Selective Coordination - Cooper IndustriesSelective coordination can be a critical aspect for electrical systems. Quite often in the design or equipment selection phase, it is ignored

89©2005 Cooper Bussmann

2005 NEC® Code:• 100 Definitions• 240.12• 517.17(B) Healthcare• 517.26 Healthcare• 620.62 Elevators• 700.27 Emergency Systems• 701.18 Legally Required Standby Systems

What Is Selective Coordination?

Today, more than ever, one of the most important parts of any installation -whether it is an office building, an industrial plant, a theater, a high-rise apartment or a hospital - is the electrical distribution system. Nothing will stopall activity, paralyze production, inconvenience and disconcert people and possibly cause a panic more effectively than a major power failure.

Isolation of a faulted circuit from the remainder of the installation is mandatoryin today’s modern electrical systems. Power blackouts cannot be tolerated.

While it's very important, it is not enough to select protective devices basedsolely on their ability to carry the system load current and interrupt the maximum fault current at their respective levels. A properly engineered systemwill allow ONLY the protective device nearest the fault to open, leaving theremainder of the system undisturbed and preserving continuity of service.

We may then define selective coordination as “THE ACT OF ISOLATING AFAULTED CIRCUIT FROM THE REMAINDER OF THE ELECTRICALSYSTEM, THEREBY ELIMINATING UNNECESSARY POWER OUTAGES.THE FAULTED CIRCUIT IS ISOLATED BY THE SELECTIVE OPERATION OF ONLY THAT OVERCURRENT PROTECTIVE DEVICE CLOSEST TO THE OVERCURRENT CONDITION.”

In Article 100 of the 2005 NEC® this definition had been added:

Coordination (Selective). Localization of an overcurrent condition to restrictoutages to the circuit or equipment affected, accomplished by the choice ofovercurrent protective devices and their ratings or settings.

The two one line diagrams in the figure below demonstrate the concept ofselective coordination.

The system represented by the one line diagram to the left is a system withoutselective coordination. A fault on the load side of one overcurrent protectivedevice unnecessarily opens other upstream overcurrent protective device(s).The result is unnecessary power loss to loads that should not be affected by thefault. This is commonly known as a "cascading effect" or lack of coordination.

The system represented by the one line diagram to the right is a system withselective coordination. For the full range of overload or fault currents possible,only the nearest upstream overcurrent protective device opens. All the otherupstream overcurrent protective devices do not open. Therefore, only the circuit with the fault is removed and the remainder of the power system isunaffected. The other loads in the system continue uninterrupted.

Selective coordination can be a critical aspect for electrical systems. Quiteoften in the design or equipment selection phase, it is ignored or overlooked.And when it is evaluated many people misinterpret the information thinkingthat selective coordination has been achieved when in fact, it has not. The following sections explain how to evaluate systems as to whether the overcurrent protective devices provide selective coordination for the full range of overcurrents.

2005 NEC® Requirements

517.26 Application of Other Articles. The essential electrical system shallmeet the requirements of Article 700, except as amended by Article 517.

700.27 Coordination. Emergency system(s) overcurrent devices shall beselectively coordinated with all supply side overcurrent protective devices.

701.18 Coordination. Legally required standby system(s) overcurrentdevices shall be selectively coordinated with all supply side overcurrent protective devices.

Popular Methods of Performing A

Selective Coordination Study

Currently three 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 hand plot on log-log paper.

2. Computer programs that utilize a PC and allow the designer to select time-currentcurves published by manufacturers and transfer to a plotter or printer, followingproper selections. Simply plotting the curves does NOT prove selective coordination. The curves must be analyzed in relation to the various available faultcurrents at various points in the system. Also, where the plot may appear to showcoordination above 0.01 second, the characteristics of the overcurrent devicesbelow 0.01 second may not be coordinated.

3. For fuse systems, 600V or less, merely use the published selectivity ratios onpage 91 for Cooper Bussmann fuses. The ratios even apply for less than 0.01second.

This text will apply to all three methods.

Overloads and Low Level Fault Currents

In the sections ahead, information is presented as an aid to understandingtime-current characteristic curves of fuses and circuit breakers. There are simple methods that will be presented to analyze coordination for most systems 600V or less.

It should be noted that the study of time-current curves indicates performanceduring overload and low level fault conditions. The performance of overcurrentdevices that operate under medium to high level fault conditions are notreflected on standard curves. Other engineering methods may be necessary.

Coordination Analysis

The next several pages cover coordination from various perspectives. Themajor areas include:

• Fuse curves• Fuse selective coordination analysis• Circuit breaker curves• Circuit breaker coordination analysis• Elevator circuits• Emergency circuits• Ground fault protection–coordination

The section on ground fault protection is included because GFP systems cancause coordination issues. The ground fault protection section discusses GFPrequirements and then discusses coordination considerations.

Selective Coordination

Introduction and Requirements

WithoutWithout Selective Coordination WithWith Selective Coordination

Selective Coordination: Avoids BlackoutsSelective Coordination: Avoids BlackoutsSelective Coordination: Avoids BlackoutsSelective Coordination: Avoids Blackouts

OPENS

NOT AFFECTED

UNNECESSARY

POWER LOSS

OPENS

NOT AFFECTED

Fault Fault

90 ©2005 Cooper Bussmann

Fuse Curves

The figure to the right illustrates the time-current characteristic curves for twosizes of time-delay, dual-element fuses in series, as depicted in the one-linediagram. The horizontal axis of the graph represents the RMS symmetricalcurrent in amps. The vertical axis represents the time, in seconds.

For example: Assume an available fault current level of 1000A RMS symmetrical on the load side of the 100A fuse. To determine the time it wouldtake this fault current to open the two fuses, first find 1000A on the horizontalaxis (Point A), follow the dotted line vertically to the intersection of the totalclear curve of the 100A time-delay dual-element fuse (Point B) and the minimum melt curve of the 400A time-delay dual-element fuse (Point C). Then,horizontally from both intersection points, follow the dotted lines to Points Dand E. At 1.75 seconds, Point D represents the maximum time the 100A time-delay dual-element fuse will take to open the 1000A fault. At 90 seconds, PointE represents the minimum time at which the 400A time-delay dual-elementfuse could open this available fault current. Thus, coordination operation isassured at this current level.

The two fuse curves can be examined by the same procedure at various current levels along the horizontal axis (for example, see Points F and G atthe 2000A fault level). It can be determined that the two fuses are coordinated, for the overcurrents corresponding to the fuse curves on thegraph. The 100A time-delay dual-element fuse will open before the 400A time-delay dual-element fuse can melt. However, it is necessary to assess coordination for the full range of overloads and fault currents that are possible.

For analyzing fuse selective coordination for higher level fault currents see thenext page, “Medium to High Level Fault Currents–Fuse Coordination.” Whenusing the published Fuse Selectivity Ratios, drawing time current curves is notnecessary for any level of overcurrent.


