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Fuseology
Circuit ProtectionElectrical distribution systems are often
quite complicated. Theycannot be absolutely fail-safe. Circuits are
subject to destructiveovercurrents. Harsh environments, general
deterioration, acciden-tal damage or damage from natural causes,
excessive expansionor overloading of the electrical distribution
system are factorswhich contribute to the occurrence of such
overcurrents. Reliableprotective devices prevent or minimize costly
damage to trans-formers, conductors, motors, and the other many
components andloads that make up the complete distribution system.
Reliable cir-cuit protection is essential to avoid the severe
monetary losseswhich can result from power blackouts and prolonged
downtime offacilities. It is the need for reliable protection,
safety, and freedomfrom fire hazards that has made the fuse a
widely used protectivedevice.
Fuses are constructed in an almost endless variety of
configurations.These photos depict the internal construction of
Bussmann Dual-Element, SEMI-TRON and LOW-PEAK Class L fuses.
OvercurrentsAn overcurrent is either an overload current or a
short-circuit cur-rent. The overload current is an excessive
current relative to nor-mal operating current, but one which is
confined to the normalconductive paths provided by the conductors
and other compo-nents and loads of the distribution system. As the
name implies, ashort-circuit current is one which flows outside the
normal conduct-ing paths.
OverloadsOverloads are most often between one and six times the
normalcurrent level. Usually, they are caused by harmless
temporarysurge currents that occur when motors start up or
transformers areenergized. Such overload currents, or transients,
are normaloccurrences. Since they are of brief duration, any
temperature riseis trivial and has no harmful effect on the circuit
components. (It isimportant that protective devices do not react to
them.)
Continuous overloads can result from defective motors (suchas
worn motor bearings), overloaded equipment, or too manyloads on one
circuit. Such sustained overloads are destructive andmust be cut
off by protective devices before they damage the dis-tribution
system or system loads. However, since they are of rela-tively low
magnitude compared to short-circuit currents, removal ofthe
overload current within a few seconds to many minutes willgenerally
prevent equipment damage. A sustained overload cur-rent results in
overheating of conductors and other componentsand will cause
deterioration of insulation, which may eventuallyresult in severe
damage and short-circuits if not interrupted.
Short-CircuitsWhereas overload currents occur at rather modest
levels, theshort-circuit or fault current can be many hundred times
largerthan the normal operating current. A high level fault may be
50,000amperes (or larger). If not cut off within a matter of a few
thou-sandths of a second, damage and destruction can become
ram-pantthere can be severe insulation damage, melting of
conduc-tors, vaporization of metal, ionization of gases, arcing,
and fires.
Simultaneously, high level short-circuit currents can develop
hugemagnetic-field stresses. The magnetic forces between bus
barsand other conductors can be many hundreds of pounds per
linearfoot; even heavy bracing may not be adequate to keep them
frombeing warped or distorted beyond repair.
FusesThe fuse is a reliable overcurrent protective device. A
fusible linkor links encapsulated in a tube and connected to
contact terminalscomprise the fundamental elements of the basic
fuse. Electricalresistance of the link is so low that it simply
acts as a conductor.However, when destructive currents occur, the
link very quicklymelts and opens the circuit to protect conductors
and other circuitcomponents and loads. Modern fuses have stable
characteristics.Fuses do not require periodic maintenance or
testing. Fuses havethree unique performance characteristics:
1. Modern fuses have an extremely high interruptingratingcan
open very high fault currents without rup-turing.
2. Properly applied, fuses prevent blackouts. Only the
fusenearest a fault opens without upstream fuses (feeders ormains)
being affectedfuses thus provide selective coordi-nation. (These
terms are precisely defined in subsequentpages.)
3. Fuses provide optimum component protection by keepingfault
currents to a low value. . .They are said to be
current-limiting.
The Louisiana Superdome in New Orleans is the worlds largest
fullyenclosed stadium. The overall electrical load exceeds
30,000,000 VA.Distribution circuits are protected with BUSS
LOW-PEAK fuses.
Voltage Rating - GeneralThis is an extremely important rating
for overcurrent protectivedevices (OCPDs). The proper application
of an overcurrent pro-tective device according to its voltage
rating requires that the volt-age rating of the device be equal to
or greater than the systemvoltage. When an overcurrent protective
device is applied beyondits voltage rating, there may not be any
initial indicators. Adverseconsequences typically result when an
improperly voltage rateddevice attempts to interrupt an
overcurrent, at which point it mayself-destruct in an unsafe
manner. There are two types of OCPDvoltage ratings: straight
voltage rated and slash voltage rated.
The proper application is straightforward for overcurrent
protectivedevices with a straight voltage rating (i.e. 600V, 480V,
240V, etc.)which have been evaluated for proper performance with
fullphase-to-phase voltage used during the testing, listing and
mark-ing. For instance, all fuses are straight voltage rated and
there isno need to be concerned about slash ratings. However,
somemechanical overcurrent protective devices are slash voltage
rated(i.e. 480/277, 240/120, 600/347, etc.). Slash voltage rated
devicesare limited in their applications and extra evaluation is
requiredwhen they are being considered for use. The next section
coversfuse voltage ratings followed by a section on slash voltage
ratingsfor other type devices.
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Fuseology
Voltage Rating-FusesMost low voltage power distribution fuses
have 250V or 600V rat-ings (other ratings are 125, 300, and 480
volts). The voltage ratingof a fuse must be at least equal to or
greater than the circuit volt-age. It can be higher but never
lower. For instance, a 600V fusecan be used in a 208V circuit. The
voltage rating of a fuse is afunction of its capability to open a
circuit under an overcurrentcondition. Specifically, the voltage
rating determines the ability ofthe fuse to suppress the internal
arcing that occurs after a fuse linkmelts and an arc is produced.
If a fuse is used with a voltage rat-ing lower than the circuit
voltage, arc suppression will be impairedand, under some
overcurrent conditions, the fuse may not clearthe overcurrent
safely. 300V rated fuses can be used to protectsingle-phase
line-to-neutral loads when supplied from three-phase,solidly
grounded, 480/277V circuits, where the single-phase line-to-neutral
voltage is 277V. This is permissible because in thisapplication, a
300V fuse will not have to interrupt a voltage greaterthan its 300V
rating. Special consideration is necessary for semi-conductor fuse
applications, where a fuse of a certain voltage rat-ing is used on
a lower voltage circuit.
Slash Voltage RatingsSome multiple-pole, mechanical overcurrent
protective devices,such as circuit breakers, self-protected
starters, and manual motorcontrollers, have a slash voltage rating
rather than a straight volt-age rating. A slash voltage rated
overcurrent protective device isone with two voltage ratings
separated by a slash and is markedsuch as 480Y/277V or 480/277V.
