
4 cal/cm2 to 8 cal/cm2
8 cal/cm2 to 25 cal/cm2
25 cal/cm2 to 40 cal/cm2
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Arcs can be caused by bridging of phases,
dielectric breakdown and mechanical failure
Severity of incident energy levels from arcs depend on:
• System voltage
• Available fault current
• Fault duration
• Arc length or gap distance
• Working distance
• Electrode materials
• Enclosure around arc
• AC or DC
• Number of phases involved
• Arc motion
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Agenda
1.
2.
3.
4.
About Kinectrics
Why should you be concerned about arc flash?
Basic information on arc flash
AC arc flash
Low voltage arc flash computations
Medium and high voltage arc flash computations
Arcpro
5.
6.
7.
8.
Comparison of arc flash assessment tools
Arc hazard mitigation
Conclusion
Q&A
life cycle management solutions
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Arc hazard assessment questions:
•
•
•
•
Is your PPE appropriate, conservative, inadequate?
What are your arc hazard criteria for clothing selection?
What are your arc hazard levels?
What is the heat (cal/cm2) on surfaces at distances from
arc?
• How do you know?
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Arc flash hazard analysis should be done to
determine:
• Incident energy level at specified working
distance
• PPE category at specified working distance
• Flash protection boundary distance
Arc hazard analysis should be reviewed at least every 5
years or whenever changes are made to the circuits or work
practices.
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Many incident energy calculation tools exist
for low voltage components.
• NFPA 70E tables
• Doughty’s equations
• IEEE 1584 equations
• Arcpro (NESC tables)
Low voltage components shouldn’t be taken lightly.
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The NFPA 70E table method provides HRC
based on task performed at live equipment.
• Restrictive current and durations:
– 240V: 25 kA and 2 cycles
– 600V MCC: 65 kA and 2 cycles (or 42 kA and 20 cycles)
– 600V Switchgear: 35 kA and 30 cycles
• Inadequate worker protection if fault current and duration
requirements are not met
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Here are some reasons why the NFPA 70E
table method is used…
• Simple to use
• Doesn`t require engineering expertise
• No calculation required (only have to determine fault
current and duration ranges)
• Better than taking a “do nothing” position
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Doughty’s equation conduct incident energy
calculations for equipment ≤ 600V.
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Doughty’s equation conduct incident energy
calculations for equipment ≤ 600V. (cont…)
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Here are some key notes for Doughty’s
equations…
• Great conservative second calculation assuming that the
arcing fault current can be as low as 38% of the bolted fault
current
• Generates reasonable results when the parameters fall
within the applicable ranges
• All bets are off when they don’t…
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The IEEE 1584 equations are widely used to
determine incident energy for LV equipment.
• Three-phase open and enclosed
• Empirical model for 208V to 15 kV
• Arc gaps of 6 inches or less
• Equipment need not be considered unless it involves at
least one 125 kVA or larger low impedance transformer in
its immediate power supply and that the equipment voltage
is less than 240 V (phase to be removed in next revision)
• Single phase AC, DC arcs not covered
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IEEE 1584’s recommended steps to conduct a
thorough arc hazard investigation.
1. Collect installation data that are readily available
i. Single line diagrams
ii. Documents
2. Determine system mode of operation
i.
Normal vs. emergency
3. Calculate bolted fault current
i.
Consider all impedance and contributions
4. Determine arcing fault current
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IEEE 1584’s recommended steps to conduct a
thorough arc hazard investigation. (cont…)
5. Attain duration from protective devices
i.