Fuses

Point BPoint D

Point F

Point A 1000A

600

400

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200

100

80

60

40

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20

108

6

4

3

2

1.8

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.1.08

.04

.06

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.02

.01

CURRENT IN AMPERES

TIM

E IN

SE

CO

ND

S

100A

400A

Minimum MeltTotal Clearing

Point G

Available FaultCurrentLevel1000A

400A

100A

Figure 3a.

Point CPoint E

100

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400

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800

1000

2000

3000

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10,0

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00


Medium to High Level Fault Currents –

Fuse Coordination

The illustrations on the next page shows the principles of selective coordination when fuses are properly applied. The available short-circuit current will reach a peak value of Ip during the first half cycle unless a protective device limits the peak fault current to a value less than Ip. A current-limiting fuse will reduce the available peak current to less than Ip, namely I'p,and will clear the fault in approximately one-half cycle or less. Note that tc isthe total clearing time of the fuse, tm the melting time and ta the arcing time ofthe fuse. Where high values of fault current are available, the sub-cycle regionbecomes the most critical region for selective operation of current-limitingfuses.

The area under the current curves is indicative of the energy let-through. If noprotective device were present, or if mechanical type overcurrent devices withopening times of one-half cycle or longer were present, the full available shortcircuit energy would be delivered to the system. The amount of energy delivered is directly proportionate to the square of the current. So we can seehow important it is to have fuses which can limit the current being delivered tothe system to a value less than the available current. The amount of energybeing produced in the circuit while the fuse is clearing is called the total clearing energy and is equal to the melting energy plus the arcing energy.

Selectivity between two fuses operating under short circuit conditions existswhen the total clearing energy of the load side fuse is less than the meltingenergy of the line side fuse.

An engineering tool has been developed to aid in the proper selection ofCooper Bussmann fuses for selective coordination. This Selectivity RatioGuide (SRG) is shown below.


Fuses: Selectivity Ratio Guide

Circuit Load-Side FuseCurrent Rating 601-6000A 601-4000A 0-600A 601-6000A 0-600A 0-1200A 0-600A 0-60A

Type Time- Time- Dual-Element Fast- Fast- Fast- Fast- Time-Delay Delay Time-Delay Acting Acting Acting Acting Delay

Trade Name Low-Peak Limitron Low-Peak Fusetron Limitron Limitron T-Tron Limitron SCClass (L) (L) (RK1) (J) (RK5) (L) (RK1) (T) (J) (G)Cooper Bussmann KRP-C_SP KLU LPN-RK_SP LPJ-SP FRN-R KTU KTN-R JJN JKS SCSymbol LPS-RK_SP TCF† FRS-R KTS-R JJS

601 to Time- Low-Peak® KRP-C_SP 2:1 2.5:1 2:1 2:1 4:1 2:1 2:1 2:1 2:1 N/A6000A Delay (L)601 to Time- Limitron® KLU 2:1 2:1 2:1 2:1 4:1 2:1 2:1 2:1 2:1 N/A4000A Delay (L)

Low-Peak LPN-RK_SP – – 2:1 2:1 8:1 – 3:1 3:1 3:1 4:10 Dual- (RK1) LPS-RK_SPto Ele- (J) LPJ-SP – – 2:1 2:1 8:1 – 3:1 3:1 3:1 4:1600A ment Fusetron® FRN-R – – 1.5:1 1.5:1 2:1 – 1.5:1 1.5:1 1.5:1 1.5:1

(RK5) FRS-R601 to Limitron KTU 2:1 2.5:1 2:1 2:1 6:1 2:1 2:1 2:1 2:1 N/A6000A (L)0 to Fast- Limitron KTN-R – – 3:1 3:1 8:1 – 3:1 3:1 3:1 4:1600A Acting (RK1) KTS-R0 to T-Tron® JJN – – 3:1 3:1 8:1 – 3:1 3:1 3:1 4:11200A (T) JJS0 to Limitron JKS – – 2:1 2:1 8:1 – 3:1 3:1 3:1 4:1600A (J)0 to Time- SC SC – – 3:1 3:1 4:1 – 2:1 2:1 2:1 2:160A Delay (G)

*Note: At some values of fault current, specified ratios may be lowered to permit closer fuse sizing. Plot fuse curves or consult with Cooper Bussmann.General Notes: Ratios given in this Table apply only to Cooper Bussmann fuses. When fuses are within the same case size, consult Cooper Bussmann.† TCF (CUBEFuse) is 1 to 100A Class J performance; dimensions and construction are unique, finger-safe IP-20 design.

Line

-Sid

e Fu

se

*Selectivity Ratio Guide for Blackout Prevention (Line-Side to Load-Side)


For the next example, the Selectivity Ratio Guide suggests that the minimumratio between line side and load side fuse should be at least 2:1. The one-lineshows Low-Peak® fuses KRP-C-1000SP feeding a LPS-RK-200SP. The ratioof amp ratings is 5:1 (1000:200) which indicates coordination between these

fuses. Continuing further into the system the LPS-RK-200SP feeds a LPJ-60SP. This ratio of amp ratings is 3.33:1 (200:60), which also indicates a selectively coordinated system.


Fuses

Line Side

Load Side

480/277V

Low-Peak

Time-Delay Fuse

KRP-C-1000SP

Low-Peak

LPS-RK-200SP

Dual-Element Fuse

Line Side

Load Side

Low-Peak

LPJ-60SP

Dual-Element Fuse

LPS-RK-200SP

Amp Fuse

Let Through Energy

KRP-C-1000SP

Amp Fuse

Let Through Energy

Ip

Available

Short-Circuit Current

I1p

tm

ta

tc

tc

tc

tm

Fault

LPJ-60SP

Amp Fuse

Let Through Energy*

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

*Area under the curves is not actual energy but is indicative of let-through energy. Actual let-through energy is I2rt.


Example — Fuse Selective Coordination

Review the one-line diagram of the fusible system. All the fuses are Low-PeakFuses. The Selectivity Ratio Guide provides the minimum amp ratio that mustbe observed between a line-side fuse and a load-side fuse in order to achieveselective coordination between the two fuses. If the entire electrical systemmaintains at least these minimum fuse amp rating ratios throughout the system, the entire electrical system will be selectively coordinated for all levelsof overcurrent. Notice, time current curves do not even need to be plotted.

Check the LPJ-400SP fuse coordination with the KRP-C-1200SP fuse. Usethe same steps as in the previous paragraph. The amp rating ratio of the twofuses in the system is 1200:400, which yields an amp rating ratio of 3:1. TheSelectivity Ratio Guide shows that the amp rating ratio must be maintained at2:1 or more to achieve selective coordination. Since the fuses used have a 3:1ratio, and all that is needed is to maintain a 2:1 ratio, these two fuses areselectively coordinated. See the following diagram.