Contrast this to a straight voltagerated overcurrent protective
device that does not have a slash volt-age rating limitation, such
as 480V. With a slash rated device, thelower of the two ratings is
for overcurrents at line-to-ground volt-ages, intended to be
cleared by one pole of the device. The high-er of the two ratings
is for overcurrents at line-to-line voltages,intended to be cleared
by two or three poles of the circuit breakeror other mechanical
overcurrent device.
Slash voltage rated overcurrent protective devices are not
intend-ed to open phase-to-phase voltages across only one pole.
Whereit is possible for full phase-to-phase voltage to appear
across onlyone pole, a full or straight rated overcurrent
protective device mustbe utilized. For example, a 480V circuit
breaker may have to openan overcurrent at 480V with only one pole,
such as might occurwhen Phase A goes to ground on a 480V, B-phase,
corner ground-ed delta system.
Slash voltage rated OCPDs must be utilized only on solidly
groundedsystems. This automatically eliminates their usage on
impedance-grounded and ungrounded systems. They can be properly
utilizedon solidly grounded, wye systems, where the voltage to
ground doesnot exceed the devices lower voltage rating and the
voltagebetween any two conductors does not exceed the devices
highervoltage rating. Slash voltage rated devices cannot be used on
cor-ner-grounded delta systems whenever the voltage to
groundexceeds the lower of the two ratings. Where slash voltage
rateddevices will not meet these requirements, straight voltage
rated over-current protective devices are required.
Overcurrent protective devices that may be slashed rated
include,but are not limited to:
Molded case circuit breakers UL489Manual motor controllers
UL508Self protected Type E combination starters UL508Supplementary
protectors UL1077 (Looks like a small circuit
breaker and sometimes referredto as mini-breaker. However,these
devices are not a circuitbreaker, they are not rated forbranch
circuit protection and cannot be a substitute where branchcircuit
protection is required.)
What about fuses, do they have slash voltage ratings? No,
fusesdo not have this limitation. Fuses by their design are full
voltagerated devices; therefore slash voltage rating concerns are
not anissue when using fuses. For instance, Bussmann LOW-PEAK
LPJ (Class J) fuses are rated at 600V. These fuses could be
uti-lized on systems of 600V or less, whether the system is
solidlygrounded, ungrounded, impedance grounded, or corner
ground-ed delta.
If a device has a slash voltage rating limitation, product
standardsrequire these devices, such as circuit breakers, manual
motor con-trollers, self protected starters, or supplementary
protectors to bemarked with the rating such as 480Y/277V. If a
machine or equip-ment electrical panel utilizes a slash voltage
rated device inside, itis recommended that the equipment nameplate
or label designatethis slash voltage rating as the equipment
voltage rating. UL508AIndustrial Control Panels requires the
electrical panel voltagemarking to be slash rated if one or more
devices in the panel areslash voltage rated. Slash voltage devices
are limited in applica-tion to solidly grounded, wye systems due to
the nature of the waythat these devices are tested, listed and
labeled. Any piece ofequipment that utilizes a slash voltage rated
overcurrent protectivedevice is therefore, limited to installation
only in a solidly grounded,wye system and should require marking
that notes this limitation.
A
B
C
480Y/277 Voltthree phase,four wire,solidlygrounded,wye
system
Circuit breaker 480Y/277 slash voltage rating 480 volts
Line-to-line
Ground
N
277 voltsLine-to-ground
Fuses are a universal protective device. They are used in power
distri-bution systems, electronic apparatus, vehicles. . .and as
illustrated, ourspace program. The Space Shuttle has over 600 fuses
installed in it pro-tecting vital equipment and circuits.
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FuseologyEquipment that utilizes straight voltage rated
overcurrent protectivedevices provides more value and utilization
to the owner or poten-tial future owners than equipment that
utilizes slash voltage rateddevices. In todays business
environment, machinery and equip-ment may be moved several times
during its useful life. Equipmentutilizing slash voltage rated
overcurrent devices is not suitable formany electrical systems
found in industrial environments.
Ampere RatingEvery fuse has a specific ampere rating. In
selecting the ampererating of a fuse, consideration must be given
to the type of loadand code requirements. The ampere rating of a
fuse normallyshould not exceed the current carrying capacity of the
circuit. Forinstance, if a conductor is rated to carry 20 amperes,
a 20 amperefuse is the largest that should be used. However, there
are somespecific circumstances in which the ampere rating is
permitted tobe greater than the current carrying capacity of the
circuit. A typi-cal example is motor circuits; dual-element fuses
generally arepermitted to be sized up to 175% and non-time-delay
fuses up to300% of the motor full-load amperes. As a rule, the
ampere ratingof a fuse and switch combination should be selected at
125% ofthe continuous load current (this usually corresponds to the
circuitcapacity, which is also selected at 125% of the load
current).There are exceptions, such as when the fuse-switch
combination isapproved for continuous operation at 100% of its
rating.
Testing Knife-Blade FusesA common practice when electricians are
testing fuses is to touchthe end caps of the fuse with their
probes. Contrary to popularbelief, fuse manufacturers do not
generally design their knife-bladefuses to have electrically
energized fuse caps during normal fuseoperation. Electrical
inclusion of the caps into the circuit occurs asa result of the
coincidental mechanical contact between the fusecap and terminal
extending through it. In most brands of knife-blade fuses, this
mechanical contact is not guaranteed; therefore,electrical contact
is not guaranteed. Thus, a resistance readingtaken across the fuse
caps is not indicative of whether or not thefuse is open.
A Continuity Test Across Any Knife-Blade Fuse ShouldBe Taken
Only Along The Fuse Blades
Do Not Test A Knife-Blade Fuse With Meter Probes ToThe Fuse
Caps
In a continuing effort to promote safer work
environments,Bussmann has introduced newly designed versions of
knife-blade FUSETRON Fuses (Class RK5) and knife-blade LOWPEAK
Fuses (Class RK1) for some of the ampere ratings. Theimprovement is
that the end caps are insulated to reduce the pos-sibility of
accidental contact with a live part. With these improvedfuses, the
informed electrician knows that the end caps are isolat-ed. With
older style non-insulated end caps, the electrician doesntreally
know if the fuse is hot or not. A portion of all
testing-relatedinjuries could be avoided by proper testing
procedures.Bussmann hopes to reduce such injuries by informing
electri-cians of proper procedures.
