electrical distribution system engineering … coordination – reading time-current curves 6...

Engineering Dependable Protection - Part II "Selective Coordination of Overcurrent Protective Devices"

Table of Contents PageBasic Considerations of Selective Coordination ………………………………………………3

- What Is Selective Coordination? ……………………………………………………………3- Regular Methods of Performing a Selective Coordination Study ………………………3- Non-Selective Coordination Resulting in a Blackout ……………………………………3- Selective Coordination ………………………………………………………………………3

Selective Coordination- Reading Time-Current Curves………………………………………………………………4

- Overloads and Low Level Fault Currents ………………………………………………4- Fuse Curves…………………………………………………………………………………4- Circuit Breaker Curves ……………………………………………………………………5

- Current Limiting Fuses ……………………………………………………………………10- Medium to High Level Fault Currents …………………………………………………10- Selectivity Ratio Guide for Blackout Prevention ………………………………………10

- Circuit Breakers ……………………………………………………………………………12- Medium to High Level Fault Currents …………………………………………………12

Recommended Procedures for Selective Coordination Study ……………………………14Examples of Selective Coordination Studies …………………………………………………15

- Time Current Curve #1 (TCC1) ……………………………………………………………16- Time Current Curve #2 (TCC2) ……………………………………………………………18- Time Current Curve #3 (TCC3) ……………………………………………………………20

Conclusions ………………………………………………………………………………………22

Electrical Distribution System

Engineering Dependable Protection

2

Part 2Selective Coordination

Of OvercurrentProtective Devices

ForLow Voltage Systems

Bulletin EDP-2(2004-2)

EngineeringDependableProtectionFor AnElectricalDistributionSystem

Bussmann

Electrical Distribution System

Basic Considerations of Selective Coordination

Engineering Dependable ProtectionPart I has provided a simple method to calculate short-

circuit currents that occur in electrical systems. With thisinformation, selective coordination studies of the systemscan be performed in order to prevent blackouts.

What Is Selective Coordination?Today, more than ever, one of the most important parts

of any installation - whether it is an office building, anindustrial plant, a theater, a high-rise apartment or ahospital - is the electrical distribution system. Nothing willstop all activity, paralyze production, inconvenience anddisconcert people and possibly cause a panic moreeffectively than a major power failure.

ISOLATION of a faulted circuit from the remainder ofthe installation is MANDATORY in today's modern electricalsystems. Power BLACKOUTS CANNOT be tolerated.

It is not enough to select protective devices basedsolely on their ability to carry the system load current andinterrupt the maximum fault current at their respectivelevels. A properly engineered system will allow ONLY theprotective device nearest the fault to open, leaving the

3

remainder of the system undisturbed and preservingcontinuity of service.

We may then define selective coordination as "THEACT OF ISOLATING A FAULTED CIRCUIT FROM THEREMAINDER OF THE ELECTRICAL SYSTEM, THEREBYELIMINATING UNNECESSARY POWER OUTAGES. THEFAULTED CIRCUIT IS ISOLATED BY THE SELECTIVEOPERATION OF ONLY THAT OVERCURRENT PROTECTIVEDEVICE CLOSEST TO THE OVERCURRENT CONDITION."

Figures 1 and 2 illustrate a non-selective system and aselectively coordinated system, respectively.

Popular Methods of Performing a Selective Coordination StudyCurrently two methods are most often used to perform

a coordination study:1. Overlays of Time-Current Curves, which utilize a

light table and manufacturers' published data, then handplot on log-log paper.

2. Computer programs that utilize a PC and allow thedesigner to select time current curves published bymanufacturers and transfer to a plotter or printer, followingproper selections.

This text will apply to both methods.

Non-Selective Coordination Resulting in a BlackoutA fault on a branch circuit opens protective devices

"D", "C" and "B". The entire power supply to the building iscompletely shut down. This non-selective operation isnormally due to a medium to high level short circuit. Thisfault may be L-L, L-G, or 3 phase bolted in nature.

Selective CoordinationA fault on a branch circuit opens protective device " D"

only. Since A, B and C are not disturbed, the remainder ofthe electrical system is still energized.

It is also possible that non-selective OPENING couldbe due to overload conditions on the branch circuit.