Fuses

One-Line For FuseSystem CoordinationAnalysis

Low-PeakKRP-C-1200SP Fuse

Low-PeakLPJ-400SP Fuses


Any Fault Level!

Any Fault Level !

Selective CoordinationOnly Faulted Circuit Cleared Low-Peak

KRP-C1200SP Fuses



Only These

Fuses Open

Opens

NotAffected

Check the LPJ 100SP fuse coordination with the LPJ-400SP fuse. The amprating ratio of these fuses is 400:100 which equals 4:1 ratio. Checking theSelectivity Ratio Guide, line-side LPJ (vertical left column) to load-side LPJ(horizontal), yields a ratio of 2:1. This indicates selective coordination for thesetwo sets of fuses. This means for any overcurrent on the load-side of the LPJ-100SP fuse, only the LPJ-100SP fuses open. The LPJ-400SP fusesremain in operation as well as the remainder of the system.

Summary — Fuse Coordination

Selective coordination is an important system criteria that is often overlookedor improperly analyzed. With modern current-limiting fuses, all that the designengineer, contractor, plan reviewer, electrical inspector or user needs to do isadhere to these ratios. It is not necessary to plot the time current curves. Justmaintain at least the minimum amp rating ratios provided in the CooperBussmann Selectivity Ratio Guide and the system will be selectively coordinated. This simple method is easy and quick.


Circuit Breaker Curves

The following curve illustrates a typical thermal magnetic molded case circuitbreaker curve with an overload region and an instantaneous trip region (twoinstantaneous trip settings are shown). Circuit breaker time-current characteristic curves are read similar to fuse curves. The horizontal axis represents the current, and the vertical axis represents the time at which thebreaker interrupts the circuit.

When using molded case circuit breakers of this type, there are four basiccurve considerations that must be understood. These are:

1. Overload Region

2. Instantaneous Region

3. Unlatching Time

4. Interrupting Rating

1. Overload Region: The opening of a molded case circuit breaker in theoverload region is generally accomplished by a thermal element, while a magnetic coil is generally used on power breakers. Electronic sensing breakers will utilize CTs. As can be seen, the overload region has a wide tolerance band, which means the breaker should open within that area for aparticular overload current.

2. Instantaneous Region: The instantaneous trip (I.T.) setting indicates themultiple of the full load rating at which the circuit breaker will open as quicklyas possible. The instantaneous region is represented in the following curveand is shown to be adjustable from 5x to 10x the breaker rating. When thebreaker coil senses an overcurrent in the instantaneous region, it releases thelatch which holds the contacts closed.

The unlatching time is represented by the curve labeled “average unlatchingtime for instantaneous tripping.” After unlatching, the overcurrent is not halteduntil the breaker contacts are mechanically separated and the arc is extinguished. Consequently, the final overcurrent termination can vary over awide range of time, as is indicated by the wide band between the unlatchingtime curve and the maximum interrupting time curve.

The instantaneous trip setting for larger molded case and power breakers canusually be adjusted by an external dial. Two instantaneous trip settings for a400A breaker are shown. The instantaneous trip region, drawn with the solidline, represents an I.T. = 5x, or five times 400A = 2000A. At this setting, thecircuit breaker will trip instantaneously on currents of approximately 2000A ormore. The ± 25% band represents the area in which it is uncertain whetherthe overload trip or the instantaneous trip will operate to clear the overcurrent.

The dashed portion represents the same 400A breaker with an I.T. = 10x, or10 times 400A = 4000A. At this setting the overload trip will operate up toapproximately 4000 amps (±10%). Overcurrents greater than 4000A (±10%)would be cleared by the instantaneous trip.

The I.T. of a circuit breaker is typically set at its lowest setting when shippedfrom the factory.

3. Unlatching Times: As explained above, the unlatching time indicates thepoint at which the breaker senses an overcurrent in the instantaneous regionand releases the latch holding the contacts. However, the fault current continues to flow through the breaker and the circuit to the point of fault untilthe contacts can physically separate and extinguish the arc. Once the unlatching mechanism has sensed an overcurrent and unlatched, the circuitbreaker will open. The final interruption of the current represented on thebreaker curve in the instantaneous region occurs after unlatching, but withinthe maximum interruption time.

The relatively long time between unlatching and the actual interruption of theovercurrent in the instantaneous region is the primary reason that moldedcase breakers are very difficult to coordinate. This is an inherent problemsince the breaking of current is accomplished by mechanical means.

4. Interrupting Rating: The interrupting rating of a circuit breaker is a criticalfactor concerning protection and safety. The interrupting rating of a circuitbreaker is the maximum fault current the breaker has been tested to interruptin accordance with testing laboratory standards. Fault currents in excess ofthe interrupting rating can result in destruction of the breaker and equipmentand possible injury to personnel. In other words, when the fault level exceedsthe circuit breaker interrupting rating, the circuit breaker is no longer a protective device.

In the example graph below, the interrupting rating at 480 volts is 30,000 amps.The interrupting ratings on circuit breakers vary according to breaker type andvoltage level. The marked interrupting on a circuit breaker is a three-pole ratingand NOT a single-pole rating (refer to pages 29 to 34 for more information).

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


Circuit Breakers

CURRENT IN AMPERES

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TIM

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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

Average Unlatching TimesBreaker Tripping Magnetically

Current in Time in RMS Amps Seconds5,000 0.004510,000 0.002915,000 0.002420,000 0.002025,000 0.0017

Interrupting Rating

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


Medium to High Level Fault Currents–Circuit Breakers

The following curve illustrates a 400A circuit breaker ahead of a 90A breaker.Any fault above 1500A on the load side of the 90A breaker will open bothbreakers. The 90A breaker will generally unlatch before the 400A breaker.However, before the 90A breaker can separate its contacts and clear the faultcurrent, the 400A breaker has unlatched and also will open.

Assume a 4000A short circuit exists on the load side of the 90A circuit breaker.The sequence of events would be as follows:

1. The 90A breaker will unlatch (Point A) and free the breaker mechanism to startthe actual opening process.

2. The 400A breaker will unlatch (Point B) and it, too, would begin the openingprocess. Once a breaker unlatches, it will open. At the unlatching point, theprocess is irreversible.

3. At Point C, the 90A breaker will have completely interrupted the fault current.

4. At Point D, the 400A breaker also will have completely opened the circuit.

Consequently, this is a non-selective system, causing a complete blackout tothe other loads protected by the 400A breaker.

As printed by one circuit breaker manufacturer, “One should not overlook thefact that when a high fault current occurs on a circuit having several circuitbreakers in series, the instantaneous trip on all breakers may operate.Therefore, in cases where several breakers are in series, the larger upstreambreaker may start to unlatch before the smaller downstream breaker hascleared the fault. This means that for faults in this range, a main breaker mayopen when it would be desirable for only the feeder breaker to open.” This istypically referred to in the industry as a "cascading effect."