Interrupting RatingA protective device must be able to withstand
the destructive ener-gy of short-circuit currents. If a fault
current exceeds a levelbeyond the capability of the protective
device, the device mayactually rupture, causing additional damage.
Thus, it is importantwhen applying a fuse or circuit breaker to use
one which can sus-tain the largest potential short-circuit
currents. The rating whichdefines the capacity of a protective
device to maintain its integritywhen reacting to fault currents is
termed its interrupting rating.The interrupting rating of most
branch-circuit, molded case, circuitbreakers typically used in
residential service entrance panels is10,000 amperes. (Please note
that a molded case circuit breakersinterrupting capacity will
typically be lower than its interrupting rat-ing.) Larger, more
expensive circuit breakers may have interrupt-ing ratings of 14,000
amperes or higher. In contrast, most modern,current-limiting fuses
have an interrupting rating of 200,000 or300,000 amperes and are
commonly used to protect the lowerrated circuit breakers. The
National Electrical Code 110.9,requires equipment intended to break
current at fault levels tohave an interrupting rating sufficient
for the current that must beinterrupted. The subjects of
interrupting rating and interruptingcapacity are treated later in
more detail.
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This photograph vividly illustratesthe effects of overcurrents
onelectrical components when protective devices are not sizedto the
ampere rating of the component.
Considerable damage to electricalequipment can result if the
inter-rupting rating of a protectivedevice is inadequate and
isexceeded by a short-circuit current.
Non Insulated
Caps
Always Test at the Blade
InsulatedCaps
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Fuseology
The table below depicts four different situations involving
anovercurrent device with a normal current rating of 100 amperesand
an interrupting rating of only 10,000 amperes.
Circuit with Overcurrent Circuit ApplicationProtective Device
Conditions And ActionCurrent Rating= 100A and of
ProtectiveInterrupting Rating= 10,000A Device
Normal Proper
Overload Proper-SafeCurrent InterruptionGreater Than of
CurrentDevicesAmpereRating
Short-Circuit Proper-SafeCurrent InterruptionWithin Device of
CurrentInterruptingRating
Short-Circuit Improper:Current Explosion orExceeds RuptureDevice
Could ResultInterruptingRating
In the first three instances above, the circuit current
conditionis within the safe operating capabilities of the
overcurrent protec-tive device. However, the fourth case involves a
misapplication ofthe overcurrent device. A short-circuit on the
load side of thedevice has resulted in a fault current of 50,000
amperes flowingthrough the overcurrent device. Because the fault
current is wellabove the interrupting rating of the device, a
violent rupture of theprotective device and resulting damage to
equipment or injury topersonnel is possible. The use of high
interrupting rated fuses (typ-ically rated at 200,000 or 300,000
amperes) would prevent thispotentially dangerous situation.
The first paragraph of NEC 110.9 requires that the overcur-rent
protective device be capable of interrupting the available
faultcurrent at its line terminals.
As depicted in the diagram that follows, when using overcur-rent
protective devices with limited interrupting rating, it
becomesnecessary to determine the available short-circuit currents
at eachlocation of a protective device. The fault currents in an
electricalsystem can be easily calculated if sufficient information
about theelectrical system is known. (See the Point-to-Point Method
forshort-circuit calculations.) With modern fuses, these
calculationsnormally are not necessary since the 200,000 or 300,000
ampereinterrupting rating is sufficient for most applications.
Also, if using circuit breakers or self-protected starters, it
maybe necessary to evaluate the devices individual pole
interruptingcapability for the level of fault current that a single
pole of a multi-pole device may have to interrupt. This is covered
in-depth in theSingle-Pole Interrupting Capability section.
50,000
Available fault current50,000 amps
Fuse must have short-circuitinterrupting rating of at
least50,000 amperes.
75,000 Amperes
75,000 Amperes
30,000 Amperes
15,000 Amperes
25,000 Amperes
Available short-circuit current (indicated by X) ateach panel
location must be determined to assureshort-circuit interrupting
rating of overcurrentprotective devices is not exceeded.
80
AMMETER100
Amperes
LOAD
200
10,000
Available fault current50,000 amps
Circuit breaker must havecapability of interrupting at
least50,000 amperes.
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General Fuse Application Data For Compliance With NEC
110.9Guideline Features Benefits Commonly Used
Fuse Types
New 1. Use modern, high interrupting 300,000 ampere interrupting
Assures proper interrupting All modern current-limiting Install-
rated fuses throughout electrical rating, on LOW-PEAK YELLOW rating
compliance currently fuses (most have 200,000ations system. fuses.
200,000 ampere inter- and future. ampere interrupting
rupting rating on other classes of Usually a short-circuit
rating). LOW-PEAK YELLOW,
modern current-limiting fuses. current calculation study is
Class R, J & L fuses have a
unnecessary. 300,000 ampere interrupting rating.
2. Use current-limiting fuses to Correct type and size
Compliance with NEC LOW-PEAK YELLOW protect low withstand rated
current-limiting fuse can 110.10 and 240.86.
Dual-Elementcomponents. protect low withstand rated T-TRON
Fast-Acting
equipment against high LIMITRON Fast-Actingshort-circuit
currents. (See CUBEFuse
fuse protection of circuitbreakers).
System 3. Where available fault current 200,000 or 300,000
ampere Assures compliance with LOW-PEAK YELLOW Up- has increased or
is questionable, interrupting rating. interrupting rating
requirements Dual-ElementGrading replace old style fuses such as
with simple direct retrofit. FUSETRON Dual-Element
One-Time and Renewable with Easily achieved since older LIMITRON
Fast-Actingmodern high interrupting rated style fuses can
physically befuses. replaced with modern fuses
with no system modification.
4. Where existing equipment may Correct type and size Improves
the level of CUBEFuse, T-TRON
have questionable withstand current-limiting fuses can be
short-circuit protection. Fast-Acting, LOW-PEAK
rating due to deterioration, or the put in switch, cut-in system
or Small size of CUBEFuse YELLOW Dual-Element,available fault
current has sometimes fuses can be cut in or T-TRON fuse permits
easy LIMITRON Fast-Acting,increased, install modern current- bus
structure. cut-in strategy. LOW-PEAK
YELLOW limiting fuses. Time-Delay
Fuseology
Interrupting RatingIt is the maximum short-circuit current that
an overcurrent protec-tive device can safely interrupt under
standard test conditions.The phrase under standard test conditions
means it is importantto know how the overcurrent protective device
is tested in order toassure it is properly applied. This can be
very important when itcomes to the application of circuit breakers,
mainly ratings of 100amperes and less.
