07 - arc flash
TRANSCRIPT
Arch Flash Analysis OTI March 2004 – Slide 2
Electrical Arc Hazards
• Electrical Arcs can occur when a conductiveobject gets too close to a high-amp currentsource (energized conductor).
• Arc Flash Burns– The arc can heat the air to temperatures as
high as 35,000 F, and vaporize metal.
– Arc flash can cause severe skin burns by directheat exposure and by igniting clothing.
Arch Flash Analysis OTI March 2004 – Slide 3
Electrical Arc Hazards
• Arc Blast Impacts– The heating of the air and vaporization of metal
creates a pressure wave that can damagehearing and cause memory loss (fromconcussion) and other injuries. Flying metalparts are also a hazard.
• Falls– Electric shocks and arc blasts can cause falls,
especially from ladders or unguardedscaffolding.
Arch Flash Analysis OTI March 2004 – Slide 4
Definitions• Limited Approach Boundary: A shock protection
boundary not to be crossed by unqualified personsunless escorted by qualified personnel
• Restricted Approach Boundary: A shock protectionboundary to be crossed by only qualified persons.Shock protection is required.
• Prohibited Approach Boundary: A shock protectionboundary to be crossed by only qualified persons. Theuse of techniques that may require direct contact withenergized equipment.
Arch Flash Analysis OTI March 2004 – Slide 5
Definitions• Flash Protection Boundary: Distance at which the
incident energy equals 1.2 Cal/cm^2• Incident Energy: The amount of energy impressed on
a surface, a certain distance from the source,generated during and electrical arc event
• Working Distance: The dimension between thepossible arc point and the head and body of a workerpositioned in place to perform the task.
• Bolted fault current: A short-circuit contact betweentwo conductors at different potentials in which theimpedance between the conductors is zero.
Arch Flash Analysis OTI March 2004 – Slide 6
Definitions• Available fault current: The electrical current that can
be provided by the serving utility and facility-ownedelectrical generating devices and large electricalmotors considering the amount of impedance in thecurrent path
• Arcing fault current: A fault current flowing through anelectrical arc-plasma, also called arc fault current andarc current.
• Voltage (Nominal): A nominal value assigned to acircuit or system for the purpose of designating itsvoltage class (I.e. 120/240 V, 480Y/277 V, 600V, etc).
Arch Flash Analysis OTI March 2004 – Slide 7
Regulating Authorities
• OSHA 29 CFR 1910.132 (d) requiresemployers to access the workplace todetermine if hazards are present, or likely to bepresent and select and have each employeeuse the types of PPE that will protect them.
• OSHA 29 CFR 1910.333 Requires employeeswho are exposed to electrical shock hazard tobe qualified for the specific task that they areperforming and use the appropriate PPE
Arch Flash Analysis OTI March 2004 – Slide 8
Regulating Authorities• OSHA 29 CFR 1910.335 (a)(1)(I): Protective
equipment for specific body parts• OSHA 29 CFR 1910.335 (a)(2)(I): use of Insulated
tools when working around energized equipment.• NEC 110.6: equipment must be marked to warn
qualified persons of potential electrical arc-flashhazards.
• NFPA 70E-2000 Part II Chapter 2, paragraph 2-1.3.3states that arc-flash analysis must be performed inorder to determine the level of hazard and appropriatePPE for given tasks.
Arch Flash Analysis OTI March 2004 – Slide 9
IEEE 1584 2002 “Guide for Performing Arc FlashHazard Calculations”
NFPA 70E 2000 “Standard for Electrical SafetyRequirements for Employee Workplaces”
Protection From Arc Flash Hazards
Arch Flash Analysis OTI March 2004 – Slide 10
18 inches +18 inches +Working Distance
No LimitNo limitArc Duration Range
Open Air, Cubic Box,Cable Bus
Open Air,Cubic Box
Installations
Cal/cm2 or J/cm2Cal/cm2 or J/cm2Unit of Measure
0.7 kA to 106 kA16 kA – 50 kACurrent Range
208 – 15 kV (Empirical)
15 kV+ (Lee Method)208 V – 600 VVoltage Range
IEEE 1584-2002NFPA 70E-2000
Comparison of Arc Flash Standards
Arch Flash Analysis OTI March 2004 – Slide 11
Incident energy exposure at a working distance of 18”
for a 19.5 kA Arc @ 600 Volts (open air equipment)
