600V and Below

8/8/2019 600V and Below

1/36

Guide to Power System Selective Coordination600V and Below

Document Number0100DB0603

2. Background2.1. What is Selective

Coordination?

1. Introduction With the inclusion o new language in the 2005 National ElectricalCode(NEC), the requirements or selective coordination o electricalpower systems are, at present, more stringent than ever beore.This paper describes the nature o selective coordination, the NECrequirements pertaining to selective coordination, and approaches orobtaining selective coordination in commonly-encountered scenariosor systems 600V and below.

The term selective coordination reers to the selection and setting oprotective devices in an electric power system in such a manner as tocause the smallest possible portion o the system to be de-energizeddue to an abnormal condition. The most commonly encounteredabnormal condition is an overcurrent condition, dened by the NECas any current in excess o the rated current o equipment, or theampacity o a conductor [1]. The NEC uses a more restricteddenition o selective coordination as ollows: Localization o anovercurrent condition to restrict outages to the circuit or equipmentaected, accomplished by the choice o overcurrent protectivedevices and their ratings or settings [1]. This is the denitionused herein.

The concept o selective coordination is best illustrated by example.In the example system o Fig. 1, all o the devices shown areovercurrent protective devices, in this case circuit breakers. Fivesystem locations, labeled A-E, have been identied. I selectivecoordination exists, an overcurrent condition at location E will onlycause the lighting panel circuit breaker CB B1 to trip. Similarly, anovercurrent ault at location D should only cause lighting panel circuitbreaker CB PM1 to trip. Table I shows the protective device thatshould operate or a ault in each labeled location in Fig. 1, assumingselective coordination exists.

8/8/2019 600V and Below

2/36

Guide to Power System Selective Coordination 600V and Below

2 2006 Schneider Electric. All rights reserved.


Fig. 1: Example System

Overcurrent conditions may be divided into two types. An overload isdened by the NEC as operation o equipment in excess o normal,ull-load rating, or o a conductor in excess o rated ampacity that,when it persists or a sucient length o time, would cause damage ordangerous overheating [1]. Similarly, a aultis dened as anunintentional connection o a power system conductor, resulting in anabnormally high fow o current. Faults typically produce higherovercurrents than do overloads, depending upon the ault impedance.A ault with no impedance in the unintentional connection is reerredto as a short circuit or bolted ault.

Faults may also be classied as to their geometry. A three-phase aultinvolves all three phases. A line-to-line aultinvolves only two phases.A short circuit involving a ground path is reerred to as a ground ault,and may be a three-phase-to-ground ault, two-line-to-ground ault,or single-line-to-ground ault(note: the typical usage o the termground-ault usually means a single-line to-ground ault).

UTILITY SERVICE

A

B

C

D

E

MAIN SWITCHBOARD

LIGHTING PANEL

"LP1"

CB M1

CB F1

CB PM1

CB B1

Table I: Protective Device Operation or System o Fig. 1

Fault location Device that should operate orselective coordination

A Utility protective device

B CB M1

C CB F1

D CB PM1

E CB B1

2.2. The Nature o Overcurrents

8/8/2019 600V and Below

3/36


2006 Schneider Electric. All rights reserved.


Statistically, ~95% o all system aults are single-line-to-ground aults.A very low percentage o aults are bolted aults. Thus, the requencyo occurrence o high-magnitude bolted aults is much lower than thato lower-magnitude aults, such as arcing ground aults. Thesestatistics should be kept in mind when considering the requirements

or selective coordination, or reasons that are outlined herein.

To urther visualize the system coordination, the system o Fig. 1 canbe divided into protective zones. A ault in a given protective zonecauses a given protective device to operate. The ideal primaryprotective zonesor the system o Fig. 1 are shown in Fig. 2. CB B1should be the only device to operate or a ault in its primaryprotective zone, CB PM1 should be the only device to operate or anovercurrent condition in its protective zone, etc. Note that the idealprimary protective zone or a given protective device includes the nextlevel o downstream protective devices, since a protective devicecannot be assumed to trip or an internal ault in the device itsel. In

other words, the ideal protective zone boundaries cannot be arbitrarilyestablished, but must take into account which overcurrent conditionseach protective device is able to sense and interrupt.

Note that the closer a protective zone is to the source o power, inthis case a utility service, the more o the system is de-energized oran overcurrent condition that zone. In act, in a radial system withonly one source o power an overcurrent condition within a protectivezone will cause all protective zones downstream rom that zone tobe aected due to the trip o the overcurrent protective device orthat zone.