Interrupting CapacityThe highest current at rated voltage that
the device can interrupt.This definition is from the IEEE Standard
Dictionary of Electricaland Electronic Terms.
Standard Test Conditions - FusesBranch circuit fuses are tested
without any additional conductor inthe test circuit. For instance,
if a fuse has an interrupting rating of300,000 amperes, the test
circuit is calibrated to have at least300,000 amperes at the rated
fuse voltage. During the test circuitcalibration, a bus bar is used
in place of the fuse to verify the prop-er short-circuit current.
Then the bus bar is removed and the fuseis inserted; the test is
then conducted. If the fuse passes the test,the fuse is marked with
this interrupting rating (300,000 amperes).In the procedures just
outlined for fuses, there are no extra con-ductors inserted into
the test circuit after the short-circuit current iscalibrated. A
major point is that the fuse interrupts an availableshort-circuit
current at least equal to or greater than its markedinterrupting
rating. In other words, because of the way fuses areshort-circuit
tested (without additional conductor impedance), theirinterrupting
capacity is equal to or greater than their marked inter-rupting
rating.
Standard Test Conditions - Circuit BreakersThis is not the case
with circuit breakers. Because of the way cir-cuit breakers are
short circuit tested (with additional conductor
impedance), their interrupting capacity can be less than their
inter-rupting rating. When the test circuit is calibrated for the
circuitbreaker interrupting rating tests, the circuit breaker is
not in the cir-cuit. After the test circuit has been verified to
the proper level ofshort-circuit current, the circuit breaker is
placed into the circuit.However, in addition to the circuit
breaker, significant lengths ofconductor are permitted to be added
to the circuit after the cali-bration. This additional conductor
impedance can result in a sig-nificantly lower short-circuit
current. So a circuit breaker markedwith an interrupting rating of
22,000 amperes may in fact have aninterrupting capacity of only
9,900 amperes.
To better understand this, it is necessary to review the
standardinterrupting rating test procedures for circuit breakers:
MoldedCase Circuit Breakers - UL 489 and CSA 5 Test Procedures.
UL489 requires a unique test set-up for testing circuit breaker
inter-rupting ratings. The diagram below illustrates a typical
calibratedtest circuit waveform for a 20 ampere, 240V, 2-pole
molded casecircuit breaker, with a marked interrupting rating of
22,000amperes, RMS symmetrical.
Amps
Time
P.F. = 20%IRMS = 22,000 Amp
Ip = 48,026A
IRMS = 22,000A
Interrupting Rating vs. Interrupting Capacity
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FuseologyThe diagram below illustrates the test circuit as
allowed by UL 489.Standard interrupting rating tests for a 22,000
ampere sym. RMS
interrupting rated circuit breaker will allow for a maximum 4
ft.rated wire on the line side for each lead, and 10 in. rated wire
onthe load side for each lead of the circuit breaker. See the
followingdiagrams and table, that provide a short-circuit analysis
of this testcircuit as seen by the circuit breaker.
Test station source impedance is adjusted to achieve a
calibrated22,000 RMS symmetrical amps at 20% or less power factor.
Thiscircuit can achieve a peak current of 48,026 amps. For the
calibra-tion test, a bus bar (shorting bar) is inserted between the
test sta-tion terminals.After the circuit calibration is verified,
the shorting bar is removed
and the circuit breaker is inserted. In addition, lengths of
ratedconductor are permitted to be added as shown. This extra
ratedconductor has a high impedance and effectively restricts the
cur-rent to 9,900 RMS symmetrical amps. The power factor
increasesto 88% due to small conductor high resistance versus its
reac-tance.
This circuit can now only achieve a peak current of 14,001
amps.
20A, 240V, 2-PoleMolded Case Circuit RMS Maximum
Breaker With 22,000A Symmetrical Instantaneous Power
FactorInterrupting Rating Amps Peak
Calibrated InterruptingRating Circuit 22,000 48,026 20%
Actual Circuit WithWire Impedance Added 9,900 14,001 88%
After Calibration
Conclusion (refer to table above and graphs below)This 22,000
ampere (with short-circuit power factor of 20%) inter-rupting rated
circuit breaker has an interrupting capacity of 9,900amperes at a
short-circuit power factor of 88%. Unless there is aguarantee that
no fault will ever occur at less than 4' 10" from theload terminals
of the circuit breaker, this circuit breaker must onlybe applied
where there are 9,900 amperes or less available on itsline
side.
A graphic analysis of this actual short-circuit follows.
Agency standards allow for a random close during the
short-circuit test, so the peak available current may be as low
as1.414 times the RMS symmetrical current.
Thus, the circuit breaker is actually tested to interrupt
9,900amperes at 88% power factor, not 22,000 amperes at 20%
powerfactor. The following graph shows the waveforms
superimposedfor comparison. Henceforth, this RMS test value will be
identifiedas the circuit breaker interrupting capacity. (Dont
confuse this withthe circuit breaker marked interrupting
rating.)
RLINE XLINERCB XCB
20A
RLOAD XLOADRS
XS
SOURCE: 4' Rated Wire (12 AWG Cu)
Note: For calculations, RCB and XCB are assum ed negligible.
10" Rated Wire (12 AWG Cu)
S.C.P.F. = 20%S.C. Avail. = 22,000A
Test station source leads
Shorting bar
Test station source leads
Each4 feet 12 AWG
Each 10 inches 12 AWG
20 A
Shorting barremoved, circuit
breaker &conductors added
20A, 240V, 2-PoleCircuit Breaker
marked 22,000 A.I.R.
Amps
Time
P.F. = 88%IRMS = 9,900 Amp
IRMS = 9,900A
Ip = 14,001A
Amps
Time
P.F. = 88%IRMS = 9,900 Amp
Ip = 48,026A
IRMS = 9,900A
Ip = 14,001A
IRMS = 22,000AP.F. = 20%IRMS = 22,000A
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FuseologyEqually important, the short-circuit power factor is
greatly affecteddue to the high R values of the small, rated wire.
This results in alower peak value that the circuit breaker must
tolerate during thefirst one-half cycle.
Following is an example of a partial table showing the actual Ip
andIRMS
values to which circuit breakers are tested.
240V - 2-Pole CB Interrupting Capacities (amps)
CB 10,000 RMS Sym. 14,000 RMS Sym. 18,000 RMS Sym.