Figure 1

Not Affected

A

B

Also Opens

C

Also Opens

De-energizedPortion of System

Opens

Branch Circuit Fault

D

Not Affected

C

Not Affected

Not Affected

A

B

FaultDe-energizedPortion of System.(This is the only part of the system affected).

Opens

D

Figure 2

Selective Coordination –

Reading Time-Current Curves

Overloads and Low Level Fault Currents

This information is presented as an aid tounderstanding time-current characteristic curves of fusesand circuit breakers, and wil l discuss the majorconsiderations in properly applying electrical protectivedevices. A thorough understanding of t ime-currentcharacteristic curves of overcurrent protective devices isessential to provide a Selectively Coordinated System.

It should be noted that the study of time-current curvesindicates performance during overload and low level faultconditions. The performance of overcurrent devices thatoperate under medium to high level fault conditions are notreflected on standard curves. Other engineering methodsmust be utilized.

Fuse CurvesFigure 3 illustrates the time-current characteristic

curves for two sizes of time-delay, dual-element fuses inseries, as depicted in the one-line diagram in Figure 3a.The horizontal axis of the graph represents the RMSsymmetrical current in amperes. The vertical axisrepresents the time, in seconds, until the fault occurs .

For example: Assume an available fault current levelof 1000 amperes RMS symmetrical on the load side of the100 ampere fuse. To determine the time it would take thisfault current to open the two fuses, first find 1000 ampereson the horizontal axis (Point A), follow the dotted linevertically to the intersection of the total clear curve of the100 ampere time-delay dual-element fuse (Point B) and theminimum melt curve of the 400 ampere time-delay dual-element fuse (Point C). Then, horizontally from bothintersection points, follow the dotted lines to Points D andE. At 1.75 seconds, Point D represents the maximum timethe 100 ampere time-delay dual-element fuse will take toopen the 1000 ampere fault. At 88 seconds, Point Erepresents the minimum time at which the 400 amperetime-delay dual-element fuse could open this available faultcurrent. Thus, selective operation is assured.

The two fuse curves can be examined by the sameprocedure at various current levels along the horizontal axis(for example, see Points F and G at the 2000 ampere faultlevel). It can be determined that the two fuses areselectively coordinated, since the 100 ampere time-delaydual-element fuse will open before the 400 ampere time-delay dual-element fuse can melt.

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Point BPoint D

Point F

Point A 1000A

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CURRENT IN AMPERES

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100A

400A

Minimum MeltTotal Clearing

Point G

Available FaultCurrentLevel1000A

400A

100A

Figure 3a.

Point CPoint E

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Figure 3



Circuit Breaker Curves

Figure 4 illustrates a typical thermal magnetic moldedcase circuit breaker curve with an overload region and aninstantaneous trip region (two instantaneous trip settingsare shown). Circuit breaker time-current characteristiccurves are read similar to fuse curves. The horizontal axisrepresents the current, and the vertical axis represents thetime at which the breaker interrupts the circuit.

When using molded case circuit breakers of this type,there are four basic curve considerations that must beunderstood. These are:

1.Overload Region2.Instantaneous Region3.Unlatching Time4.Interrupting Rating

1. Overload Region - The opening of a molded case circuitbreaker in the overload region (see Figure 4) is generallyaccomplished by a thermal element, while a magnetic coilis generally used on power breakers. Electronic sensingbreakers will utilize CT's. As can be seen, the overloadregion has a wide tolerance band, which means thebreaker should open within that area for a particularoverload current.2. Instantaneous Region - The instantaneous trip settingindicates the multiple of the full load rating at which thecircuit breaker will open as quickly as possible. Theinstantaneous region is represented in Figure 4 and isshown to be adjustable from 5x to 10x the breaker rating.When the breaker coil senses an overcurrent in theinstantaneous region, it releases the latch which holds thecontacts closed.

In Figure 4, the unlatching time is represented by thecurve labeled "average unlatching time for instantaneoustripping". After unlatching, the overcurrent is not halted untilthe breaker contacts are mechanically separated and thearc is extinguished. Consequently, the final overcurrenttermination can vary over a wide range of time, as isindicated by the wide band between the unlatching timecurve and the maximum interrupting time curve in Figure 4.