Maximum of 2 seconds (as applicable)
6. Document system voltages and classes of
equipment to determine arc gap distances
7. Select working distances
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IEEE 1584’s recommended steps to conduct a
thorough arc hazard investigation. (cont…)
8. Determine incident energy level
9. Determine flash protection boundary distance
10.Select appropriate PPE
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IEEE 1584 arcing current equation varies with
voltage, bolted current, gap and configuration.
log(Ia) = K + 0.662 log(Ib) + 0.0966 Voltage
+ 0.000526 G + 0.5588 log(Ib) Voltage
– 0.00304 log(Ib) G
Where:
log(Ia) = log in base 10 of arcing current (kA)
K = – 0.153 for open configuration
or – 0.097 for box configuration
log(Ib) = log10 of symmetrical RMS bolted
fault current (kA)
Voltage = system voltage (kV)
G = distance between buses (mm)
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IEEE 1584 incident energy equation varies
with arcing current, gap and configurations.
log(En) = K1 + K2 + 1.081 log(Ia) + 0.0011 G
Where:
log(En) = log10 of incident energy normalized to 0.2 seconds and
610 mm distance
K1 = -0.792 for open configurations or -0.555 for box
configurations
K2 = 0 for ungrounded and high resistance grounded
systems, or -0.113 for grounded systems
G = bus bar spacing or gap (mm)
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For specific working distance and duration,
the following equation is used to attain Ei.
E = Cf10log(En)(t /0.2)( 610X / DX)
Where:
E = Incident energy in cal/cm2
Cf = Calculation factor of 1.0 for Voltage > 1kV,or 1.5 for
Voltage < 1kV
t = Time in seconds
D = Working distance in mm
X = Distance exponent from table based on enclosure size
and voltage
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Conduct a second calculation at 85% of the
arcing fault current.
• Lower current may cause a longer tripping time of the
upstream protective device and may result in higher incident
energy levels
• Conservative approach is to calculate the energy at a
reduced arcing current of 85% of the calculated value and
use the higher resultant energy
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Here are the advantages of using IEEE 1584
equations…
• Empirically derived equations are based on a large number
of tests
• Great conservative second calculation assuming that
• Accessibility – Spreadsheet calculations
• International standard and method
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IEEE 1584 equations also have limitations.
• Distance is from the electrodes and not the actual arc
• Requires detailed information about the equipment
installation
• Requires in-depth knowledge of short circuit and protective
device’s time-current curves
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IEEE 1584 equations also have limitations.
(cont…)
•
•
•
•
•
•
208 V to 15 kV
3-phase faults
50 Hz and 60 Hz
Bolted fault of 700A to 106 kA
Arcing fault up to 50 kA
Gaps of ½” to 6”
<6”
<15kV
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There are only a couple of widely-used tools
to predict incident energy from MV & HV arcs.
• Ralph Lee’s theoretical equation
• Arcpro, recommended by OSHA
– NESC tables
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IEEE 1584 recommends Lee’s equation for
equipment > 15 kV or arc gap > 6 inches.
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Lee’s equation is known to provide overly
conservative results.
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Agenda
1.
2.
3.
4.
About Kinectrics
Why should you be concerned about arc flash?
Basic information on arc flash
AC arc flash
Low voltage arc flash computations
Medium and high voltage arc flash computations
Arcpro
5.
6.
7.
8.
Comparison of arc flash assessment tools
Arc hazard mitigation
Conclusion
Q&A
life cycle management solutions
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Arcpro can be used to conduct arc hazard
analysis.
• Assist user in the selection of protective clothing by
calculating the thermal parameters of electric arcs
• Quantify the total arc energy
• Quantify heat at various distances from arc
• Consider different currents, arc lengths, materials, etc.
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Arcpro accepts the following inputs:
• Arcing fault current magnitude (kA rms)
• Duration (cycles)
• Arc length
• Electrode materials
• Distance from the arc
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Arcpro provides the following output:
• Total arc energy
• Heat flux on a surface (cal/cm2/sec)
• Heat energy at receiving surface (cal/cm2)
• Radiated and convected components of heat flux
• Voltage and current waveforms
• Arc diameter and resistance
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Arcpro has the following features:
• Windows based, user friendly
• Cautions against inappropriate input
• Graphical and tabulated results
• 2 system of units
• User guide
• Clothing database
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Arcpro also has its limitations and
assumptions.