AMP Interrupting Rating Interrupting Rating Interrupting
RatingRATING Ip IRMS Ip IRMS Ip IRMS
15A 7,200 5,100 8,700 6,100 9,300 6,600
20A 8,900 6,300 11,400 8,100 12,600 8,900
25A 10,700 7,500 14,200 10,100 16,500 11,700
30A 10,700 7,500 14,200 10,100 16,500 11,700
40A 11,700 8,300 16,000 11,300 19,200 13,600
50A 11,700 8,300 16,000 11,300 19,200 13,600
60A 12,500 8,800 17,300 12,200 21,300 15,100
70A 13,000 9,200 18,100 12,800 22,600 16,000
80A 13,000 9,200 18,100 12,800 22,600 16,000
90A 13,200 9,300 18,300 12,900 23,000 16,300
100A 13,200 9,300 18,300 12,900 23,000 16,300
These values are known as the circuit breakers
interruptingcapacities.
Bus Bar Conditions- Circuit BreakersBeginning 10/31/2000, UL 489
requires circuit breakers rated 100Aand less to additionally be
tested under bus bar conditions.However, this does not assure that
the circuit breakers interruptingcapacity equals its interrupting
rating nor even that the circuitbreaker is reusable. In this test,
line and load terminals are con-nected to 10" of rated conductor.
For single pole circuit breakers,these 10" leads are then connected
to 4' of 1 AWG for connection tothe test station. For multi-pole
circuit breakers, the 10" line sideleads are connected to the test
station through 4' of 1 AWG. Theload side is shorted by 10" leads
of rated conductor per pole.These bus bar condition tests still do
not fully address the situa-tion where a fault can occur less than
4'10" from the circuit breaker.
One point to be made is that acceptable bus shot test results
perthe product standard do not meet the NEC definition for a
circuitbreaker. For example, 7.1.11.6.3.1 of UL 489 states The
inability torelatch, reclose, or otherwise reestablish continuity
... shall be con-sidered acceptable for circuit breakers which are
tested under busbar conditions. In practical terms, this means the
circuit breakerdoesnt have to work after a fault near the circuit
breaker occurs.This is in violation of the 2002 NEC definition for
a circuit breaker:A device designed to open and close a circuit by
nonautomaticmeans and to open the circuit automatically on a
predeterminedovercurrent without damage to itself when properly
applied withinits rating. In addition, under bus bar condition
tests the circuitbreaker is required to only interrupt one
short-circuit current. Forthis one short-circuit test shot, the
circuit breaker is in its closedposition and the short-circuit
current is initiated by the test stationswitch closing randomly.
The bus bar conditions test proceduresdo not evaluate the circuit
breaker for closing-on the short-circuit.Closing-on a short-circuit
is an important criteria for safety.
Single-Pole Interrupting CapabilityAn overcurrent protective
device must have an interrupting ratingequal to or greater than the
fault current available at its line terminalsfor both three-phase
bolted faults and for one or more phase-to-ground faults (110.9).
Although most electrical systems are designedwith overcurrent
devices having adequate three-phase interruptingratings, the
single-pole interrupting capabilities are easily over-looked. This
section will examine single-pole interrupting capability(also
referred to as individual pole interrupting capability).
This section will show how single-pole interrupting capabilities
mustbe considered in some applications. It will also show there are
simplesolutions that exist to provide adequate interrupting ratings
if moldedcase circuit breakers, self protected starters and other
overcurrentprotective devices are found to have insufficient
single-pole interrupt-ing capabilities.
A Fine Print Note was added to 240.85 of the 2002 NEC to
alertusers that circuit breakers have single-pole interrupting
capabili-ties that must be considered for proper application. It
states:
240.85 FPN: Proper application of molded case circuit breakerson
3-phase systems, other than solidly grounded wye, particu-larly on
corner grounded delta systems, considers the circuitbreakers
individual pole interrupting capability.
As will be shown, there are other overcurrent device types
andother grounding system types where individual pole
interruptingcapability must be analyzed.
The single-pole interrupting capability of a circuit breaker,
self pro-tected starter and other similar mechanical overcurrent
protective
device is its ability to open an overcurrent at a specified
voltage uti-lizing only one pole of the multi-pole device.
Multi-pole mechanicalovercurrent protective devices are typically
marked with an interrupt-ing rating. This marked interrupting
rating applies to all three polesinterrupting a three-phase fault
for a three-pole device. The markedinterrupting rating of a
three-pole device does not apply to a singlepole that must
interrupt a fault current at rated voltage.
Single-Pole Interrupting Capabilities For Overcurrent
DevicesCurrent-limiting fuses: the marked interrupting rating is
the testedsingle-pole interrupting rating. So single-pole
interrupting capabili-ty is not an issue with fuse
applications.
Airframe/power circuit breaker: per ANSI C37.13 and C37.16 the
sin-gle-pole interrupting rating is 87% of its three-pole
interrupting rating.
Molded case circuit breakers: Listed three-pole molded case
circuitbreakers have minimum single-pole interrupting capabilities
accord-ing to Table 7.1.7.2 of UL 489. Table 1 on the next page
indicates thesingle-pole test value for various three-pole molded
case circuit break-ers taken from Table 7.1.7.2 of UL 489. A
similar table is shown on
Single Pole Interrupting Capabilities
A circuit breakers, self protected starters, or othermechanical
protective devices ability to open an overcurrent
at a specified voltage utilizing only one pole of the
device.
-
8
Fuseologypage 54 of the IEEE Blue Book, Recommended Practice
forApplying Low-Voltage Circuit Breakers Used in Industrial
andCommercial Power Systems, (Std 1015-1997).
Self protected starters: UL 508, Table 82A.3 specifies the short
circuittest values on one pole as 4320 amps for 0 to 10 hp devices
rated 200to 250 volts and 8660 amperes for 0 to 200 hp devices
rated 600 voltsmaximum.
Molded case circuit breakers and self protected starters may not
beable to safely interrupt single-pole faults above these
respective valuesshown in previous paragraphs. Per 110.9, all
overcurrent protectivedevices that are intended to interrupt fault
currents must have single-pole interrupting capabilities for the
maximum single-pole fault currentthat is available. And typically,
engineers, contractors and inspectors(AHJs) rely on the applicable
product standard testing and listing crite-ria to verify device
ratings as being suitable for specific applications.
TABLE 1 Standard UL 489 Interrupting Tests For 3-Pole Molded
Case Circuit Breakers
Standard3-Pole Standard Single-Pole Interrupting
Interrupting Tests ValuesValues
FRAME RATING 240V 480/277V 480V 600/347V 600V
100A Maximum250V Maximum 4,330 -- -- -- -- 5,000
100A Maximum251-600V -- 10,000 8,660 10,000 8,660 10,000
101 800 8,660 10,000 8,660 10,000 8,660 14,000
801 1200 12,120 14,000 12,120 14,000 12,120 20,000
1201 2000 14,000 14,000 14,000 14,000 14,000 25,000
2001 2500 20,000 20,000 20,000 20,000 20,000 30,000
2501 3000 25,000 25,000 25,000 25,000 25,000 35,000
3001 4000 30,000 30,000 30,000 30,000 30,000 45,000
4001 5000 40,000 40,000 40,000 40,000 40,000 60,000
5001 6000 50,000 50,000 50,000 50,000 50,000 70,000
Note: This data is from UL 489 Table 7.1.7.2.