Typically circuit breaker manufacturers do not publish the unlatching times orunlatching curves for their products.


Circuit Breakers

80

TIM

E IN

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••

•

•

CURRENT IN AMPERES

1,500A

A

C

D

B

30,000AI.R.

14,000AI.R.

90AmpCircuit Breaker

400Amp Circuit BreakerI.T. = 5X

400A

90A

4000A

4,000A

1000

600

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Simple Method To Check

Circuit Breaker Coordination

The previous discussion and curve illustrated two molded case circuit breakers(90A and 400A) with the unlatching characteristics for both shown on onecurve. This illustrated that two circuit breakers with instantaneous trips can notbe selectively coordinated for fault currents above a certain level. That level isthe fault current at which the upstream circuit breaker operates in its instantaneous trip region. When a fault above that level occurs, the lower circuit breaker (90A in this case) unlatches. However, before it clears the circuit, the upstream circuit breaker(s) (400A in this case) also unlatches.Once a circuit breaker unlatches, it will open, thereby disconnecting the circuitfrom the power source. In the case shown, the curves show the 400A circuitbreaker needlessly opens for a fault on the load side of the 90A circuit breaker. For any fault current greater than where the two circuit breakercurves intersect (in this case 1500A) the upstream circuit breaker does notcoordinate with the down stream circuit breaker.

However, in most cases, manufacturers do not publish unlatching times orunlatching curves for their circuit breakers. Therefore, even the most detailedcoordination study from a software program will NOT show whether or not acircuit breaker system is "selectively coordinated." So then, how can coordination of circuit breakers be assessed when all the circuit breakers usedin a system have instantaneous trip settings? There is a very simple methodthat does not even require drawing the circuit breaker time current curves.This method can be used to analyze software program coordination plots inmost cases. The following paragraphs present this method.

1. This simple method will be shown with the time current curves (but without theunlatching time curves included).

2. However, normally it is not even necessary to draw the time current curves inorder to evaluate coordination of circuit breakers having instantaneous trip units.So another section provides this simplified method without needing a time currentcurve.

Below is the one line diagram that will be used for learning these simple methods. Review the one-line diagram below that has three molded casecircuit breakers in series: from the main 1200A to the 100A branch circuit with

the 400A feeder in between. The other circuit breakers on the one-line diagram supply other circuits and loads. The fault current path from the powersource is depicted by the red arrows/lines superseded on the one-line diagram.

Circuit Breaker Coordination —

Simplified Method With Time Current Curve

With the simplified method, there is no need to have the unlatching times ordraw the unlatching curves. The following curve illustrates the time currentcharacteristics for the 1200A circuit breaker, the 400A circuit breaker and 100Acircuit breaker. The instantaneous trip settings for each of these three moldedcase circuit breakers are provided on the one-line diagram. The 100A circuitbreaker has a non-adjustable instantaneous trip setting and the curve is asdepicted. The 400A circuit breaker has an instantaneous trip set at 10 times itsamp rating (10X) which is 10 times 400A or 4000A. The 1200A circuit breakerhas an instantaneous trip set at six times its amp rating (6X) which is six times1200A rating or 7200A.. Remember from a previous section “2. InstantaneousRegion” that there is a tolerance associated with the instantaneous trip regionfor adjustable instantaneous trip settings; the curve shown for the 400A and1200A circuit breakers are drawn with the tolerances included.


Circuit Breakers

One-Line For CircuitBreaker SystemCoordinationAnalysis 1200A MCCB

It @ 6 X = 7,200A

400A MCCBIt @ 10 X = 4,000A

100A MCCBIt Non-Adjustable

Fault > 7,200A

When the curves of two circuit breakers cross over in their instantaneous tripregion, then the drawing indicates that the two circuit breakers do not coordinate for fault currents greater than this cross over point.

For instance, interpreting the coordination curves for the 100A circuit breakerand the 400A circuit breaker: their curves intersect in the instantaneous regionstarting at approximately 3600A. That means for a fault current greater than3600A on the load side of the 100A circuit breaker, the 400A circuit breakerwill open as well as the 100A circuit breaker. This demonstrates a lack of coordination and results in a "cascading effect" that will cause a partial blackout.

This curve also shows that for any fault greater than approximately 6500 ampson the load side of the 100A circuit breaker, the 400A and 1200A circuit breakers will open as well as the 100A circuit breaker. The reason: for a faultof greater than 6500A, all three of these circuit breakers are in their instantaneous trip region. Both the 400A and 1200A circuit breakers can


unlatch before the 100A circuit breaker clears the fault current. If this is notunderstood, re-read the previous section “Circuit Breaker Coordination -Medium to High Level Fault Currents.”

How does this affect the electrical system? Look at the one-line diagrambelow. For any fault current greater than approximately 6500A on the load sideof the 100A circuit breaker, the 1200A and 400A circuit breakers open as wellas the 100A circuit breaker. The yellow shading indicates that all three circuitbreakers opened - 100A branch circuit, 400A feeder and the 1200A main. Inaddition, all the loads fed by the other circuit breakers, denoted by the hashshading, are blacked out unnecessarily. This is due to the lack of coordinationbetween the 100A, 400A and 1200A circuit breakers.

The coordination analysis of the circuit breakers merely requires knowing whatthe numbers mean:

1. Any fault on the loadside of the 100A circuit breaker greater than 4000A will openthe 400A circuit breaker as well as the 100A circuit breaker. Reason: the 400Acircuit breaker with an instantaneous trip set at 10 times opens instantaneously for any fault current greater than 4000A.

2. Any fault on the loadside of the 100A circuit breaker greater than 7200A will openthe 1200A circuit breaker as well as the 100A and 400A circuit breakers. Reason:the 1200A circuit breaker with an instantaneous trip set at six times opens instantaneously for any fault current greater than 7200A.

3. Any fault on the loadside of the 400A circuit breaker greater than 7200A will openthe 1200A circuit breaker as well as the 400A circuit breaker. Reason: the 1200Acircuit breaker with an instantaneous trip set at six times opens instantaneouslyfor any fault current greater than 7200A.

So it becomes apparent, to evaluate coordination of circuit breakers withinstantaneous trips, the time current curves do not have to be drawn. All that isnecessary is to use simple multiplication of the instantaneous trip settingstimes the circuit breaker amp ratings,and evaluate this in conjunction with theavailable fault current.