• Vertical free burning arc in air
• Arc length >>> arc diameter
• Optically thin plasma and gas
• Melting and shortening of electrodes not considered
• Wire and gas explosion not considered
• Resistive electric current
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Arcpro perform calculations based on arc
physics, not empirically derived equations.
• Thermodynamic and transport properties
• Unit length divided into cells across radius
• Temperature at arc boundary is 6,000K
• Arc radius is dependent on total power, radiation loss,
thermal conduction, and convection
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Arcpro computations…
• Numerical convergence to obtain radius and temperature
profile for the current at each time step
• Radiation and convection computed for each unit length
• Arc resistance and voltage computed based on arc radius
• Thermal radiation to surfaces from line element of arc
• Boundary layer convection calculation
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These are the fundamental equations in
physical arc model that Arcpro uses…
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Arcpro’s result has been verified with
laboratory test data.
Verified with over 300 points for arc energy and surface heat
at:
• Currents 3 kA to 25 kA
• Arc duration from 4 cycles to 35 cycles
• Working distance from 8 inches to 24 inches
• Arc gap from 1 inch to 12 inches
Over these ranges Arcpro averages:
• About 10% above measured energy
• About 20% above average measured heat
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Arcpro generates accurate and slightly
conservative results.
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Arcpro generates accurate and slightly
conservative results. (cont...)
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Arcpro generates accurate and slightly
conservative results. (cont...)
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Arcpro generates accurate and slightly
conservative results. (cont...)
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Kinectrics generated factors for Arcpro to
covert results to in-box and three-phase.
The factors were derived by comparing Arcpro results to
IEEE 1584 test data.
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NESC scope covers utility installations related
to generation, transmission and distribution.
• Jan 1, 2009 – assess to determine exposure to electric arc
for employees who work on or near to electric arcs
• If above 2 cal/cm2, employees are required to wear clothing
system that has arc rating not less than the anticipated level
• When exposed to arc, the following must not be worn:
acetate, nylon, polyester, polypropylene
• Results from the NESC tables are generated from Arcpro
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NESC tables is intended to reduce injury and
not prevent all burns.
Exception #1: If clothing for arc hazard would create an
additional or greater hazard, lower rated clothing can be worn
Exception #2: For < 1000 V, in place of arc hazard analysis,
minimum of a 4 cal/cm2 clothing system can be implemented
to limit ignition.
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The NESC tables were generated according to
the following…
Maximum clearing times specified for 4 cal, 8 cal and 12 cal
clothing systems
1 – 46 kV table:
• Open air phase-to-ground arc
• 15 inch working distance
• Arc gap: 1-15 kV, 2”; 15-25 kV, 4”; 25-36kV, 6”; 36-46 kV, 9”
46 – 800 kV table:
• Open air phase-to-ground arc
• Gap calculated as phase-ground voltage / 10 (dielectric
strength of air = 10 kV per inch)
• WD = minimum approach distance – 2 X gap length
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Here is a sample of the NESC tables.
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Here are the limitations of the NESC tables.
• Four discrete fault values per voltage range
• No fault currents under 5 kA
• No multi-phase events
• Calculations based on small arc gaps, unrealistic for
overhead systems
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Agenda
1.
2.
3.
4.
About Kinectrics
Why should you be concerned about arc flash?
Basic information on arc flash
AC arc flash
Low voltage arc flash computations
Medium and high voltage arc flash computations
Arcpro
5.
6.
7.
8.
Comparison of arc flash assessment tools
Arc hazard mitigation
Conclusion
Q&A
life cycle management solutions
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Here is how the computation tools compare
against one another…
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Here is how the computation tools compare
against one another…
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Here is how the computation tools compare
against one another…
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Results from Lee’s equation exceeds those
from other methods at higher voltage.
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Results from IEEE 1584 equation exceeds
those from other methods at larger arc gaps.
