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Arc Flash with EasyPower
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Topics:
EasyPower, LLC
Molded-case Circuit Breaker vsLow Voltage Power Circuit
Breakers Coordination of MCCBs
Transformer Damage Curves
Transformer Protection TCC Clipping & Tick Marks
I2T Settings What and Why?
Questions2
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Molded Case Circuit Breakers vs LowVoltage Power Circuit Breakers
Designed, Built and Tested to MeetDifferent Standards:
MCCBs & ICCBs UL489
LVPCBs UL1066 (ANSI C37)
In general:
MCCBs Switchboards
LVPCBs Switchgear (Drawout)
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Comparison
Characteristic MCCB LVPCB
Maintenance No user-serviceableparts
Can be inspected andmaintained, including
contacts, arc-chutes,etc.
Short-Time Duty Cycle Not Required (But canbe tested)
Carry Fault Current fortwo 30-cycle periods
Short Circuit Testpower factor
15 to 20% (> 20kA) 15% max
Instantaneous Trip Required Not Required
Normal Use Switchboards (UL891) Switchgear (ANSIC37.20.1)
Cont Current Rating 80% (unless 100%
rated)
Always 100%
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Look at Short Circuit Example
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3 PHASE Fault Total Fault Currents Equipment DutiesBus Name Bus kV Sym Amps X/R Ratio Mult Factor Asym Amps Equip Type Duty Amps
SUB 8A 0.480 39609.5 6.89 1.35 53629.1 LVPCBMCCB 10-20 kAMCCB > 20 kA
39890.246332.142129.4
Calculated SC
Current= pf of 14.4%
Adjusted Duty de-rated
So, 42 kA LVPCB OK, 42 kA MCCB NOT OK
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Impact on Coordination: Short-time withstand rating allows
LVPCB to stay closed, allowing
downstream equipment to clear fault.
Lack of short-time rating means thatMCCB must have instantaneous trip to
trip breaker at the short circuitwithstand rating
Newer MCCBs may be tested for short-
time withstand. EasyPower, LLC 6
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Coordinating MCCBs and LVPCBs
In general, MCCBs in series will often not coordinatewith each other due to instantaneous trip elements.
Coordination is much more reliable with LVPCBs IFthe proper trip units and settings are used.
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Short-time Withstand
Improves Coordination Specify LVPCBs with no instantaneous or switchable
Will coordinate with downstream MCCBs
Use Short-time delay to coordinate between mainand feeder breakers
Can also use Zone Selective Interlocking withLVPCBs.
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MCCBs in Series General case do not coordinate
May coordinate below certain levels offault current.
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Typical MCCBCoordination
Overlapindicatescoordinationproblem
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TypicalLVPCB
Coordination
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Selective Coordination of
MCCBs In 2005 NEC required selective
coordination of emergency power
systems This spurred MCCB manufacturers to
improve testing and documentation ofMCCB coordination
Fuses, if applied per manufacturersinstructions (2:1) have been shown tocoordinate by test
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Two techniques to coordinate
MCCBs Use upstream breaker with high short-
circuit withstand rating and set
instantaneous above actual maximumfault current available
Use MCCBs that are tested combination
and shown to coordinate up to certainfault current
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Series Coordination of MCCBsValid only by test
Test data shows MCCBs will coordinateeven though curves overlap on TCC
Limited to same manufacturer
Tracked in EasyPower Library Data
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Set Option in PDC Options
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SeriesCoordinationTest Data in
EasyPower
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Coordination Time Intervals
Low Voltage Breakers and Trip Units TCCs include all breaker operating time
and tolerances as long as they do nottouch, they should coordinate
Relay curves show only nominal relay
operate time need to allow additionaltime separation
Traditionally we have used 0.3 to 0.4
seconds EasyPower, LLC 18
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CTI Limits Number of Relays
in Series Since we must allow this CTI between
each pair of relays, clearing times get
very slow when we have more than twoor three relays in series.