The instantaneous trip setting for larger molded caseand power breakers can usually be adjusted by an externaldial. Figure 4 shows two instantaneous trip settings for a400 amp breaker. The instantaneous trip region, drawn withthe solid line, represents an I.T. = 5x, or five times 400amperes = 2000 amperes. At this setting, the circuitbreaker wil l tr ip instantaneously on currents ofapproximately 2000 amperes or more. The ± 25% bandrepresents the area in which it is uncertain whether theoverload trip or the instantaneous trip will operate to clearthe overcurrent.

5

The dashed portion of Figure 4 represents the same400 ampere breaker with an I.T. = 10x, or 10 times 400amperes = 4000 amperes. At this setting the overload tripwill operate up to approximately 4000 amperes (±10%).Overcurrents greater than 4000 amperes (±10%) would becleared by the instantaneous trip.3. Unlatching Times - As explained above, the unlatchingtime indicates the point at which the breaker senses anovercurrent in the instantaneous region and releases thelatch holding the contacts. However, the fault currentcontinues to flow through the breaker and the circuit to thepoint of fault until the contacts can physically separate andextinguish the arc. Once the unlatching mechanism hassensed an overcurrent and unlatched, the circuit breakerwill open. The final interruption of the current representedon the breaker curve in the instantaneous region occursafter unlatching, but within the maximum interruption time.

The relatively long delay between unlatching and theactual interruption of the overcurrent in the instantaneousregion is the primary reason that molded case breakersare very difficult to coordinate. This is an inherent problemsince the breaking of current is accomplished bymechanical means.4. Interrupting Rating - The interrupting rating of a circuitbreaker is a critical factor concerning protection andsafety. The interrupting rating of a circuit breaker is themaximum fault current the breaker has been tested tointerrupt in accordance with testing laboratory standards.Fault currents in excess of the interrupting rating can resultin destruction of the breaker and equipment and possibleinjury to personnel. In other words, when the fault levelexceeds the circuit breaker interrupting rating, the circuitbreaker is no longer a protective device.

Looking at Figure 10, the interrupting ratings at 480volts are 14,000 amperes for the 90 ampere breaker and30,000 amperes for the 400 ampere breaker. Theinterrupting ratings on circuit breakers vary according tobreaker type and voltage level.

When drawing circuit breaker time-current curves,determine the proper interrupting rating from themanufacturer's literature and represent this interruptingrating on the drawing by a vertical line at the right end ofthe curve.



CURRENT IN AMPERES

100

200

300

400

600

800

1000

2000

3000

4000

6000

8000

10,0

00

20,0

00

30,0

00

40,0

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100,

000

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8001000

.008

.006

.004

.003

.002

.001

InterruptingRatingat 480 Volt

Instantanous Region

MinimumUnlatchingTime

Overload Region MaximumInterrrupting Time

400 Ampere Circuit Breaker

AdjustableInstantaneous TripSet at 5 TimesI.T. = 5X(± 25% Band)

Adjustable MagneticInstantaneous TripSet at 10 TimesI.T. = 10X(± 10% Band)

MaximumInterruptingTime

Average UnlatchingTimes for Instantaneous Tripping

Figure 4. Typical Circuit Breaker Time-Current Characteristic Curve

6

Average Unlatching TimesBreaker Tripping Magnetically

Current in Time in RMS Amps Seconds5,000 .004510,000 .002915,000 .002420,000 .002025,000 .0017

Interrupting Rating

RMS Sym. Amps240V 42,000480V 30,000600V 22,000


5. Short Time Delay And Instantaneous Override - Circuitbreaker short-time-delay (STD) mechanisms allow anintentional delay to be installed on Low Voltage PowerCircuit Breakers (Figure 5). Short-time-delays allow the faultcurrent to flow for several cycles, which subjects theelectrical equipment being protected to unnecessarily highmechanical and thermal stress. Most equipment ratings,such as short-circuit ratings for bus duct and switchboardbus, do not apply when short-time-delay settings areemployed. The use of short-time-delay settings on circuitbreakers requires the system equipment to be reinforced towithstand the available fault current for the duration of theshort-time-delay. Ignoring equipment ratings in relation tothe protective device opening time and let-thrucharacteristics can be disastrous.
An Insulated Case Circuit Breaker (ICCB) may also beequipped with short-time-delay. However, ICCB's will havea built-in override mechanism (Figure 6). This is called theinstantaneous override function, and will override the STDfor medium to high level faults. This override may "kick in"for faults as low as 12x the breaker's ampere rating. Thiscan result in non-selective tripping of the breaker and loadside breakers where overlaps occur. This can be seen inthe example given in Figure 7. As the overlap suggests, forany fault condition greater than 21,000 amperes, bothdevices will open, causing a blackout.