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Here is a summary of the arc hazard analysis
toolkit…
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Agenda
1.
2.
3.
4.
About Kinectrics
Why should you be concerned about arc flash?
Basic information on arc flash
AC arc flash
Low voltage arc flash computations
Medium and high voltage arc flash computations
Arcpro
5.
6.
7.
8.
Comparison of arc flash assessment tools
Arc hazard mitigation
Conclusion
Q&A
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Mitigation measures should be implemented
for component locations with high Ei.
• Job Briefing and Work Permit
• De-energize and isolate where possible
– portable grounds
– Interlocks
• Increase working distance (Ei α 1/ Distance2)
– Longer operation arms
– Remote operation
• Decrease arc duration and reduce fault level
–
–
–
–
Protection changes – faster operation, current limiting
Arc sensing technologies – fibre optic sensors
Update old equipment – breakers, relays, reclosers
Change switching sequence
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Mitigation measures should be implemented
for component locations with high Ei. (cont…)
• Shield from incident energy
– Blast blanket
– Portable shield
– Arc proof switchgear – prevent
radiation and flying particles
– Relocate circuit breaker control
switches to remote locations
(electrically operated circuit
breakers)
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Agenda
1.
2.
3.
4.
About Kinectrics
Why should you be concerned about arc flash?
Basic information on arc flash
AC arc flash
Low voltage arc flash computations
Medium and high voltage arc flash computations
Arcpro
5.
6.
7.
8.
Comparison of arc flash assessment tools
Arc hazard mitigation
Conclusion
Q&A
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Conclusions…
• There are numerous useful tools in the arc hazard
assessment tool kit – the kit is half full
• At present, there is no one all inclusive model
• Industry practice is to use IEEE 1584 equations for
LV, in-box 3-phase small gaps and Arcpro for HV in
air, single-phase longer gaps
• Currently no verified model for HV 3-phase, in-box
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Conclusions…(cont…)
• Be certain that the most correct tool is being used
for the job
• Understand the applications and limitations of
existing models
• Extrapolation may produce misleading results
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Agenda
1.
2.
3.
4.
About Kinectrics
Why should you be concerned about arc flash?
Basic information on arc flash
AC arc flash
Low voltage arc flash computations
Medium and high voltage arc flash computations
Arcpro
5.
6.
7.
8.
Comparison of arc flash assessment tools
Arc hazard mitigation
Conclusion
Q&A
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The most important daily objective at work is
to go home safely!
Thank you for your
time!
Questions?
Ken Cheng
416-903-5313
Ken.Cheng@Kinectrics.com
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DC Arc Flash
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DC arc hazard analysis is required when live
work is performed at the following…
• Battery installations at substations and renewable
storage locations
• Inverters
• UPS
• DC supply
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There are considerable differences between
AC and DC arcs
AC Arc Current Waveform
DC Arc Current Waveform
AC Arc Voltage Waveform
DC Arc Voltage Waveform
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Slow-motion DC arc flash test
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There are considerable differences between
AC and DC arcs (cont…)
• AC arcs encounter zero-crossing, DC arcs do not
• Diameter of DC arc’s plasma column remain constant,
AC arc’s plasma column expands and contracts
• DC arcs are more difficult to extinguish
• DC arcs decay characteristics are dependent on the
source (ie. battery systems have a finite capacity to
sustain the arc)
• Fewer upstream protections for DC systems
With all parameters being equal, DC arcs generate more
energy than AC arcs
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Kinectrics was the first to conduct DC arc
flash test in Year 2007 (client: Bruce Power)
Test range:
• 125 VDC and 260 VDC
• 1 kA to 10 kA
• 1” to 5” arc gaps
• 0.01 to 2.00 seconds
• Working distance of 12” and 24”
• Electrodes pointed downwards
• Open arcs
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Total arc energy, whether from an AC or a DC
source, produces similar incident energy.