600 Volt Arc in Open Air Incident energy Exposure @ 18 in.
0
5
10
15
20
0 10 20Fault clearing time (Cycles)
Cal
orie
/cm
^2
NFPA 70E-2000IEEE 1584-2002
Incident Energy Comparison
Arch Flash Analysis OTI March 2004 – Slide 12
600 Volt Arc in Closed Box Incident energy Exposure @ 18 in.
0
5
10
15
20
0 10 20Fault clearing time (Cycles)
Cal
orie
/cm
^2
NFPA 70E-2000IEEE 1584-2002
Incident energy exposure at a working distance of 18”
for a 19.5 kA Arc @ 600 Volts (enclosed equipment)
Arch Flash Analysis OTI March 2004 – Slide 13
NFPA Hazard Risk DeterminationQuick Table (Table 3-3.9.1 of 2000 Ed)
• Can you use them exclusively and still be incompliance for Arc-Flash safety?
• Developed based on outdated standard that onlycovers 600 V systems
• May result in unnecessary overprotection / underprotection
• Best when used only in emergency situation for quickevaluation of hazard level
• Standard mandates a detail arc-flash analysis beperformed when the task is not specifically covered bythis table
Arch Flash Analysis OTI March 2004 – Slide 14
General Steps for PerformingArc Flash Analysis
• Collect system information required for the ArcFlash Calculation
• Determine the system operating configuration
• Calculate 3-Phase bolted fault currents
• Calculate arcing fault current (IEEE only)
• Determine arc clearing time (arc duration) -TCC
Arch Flash Analysis OTI March 2004 – Slide 15
• Calculate Incident Energy
• Determine Flash Protection Boundary
• Determine Hazard/Risk Category based onNFPA 70E requirements
• Select appropriate protective equipment(PPE Matrix)
General Steps for PerformingArc Flash Analysis
Arch Flash Analysis OTI March 2004 – Slide 16
XXWorking Distance
XXOpen/Enclosed Equipment
XSystem Grounding(Grounded/Ungrounded)
XXCoordination Information (TCC)
XXSystem Nominal Voltage
XGap Between Conductors
XDistance X Factor
IEEE1584
NFPA70ERequired Parameter
Data Collection for Arc Flash
Arch Flash Analysis OTI March 2004 – Slide 18
Additional Considerations
• Up to date one-line-diagrams
• Data similar to information required for Short-circuit studies like MVAsc values of Utilitiyincluding X/R, subtransient and transientreactance, cable impedance, etc.
• Include low voltage equipment which is oftennot included in large systems
Arch Flash Analysis OTI March 2004 – Slide 19
3-Phase Bolted Fault Current• Perform ANSI/IEC short circuit study that considers
the following:– 3-phase bolted fault– ½ cycle or 1½-4 cycle fault current depending on the
type of device or system voltage– Include all cables & Overload heaters– Prefault voltage (nominal circuit voltage)– Short-circuit Calculation should be more accurate rather
than too conservative (faults may persist longer at lowercurrent levels which may translate into higher energy)
Arch Flash Analysis OTI March 2004 – Slide 20
System Modes of Operation• Open or looped• One or more utility feeders in service• Utility interface substation secondary bus tie breaker
open or closed• Unit substation with one or two primary feeders• Unit Substation with two transformers with secondary
tie opened or closed• MCC with one or two feeders, one or both energized.• Generators running in parallel with the utility supply or
in standby mode
Arch Flash Analysis OTI March 2004 – Slide 21
Why use 3-Phase Faults
• Line to Line faults quickly escalate into three- phasefaults
• LV L-G faults in solidly grounded systems quicklyescalate into three phase faults
• LV L-G faults in Ungrounded / High resistancegrounded systems do not release enough energy.