Especially true in industrial applications
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Coordination Time Interval
(CTI) Accounts For: Breaker Operating Time (2 to 8 cycles)
Relay Overtravel (Electro-mechanical)
Relay Error
CT Error
Lockout Relay Operate Time
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What has changed? Newer Digital Relays have no overtravel
and are extremely accurate can we
reduce the CTI?Yes we can, but keep in mind:
Still have breaker operate time and CT
error CT Saturation must still be considered as
well
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UPSTREAM DEVICE DOWNSTREAM DEVICE(TIMEINSECONDS)
Electro-Mech. Relay Digital Relay Direct-Acting Trip Fuse
Electro-Mech.Relay 0.30 0.25 0.20 0.20
Digital Relay 0.25 0.20 0.15 0.20Direct-Acting Trip 0.20 .20 Separation Separation
Fuse 0.30 .25 0.10 0.10
TYPICAL COORDINATION TIMEINTERVALS
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Protecting Transformers Overloads
Internal Faults
Through Faults
Transformer Damage Curves only dealwith through faults not internal faults,not overloads
If you have an internal fault thetransformer is already damaged
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f
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Transformer Protection ANSIDamage Curve Damage Curve for
Through-Faults
ANSI C57.109
Oil Frequent vs
Infrequent faults
Mechanical
Damage =Frequent Fault
Mechanical
Thermal
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ANSI C57.109Category Single-Phase
(kVA)Three-Phase
(kVA)Frequent
Fault
I 5 to 500 15 to 500 n/a
II 501 to 1667 501 to 5000 10
III 1668 to 10,000 5001 to 30,000 5
IV Above 10,000 Above 30,000 5
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Damage Curve Shape Varies Based onCategory, Transformer Impedance
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Protecting Delta-Wye
Transformers Per-Unit Current Seen by Primary
Protection Not the Same as Seen by
Secondary Winding. Winding Damage on Through-Faults a
function of current through any one
winding. We must adjust damage curve or
primary protection curve to compensate
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Protecting Delta-Wye Xfmr
1 pu
1 pu
1 pu
1 pu
.58 pu
1 pu
1 pu
Currents at Full Load -Normal
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Protecting Delta-Wye Xfmr Ignoring Load
Current, PrimaryRelay or Fuse willsee only 58% of
the per unit currentin the secondarywinding for groundfault on secondaryside.
1 pu
.58 pu
.58 pu
Currents For SLG Fault on Wye Side
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We Must Adjust Damage Curve
Adjusted ForGround FaultSeen FromPrimary
Applies Only for GroundFaults on Secondary onDelta-Wye Transformers
Applies Only forPrimary Protection
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NEC Requirements for
Transformer ProtectionNEC Table 450-3(A)Primary > 600V
Primary Secondary
Over 600V Over 600V 6% 400% 300% 250% 225% 125%
Supervised Any 300% 250% N.R. N.R. N.R.
6% 600% 300% 300% 250% 250%
>6% 400% 300% 250% 225% 250%
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Transformer Inrush
Transformer PrimaryProtection Must Ride
through Inrush Inrush hard to
calculate, high inharmonics
Assume 8 to 10 x FLCfor 0.1 sec
Based on Self-CooledRating
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TCC Clipping and Tick Marks Intended to aid in understanding of
impact of short circuit current on
coordination Well look at why we want to use these
features and how they behave in
EasyPower
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TCC Clipping and Tick Marks Consider TCC using manufacturers TCC
and no tick marks
Curve may extend to maximum faultcurrent grid line on TCC
If we do not know what the maximum
fault current through each device is, itis difficult to determine selectivity
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TCC Tick Marks Tick Marks Can be Added to Indicate
Maximum Fault Current Possible For
Each Device Selected under Short Circuit tab for
each protective device
Can select cycle, 5 cycle, or 30 cycle
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TCC Curve Clipping In addition to Tick Marks, we can stop
(clip) TCC at the maximum fault
current it can see.Also selected under Short Circuit tab
for each protective device
Can select cycle, 5 cycle, or 30 cycle
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TCC Defaults EasyPower Defaults as delivered:
TCC curves clipped cycle
Tick Marks for each device cycle
When TCC is opened or created, EasyPower faults every busand uses maximum current through each device to determineclipping and tick marks.
But if a single bus is faulted tick marks and clipping canchange.
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Last and Least I2
t Function 480 V Trip Units Generally Have I2t
function on Short-time and (sometimes)
Ground FunctionCuts off the corner of Short-Time
curve
In = ON, Out = OFF Intent is to improve coordination with
downstream fuses
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Last and Least I2
t Function In many situations, use of I2t function
will increase arc-flash incident energy.
Generally not necessary for coordinationwith other trip units or MCCBs.
Not widely used.
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