Date post: | 20-Aug-2015 |
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Breakers and Fuses
Overall, Circuit BreakersProvide Better Protection
than Fusesfor Low Voltage
Circuit Protection
…and here’s why.
Breakers and Fuses
… Technical
The Issues
Equipment protection. Coordination. Downtime. Current limiting
fuse characteristics.- Fuse let-through chart.- Up-Over-Down Method.
Series ratings.
Short circuit capacity. Motor circuit
protectors. Single-phase
protection. High/low level
fault protection.
Breakers and Fuses
Short Circuit Calculations
Unlimited Fault Current
2,500 kVA, Z=5.75%
Calculated Fault 52,296A
12,470V
480Y/277V
Isc = = = 52,296AkVA 2,500 x 1,000 x 100
VxZ 480 x 3 x 5.75
Breakers and Fuses
Secondary Short Circuit Capacity ofTypical Power Transformers
3005%
5005%
7505.75%
10005.75%
15005.75%
20005.75%
25005.75%
208 Volts, 3-Phase 240 Volts, 3-Phase 480 Volts, 3-PhaseTransformer Maximum Rating 3-Phase Short CircuitkVA and kVA Available
Impedance from PrimaryPercent System
Short Circuit Current rmsSymmetrical Amps
Combined Transformer and Motor
Unlimited 18400 17300 8600
Unlimited 30600 28900 14400
Unlimited 40400 38600 19300
Unlimited 53900 51400 25700
Unlimited 80800 77200 38600
Unlimited 51400
Unlimited 64300
Breakers and Fuses
1500 kVA Transformer/5.75% Impedance/480 Volts
Distance in Feet from Transformer to Breaker Location
5000200010005002001005020105200
60
50
40
30
20
10
UTILITY KVA500,000
4 - 750 MCM2 - 500 MCM250 MCM#1/0 AWG#4 AWG
Fau
lt C
urre
nt in
Tho
usan
ds o
f A
mpe
res
(Sym
.)Effects of Impedance
Breakers and Fuses
Short Circuit CalculationsUnlimited Fault Current
2,500 kVA, Z = 5.75%
Calculated Fault
12 Feet, 3,200A Copper Feeder Busway
3,200A
3,200A Bus
12,470V
480Y/277V
60 Feet(3) 1-Conductor#4/0 CopperTHW InsulationSteel Conduit
200A 200A 150A 100 Feet(3) 1-Conductor#2 CopperTHW InsulationSteel Conduit
Bus Plug
Note:Obtain specific impedancevalues for each system.Do not assume the valuesshown here will be typical.
MainControlPanelCalculated at 100% Motor Contributions
10,968
29,560
64,328
52,296
Breakers and Fuses
Bolted Fault Arcing Fault
Systems must be designed However, the majority of faultsfor worst case conditions. will be arcing type.
Line-to-Line-to-Line Fault
Breakers and Fuses
Frequency of Faults
Least Likelihoodof Fault
Greatest Likelihoodof Fault
UTILITY
M M
Breakers and Fuses
Frequency of Faults
TYPE OF FAULTS INCIDENCE % FAULT MAGNITUDE
Three-phase bolted ? Approaches fault available
Single-phase bolted 5% 30-60% of fault available
Line-to-line arcing 15% Low to medium (less than 30%)
Line-to-ground arcing 80% Very low to low (less than 10%)
Breakers and Fuses
Time
Current
Starting Curve
Typical MotorCapability Curve
LockedRotor Time
Motor Circuit Protectors (HMCP)
Breakers and Fuses
Single-Phase Protection
Single-phasing on three-phase loads cannot occur with circuit breakers as all three poles open on a single-phase fault.
Single-phasing on motor loads when a single fuse blows can cause heating and eventual damage to motor windings.
To eliminate single-phasing when usingfuses is both costly and inefficient.
Low level arcing faults can continue to be fedthrough the load.
Breakers and Fuses
Single-Phasing Condition
A
BC
Protective Device Starter
10A
10A
10A
Motor
“Normal Condition”
Dual Element Fuse
A
BC Fault
18A
0
18A
10A x 180%
10A x 180%
Motor
A
B
C
Pha
se
Clears after multiple cycles.