Note: Circuit breakers that provide the use of a short time delay do not alwaysassure coordination. The reason is that molded case circuit breakers and insulated case circuit breakers that have a short-time delay will also have aninstantaneous trip setting that overrides the short-time delay at some faultlevel. Molded case circuit breakers with short time delay settings will have aninstantaneous trip that overrides the short time delay, typically at a maximumof 10 times the amp rating. These instantaneous overrides are necessary toprotect the circuit breakers for higher faults. The same simple procedure forevaluating circuit breakers with instantaneous trips can be used for this typecircuit breaker, also. Merely read the manufacturer’s literature to determinethis instantaneous trip override setting. However, be certain to establish if theinstantaneous trip pickup is given in symmetrical amps or asymmetrical amps.Some manufacturers specify the instantaneous override in asymmetrical ampswhich for practical evaluation purposes moves the instantaneous trip pickupsetting to the left (picks up at lower symmetrical fault currents than perceived).

See the next two pages for a brief discussion and curves of short-time delaysettings and instantaneous overrides.


Circuit Breakers

Circuit Breaker Coordination — Simplified Method

Without Time Current Curve

It is not even necessary to draw the curves to assess circuit breaker coordination. All that is necessary is to use some simple multiplication. Multiplythe instantaneous trip setting times the circuit breaker amp rating (the instantaneous trip setting is usually adjustable but can vary depending uponframe size and circuit breaker type - some have adjustable settings of four to10 times the amp rating - check specifications of specific circuit breaker). Theproduct of these two is the approximate point at which a circuit breaker entersits instantaneous trip region. (As explained in a previous section “2.Instantaneous Region”, there is a tolerance associated with where the instantaneous trip initially picks up. A vertical band depicts the instantaneoustrip pickup tolerance. For this easy method, we will ignore the tolerance band;therefore the results differ somewhat from the time current curve example justgiven.)

For instance, the 400A circuit breaker in this example has its instantaneoustrip (IT) set at 10 times its amp rating (10X). Therefore for fault currents above10 x 400A or 4000A, the 400A circuit breaker will unlatch in its instantaneous trip region, thereby opening.

The same could be determined for the 1200A circuit breaker, which has itsinstantaneous trip set at 6X its amp rating. Therefore, for fault currents above7200A, the 1200A circuit breaker unlatches in its instantaneous trip region,thereby opening.


Short-Time-Delay and Instantaneous Override

Some circuit breakers are equipped with short-time delay settings for the solepurpose of improving system coordination. Review the three curves on thispage and the next page.

Circuit breaker short-time-delay (STD) mechanisms allow an intentional delayto be installed on low voltage power circuit breakers. Short-time-delays allowthe fault current to flow for several cycles, which subjects the electrical equipment to unnecessarily high mechanical and thermal stress. Most equipment ratings, such as short circuit ratings for bus duct and switchboardbus, do not apply when short-time-delay settings are employed. The use ofshort-time-delay settings on circuit breakers requires the system equipment tobe reinforced to withstand the available fault current for the duration of theshort-time-delay. Ignoring equipment ratings in relation to the protective deviceopening time and let-through characteristics can be disastrous. Following is atime-current curve plot for two low voltage power circuit breaker with short-time delay and a 20A MCCB. The 100A CB has a STD set at 6 cyclesand the 800A CB has a STD set at 24 cycles. This type of separation of thecurves should allow for selective coordination, assuming that the breakershave been serviced and maintained per the manufacturer's requirements. Thisis an approach to achieve selective coordination that can diminish electricalsafety and component protection.

An insulated case circuit breaker (ICCB) may also be equipped with short-time-delay. However, ICCBs will have a built-in override mechanism. This iscalled the instantaneous override function, and will override the STD for medium to high level faults. This override may “kick in” for faults as low as 12times (12x) the breaker’s amp rating. (See curve in left column on next page.)This can result in non-selective tripping of the breaker and load side breakerswhere overlaps occur. This can be seen in the example. (See curve in rightcolumn on next page.) As the overlap suggests, for any fault condition greaterthan 21,000A, both devices will open, causing a blackout.

Zone-Selective Interlocking

Zone-Selective Interlocking (ZSI), or zone restraint, has been available sincethe early 1990s. ZSI is designed to limit thermal stress caused by short-circuits on a distribution system. ZSI will enhance the coordination of theupstream and downstream molded case circuit breakers for all values of available short-circuit current up to the instantaneous override of the upstreamcircuit breaker.

Caution: Use of Circuit Breaker Short-Time Delay Settings May Negate Protection and

Increase Arc-Flash Hazard

The longer an overcurrent is permitted to flow the greater the potential forcomponent damage. The primary function of an overcurrent protective deviceis to provide protection to circuit components and equipment. A short-timedelay (STD) setting on a circuit breaker can negate the function of protectingthe circuit components. A low voltage power circuit breaker with a short-timedelay and without instantaneous trip, permits a fault to flow for the length oftime of the STD setting, which might be 6, 12, 18, 24 or 30 cycles. This typically is done to achieve fault coordination with downstream circuit breakers. However, there is an adverse consequence associated with usingcircuit breaker short-time delay settings. If a fault occurs on the circuit protected by a short time delay setting, a tremendous amount of damagingfault energy can be released while the system waits for the circuit breakershort-time delay to time out.

In addition, circuit breakers with short-time delay settings can drasticallyincrease the arc-flash hazard for a worker. The longer an overcurrent protective device takes to open, the greater the flash hazard due to arcingfaults. Research has shown that the arc-flash hazard can increase with themagnitude of the current and the time duration the current is permitted to flow.System designers and users should understand that using circuit breakerswith short-time delay settings will greatly increase the arc-flash energy if anarcing fault incident occurs. If an incident occurs when a worker is at or nearthe arc-flash, the worker may be subjected to considerably more arc-flashenergy than if an instantaneous trip circuit breaker or better yet a current-limiting circuit breaker or current-limiting fuses were protecting the circuit. Therequirements for doing flash hazard analysis for worker safety are found inNFPA 70E “Electrical Safety Requirements for Employee Workplaces."

As an example, compare the photos resulting from investigative testing of arcing faults. Further information is provided in “Electrical Safety & Arc-FlashProtection” in this bulletin. A couple of comparison photos are shown on thenext page. These tests and others are detailed in “Staged Tests IncreaseAwareness of Arc-Fault Hazards in Electrical Equipment”, IEEE Petroleum andChemical Industry Conference Record, September, 1997, pp. 313-322. Thispaper can be found on the Cooper Bussmann web site at www.cooperbussmann.com/services/safetybasics. One finding of this IEEEpaper is that current-limiting overcurrent protective devices reduce damageand arc-fault energy (provided the fault current is within the current-limitingrange).