• MV faults in low resistance or reactance groundedsystems should be cleared quickly, but worst casescenario 3-phase fault should be considered
Arch Flash Analysis OTI March 2004 – Slide 22
Standards for Short-Circuit
• IEEE Std 141-1993 (IEEE Red Book)
• IEEE Std 242-2001 (IEEE Buff Book)
• ANSI (different standards like C37, etc)
• IEC (60909, 60363, etc)
• See ETAP help file for more standards
Arch Flash Analysis OTI March 2004 – Slide 23
))(lg(**00304.0))(lg(**5588.0
*000526.0*0966.0)lg(*662.0)lg(
bfbf
bf
IGIVGVIKIa
−+
+++=
For buses with nominal kV in the range of 0.208 to 1.0 kV:
In general, arcing current in systems below 15.0 kV will be lessthan the 3-phase fault current because of arc impedance.
Arcing Current
Arch Flash Analysis OTI March 2004 – Slide 24
For buses with nominal kV rating greater than 15 kV, thearcing current can be considered to be the same as thebolted fault current:
For buses with nominal kV rating in the range of 1 to 15.0 kV:
)Ilg(*983.000402.0)Ialg( bf+=
bfIIa =
Arcing Current
Arch Flash Analysis OTI March 2004 – Slide 28
Incident Energy
Empirical method (1.0 to 15.0 kV)
= x
x
nf DtECE 610*2.0
**184.4
Lee method (higher than 15.0 kV)
= 2
6 **10*142.2DtIVE bf
Arch Flash Analysis OTI March 2004 – Slide 29
Flash Protection Boundary
Empirical method (1.0 to 15.0 kV)
= x
x
nf DtEC 610*2.0
**184.42.1
Lee method (higher than 15.0 kV)
= 2
6 **10*142.22.1DtIV bf
Arch Flash Analysis OTI March 2004 – Slide 30
9 – 1228 > cal/cm2 ≥ 5
4.5 – 815 > cal/cm2 ≥ 1.24.5 – 701.2 > cal/cm2 ≥ 0
24-304cal/cm2 ≥ 2516-20325> cal/cm2 ≥ 8
Total WeightOz/yd2
Hazard RiskCategory
Incident EnergyExposure cal/cm2
Hazard / Risk CategoriesNFPA 70E 2000
Arch Flash Analysis OTI March 2004 – Slide 31
Categories 0 and 1 Personal Clothing/Equipment Requirementsper Table 3-3.9.2 of NFPA 70E 2000
Personal Protective EquipmentPPE Matrix
Arch Flash Analysis OTI March 2004 – Slide 32
Category 0 (up to 1.2 Cal/cm2)
• Shirt (Long-Sleeve)
• Pants (Long)
• Safety Glasses
• V-Rated Gloves
• Insulated Tools
Arch Flash Analysis OTI March 2004 – Slide 33
Category 1 (1.2 up to 5.0 Cal/cm2)
• Shirt (Long-Sleeve) FR
• Pants (Long) FR
• Safety Glasses FR
• V-Rated Gloves
• Insulated Tools
• Hard Hat FR
Arch Flash Analysis OTI March 2004 – Slide 34
Category 2 (5.0 up to 8.0 Cal/cm2)
• Category 1 Requirements
plus
• Extra Layer of UntreatedNatural fiber (Shirt &Pants)
• Leather Work Shoes
FR FR
Arch Flash Analysis OTI March 2004 – Slide 35
Category 3 (8 up to 25 Cal/cm2)
• Category 2 Requirements
plus
• Coveralls up to 2 Sets
• Double Layer SwitchingHood
• Hearing Protection
Arch Flash Analysis OTI March 2004 – Slide 36
Category 4 (higher than 25 Cal/cm2)
• Category 3 Requirements
plus
• Flash Suit
Arch Flash Analysis OTI March 2004 – Slide 37
PPE Incident Energy Rating
• ATPV: is the defined as the incident energy on a fabric ormaterial that results in sufficient heat transfer through thefabric or material to cause the onset of a second degreeburn.