CLEARING TIMEFOR A LOW LEVEL FAULT
ABC
Motor Circuit Protector (MCP)
Fault
0
0
0
Motor
A
B
CClears in less than one cycle.
Pha
se
Breakers and Fuses
High/Low Level Fault Protection
Circuit breakers will clear high level(short circuit) faults safely and effectively.
Circuit breakers in general will clear low level (overload) faults more effectively than fuses (e.g., a motor circuit protector will clear a low current motor fault (X6) in a shorter time than a fuse). Plus prevent single-phasing.
Circuit breakers can include ground fault protection. They quickly interrupt dangerous ground faults before they escalate.
Breakers and Fuses
IEC 947-4-1 defines two levels of coordination for the motorstarter under short circuit conditions.
TYPE 1 COORDINATION
Under short circuit conditions, the contactor or starter shall cause no danger to persons or installation and may not be suitablefor further service without repair and replacement of parts.
TYPE 2 COORDINATION
Under short circuit conditions, the contactor or starter shallcause no danger to persons or installation and shall be suitablefor further use. The risk of contact welding is recognized, inwhich case the manufacturer shall indicate the measures tobe taken in regards to equipment maintenance.
Equipment Protection
Breakers and Fuses
FACTS
Type 1 coordinated motor branch circuits are capableof clearing low level faults without damage.
Type 1 may not prevent damage to the motor starter components in high level faults.
Type 2 does not permit damage to the starter (as noted).
Type 1 or Type 2 does not cover protection of the motor.
Breakers or fuses may be utilized in Type 1 or Type 2.
Type 2 prohibits replacement of parts (except fuses).
Most faults in electrical systems are low level.
Fuses can nuisance trip on startup under Type 2.
Equipment Protection
Breakers and Fuses
CHOICES
Type 1 Must replace heaters.May need to replace
starter.
Type 2 with fuses Must replace fuses.
Type 2 with breaker No replacement required.
Reduction in downtime is critical to manufacturing facilities.
Type 2 intended to keep production running.Repair or replacement is recommended.
Equipment Protection
Breakers and Fuses
Why do we have molded case, insulatedcase and metal enclosed breakers? Molded
CaseCircuitBreakers
InsulatedCaseCircuitBreakers
Metal EnclosedAC PowerCircuitBreakers
Westinghouse Series CMolded Case Circuit Breakers70A - 2,500A
Westinghouse SPB SystemsPow-R Circuit Breakers200A - 5,000A
Westinghouse Types DSII/DSLIILow Voltage AC PowerCircuit Breakers100A - 5,000A
Breakers and Fuses
Time-Current Curve Coordination StudyA B C D EMOTOR
Ansi 3-PhaseThrough FaultProtectionCurve(MoreThan 10In Lifetime)
TransformerInrush
A B
GroundProtection
PhaseProtection
.5 1 10 100 1,000 10,000.01
.1
1
10
100
1,000
Scale X 100 = Current in Amperes at 480 Volts
Tim
e in
Sec
onds
M
A
B
C
D
E
4.16 kV 250 MVA
250A
19,600A
1,600A
1,000A
20,000A
175A
100 HP-124A FLC
= Available fault current including motor contribution.
24,400A
1,000kVA5.75%
4,160 V 480Y/277 V
Breakers and Fuses
Electronic Trip Unit
Zone SelectiveInterlocking- Short Time Delay
- Ground FaultTime Delay
- InstantaneousTrip Regardlessof Short TimeDelay
- MinimizeDamage
Zone 1
Zone 2
Zone 3
Ground Fault Setting:1,200A Pickup0.5 Sec. Time Delay
Ground Fault Setting:600A Pickup0.3 Sec. Time Delay
Ground Fault Setting:300A PickupNo Time Delay
Zone Interlock Wiring
LOAD
Fault 2
Fault 1
Breaker 1
Breaker 2
Breakers and Fuses
Coordination Fuses coordinate well between each other as
the 2:1 ratio of the current rating is maintained.