Heat Flux at 12 inches vs Arc Current
260 V DC, 1" and 2" arc gap
Incident energy (Ei) at 12" Distance vs Arc Energy (kJ)
12.0
18
2” arc gap
16
H e a t- F lu x , C a l/c m ² /s
Inc ident Energy , Cal/c m ²
20
14
12
1” arc gap
10
8
6
10.0
8.0
6.0
4.0
2.0
4
0.0
2
0
0
0
2
4
6
8
10
12
500
1000
1500
Arc Energy, kJ
14
Arc Current, (kA)
DC, 1" gap
DC, 2" gap
AC 1" Gap
Arcpro
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Heat flux from an average DC current is 1.25X
the heat flux from an equal rms AC current
Comparison of AC vs DC Heat-Flux, 24" away
6
Heat-Flux, Cal/cm²/s
5
Computed AC
4
DC, 2" gap
AC, 2" gap
3
DC, 1” gap
AC, 1” gap
2
1
0
0
2000
4000
6000
8000
10000
12000
14000
Current, DC Avg or AC RMS
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Kinectrics conducted DC arc flash tests at
600 VDC for CMBC in Year 2012
Test range:
• 600 VDC
• 2 kA, 15 kA, 25 kA
• 1”, 3” and 6” arc gaps
• 0.1 and 0.5 seconds
• Working distance of 6”, 12” and 24”
• Electrodes pointed downwards and
outwards
• Open and enclosed arcs
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An equation was empirically derived to
compute arcing fault currents for CMBC.
• At 600 VDC, Ibolted ≠ Iarcing
• Iarcing α Ibolted,
Iarcing α 1/G
where G is the arc gap
distance
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Kinectrics-derived incident energy equation
generates accurate results at 600 VDC.
Measured vs. Calculated
Incident Energy Level
(Enclosed Arcs with
Electrodes Pointing Out)
Measured vs. Calculated
Incident Energy Level
(Enclosed Arcs with
Electrodes Pointing Down)
Measured
Measured
Calculated (Open-Air Equation)
Calculated (Open-Air Equation)
Calculated (Open-Air Equation with Factor)
Calculated (Open-Air Equation with Factor)
6
Incident Energy (cal/cm2)
Incident Energy (cal/cm2)
35
30
25
20
15
10
5
0
5
4
3
2
1
0
0
5
10
Comparison Number
15
0
20
2
4
6
Comparison Number
8
10
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The parameters Kinectrics considered for the
DC arc flash tests can be summarized below.
Voltage
Bolted
Fault
Current
Arc
Calorimeter
Lengths
Distance
Duration
Electrodes Enclosure
125 V
DC
4 & 20kA
0.5”
12, 24, 36”
Up to 1 sec
Vertical
Open
260V
DC
2 to 25 kA
1 & 2”
12, 24, 36”
Up to 2 sec
Vertical
Open
600V
DC
2, 15 & 25 1, 3, & 6”
kA
6, 12, 24”
0.1 & 0.5
sec
Vertical
Open
12, 24”
0.1 & 0.5
sec
Pointed
Down
In-Box
12, 24”
0.1 & 0.5
sec
Pointed at
Calorimeter
In-Box
15 & 25kA
15 & 25kA
3”
3”
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At the time of writing, there are only a few DC
arc flash calculation methodologies.
• Arcpro with empirically derived factors
• Kinectrics empirical model for open-air and
enclosed (with factors) DC arcs
• NFPA 70 E equation
• NFPA 70 E tables
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The NFPA 70E equation assumes a certain
percentage of power is released as Ei.
…but arc energy should not be dependent on system
voltage
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NFPA 70E tables are task-specific, are prone
to confusion and human errors.
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NFPA 70E equations generate overlyconservative results for open air DC arcs.
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NFPA 70E equation and Arcpro do not produce
realistic results for enclosed arcs at 600 VDC.
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Kinectrics recommends using the following
approach for DC arc hazard calculations.