• EBT: is defined as the average of the five highest incidentenergy exposures values below the Stoll curve where thespecimens do not exhibit breakopen. EBT is reported whenthe ATPV cannot be determined due to FR fabricbreakopen.
• HAF%: is the heat transfer capability of the fabric ormaterial
Arch Flash Analysis OTI March 2004 – Slide 40
Example of Layered System
100%)100(*' )/( 2
HAFEE cmcalcalculated
−=
• Proposed PPE for Arc Fault with E = 22 Cal/cm^2
ProposedEquipment
ATPV Rating(cal/cm^2)
EBT(cal/cm^2)
HAF %
FR Shirt (longSleeve)
5 9 85
FR Raincoat 10 18 70
Arch Flash Analysis OTI March 2004 – Slide 41
Example of Layered System
• Energy that passes to second layer is higher than ATPV
• EBT is too low for outer layer (possible breakopen)
ModifiedEquipment
ATPV Rating(cal/cm^2)
EBT(cal/cm^2)
HAF %
FR Shirt (longSleeve)
9 9 85
FR Raincoat 15 22 70
2/6.6100
)70100(*22' cmcalE =−=
Arch Flash Analysis OTI March 2004 – Slide 42
Considerations for layering
• ATPV rating of the equipment must be abovethe calculated incident energy of the Arc forsingle layer FR system
• In multiple layer FR system there must be nobreakopen that reaches the innermost layer toprevent possible ignition of such
• NFPA example recommends
Arch Flash Analysis OTI March 2004 – Slide 43
Arc Fault atLocation B
Arc Fault atLocation A
Example1
Arch Flash Analysis OTI March 2004 – Slide 45
Example1
• Fault at location B
Calculated incident energy = 0.784 Cal/cm2
(Relay B operates at 1.206 cycles + 5 cycles HVCB)
• For a fault at location A
Calculated incident energy = 0.945 Cal/cm2
(Relay A operates at 2.406 cycles + 5 cycles HVCB)
• Hence the Incident Energy to be considered for this systemshould be 0.945 Cal/cm2 (the most conservative value).
Arch Flash Analysis OTI March 2004 – Slide 46
Arc Fault atLocation C
Example 2
Arc Fault atLocation D
Arch Flash Analysis OTI March 2004 – Slide 48
Example 2
• Fault at location C:
Calculated incident energy = 7.604 Cal/cm2
(LVCB 15 operates in 0.150 sec.)
• For a fault at location D:
Calculated incident energy = 5.576 Cal/cm2
(LVCB 16, 17 & 18 operate in 0.115 sec.)
• Hence the Incident Energy to be considered for this systemshould be 7.604 Cal/cm2 (the most conservative value).
Arch Flash Analysis OTI March 2004 – Slide 49
Arc Flash Hazard Labels
• Place labels at each location (cubicle)
• Contain information that is clear andcommunicates the danger level
• Meet current format per ANSI Z535 2002(safety symbols)
Arch Flash Analysis OTI March 2004 – Slide 53
26.5 kV < Bus kV ≤ 36.0 kV436000
17.0 kV < Bus kV ≤ 26.5 kV326500
7.5 kV < Bus kV ≤ 17.0 kV217000
1.0 kV < Bus kV ≤ 7.5 kV17500
High Voltage Gloves
0.500 kV < Bus kV ≤ 1.0 kV01000
kV ≤ 0.500 Bus kV ≤≥00500
Low Voltage Gloves
Bus nominal kV rangeClassMax. use voltage AC
(L-L) (V-Ratingfield)
Types of Insulating Glove
ASTM Insulating Glove Voltage Classes