Circuit breakers with thermal magnetic trip units will also coordinate well under the same conditions.
Circuit breakers with adjustable electronic tripunits will coordinate below the 2:1 ratio. Trip unit settings need to be determined by carrying outa coordination study.
Fuses cannot do zone selective interlocking.
Breakers and Fuses
Coordination
Circuit breaker flexibility.
- Adjustable pickup settings give improvedcurrent coordination.
- Adjustable time delay setting gives improvedtime coordination.
- Zone selective interlocking.• Improving time coordination.• Reducing damage to equipment.• Reducing stress on upstream devices.
Breakers and Fuses
The Issues
…Technical …The Advantages of Using Breakers
Resettable and reusable devices.
Better coordination.
Closer equipment protection.
Early warning (alarms).
No time lost in searching for replacement fuses.
Plus ground fault option.
Downtime
Breakers and Fuses
Current Limiting
I2t = (IRMS)2t
PeakLet-ThroughCurrent (Ip)
IRMS
rmsLet-ThroughCurrent(Calculated)
TotalClearing Time
t
tmelt tarc
Ip
AvailableShort CircuitCurrent
Breakers and FusesT
hous
ands
Ten
Tho
usan
ds
Hun
dred
Tho
usan
ds
Fuse Let-Through Chart
AvailableFaultCurrent
Let-ThroughCurrent
45 kA
40 kA
32 kA
30 kA
600A600A 600A 600A
100 kA 65 kA 35 kA 30 kA
X/R Ratio = 6.6
Fuses Tested at 15% Power Factor
Available Current in rms Symmetrical Amperes
Max
imum
Ins
tant
aneo
us P
eak
Let-
Thr
ough
Am
pere
s
Thousands Ten Thousands Hundred Thousands
CurrentLimitingThreshold 600A Fuse
BIp = 2.3 x IRMS SYM
1 5 10 5 10 5 101
5
10
5
10
5
10
Breakers and Fuses
Up-Over-Down Method100 kAAvailableFaultCurrent
200AFuse
Let-ThroughFaultCurrent
Available Current in rms Symmetrical Amperes17 kA 100 kA
Fuses Tested at15% Power Factor
X/R Ratio = 6.6
This methodassumes thatthere is nodownstreamfast actinginterruptingdevice.
CurrentLimitingThreshold
200A Fuse
Ma
xim
um
In
sta
nta
ne
ou
s P
ea
k L
et-
Th
rou
gh
Am
pe
res
Th
ousa
nds
Te
n T
hou
sand
s
H
und
red
Th
ou
san
ds
Thousands Ten Thousands Hundred Thousands
BIp = 2.3 x IRMS SYM
1 5 10
5 10 5 101
5
10
5
10
5
10
Breakers and Fuses
Current Limiting Fuse Characteristics
Stand alone current limiting fuse. Fuse is current limiting and clears
the fault in the first 1/4 cycle.
Current limiting fuse with modernfast acting circuit breakers. Breaker begins to open.
Breaker tries to clear the fault.
Dynamic impedance is introduced.
The current limiting fuse thenbecomes a slow acting device.
FuseAction
FAST
SLOW
FuseAction
FAST
SLOW
Breakers and Fuses
The Circuit Breaker Sees the FaultBefore the Fuse
…Therefore, the Up-Over-Down Method DOES NOT WORK because it’s only a theoretical calculation.
NEC requires that fuse/breaker series combinations must betested per UL test procedures.
One test is better thana million calculations.
FuseAction
FAST
SLOW
Breakers and Fuses
Fuse manufacturers have acknowledged that theUp-Over-Down MethodDOES NOT WORK withtoday’s modern highinterrupting circuit breakers.
Breakers and Fuses
Three Types of Systems
1. Selectively Coordinated System
2. Fully Rated System
3. Series Rated System
Breakers and Fuses
This system allows or selects the breaker closest to the overcurrent source to open, thus most closely isolating the problem.
CONTINUITY OF SERVICE
PROTECTION
COST
1. Selectively Coordinated System
High continuity.
All breakers fully rated.
Most costly of all three systems.