Circuit Breakers

Low Voltage Power Circuit Breaker with Short-Time-Delay


Current-limiting fuses or current-limiting circuit breakers can reduce the risksassociated with arc-flash hazards by limiting the magnitude of the fault currents (provided the fault current is within the current-limiting range) andreducing the time duration of the fault. Test 3 photos, to the right, are fromtests with the same test setup as shown in Test 4 above, except that KRP-C-601SP Low-Peak current-limiting fuses protect the circuit and clear the arcingfault in less than 1⁄2 cycle. The arc-flash was greatly reduced because thesefuses were in their current-limiting range. Also, the thermal and mechanicalstresses on the circuit components that conducted the fault current were

greatly reduced. Recent arc-flash research has shown that arc-flash energy islinearly proportional to the time duration of the fault (given the fault currentsare the same). Ignoring the fact that the KRP-C-601SP Low-Peak fuses inTest 3 limited the current let-through, the arc-flash energy released in Test 3was approximately 1⁄12 that of Test 4 just due to the faster operation of the KRP-C-601SP Low-Peak fuses (less than 1⁄2 cycle clearing in Test 3 vs. 6 cyclesclearing in Test 4). The actual arc-flash energy was reduced even more in Test3 because of the current-limiting effect of the KRP-C-601SP Low-Peak fuses.


Circuit Breakers

Test 4 shows sequential photos of a circuit protected by a circuit breaker with a short-time delay: interrupted at 6 cycles, so this incident lasted 1⁄10 of a second. Thearcing fault was initiated on a three phase, 480V system with 22,600A short circuit available.

Test 3 – Same test circuit as the prior photos, to the left, except the circuit is protected by KRP-C-601SP Cooper Bussmann Low-Peak® Current-Limiting Fuses. Inthis case these fuses limited the current and the fuses cleared in less than a 1⁄2 cycle.

Insulated Case Circuit Breaker–Instantaneous Override Instantaneous Override Opens at 21,000 Amps

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New Article 100 Definition

Coordination (Selective). Localization of an overcurrent condition to restrictoutages to the circuit or equipment affected, accomplished by the choice ofovercurrent protective devices and their ratings or settings.

NEC® 700.27 (Emergency Systems) and 701.18 (Legally Required StandbySystems) have added selective coordination of overcurrent protective devicesto help ensure life safety. Examples of circuits that require selective coordination would be emergency and egress lighting for the safe evacuationfrom a building and to assist in crowd and panic control. Also, in many cases,some elevators and ventilation equipment may be classified as an emergencyor legally required standby system by the “Authority Having Jurisdiction” (AHJ)or the locally adopted building code. Also, 527.26 requires the essential electrical systems of healthcare facilities have overcurrent protective devicesthat are selectively coordinated.

The New NEC® Requirements

517.26 Application of Other Articles. The essential electrical system shall meet the requirements of Article 700, except asamended by Article 517.700.27 Coordination. Emergency system(s) overcurrent devicesshall be selectively coordinated with all supply side overcurrent protective devices.701.18 Coordination. Legally required standby system(s) overcurrent devices shall be selectively coordinated with all supplyside overcurrent protective devices.

System Requirement

As stated in the NEC® requirements above, selective coordination is not justbetween the branch circuit and feeder. It is the entire circuit path from themain overcurrent protective device to the branch circuit device. See figurebelow.

In recent history, there has been an increasing demand for reliability of electrical power for buildings spanning the entire spectrum from single-familyresidences to places of assembly to industrial facilities. Many building systems have gone to the extent of installing emergency back-up or standbysystems to ensure maximum possible reliability and continuity of their electrical system. There are some types of buildings that require an emer-gency system or legally required standby system such as hotels, theatres,sports arenas, healthcare facilities and many more similar institutions. Theselife safety electrical systems are used to ensure electrical power in the case ofpower loss due to an outside source (utility loss) or an inside source (overcurrent condition). Very important loads must remain energized as longas possible during times of normal power outage and emergencies that maybe caused by equipment failures, nature or man. However, simply installing aback-up system is not the entire solution, the electrical system must bedesigned to maximize power to these important loads and minimize outageseven under physical catastrophes. It is very important to analyze the characteristics of the overcurrent protective devices (OCPDs) of these lifesafety systems to ensure they will perform as desired.

The NEC® has special requirements for these life safety electrical systems.These include several requirements that are based upon providing a systemwith reliable operation, reducing the probability of faults, and minimizing theeffects of an outage to the smallest portion of the system as possible. Beloware a few of these sections:

• 700.4 maintenance and testing requirements• 700.9(B) emergency circuits separated from normal supply circuits• 700.9(C) wiring specifically located to minimize system hazards• 700.16 failure of one component must not result in a condition where a

means of egress will be in total darkness.

The objective of these requirements is to ensure system uptime with the goalof safety of human life during emergencies or for essential health care functions.

The 2005 NEC® has adapted to this demand and has modified a few articlesto address the issue of selective coordination and to ensure by design andinstallation that the intended life safety electrical systems will stay in servicefor the longest possible period of time.

The installation of a back-up system could easily be negated by the use ofovercurrent protective devices that are not selectively coordinated. Selectivecoordination helps ensure by design and installation that some life safety electrical systems will stay in service for the longest possible period of time.There has been a requirement for selective coordination in the NEC® for manyyears. NEC® 620.62 has had a selective coordination requirement for multipleelevators in a building. This requirement is crucial for a few specific reasonssuch as not stranding passengers for normal operation and for emergencyegress as well as keeping elevators in use for emergency firefighting operations. The 2005 NEC® selective coordination requirements have beenexpanded and a definition Coordination (Selective) has been added to Article100:



Essential Electrical Systems In Healthcare Facilities, Emergency Systems and LegallyRequired Standby Systems




A given emergency branch circuit load has a circuit path up through to themain on the normal source and another circuit path up through to the main onthe emergency source. The requirement for selective coordination means thatall the overcurrent protective devices must be selectively coordinated witheach other, from each emergency branch circuit up through to the main of thenormal power supply and up through to the main on the emergency powersupply. See following example:

How Can Selective Coordination Be Achieved?

Selective coordination of the overcurrent protective devices for these life safety systems for both the normal power source path and the alternate powersource path can be achieved with either modern current-limiting fuse technology or modern circuit breaker technology. However, not all fuse typesor circuit breaker types can be used in any and all situations to ensure selective coordination. The ability to achieve selective coordination dependson the specific circuit parameters (short-circuit currents available at variouspoints in the system) and the fuse or circuit breaker opening characteristicsand possible setting options. Proper overcurrent protective device choice andselective coordination analysis is absolutely necessary. However, if the properchoices and analysis are not made so that selective coordination is ensured,the system may be built with deficiencies that may negatively impact life safety.

Other Important Considerations

1. Either a system has overcurrent protective devices that are selectively coordinated or the system does not. There is no middle ground. When termssuch as “enhancing” coordination, “optimizing” coordination, coordination "to thebest degree possible," or similar terms are used, it generally means the overcurrent protective devices are not selectively coordinated over the full rangeof short-circuit currents that are available in the application.

2. Selective coordination is for the full range of overcurrents available at allmains, feeders and branch circuits. It is not acceptable to arbitrarily considerselective coordination is applicable only above certain times such as 0.01, 0.1, or1.0 seconds. The coverage of selective coordination in the sections on SelectiveCoordination for Fuses and Circuit Breakers of this SPD publication explains howto assess fuses and circuit breakers for the full range of overcurrents irrespectiveof the times. Also, these preceding sections provide some easy, quick methods toassess selective coordination for both fuse and circuit breaker systems.