Note: Choosing overcurrent protective devices strictly onthe basis of voltage, current, and interrupting rating will notassure component protection from short-circuit currents.The interrupting rating of a protective device pertains onlyto that device and has absolutely no bearing on its ability toprotect connected downstream components. Highinterrupting rated electro-mechanical overcurrent protectivedevices, such as circuit breakers, especially those that arenot current-limiting, may not be capable of protecting wire,cable or other components within the higher short-circuitranges. Quite often, the component is completelydestroyed under short-circuit conditions while theprotective device is opening the faulted circuit.

7

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CURRENT IN AMPERES

LVPCB

STD = 21 Cycles

600

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Figure 5


T
IME

IN S

EC

ON

DS

CURRENT IN AMPERES

ICCB

InstantaneousOverride= 12X

600

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Figure 6

8



CURRENT IN AMPERES

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2000A ICCB100A CB

2000A

100A

BLACKOUT!

80

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1

.1

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Figure 7

9


Current Limiting Fuses
Medium to High Level Fault Currents
Figure 8 shows that the available short-circuit currentwill reach a peak value of Ip during the first half cycleunless a protective device limits the peak fault current to avalue less than Ip. A current-limiting fuse will reduce theavailable peak current to less than Ip, namely I'p, and willclear the fault in approximately one-half cycle or less. Notethat tc is the total clearing time of the fuse, tm the meltingtime and ta the arcing time of the fuse. Where high valuesof fault current are available, the sub-cycle region becomesthe most critical region for selective operation of current-limiting fuses.

The area under the current curves indicates the energylet-thru. If no protective device were present, or ifmechanical type overcurrent devices with opening times ofone-half cycle or longer were present, the full availableshort-circuit energy would be delivered to the system. Theamount of energy delivered is directly proportionate to thesquare of the current. So we can see how important it is tohave fuses which can limit the current being delivered tothe system to a value less than the available current. Theamount of energy being produced in the circuit while thefuse is clearing is called the total clearing energy and isequal to the melting energy plus the arcing energy.

Selectivity between two fuses operating under short-circuit conditions exists when the total clearing energy ofthe load side fuse is less than the melting energy of the lineside fuse (See Figure 9).

10

Ip

I'p

AvailableShort-Circuit Current

Limited Current ResultsWhen Fuse Clears

tm taCu

rren

tTime

Fault is Initiated Here

tc

Figure 8An engineering tool has been developed to aid in the

proper selection of fuses for selective coordination. ThisSelectivity Ratio Guide (SRG) is shown below.

Selectivity Ratio Guide (Line-Side to Load-Side) for Blackout PreventioCircuit Load-Side Fuse

Current Rating 601-6000A 601-4000A 0-600A Type Time- Time- Dual-Element

Delay Delay Time-Delay Trade Name & LOW-PEAK LIMITRON LOW-PEAKClass (L) (L) (RK1) (J)**

Buss KRP-CSP KLU LPN-RKSP LPJSPSymbol LPS-RKSP

601 to Time- LOW-PEAK KRP-CSP 2:1 2.5:1 2:1 2:1 6000A Delay (L)601 to Time- LIMITRON KLU 2:1 2:1 2:1 2:14000A Delay (L)

LOW-PEAK LPN-RKSP – – 2:1 2:1 0 Dual (RK1) LPS-RKSPto Ele- (J) LPJSP** – – 2:1 2:1 600A ment FUSETRON FRN-R – – 1.5:1 1.5:1