Breakers and Fuses
2. Fully Rated System
In this system, all of the breakers must be fully rated for thesystem’s available fault current. This allows for quick selectionof equipment, but allows for less continuity of service in general.
CONTINUITY OF SERVICE
PROTECTION
COST
Lower than selectively coordinated system. Usually higher than series connected system.
All breakers fully rated.
Lower than selectively coordinated system. Usually higher than series rated system.
Breakers and Fuses
3. Series Rated SystemThis is a system of series connected breakers which havebeen tested in combination and shown to effectively protect the system. Downstream breakers are not fully rated for the system’s available fault current but the upstream breaker, which is testedin combination, protects the downstream breaker by operatingbefore damage occurs.
CONTINUITY OF SERVICE
PROTECTION
COST
Continuity of service may suffer.
Downstream breakers not fully rated.
Usually the least costly system.
Breakers and Fuses
Fuses Circuit Breakers
FULLY RATED SERIES RATING
Series ratings can Circuit breakers are not be done with fuses- tested as a component and
there’s no advantage. tested in an assembly.
Series Ratings
100 kA
100 kA
100 kA
100 kA
100 kA
100 kA
100 kA
100 kA
100 kA
14 kA
14 kA
14 kA
14 kA
14 kA
14 kA
14 kA
14 kA
65 kA
Breakers and Fuses
… Practical
The Issues
Safety. Monitoring and
communications. Testability. Accessorization.
Size. Economics. Cost.
Breakers and Fuses
The Issues
Safety
…Practical …The Advantages of Using Breakers
Breakers are dead front devices.
Fused switches have exposed live parts.
Terminal shields and end covers available.
Fuses can be easily replaced withdevices that are improper and havedifferent characteristics.
Handle mechanism allows resettabilityof breaker without needing access.
Breakers and Fuses
…Practical …The Advantages of Using Breakers
The Issues
Monitoring andCommunications
Many functions can be integralto electronic trip units.
- Earth leakage/ground fault.
- Monitoring functions.
- Metering.
- Energy.
- Power factor.
- Communications.
Breakers and Fuses
The Issues
Testability
…Practical …The Advantages of Using Breakers
Electronic trip units are field testable.
Fuses are not field testable.
Push to test in the field.
Verify each unit off the assembly line.
Breakers and Fuses
…Practical …The Advantages of Using Breakers
The Issues
Accessorization Accessories are difficult to apply to fuseswitches and are very costly.
Both internal and external circuit breaker accessories are easy to install and arecost effective.
Internal.- Shunt trip. - Auxiliary
contacts.- Undervoltage release. - Bell alarms.
External.- Motor operators. - Interlocks.- Handle mechanisms. - Cylinder locks.
Breakers and Fuses
…Practical …The Advantages of Using Breakers
The Issues
Size Smaller devices allow for more room in an assembly.
Assemblies with fusible devices are larger.
Space saving in control panel.
Breakers and Fuses
Practical Issues Space and Cost Comparisons
SAVINGS… 59% Wall Space
Equipment CostSavings
Installation Time
Cost of Fuses andLabor for Replacement
Downtime
69% Floor Space
WallSpace
FloorSpace
400A KD400A KD
400A KD400A KD
1200A ND
PRL 4BBreaker
PRL 4F Fusible
$1,410 Additional for FusesPLUS Spares
9 Sq. Ft.2.83 Sq. Ft.
1200AFDP
VERT.
400AFDPW
400AFDPW
Thru-FeedLugs
400AFDPW
400AFDPW
11.3” 18”
90”73.5”
36” 72”
Breakers and Fuses
…Practical …The Advantages of Using Breakers
The Issues
Economics
Cost
No spares required. Fuses need to be replaced. Breakers are more electrically efficient.
(Fuses have a higher wattage dissipation.)
Less downtime. Less contractor labor to handle and install. $ $ saved in floor and wall space. Saves panel space.
Breakers and Fuses
Why Use a Breaker in a Distribution System:Prevents single-phasing.Motor protection.Coordination.Zone interlocking.Resettable.Dead front, no exposed parts.Space savings.Prevents downtime.Accessorization.Testable.