Molded Case Circuit Breaker Testing - UL 489Devices must be
applied within the limitations of their listing. UL 489 isthe
standard for molded case circuit breakers. UL 489 has tests whichit
refers to as standard interrupting tests for molded case
circuitbreakers. A more appropriate term would be base or lowest
inter-rupting level that any circuit breaker of a given rated
voltage andampere rating must meet. There are circuit breakers on
the marketthat just test to these standard or base interrupting
tests. However,because these base interrupting ratings are rather
modest values,some circuit breakers are listed with higher
interrupting ratings thanthe standard or base levels; for these
circuit breakers, there are addi-tional procedures for higher level
interrupting tests.
These standard or base interrupting tests for three-pole
circuitbreakers involve individual single-pole interrupting tests
and multi-pole interrupting tests. Table 7.1.7.2 of UL 489 provides
the single-pole (individual) and multi-pole interrupting current
values for variousvoltage rating and ampere rating circuit
breakers. Table 1 showsthe single-pole short-circuit current values
(from Table 7.1.7.2 of UL489) for which all three-pole circuit
breakers are tested and evaluat-ed under single-pole interrupting
capabilities. The far right column ofTable 1 shows the three-phase
short-circuit current values (fromTable 7.1.7.2 of UL 489) for
which all three-pole circuit breakers aretested and evaluated.
These standard circuit breakers would bemarked with an Interrupting
Rating (if above 5,000 amperes) corre-sponding to the three-phase
short-circuit current value. The stan-dard circuit breaker is not
marked with a single-pole interrupting rat-ing which would
correspond to the single-pole interrupting test value.
Circuit breakers with interrupting ratings higher than the
standardinterrupting values are needed in todays systems, so
additional provi-sions are in UL 489. Higher interrupting rated
molded case circuitbreakers are additionally tested and evaluated
per 7.1.11 of UL 489 toa High Short-Circuit Test procedure in order
to be marked with ahigher interrupting rating. This test procedure
does not include a sin-gle-pole test of higher short-circuit
current value than the standardtest provisions. The three-pole test
current value can be equal to anyvalue listed in Table 8.1 of UL
489, from 7,500A to 200,000A. Thisthree-phase test value must be
greater than the values in the far rightcolumn of Table 1. If a
circuit breaker successfully tested to a higherthree-pole
interrupting value per the High Short-Circuit Tests, themolded case
circuit breaker is marked with this higher interrupting rat-ing
which corresponds to the three-pole high short-circuit current
testvalue.
As mentioned, a single-pole interrupting test at a higher value
thanshown in Table 1 is not required in these optional High
Short-Circuit Test procedures. Because of this, the marked
three-poleinterrupting rating can be much higher than the tested
individualpole interrupting capability. In addition, the
single-pole capabilityis not required to be marked on the molded
case circuit breaker; itcan only be determined by reviewing the UL
489 standard.
Actual ExampleThe diagram below illustrates the UL 489 test
procedure for a 100A,480V, three phase circuit breaker that gets
listed for a high interrupt-ing rating. Test A and B are the
required standard or base interrupt-ing tests. Test A is a
three-pole interrupting test at 10,000 A (Table1, right column),
which is a modest three-phase available short-cir-cuit current, and
Test B is a single-pole interrupting test at a modestsingle-phase
available short-circuit current of 8660A (Table 1).Then, in
addition, to be listed and marked with the higher
65,000Ainterrupting rating, the circuit breaker must pass the
criteria for TestC. Test C is a three-pole interrupting test at
65,000A three-phaseavailable short-circuit current. Test D is not
conducted; it is not partof the UL 489 evaluation procedure. This
higher three-phase inter-rupting rated circuit breaker does not
have to undergo any test cri-teria at a corresponding higher
single-pole short-circuit current.
Three-Pole Interrupting Rating & Single-Pole Interrupting
CapabilitiesTest Procedures for Molded Case Circuit Breakers UL
489
As an example of single-pole interrupting capability in a
typicalinstallation, consider this three-pole, 100 amp, 480V
circuit breakerwith a three-pole interrupting rating of 65,000
amperes. Referring toTable 1, this breaker has an 8,660 ampere
single-pole interruptingcapability for 480V faults across one pole.
If the available line-to-ground fault current exceeds 8,660 amps at
480V, such as mightoccur on the secondary of a 1000 KVA, 480V,
corner-grounded,delta transformer, the circuit breaker may be
misapplied.
100A, 480V, 3-Pole CB Interrupting Rating = 65,000 A
(3-Pole)
Base Interrupting Rating Procedure
High Interrupting Rating Procedure
1-Pole Test8,660A
From Table 1
1-Pole TestNONE
Test D
Test B
3-Phase Test65,000A
From Table 1
Test A
Test C
3-Phase Test10,000A
-
9
FuseologyShown below are three still photos from a videotaping
of a single-pole fault interruption test on a three-pole circuit
breaker rated480V. This circuit breaker is marked with a three-pole
interruptingrating of 35,000 amperes at 480V. This marked
interrupting rating isper UL 489 test procedures. This circuit
breaker is tested for individ-ual single-pole interrupting
capabilities in UL 489 at an availablefault current of 8,660 amps
(Table 1 prior page). The test that isshown below is with an
available fault current of 25,000 amperes.
Test set up prior to closure of test station switch.
Photo of 3-pole circuit breaker during test of individual
single-poleinterruption of a fault current beyond the value in
Table 1.Magnetic forces of short-circuit current caused test board
to move.
Photo (later in sequence) of 3-pole circuit breaker during test
ofindividual single-pole interruption of a fault current beyond its
single-pole interrupting capability - it violently exploded.
Possible Fault Currents During A Ground Fault ConditionThe
magnitude of a ground fault current is dependent upon thelocation
of the fault with respect to the transformer secondary.Referring to
Figure 2, the ground fault current flows through onecoil of the wye
transformer secondary and through the phase con-ductor to the point
of the fault. The return path is through the enclo-sure and conduit
to the bonding jumper and back to the sec-ondary through the
grounded neutral. Unlike three-phase faults,the impedance of the
return path must be used in determining themagnitude of ground
fault current. This ground return impedanceis usually difficult to
calculate. If the ground return path is relativelyshort (i.e. close
to the center tap of the transformer), the groundfault current will
approach the three phase short-circuit current.