(RK5) FRS-R601 to LIMITRON KTU 2:1 2.5:1 2:1 2:1 6000A (L)0 to Fast- LIMITRON KTN-R – – 3:1 3:1 600A Acting (RK1) KTS-R0 to T-TRON JJN – – 3:1 3:1 1200A (T) JJS0 to LIMITRON JKS – – 2:1 2:1 600A (J)0 to Time- SC SC – – 3:1 3:1 60A Delay (G)

Line

-Sid

e F

use

*
n
601-6000A 0-600A 0-1200A 0-600A 0-60AFast-Acting Fast-Acting Time-

DelayFUSETRON LIMITRON LIMITRON T-TRON LIMITRON SC(RK5) (L) (RK1) (T) (J) (G)

FRN-R KTU KTN-R JJN JKS SCFRS-R KTS-R JJS4:1 2:1 2:1 2:1 2:1 N/A

4:1 2:1 2:1 2:1 2:1 N/A

8:1 – 3:1 3:1 3:1 4:1

8:1 – 3:1 3:1 3:1 4:12:1 – 1.5:1 1.5:1 1.5:1 1.5:1

6:1 2:1 2:1 2:1 2:1 N/A

8:1 – 3:1 3:1 3:1 4:1

8:1 – 3:1 3:1 3:1 4:1

8:1 – 3:1 3:1 3:1 4:1

4:1 – 2:1 2:1 2:1 2:1

Note: At some values of fault current, specified ratios may be lowered to permit closer fGeneral Notes: Ratios given in this Table apply only to Buss fuses. When fuses are withiConsult Bussmann for latest LPJSP ratios.

As an example, refer to Figure 9 and the SRG for LowPeak fuses. The SRG suggests that the minimum ratiobetween line side and load side fuse should be at least 2:1.The one-line illustrated in Figure 9 shows Low Peak fusesKRP-C1000SP feeding a LPS-RK200SP. The ratio of

*

**

use sizing. Plot fuse curves or consult with Bussmann.n the same case size, consult Bussmann.

ampere ratings is 5:1 (1000:200) which indicatescoordination between these fuses. Continuing further intothe system the LPS-RK-200SP feeds a LPJ60SP. This ratioof ampere ratings is 3.33:1 (200:60), which also indicates aselectively coordinated system.


Current Limiting Fuses

AvailableShort-Circuit Current

tm

tc

480/277Volts

LOW-PEAK®

Time-Delay FuseKRP-C-1000SP

Line SideLoad Side

KRP-C-1000SP Amp FuseLet-Thru Energy*

tmtc

LPS-RK-200SP Amp FuseLet-Thru Energy*

LOW-PEAK®

LPS-RK-200SPDual-Element Fuse

Load Side

Line Side

tc

LPJ-60SP Amp FuseLet-Thru Energy*

LOW-PEAK®

LPJ-60SPDual-Element Fuse

Fault

Figure 9Requirements for selectivity—Total clearing energy of load side fuse is less than melting energy of line side fuse.

*Area under the curves indicates let-thru energy.

11


Circuit Breakers
Medium to High Level Fault Currents
Figure 10 illustrates a 400 ampere circuit breakerahead of a 90 ampere breaker. Any fault above 1500amperes on the load side of the 90 ampere breaker willopen both breakers. The 90 ampere breaker will generallyunlatch before the 400 ampere breaker. However, beforethe 90 ampere breaker can separate its contacts and clearthe fault current, the 400 ampere breaker has unlatchedand also will open.

Assume a 4000 ampere short circuit exists on the loadside of the 90 ampere circuit breaker. The sequence ofevents would be as follows:

1. The 90 ampere breaker will unlatch (Point A) andfree the breaker mechanism to start the actual openingprocess.

2. The 400 ampere breaker will unlatch (Point B) and it,too, would begin the opening process. Once a breakerunlatches, it will open. At the unlatching point, the processis irreversible.

3. At Point C, the 90 ampere breaker wil l havecompletely interrupted the fault current.

4. At Point D, the 400 ampere breaker also will havecompletely opened the circuit.

Consequently, this is a non-selective system, causinga complete blackout to the load protected by the 400ampere breaker.