3. A coordination study should fully investigate, interpret, and present thelevel of coordination achieved for the full system and for the determinedamount of available short-circuit currents. This requires a person qualified forthis task. Too often, coordination studies are performed where the deliverable isjust time current curve plots with no interpretation or discussion on the level ofcoordination achieved (or sacrificed). Also, the studies should be performed in thedesign phase so that if the study results prompt a change, the change can bewithout major ramifications.

Fusible Systems

Cooper Bussmann makes it easy to design fusible systems that are selectivelycoordinated. For the modern current-limiting fuses, selectivity ratios are published (see Fuse Selectivity Ratio Guide section). It is not necessary to plottime current curves or do a short-circuit current analysis; all that is necessaryis to make sure the fuse types and ampere rating ratios for the mains, feedersand branch circuit meet or exceed the selectivity ratio. These selectivity ratiosare for all levels of overcurrent up to the interrupting ratings of the respectivefuses. The ratios are valid even for fuse opening times less than 0.01 seconds. This means with current-limiting fuses, it is not necessary to do anyanalysis for less than 0.01 seconds when the fuse types and ampere ratingratios adhere to the selectivity ratios. The industry standard for publishing fusetime current curves is to plot the times from 0.01 seconds and longer. The following example illustrates all that is necessary to achieve selective coordination with a fusible system.


Frequently Asked Questions: Fuse System

Q. How about when two different current-limiting fuses are plotted on a timecurrent curve and there is a space between the lower ampere fuse total clearcurve and the larger ampere fuse minimum melt curve – does this ensureselective coordination (if the selectivity ratio is not known)?

A. In this case, without knowing the selectivity ratio and having only time current curves, selective coordination can only be ensured for the level ofovercurrent up to the point where the larger fuse curve crosses 0.01 seconds.For times where the fuse opening time is less than 0.01 seconds, selectivityratios are necessary. It is not necessary to plot fuse time current curves; justadhere to the selectivity ratios.

Q. What about lighting branch circuits? There has not been a commerciallyavailable fusible lighting panelboards, so how can selective coordination beachieved for the 20A panelboard circuits?

A. In order to achieve a selectively coordinated fusible system, CooperBussmann has the Coordination Module™, which is a fusible branch circuitpanelboard. So now it is easy to achieve selective coordination from main tobranch circuit with an all fusible system, including branch circuit lighting panelboards, by adhering to the simple selectivity ratios. Information onCoordination Module on www.cooperbussmann.com data sheet 3115.



Load-Side Fuse

LP-CC

FNQ-R

KTK-R

LPJ_SP 2:1

LPN-RK_SP 2:1

LPS-RK_SP 2:1

FRN-R 2:1

FRS-R 2:1

Ratios only apply to Cooper Bussmann Fuses. When fuses are in the same case size, consultCooper Bussmann.

Selectivity Ratio Guide For Coordination Module

Line

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Circuit Breaker Systems

As stated earlier, if a designer wants to use circuit breakers, it may be possibleto achieve a selectively coordinated system. There are a host of circuit breaker alternatives in the form of circuit breaker types, optional features, andpossible settings. See Circuit Breaker Coordination section in this publicationfor more in-depth discussion on some of the options and tradeoffs. Thedesigner must understand the characteristics, setting options and override particulars for each circuit breaker considered and in some cases, factor in thespecific circuit parameters such as the available short-circuit currents at eachpoint in the system.

If circuit breakers are considered, typically the following become important:

1. A short-circuit current study with a coordination study (interpreted properly) is necessary to determine if selective coordination is achieved for each circuit andfor the entire circuit path. It is necessary to have the available short-circuit currents and circuit breakers’ characteristics and settings. Depending on the circumstances, the designer may have to use different type circuit breakers oroptions in order to achieve selective coordination.

2. Typically the designer will have to utilize circuit breakers with short-time delaysand possibly larger frame sizes. This may increase the cost and equipment size.

3. Tradeoffs in system protection: These decisions that may improve the level ofcoordination may negatively impact the level of component protection.

4. Arc flash hazard level: These decisions that may improve the level of coordinationmay negatively impact the arc flash hazard level.

5. In many cases, the circuit breaker solution may only be selectively coordinated forthe exact designed system (for specific circuit breaker settings and available shortcircuit currents of a specific system). Most systems get upgraded or change atsome point in time and this can significantly increase the available short-circuitcurrents. This may negate the selective coordination for some circuit breaker systems that may have originally been selectively coordinated.

6. Testing and maintenance: periodic exercising and testing are important for circuitbreakers to ensure they operate as intended. If circuit breakers are found not tooperate as specified, maintenance or replacement of these circuit breakers is necessary. For instance, if a feeder circuit breaker fails to operate or operatesslower than as specified, it may cause the main circuit breaker to open due to afeeder fault. The result would be a much larger portion of these critical systemloads being without power.

System with Mixture of Fuses and Circuit Breakers

Fuses and circuit breakers operate in totally different ways. Fuses are thermaldevices that have encased fusible elements that operate under overcurrentconditions by melting or vaporizing at some points of the element, arcing, andclearing the circuit. All circuit breakers have three operating functions to cleara circuit: (1) overcurrent sensing, (2) unlatching, and (3) parting the contacts,arcing, and clearing. There are numerous technologies used for circuit breakerovercurrent sensing.

For downstream fuse and upstream circuit breaker, it is not a simple matter todetermine if a fuse and circuit breaker will be selectively coordinated. Even ifthe plot of the time current curves for a downstream fuse and an upstream circuit breaker show that the curves do not cross, selective coordination maynot be possible.

If a fuse is upstream and a circuit breaker is downstream, at some point thefuse time current characteristic crosses the circuit breaker time current characteristic. For short-circuit currents at that point and higher, the upstreamfuse is not coordinated with the down stream circuit breaker.

New Requirement Compliance

Achieving overcurrent protective device selective coordination for a systemrequires the proper engineering, specification and installation of the requireddevices, and in addition, knowledgeable plan review and inspection to ensurecompliance. The designer, contractor, and plan review/inspector each havetheir role in compliance to selective coordination in order to ensure safety tohuman life.

Role of Designers

For these vital systems, the designer must select, specify, and document overcurrent protective devices that achieve selective coordination for the fullrange of possible overcurrents and for faults at all possible points in the system (faults can occur in the branch circuits, sub-feeders, and feeders). Thedesigner should provide the plan reviewer a stamped analysis verifying thatselective coordination is achieved as designed for the required electrical systems. Documentation of the basis should be included. Also, this informationis necessary so that the contractor quotes and installs the overcurrent protective devices as designed.