Theoretically, a bolted line-to-ground fault may be higher than
athree-phase bolted fault since the zero-sequence impedance canbe
less than the positive sequence impedance. The ground faultlocation
will determine the level of short-circuit current. The
prudentdesign engineer assumes that the ground fault current equals
atleast the available three-phase bolted fault current and makes
surethat the overcurrent devices are rated accordingly.
TYPE OF GROUND SYSTEM AFFECT ON SINGLE-POLE INTERRUPTIONThe
method in which a system is grounded can be a significantfactor in
the performance of multi-pole, mechanical overcurrentprotective
devices used in three phase systems. To illustrate this,several
different grounding systems with molded case circuitbreakers will
be analyzed.
Solidly Grounded WYE SystemsThe solidly grounded, wye system
shown in Figure 1 is by far themost common type of electrical
system. This system is typicallydelta connected on the primary and
has an intentional solid con-nection between the ground and the
center of the wye connectedsecondary (neutral). The grounded
neutral conductor carries sin-gle-phase or unbalanced three-phase
current. This system lendsitself well to commercial and industrial
applications where 480V (L-L-L) three-phase motor loads and 277V
(L-N) lighting is needed.
Figure 1 - Solidly Grounded WYE System
If a fault occurs between any phase conductor and ground
(Figure2), the available short-circuit current is limited only by
the com-bined impedance of the transformer winding, the phase
conductorand the equipment ground path from the point of the fault
back tothe source. Some current (typically 5%) will flow in the
parallelearth ground path. Since the earth impedance is typically
muchgreater than the equipment ground path, current flow
throughearth ground is generally negligible.
B
A
C
SERVICE
PANEL
BRANCH
PANELSteel Conduit
A
B
C
NN
480V
480V
Solidly Grounded WYE System
277V
277V277
V
-
10
Figure 2 - Single-Pole Fault to Ground in Solidly Grounded
WyeSystem
In solidly grounded wye systems, the first low impedance fault
toground is generally sufficient to open the overcurrent device on
thefaulted leg. In Figure 2, this fault current causes the branch
circuitovercurrent device to clear the 277V fault. This system
requirescompliance with single-pole interrupting capability for
277V faultson one pole. If the overcurrent devices have a
single-pole inter-rupting capability adequate for the available
short-circuit current,then the system meets NEC 110.9.
Although not as common as the solidly grounded wye
connection,the following three systems are typically found in
industrial installa-tions where continuous operation is essential.
Whenever these sys-tems are encountered, it is absolutely essential
that the properapplication of single-pole interrupting capabilities
be assured. Thisis due to the fact that full phase-to-phase voltage
can appearacross just one pole. Phase-to-phase voltage across one
pole ismuch more difficult for an overcurrent device to clear than
the line-to-neutral voltage associated with the solidly grounded
wye sys-tems.
Corner-Grounded-Delta Systems (Solidly Grounded)The system of
Figure 3 has a delta-connected secondary and issolidly grounded on
the B-phase. If the B-phase should short toground, no fault current
will flow because it is already solidlygrounded.
Figure 3 - Corner-Grounded Delta System (Solidly Grounded)
FuseologyIf either Phase A or C is shorted to ground, only one
pole of thebranch-circuit overcurrent device will see the 480V
fault as shownin Figure 4. This system requires compliance with
single-pole inter-rupting capabilities for 480V faults on one pole
because thebranch-circuit circuit breaker would be required to
interrupt480V with only one pole.
Figure 4 - Fault to Ground on a Corner-Grounded Delta System
A disadvantage of corner-grounded delta systems is the inability
toreadily supply voltage levels for fluorescent or HID lighting
(277V).Installations with this system require a 480-120V
transformer tosupply 120V lighting. Another disadvantage, as given
on page 33of IEEE Std 142-1991, Section 1.5.1(4) (Green Book) is
the possi-bility of exceeding interrupting capabilities of
marginally appliedcircuit breakers, because for a ground fault, the
interrupting dutyon the affected circuit breaker pole exceeds the
three-phase faultduty. A line-to-ground fault with this type
grounding system isessentially a line-to-line fault where the one
line is grounded. Themaximum line-to-line bolted short-circuit
current is 87% of the threephase bolted short-circuit current.
Review the prior page photosequence testing of the 225 amp, three
phase circuit breaker witha 35,000 ampere interrupting rating
(three-pole rating). 87% of35,000 amperes is 30,450 amperes. The
single-pole test was runwith an available of only 25,000
amperes.
Impedance Grounded SystemLow or High impedance grounding schemes
are found primarilyin industrial installations. These systems are
used to limit, to vary-ing degrees, the amount of current that will
flow in a phase toground fault. Low impedance grounding is used to
limit groundfault current to values acceptable for relaying
schemes. This typeof grounding is used mainly in medium voltage
systems and is notwidely installed in low voltage applications
(600V or below). TheHigh impedance grounded system offers the
advantage that thefirst fault to ground will not draw enough
current to cause the over-current device to open. This system will
reduce the stresses, volt-age dips, heating effects, etc. normally
associated with high short-circuit current. Referring to Figure 5,
high impedance groundedsystems have a resistor between the center
tap of the wye trans-former and ground. High impedance grounding
systems are usedin low voltage systems (600V or less). With high
impedancegrounded systems, line-to-neutral loads are not permitted
perNational Electrical Code, 250.36(4).
B
A
480V
SERVICE
PANEL
BRANCH
PANELSteel Conduit
A
B
C
C
Single pole must
interrupt fault current
Fault to
conduit
Corner Grounded Delta System
Solidly Grounded WYE System
B
A
C
SERVICE
PANEL
BRANCH
PANELSteel Conduit
A
B
C
N N
480V
480V
Fault to
conduit
Single pole must
interrupt fault current
Corner Grounded Delta System
B
A
480V
SERVICE
PANEL
BRANCH
PANELSteel Conduit
A
B
C
C
480V
480V
-
11
Figure 5 - Impedance Grounded System
When the first fault occurs from phase to ground as shown
inFigure 6, the current path is through the grounding
resistor.Because of this inserted resistance, the fault current is
not highenough to open protective devices. This allows the plant to
contin-ue on line. NEC 250.36(3) requires ground detectors to
beinstalled on these systems, so that the first fault can be found
andfixed before a second fault occurs on another phase.