As printed by one circuit breaker manufacturer, "Oneshould not overlook the fact that when a high fault currentoccurs on a circuit having several circuit breakers in series,the instantaneous trip on all breakers may operate.Therefore, in cases where several breakers are in series,the larger upstream breaker may start to unlatch before thesmaller downstream breaker has cleared the fault. Thismeans that for faults in this range, a main breaker mayopen when it would be desirable for only the feeder breakerto open."

12


Circuit Breakers

13

80

TIM

E IN

SE

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30

••

•

•

CURRENT IN AMPERES1,500A

A

C

D

B

30,000AI.R.

14,000AI.R.

90AmpCircuit Breaker

400Amp Circuit BreakerI.T. = 5X

400A

90A

4000A

4,000A

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.001

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Figure 10

Selective Coordination Study –

Recommended Procedures
The following steps are recommended when
conducting a selective coordination study.1. One-Line DiagramObtain the electrical system one-line diagram that identifiesimportant system components, as given below.a. TransformersObtain the following data for protection and coordination infor-mation of transformers:

- KVA rating- Inrush points- Primary and secondary connections- Impedance- Damage curves- Primary and secondary voltages- Liquid or dry type

b. Conductors - Check phase, neutral, and equipmentgrounding. The one-line diagram should include infor-mation such as:

- Conductor size- Number of conductors per phase- Material (copper or aluminum)- Insulation- Conduit (magnetic or non-magnetic)

From this information, short circuit withstand curves can bedeveloped. This provides information on how overcurrentdevices will protect conductors from overload and shortcircuit damage.c. MotorsThe system one-line diagram should include motorinformation such as:

- Full load currents- Horsepower- Voltage- Type of starting characteristic

(across the line, etc.)- Type of overload relay

(Class 10, 20, 30)Overload protection of the motor and motor circuit can bedetermined from this data.d. Fuse CharacteristicsFuse Types/Classes should be identified on the one-linediagram.e. Circuit Breaker CharacteristicsCircuit Breaker Types should be identified on the one-linediagram.f. Relay CharacteristicsRelay Types should be identified on the one-line diagram.

14

2. Short Circuit StudyPerform a short circuit analysis, calculating maximumavailable short circuit currents at critical points in thedistribution system (such as transformers, main switchgear,panelboards, motor control centers, load centers, and largemotors and generators.) (Reference: Bussmann Bulletin,Engineering Dependable Protection - EDPI.)

3. Helpful Hintsa. Determine the Ampere Scale Selection. It is mostconvenient to place the time current curves in the center ofthe log-log paper. This is accomplished by multiplying ordividing the ampere scale by a factor of 10.b. Determine the Reference (Base) Voltage. The bestreference voltage is the voltage level at which most of thedevices being studied fall. (On most low voltage industrialand commercial studies, the reference voltage will be 208,240, or 480 volts). Devices at other voltage levels will beshifted by a multiplier based on the transformer turn ratio.The best reference voltage will require the least amount ofmanipulation. Modern computer programs will automat-ically make these adjustments when the voltage levels ofdevices are identified by the input data.c. Commencing the Analysis. The starting point can bedetermined by the designer. Typically, studies begin withthe main circuit devices and work down through thefeeders and branches. (Right to left on your log-log paper.)d. Multiple Branches. If many branches are taken off onefeeder, and the branch loads are similar, the largest ratedbranch circuit should be checked for coordination withupstream devices. If the largest branch will coordinate, andthe branch devices are similar, they generally wil lcoordinate as well. (The designer may wish to verify otherareas of protection on those branches, conductors, etc.)e. Don't Overcrowd the Study. Many computer generatedstudies will allow a maximum of ten device characteristicsper page.f. One-Line Diagram. A one-line diagram of the studyshould be drawn for future reference.

Examples of Selective Coordination Studies
The following pages will analyze in detail the system
shown in Figure 11. It is understood that a short circuitstudy has been completed, and all devices have adequateinterrupting ratings. A Selective Coordination Analysis is thenext step.

15

This simple radial system will involve three separatetime current curve studies, applicable to the three feeder/branches shown.