Role of Plan Review/Inspectors

The plan review process is a critical phase. If a non-compliant design does notget caught and corrected in the plan review, it could get red tagged in theinspection phase, which may require a very expensive gear change out.During the plan review process, the engineer must provide the stamped substantiation in the form of documentation that his design achieves selectivecoordination for these vital circuits. The plan reviewer should not have toprove or disprove that selective coordination is achieved. The engineer’s documentation should be clear enough to demonstrate that the design workincluded the proper selective coordination analysis and that the plans andspecifications clearly articulate the required details on type of overcurrent protective devices, ampere ratings, and settings (if circuit breakers). The engineer’s documentation should clearly state that the plans specify overcurrent protective devices that achieve selective coordination for thesevital systems.

During the inspection process, prior to energizing the system, the field inspection should include verifying that the overcurrent protective deviceshave been installed as specified. If circuit breakers are used, the settingsshould be verified as per plan.

Role of Contractors

Contractors must install the system as designed to ensure selective coordination for the system. If a fusible system, install the fuse types andampere ratings as called for in the design. If a circuit breaker system, install the circuit breaker types with the specified settings.

If the contractor opts to suggest a value engineering option for these vital systems, it is critical that the engineer evaluates and approves of any changesas well as the plan reviewer.




Elevator Circuits and Required

Shunt Trip Disconnect — A Simple Solution.

When sprinklers are installed in elevator hoistways, machine rooms, ormachinery spaces, ANSI/ASME A17.1 requires that the power be removed tothe affected elevator upon or prior to the activation of these sprinklers. This isan elevator code requirement that affects the electrical installation. The electrical installation allows this requirement to be implemented at the disconnecting means for the elevator in NEC® 620.51(B). This requirement ismost commonly accomplished through the use of a shunt trip disconnect andits own control power. To make this situation even more complicated, interfacewith the fire alarm system along with the monitoring of components requiredby NFPA 72 must be accomplished in order to activate the shunt trip actionwhen appropriate and as well as making sure that the system is functionalduring normal operation. This requires the use of interposing relays that mustbe supplied in an additional enclosure. Other requirements that have to be metinclude selective coordination for multiple elevators (620.62) and hydraulic elevators with battery lowering [620.91(C)].

There is a simple solution available for engineering consultants, contractors,and inspectors to help comply with all of these requirements in one enclosurecalled the Cooper Bussmann Power Module™.

Elevator Selective Coordination Requirement

In the 2005 NEC®, 620.62 states:

Where more than one driving machine disconnecting means is suppliedby a single feeder, the overcurrent protective devices in each disconnecting means shall be selectively coordinated with any othersupply side overcurrent protective devices.

A design engineer must specify and the contractor must install main, feeder,sub-feeder, and branch circuit protective devices that are selectively coordinated for all values of overloads and short circuits.

To better understand how to assess if the overcurrent protective devices in anelectrical system are selectively coordinated refer to the SelectiveCoordination Section of this booklet. Below is a brief coordination assessmentof an elevator system in a circuit breaker system (example 1) and in a fusesystem (Example 2).

Elevator Circuit


NEC®

• Selective Coordination• Hydraulic Elevators• Traction Elevators

NFPA 72• Fire Safety Interface

• Component Monitoring

ANSI/ASME A17.1• Shunt TripRequirement

Power Module™ Elevator DisconnectAll-in-One Solution for Three Disciplines

The Power Module contains a shunt trip fusible switch together with the components necessary to comply with the fire alarm system requirements andshunt trip control power all in one package. For engineering consultants thismeans a simplified specification. For contractors this means a simplified installation because all that has to be done is connecting the appropriatewires. For inspectors this becomes simplified because everything is in oneplace with the same wiring every time. The fusible portion of the switch utilizesLow-Peak LPJ-(amp)SP fuses that protect the elevator branch circuit from thedamaging effects of short-circuit currents as well as helping to provide an easymethod of selective coordination when supplied with an upstream Low-Peakfuse with at least a 2:1 amp rating ratio. More information about the CooperBussmann Power Module can be found at www.cooperbussmann.com.

Using the one-line diagram above, a coordination study must be done to seethat the system complies with the 620.62 selective coordination requirement ifEL-1, EL-2, and EL-3 are elevator motors.

Go to the Selective Coordination section for a more indepth discussion on howto analyze systems to determine if selective coordination can be achieved.


Example 1 Circuit Breaker System

In this example, molded case circuit breakers (MCCB) will be used for thebranch and feeder protective devices and an insulated case circuit breaker(ICCB) will be used for the main protective device.

Example 2 Fusible System

In our second example, LPJ-(amp)SP fuses will be used for the branch protection, LPS-RK-(amp)SP fuses will be used for the feeder protection, andKRP-C-(amp)SP fuses will be used for the main protection.

Elevator Circuit


1600A ICCB

200A MCCB

400A MCCB

BLACKOUT(TOTAL)

100A MCCB

BLACKOUT(PARTIAL)

CURRENT IN AMPERES

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LPS-RK-200SP

LPS-RK-400SP

KRP-C-1600SP

LPJ-100SP

Looking at the time current curves for the circuit breaker in the figure above,where any two circuit breaker curves overlap is a lack of selective coordination. The overlap indicates both devices open. If any fault currentgreater than 750A and less than 3100A occurs at EL-1, EL-2 or EL-3, the200A circuit breaker will open as well as the 100A branch circuit breaker - thisis not a selectively coordinated system and does not meet the requirements of620.62. This lack of selective coordination could result in stranding passen-gers in elevators or not having elevators available for fire fighters. Fault cur-rents above 3100A will open the 400A circuit breaker as well and faults aboveapproximately 16,000A will open the 1600A circuit breaker - which further illustrates the lack of coordination. For a better understanding of how toassess circuit breaker coordination, see the section on Circuit BreakerCoordination in this book. A system that is not in compliance may result inneedlessly stranding passengers and creating a serious safety hazard.

To verify selective coordination, go no further than the Fuse Selectivity RatioGuide in the Fuse Selective Coordination section in this book. The Low-Peakfuses just require a 2:1 amp rating ratio to assure selective coordination. Inthis example, there is a 4:1 ratio between the main fuse (1600A) and the firstlevel feeder fuse (400A) and a 2:1 ratio between the first level feeder fuse andthe second level feeder fuse (200A). As well, there is a 2:1 ratio between thesecond level feeder fuse and the branch circuit fuse (100A). Since a minimumof a 2:1 ratio is satisfied at all levels for this system, selective coordination isachieved and 620.62 is met.

As just demonstrated in the prior paragraph, the fuse time current curves donot have to be drawn to assess selective coordination. For illustrative purposes, the time current curves for this example are shown above.

Documents

Selective Coordination - Cooper IndustriesSelective coordination can be a critical aspect for electrical systems. Quite often in the design or equipment selection phase, it is ignored