Voltage
(VDC)
NFPA 70E
Equation
NFPA 70E
Table
Arcpro (with
factors)
Kinectrics’
Equation
125 VDC
260 VDC
600 VDC
Other
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To complete a credible DC arc hazard analysis,
the following steps should be followed.
1.
Specify component locations and work procedures
2.
Determine source voltages
3.
Determine bolted currents from SC study at
component locations, generally requires:
–
Source kA or kVA
–
Transformer size, impedance, X/R ratio
–
Cable impedance (type, size and length)
4.
Determine arcing currents (max and min) as
opposed to bolted current
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To complete a credible DC arc hazard analysis,
the following steps should be followed. (cont…)
5.
Determine duration of arc (at max and min
arcing current) from protective device
coordination study
–
Relay and breaker - Type, continuous
rating, settings (LTPU, STPU, etc.)
–
Time-current characteristics (include
relay time, aux contact time and clearing
time of CBs)
–
Consider decay of battery current
–
Consider max duration of exposure to
worker (2 sec?)
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The battery discharge magnitude and correlated
duration impact the incident energy level.
Always try and determine the worst case scenario.
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To complete a credible DC arc hazard analysis,
the following steps should be followed. (cont…)
6. Specify arc gap length
7. Calculate incident energy (max) with
appropriate equation – Arcpro and factors,
NFPA 70E, Kinectrics’ empirical equation
8.
Select appropriate PPE and arc flash
boundary
9.
Equipment labeling and awareness
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Conclusions
• There are useful tools in the DC arc hazard
assessment tool kit – the kit is half full
• At present these is no one all inclusive model
• NFPA 70E formula is conservative in most cases
compared to measurement
• Need for additional development of DC models
• Understand the applications and limitation of existing
models
• Extrapolation may produce misleading results
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AC Arc Flash at
Generation,
Transmission and
Distribution Systems
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Arc Hazard Assessment Locations
Distribution Lines
Distribution Lines
Distribution Station
Distribution Station
Transmission Stations
Transmission Stations
Transmission and
SubTransmission
Generation Plant
SYSTEM LOCATIONS
OH Lines, Switches, OH Transformers
Padmounted Transformers & Switchgear
Air Insulated Bus and Switches
Transformers, Breakers, Regulators,
Reclosers, Switchgear
Air Insulated Bus and Switches
Transformers, Breakers, CTs
OH Lines
Switchgear
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Distribution System – OH Lines
•
Current
• Line SC study
• changes with distance from the station
•
Phases
• Phase-phase or phase-ground
•
Duration
•
•
•
•
•
Upstream protective devices, recloser, CB
Changes downstream of each protective device
Consider reclosing
Consider work practice (eg cannot get out of bucket)
Gap
• Across failed insulation
• Arc extension
•
Distance
• According to work practice
•
Enclosure
• No
•
Tools
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• ArcPro
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Application Issues – Arc Length vs
Electrode Gap
Voltage Classification
(Nominal System Voltage)
Rated Lightning Impulse
Withstand Voltage (LIL)
Minimum Striking Distance in Air (Metalto-Metal)
Phase-to-Ground
Phase-to-Phase
Phase-to-Phase
Phase-to-Ground
Indoor
Outdoor
Indoor
Outdoor
kV rms
kV crest
mm
mm
mm
mm
15 (13.8)
95
180
250
250
300
25 (23)
150
260
310
330
380
35 (27.6)
200
380
380
480
480
50 (44)
250
480
480
610
610
72.5 (69)
350
710
710
860
860
123 (115)
550
1100
1100
1300
1300
145 (138)
650
1300
1300
1600
1600
250 (230)
900
1800
1800
2100
2100
362 (346)
1300
N/A
2900
N/A
3400
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Distribution OH Line Example
• Working within
restricted approach
distance (NFPA - 26”)
• 13.8 kV
• Current – 10 kA
• Duration – station
breaker - 6 cycles
• Gap – 12”
• Distance - 18” distance
to the arc
• Incident Energy = 3.2
cal/cm2
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Boundary Concept
• Arc flash incident energy changes after feeder passes
through a protective device.