M

JCN80EIFLA=42A

5.75% Z1000KVA

∆-Y480/277V

LOW-PEAK®

KRP-C-1600SPFault X1 20,000A RMS Sym

Main Switchboard

1

LOW PEAK®

LPS-RK-200SPLOW-PEAK®

LPS-RK-400SPLOW-PEAK®

LPS-RK-225SP

200A Feeder 400A Feeder 150KVA∆-Y

208/120V2% Z

LOW-PEAK®

LPN-RK-500SPLOW-PEAK®

LPS-RK-100SP

20A Branch

20A CB

20A CB

LP1

60HP 3Ø77A FLA

1600A Main Bus

PDP

13.8KV

Overcurrent Relay

#6 XLP

#3/0 THW

100A Motor Branch

#1 THW

250 kcmil2/Ø THW

#12 THW

Figure 11

Example –

Time Current Curve #1 (TCC1)

Notes:1. TCC1 includes the primary fuse, secondary main fuse,200 ampere feeder fuse, and 20 ampere branch circuitbreaker from LP1.2. Analysis will begin at the main devices and proceeddown through the system.3. Reference (base) voltage will be 480 volts, arbitrarilychosen since most of the devices are at this level.4. Selective coordination between the feeder and branchcircuit is not attainable for faults above 2500 amperes thatoccur on the 20 amp branch circuit, from LP1. Notice theoverlap of the 200 ampere fuse and 20 ampere circuitbreaker.5. The required minimum ratio of 2:1 is easily met betweenthe KRP-C-1600SP and the LPS-RK-200SP.

16

Device ID Description Comments

1 1000KVA XFMR 12 x FLAInrush Point @ .1 Seconds

2 1000KVA XFMR 5.75%Z, liquidDamage Curves filled

(Footnote 1)(Footnote 2)

3 JCN 80E E-Rated Fuse

4 #6 Conductor Copper, XLPDamage Curve Insulation

5 Medium Voltage Needed for XFMRRelay Primary Overload

Protection

6 KRP-C-1600SP Class L Fuse

11 LPS-RK-200SP Class RK1 Fuse

12 3/0 Conductor Copper THWDamage Curve Insulation

13 20A CB Thermal MagneticCircuit Breaker

14 #12 Conductor Copper THWDamage Curve Insulation

Footnote 1: Transformer damage curves indicate when it will be damaged,thermally and/or mechanically, under overcurrent conditions.

Transformer impedance, as well as primary and secondaryconnections, and type, all will determine their damagecharacteristics.

Footnote 2: A ∆-Y transformer connection requires a 15% shift, to the right,of the L-L thermal damage curve. This is due to a L-Lsecondary fault condition, which will cause 1.0 p.u. to flowthrough one primary phase, and .866 p.u. through the twofaulted secondary phases. (These currents are p.u. of 3-phasefault current.)

Example –


TIM

E IN

SE

CO

ND

S

FLA

11

2

2

3

5

12

14

13

1

6

#12 DAMAGE

3/0 DAMAGE

20A MCCB

XFMRDAMAGE

4

JCN 80E

MV OLR

200A Feeder

KRP-C-1600SP

TXINRUSH

600

400

300

200

100

80

60

40

30

20

10

8

6

4

3

2

1

.8

.6

.4

.3

.2

.1

.08

.04

.06

.03

.02

.01

8001000

1 2 3 4 6 8 10 20 30 40 60 80 100

200

300

400

600

800

1000

2000

3000

4000

6000

8000

10,0

00

LPS-RK-200SP

#6 DAMAGE

CURRENT IN AMPERES X 10 @ 480V

1000KVA5.75%Z∆-Y480/277V

JCN80E

13.8KV

KRP-C-1600SP

LPS-RK-200SP

#6 XLP

#3/0 THW

OvercurrentRelay

20A CB#12 THW

20A CB

17

Example –


Notes:1. TCC2 includes the primary fuse, secondary main fuse,400 ampere feeder fuse, 100 ampere motor branch fuse,77 ampere motor and overload relaying.2. Analysis will begin at the main devices and proceeddown through the system.3. Reference (base) voltage will be 480 volts, arbitrarilychosen since most of the devices are at this level.