Figure 6 - First Fault in Impedance Grounded System
Even though the system is equipped with a ground alarm, theexact
location of the ground fault may be difficult to determine.
Thefirst fault to ground MUST be removed before a second phasegoes
to ground, creating a 480V fault across only one pole of
theaffected branch circuit device. Figure 7 shows how the 480V
faultcan occur across one pole of the branch circuit device. It is
exact-ly because of this possibility that single-pole interrupting
capabili-ties must be considered for mechanical overcurrent
protectivedevices.
Figure 7 - Second Fault in Impedance Grounded System
The magnitude of this fault current can approach 87% of the
L-L-Lshort-circuit current. Because of the possibility that a
second faultwill occur, single-pole interrupting capability must be
investigated.The IEEE Red Book, Std 141-1993, page 367, supports
thisrequirement, One final consideration for
impedance-groundedsystems is the necessity to apply overcurrent
devices based upontheir single-pole short-circuit interrupting
rating, which can beequal to or in some cases less than their
normal rating.
Ungrounded Systems The Ungrounded System of Figure 8 offers the
same advantage forcontinuity of service that is characteristic of
high impedancegrounded systems.
Figure 8 - Ungrounded System
Although not physically connected, the phase conductors
arecapacitively coupled to ground. The first fault to ground is
limited bythe large impedance through which the current has to flow
(Figure9). Since the fault current is reduced to such a low level,
the overcur-rent devices do not open and the plant continues to
run.
Figure 9 - First Fault to Conduit in Ungrounded System
As with High Impedance Grounded Systems, ground detectorsshould
be installed (but are not required by the 2002 NEC), towarn the
maintenance crew to find and fix the fault before a sec-ond fault
from another phase also goes to ground (Figure 10).
The second fault from Phase B to ground (in Figure 10) will
create a 480volt fault across only one pole at the branch circuit
overcurrent device.Again, the values from Table 1 for single pole
interrupting capabilitiesmust be used for molded case circuit
breaker systems as the tradeofffor the increased continuity of
service. The IEEE Red Book, Std 141-1993, page 366, supports this
requirement, One final consideration forungrounded systems is the
necessity to apply overcurrent devicesbased upon their single-pole
short circuit interrupting rating, whichcan be equal to or in some
cases less than their normal rating.
Fuseology
Resistor
B
A
C
SERVICE
PANEL
BRANCH
PANELSteel Conduit
A
B
C
277V
480V
High Impedance Grounded System
480V
277V
27
7V
B
A
SERVICE
PANEL
BRANCH
PANELSteel Conduit
A
B
C
480V
480VFirst fault
to steel
conduit
Resistor keeps first
fault current low:
5 Amps or so
Low Value of Fault Current
Because of Ground Resistor in
Short-Circuit Path
High Impedance Grounded System
C 277V
277V
27
7V
B
A
C
SERVICE
PANEL
BRANCH
PANELSteel Conduit
A
B
C
480V
480VFirst fault
to steel
conduit
Second Fault
To Enclosure
High Value of Fault
Current Because
Ground Resistor No
Longer in Path
High Impedance Grounded System
Single pole must
interrupt fault current
277V
27
7V
277V
Ungrounded System
B
A
SERVICE
PANEL
BRANCH
PANEL Steel
Conduit
A
B
C
C
480V
480V
Low Value of Fault Current
Because of Large Capacitively
Coupled Impedance to Ground
Ungrounded System
B
A
SERVICE
PANEL
BRANCH
PANEL Steel
Conduit
A
B
C
C
480V
480V
First fault
to steel
conduit
-
12
Fuseology
Figure 10 - Second Fault to Conduit in Ungrounded System
In the 2002 NEC 250.4(B) Ungrounded Systems (4) Path for
FaultCurrent, it is required that the impedance path through the
equip-ment be low so that the fault current is high when a second
faultoccurs on an ungrounded system.
What Are Single-Pole Interrupting Capabilities For Fuses?By
their inherent design a fuses marked interrupting rating is
itssingle-pole interrupting rating. Per UL/CSA/ANCE 248
FuseStandards, fuses are tested and evaluated as single-pole
devices.Therefore, a fuses marked interrupting rating is its
single-poleinterrupting rating. So it is simple, fuses can be
applied on singlephase or three phase circuits without extra
concern for single-poleinterrupting capabilities. There is no need
to perform any specialcalculations because of the grounding system
utilized. Just besure the fuses interrupting ratings are equal to
or greater than theavailable short-circuit currents. Modern
current-limiting fuses areavailable with tested and marked
single-pole interrupting ratings of200,000 or 300,000 amperes.
LOW-PEAK LPJ_SP, KRP-C_SP,LPS-RK_SP and LPN-RK_SP fuses all have UL
Listed 300,000ampere single-pole interrupting ratings. This is a
simple solution toassure adequate interrupting ratings for present
and future sys-tems no matter what the grounding scheme. Review the
threedrawings for a fusible, high impedance grounded system.
Figure 11 - Fusible high impedance grounded system.
Figure 12 - Upon first fault, the fault current is low due to
resistor.As intended the fuse does not open.
Figure 13- Upon the second fault, the fault is essentially a
line-linefault with the impedance of the conductors and theground
path. The fuse must interrupt this fault. Since afuses interrupting
rating is the same as its single-poleinterrupting capability,
modern fuses with 200,000A or300,000A interrupting rating can be
applied without fur-ther analysis for single pole interrupting
capabilities.
Resistor
B
A
C
SERVICEPANEL
BRANCHPANEL
A
B
C
480V
480V
Steel Conduit
High Impedance Grounded System
277V
277V
SERVICE
PANEL
BRANCH
PANEL
Single Pole Must Interrupt Fault Current:Fuses Marked
Interrupting Rating Is Its Single-
Pole Interrupting Rating: Simple Solution
B
A
C
A
480V
480V
Steel Conduit
Second Faultto Enclosure
High Value of FaultCurrent Because
Ground Resistor NoLonger in Path
First Faultto Steel ConduitC
B
High Impedance Grounded System
277V
277V
B
A
C
SERVICEPANEL
BRANCHPANEL
A
B
C
480V
480V
Steel Conduit
Resistor Keeps FirstFault Current Low:
5 Amps or So
First Faultto Steel Conduit
High Impedance Grounded System
277V
277V
Ungrounded System
B
A
SERVICE
PANEL
BRANCH
PANEL Steel
Conduit
A
B
C
C
480V
480V
Second Fault
To Enclosure
High Value of Fault
Current Because
Large Impedance is
No Longer in Path
First fault
to steel
conduit
Single pole must
interrupt fault current