• Arc Flash incident energy typically increases further
from the source due to the inverse time vs. current
characteristics of protective devices
• Distribution systems start with large conductors and
then reduce in size further away from the source.
• Recloser, sectionalizer, fuse provide transitions
• Using the existing protective devices, hazard zones
bounded by protective devices can be described.
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Boundary Concept
• The system drawing provided shows a substation
with two feeders.
• Each feeder is similar with branches at various
locations.
• At each branch a protective device is shown.
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Steps in Applying the Boundary Concept
•
•
•
•
•
Obtain feeder operating diagrams
Identify overcurrent protection devices and protection zones
Determined bolted fault levels
Determine arcing fault levels
Determined protection device clearing times TCCs
– Beginning of zone
– End of zone
•
Calculate incident heat energy
– worker distances according to limits of approach
• 1.5’, 2.5’, 4’ and 10’
– arc gaps according to Minimum Striking chart (ie12”, 18”, etc)
– multiplier to consider 3 phase arc flash event
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Boundary Concept – Breaker Zone Example
(example courtesy BC Hydro)
Beginning
•Clearing times from 6 cycles (instantaneous) to 23 cycles depending
on current
•Without breaker instantaneous, incident energy values range from 2.5
to 13 cal/cm2 at 18” worker distance
•With breaker instantaneous trips, incident energy values ranged from
1 to 5 cal/cm2 at 18” worker distance
•For 30” distance, highest energy was found to be 5 cal/cm2
•Beyond 4 ft energy is below 2 cal/cm2
End
•Clearing times from 30 to 107 cycles depending on branch location
•Incident energy is from 2.2 to 12 cal/cm2 at 18” worker distance
•For 30” worker distance highest energy was found to be 4.6 cal/cm2
Beyond 4 ft energy is below 2 cal/cm2
Need to use instantaneous trip during live line work
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Breaker Zone
(example courtesy BC Hydro)
20
Three Phase Incident - Worst Case Incident Energies for Westinghouse CO-9 Protected Zones (TD - 2,
540A Pickup, 4800 Instantaneous, 18" Arc Gap)
18
Col 1 (1.5')
Incident Energy (Cal/cm^2)
16
Col 2 (2.5')
14
Col 3 (4')
12
Col 4 (10')
10
8
6
4
2
0
0
1000
2000
3000
4000
5000
6000
7000
8000
Fault Current
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Fuses and Breakers – Max I2t – Max cal/cm2
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Boundary Concept – Fuse Zone Example
(example courtesy BC Hydro)
Beginning
•Clearing times from 6 cycles to 13 cycles depending on fault level
•Incident energy is below 2 cal/cm2
End
•
•Clearing times from 7 to 14 cycles
•Incident energy is below 2 cal/cm2 at 18” worker distance
fuse TCC shows heat energy to be below 3 cal for range of 3 to 5 kA.