1

Device ID Description Comment

1 1000KVA XFMR 12 x FLAInrush Point @ .1 seconds






Protection



22 Motor Starting Curve Across the Line Start

23 Motor Overload Relay Class 10

24 Motor Stall Point Part of a Motor Damage Curve

25 #1 Conductor Copper THWDamage Curve Insulation




8

Example –



TIM

E IN

SE

CO

ND

S

MV OLR

2

2

3

5

1

6

4

21

2225

2324

FLA

MS

JCN80EMTR START

LPS-RK-100SP

MTR OLRXFMR DAMAGE

TXINRUSH

600

400

300

200

100

80

60

40

30

20

10

8

6

4

3

2

1

.8

.6

.4

.3

.2

.1

.08

.04

.06

.03

.02

.01

8001000

1 2 3 4 6 8 10 20 30 40 60 80 100

200

300

400

600

800

1000

2000

3000

4000

6000

8000

10,0

00

KRP-C-1600SP

#1 DAMAGE

#6 DAMAGE

KRP-C-1600SP

LPS-RK-400SP

LPS-RK-100SP

60HP

400A Feeder

M

#1 THW

#6 XLP

13.8KV

OvercurrentRelay

JCN 80E

1000KVA5.75%Z∆-Y480/277V

19

Example –


Notes:1. TCC3 includes the primary fuse, secondary main fuse,225 ampere feeder/transformer primary and secondaryfuses.2. Analysis will begin at the main devices and proceeddown through the system.3. Reference (base) voltage will be 480 volts, arbitrarilychosen since most of the devices are at this level.4. Relative to the 225 ampere feeder, coordination betweenprimary and secondary fuses is not attainable, noted byoverlap of curves.5. Overload and short circuit protection for the 150 KVAtransformer is afforded by the LPS-RK-225SP fuse.

20

Device ID Description Comment

1 1000KVA XFMR 12 x FLAInrush Point @ .1 seconds






Protection



32 150 KVA XFMR 12 x FLAInrush Point @.1 Seconds

33 150 KVA XFMR 2.00% Dry TypeDamage Curves (Footnote 3)

34 LPN-RK-500SP Class RK1 Fuse

35 2-250kcmil Conductors Copper THWDamage Curve Insulation




Footnote 3: Damage curves for a small KVA (<500KVA) transformer,illustrate thermal damage characteristics for ∆-Y connected.From right to left, these reflect damage characteristics, for aline-line fault, 3Ø fault, and L-G fault condition.

Example –



600

400

300

200

100

80

60

40

30

20

10

8

6

4

3

2

1

.8

.6

.4

.3

.2

.1

.08

.04

.06

.03

.02

.01

TIM

E IN

SE

CO

ND

S

8001000

4

5

2

FLA FLA

MV OLR

XFMR DAMAGE

JCN80E

XFMR DAMAGE 33

35

-225SP

-500SP

TXINRUSH

TXINRUSH

132

6

cmilHW

31

34

KRP-C1600SP

23

VAZ

120V

1 2 3 4 6 8 10 20 30 40 60 80 100

200

300

400

600

800

1000

2000

3000

4000

6000

8000

10,0

00

LPS-RK-225SP

LPN-RK-500SP

2-250 DAMAGE

#6 DAMAGE

LPS-RK

LPN-RK

KRP-C-1600SP

#6 XLP

JCN 80E

13.8KV

OvercurrentRelay

250 k2/Ø T

1000KVA5.75%Z∆-Y480/277V

150K2.0%∆-Y208/

21

Conclusions

Unnecessary power OUTAGES, such as theBLACKOUTS we so often experience, can be stopped byisolating a faulted circuit from the remainder of the systemthrough the proper selection of MODERN CURRENT-LIMITING FUSES.

Time-Delay type current-limiting fuses can be sizedclose to the load current and still hold motor-startingcurrents or other harmless transients, therebyELIMINATING nuisance OUTAGES.

The SELECTIVITY GUIDE on page 10 may be used foran easy check on fuse selectivity regardless of the short-circuit current levels involved. Where medium and highvoltage primary fuses are involved, the time-currentcharacteristic curves of the fuses in question should beplotted on standard NEMA log-log graph paper for properstudy.

The time saved by using the SELECTIVITY GUIDE willallow the electrical systems designer to pursue other areasfor improved systems design.

22

electrical distribution system engineering … coordination – reading time-current curves 6...

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