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Boundary Concept – Recloser Zone Example
(example courtesy BC Hydro)
Fast curve blocked during live line work
Beginning
•Clearing times from 21 to 39 cycles depending on fault level
•Incident energy from 1 to 3.6 cal/cm2 at 18” worker distance
•For 30” worker distance, max energy was 1.4 cal/cm2
•Beyond 4 ft energy is below 1 cal/cm2
End
•Clearing times from 34 to 233 cycles depending on branch location
•Incident energy from 1 to 5 cal/cm2 at 18” worker distance
•For 30” worker distance, highest energy is 2 cal/cm2
•Beyond 4 ft energy is below 1 cal/cm2
General
•If recloser is operated on fast curve for permit work then incident
energy can be below 1 cal/cm2 for entire zone
•Hot Line Tag Option
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It Charts for PPE Categories
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Distribution Lines – Padmounted Transformers and Switchgear
•
Current
• Line SC study
• Reduces with distance from the station
•
Phases
• 3-phase or 1-phase
•
Duration
• Upstream protective devices, breaker, recloser, sectionalizer, fuse
• Consider reclosing
•
Gap
• Across failed insulation
• Between bus
•
Distance
• According to work practice
•
Enclosure
• Yes
•
Tools
• ArcPro, 1584 below 15kV
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Distribution Lines – Padmounted Transformers and
Switchgear Example
• Working on energized
padmount transformers
• 13.8 kV
• 24” distance to the arc
• Gap 12”
• Fault Current – varies
• Clearing time –varies
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Distribution Lines – Padmounted Transformers and
Switchgear Example
Energized Primary Switching –
Enclosed Space
13.8 kV
24” distance to the arc
Fault Current – 10 kA
Clearing time – 6 cycles
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Distribution Station– Bus and Switches
•
Current
• Station SC study
•
Phases
• Phase-phase or phase-ground
•
Duration
• Station protection schemes if any
• High side fuse – consider ability to clear arcing fault
•
Gap
• Across failed insulation
• Arc extension
•
Distance
• According to work practice
•
Enclosure
• No
•
Tools
• ArcPro
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Distribution Station– Bus and Switches
Example
Substation Switching
44 kV
Distance to Arc - 72”
Gap – 24”
Fault Current –varies
Clearing time – varies (high side
fuses)
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Distribution Station – Transformers, Breakers,
Regulators
•
Current
• Station SC study
•
Phases
• 3-phase or 1-phase
•
Duration
• Station protection schemes if any
• High side fuse – consider ability to
clear arcing fault
•
Gap
• Across failed insulation
• Between bus
•
Distance
• According to work practice
•
Enclosure
• Yes
•
Tools
• ArcPro, 1584 below 15kV
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Distribution Station – Transformers, Breakers,
Regulators Example
•
•
•
•
•
•
Racking of 3 phase breakers
13.8 kV
Gap – 12”
36” distance to the arc
Fault Current – varies
Clearing time – varies
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Transmission Station– Bus and Switches
•
Current
• Station SC study
•
Phases
• Phase-phase or phase-ground
•
Duration
• Station protection schemes
• Consider bus protection
•
Gap
• Across failed insulation
• Arc extension
•
Distance
• According to work practice
•
Enclosure
• No
•
Tools
• ArcPro
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Transmission Station – Transformers, Breakers, CTs,
Reactors
•
Current
• Station SC study
•
Phases
• 3-phase or 1-phase
•
Duration
• Station protection schemes
• Consider transformer protection, bus protection, breaker protection
•
Gap
• Across failed insulation
• Between bus
•
Distance
• According to work practice
•
Enclosure
• Yes
•
Tools
• ArcPro with factors
life cycle management solutions
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Transmission Station – Transformers, Breakers,
CTs, Reactors
life cycle management solutions
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Transmission System – OH Lines
•
Current
• Line SC study
•
Phases
• Phase-phase or phase-ground
•
Duration
• Line protection – distance
• Consider work practice (e.g. cannot move from location)
•
Gap
• Across failed insulation
• Arc extension
•
Distance
• According to work practice
•
Enclosure
• No
•
Tools
• ArcPro
life cycle management solutions
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Generation Plant – Switchgear, MCC, UPS,
Panelboards
•
Current
• Line SC study
• Reduces with distance from transformation - dependent on cable
length
•
Phases
• 3-phase or 1-phase
•
Duration
• Upstream protective devices, breaker, relay, fuse
•
Gap
• Across failed insulation
• Between bus
•
Distance
• According to work practice
•
Enclosure
• Yes
•
Tools
• ArcPro, 1584 below 15kV
life cycle management solutions
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Generating Station – Single Line Diagram
life cycle management solutions
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Generation Station – Coordination Study
life cycle management solutions
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