2.. The Protective Zone Concept

UTILITY SERVICE

CB M1

CB F1

CB PM1

CB B1

CB M1 PRIMARY

PROTECTIVE

ZONE

CB F1 PRIMARY

PROTECTIVE

ZONE

CB PM1 PRIMARY PROTECTIVE ZONE

CB B1 PRIMARY PROTECTIVE ZONE

Fig. 2: Ideal Primary Protective Zones or theExample System o Fig. 1

8/8/2019 600V and Below

4/36




Note also that, or an overcurrent condition in CB B1s primaryprotective zone, i CB B1 ails to operate CB PM1 should operateas a backup. Thus, CB B1s protective zone may be said to be in thebackup protective zoneor CB PM1. This same relationship ollowsto upstream devices as well. Each backup protective zone is limited

by the lowest level overcurrent condition the protective device cansense. This limit is reerred to as the reacho the device and isdependent upon the size and characteristics o the device, itssettings (i applicable), and the available ault currents at variouspoints downstream rom the device. In practice, however, the backupprotective zones should at least overlap the primary protective zoneor the next downstream device, to allow each portion o thesystem to have backup protection should its primary protectivedevice ail to operate.

Typical backup protective zones or the system o Fig. 1 are shown inFig. 3. (based upon the time-current characteristics and available aultcurrents or this system). Note that although the backup protective

zones overlap in a way determined by the reach o the protectivedevices, the next upstream deviceshould operate upon ailure o theprimary protective device. For example, or a ault on the branchcircuit supplied by CB B1, CB PM1 should operate i CB B1 ails tooperate. For a ault on this circuit close to CB B1, the backupprotective zones or CB M1, and CB F1 overlap, as dictated by thereach o these circuit breakers. However, i CB PM1, CB F1, and CBM1 are selectively coordinated, even in the region where the backupprotective zones overlap CB PM1 will trip should CB B1 ail tooperate. I CB PM1 ails to operate, CB F1 will operate so long as theault is within its backup protective zone. Should CB F1 ail to operate,then CB M1 will operate, again so long as the ault is within its backupprotective zone. In this case a ault on the CB B1 branch circuit, evenclose to CB B1, is beyond the reach o the utility protective device, soCB M1 is the last line o deense to clear a ault on this circuit closeto CB B1. Only CB PM1, however, provides backup protection or theentire circuit, since its backup protective zone is the only one whichextends around the entire circuit.

8/8/2019 600V and Below

5/36




Fig. : Backup Protective Zones or the ExampleSystem o Fig. 1

A more specic denition o selective coordination between twodevices in series may now be stated: Selective coordination existsbetween two overcurrent protective devices in series if and only ifeach device is the only device which operates or aults within its idealprimary protective zone, where the ideal primary protective zone

begins at the load terminals o that device and ends at the loadterminals o the next level o downstream devices. Operation o aprotective device in its backup zone o protection may indicate a lacko coordination or may indicate that a protective device has ailed.

Using this denition, the term system selective coordinationmaybe applied to an entire electric power system as ollows: Systemselective coordinationor an electric power system exists if andonly ifany outage due to an overcurrent condition is restricted to thesmallest possible number o loads, as dened by the overcurrentdevice placement and the ideal protective zone or each device.While not an ocial industry term, system selective coordinationisan important concept as it is the ideal condition or protective device

coordination in the context o the over-all system.

UTILITY SERVICE

UTILITY PROTECTION

BACKUP PROTECTIVE ZONECB M1

CB F1

CB PM1

CB B1

CB M1 BACKUP

PROTECTIVE

ZONE

CB F1 BACKUP PROTECTIVE ZONE

CB PM1 BACKUP

PROTECTIVE ZONECB PM1 BACKUP

PROTECTIVE ZONE

8/8/2019 600V and Below

6/36




CB M1

CB F1

10100 1

K10K

100K

10100

1K

10K

100K

0.01

0.10

1

10

100

1000

0.01

0.10

1

10

100

1000

CURRENT IN AMPERES

TIMEIN

SECONDS

30kA Available Fault

In most cases selective coordination is achieved via the timingcharacteristics o the devices to be coordinated. For example, each othe circuit breakers or the system o Fig. 1 has its own time-currentcharacteristic; by coordinating these, selective coordination may beachieved. This is usually accomplished by comparing the device

time-current characteristics graphically. An example is shown inFig. 4, which illustrates the time-current coordination between circuitbreakers CB M1 and CB F1 rom Fig. 1. Note that a log-log scale isused to display the device time-current characteristics. The curves orboth devices end at the available ault current or their respectivebusses, in this case 30kA. Because there is no overlap in thetime-current characteristics up to 30kA, selective coordination existsbetween these two devices. For example, or the 30kA available ault,CB F1 will operate in 0.01 0.02s and CB-M1 will operate in0.22 0.31s. CB F1 will thereore operate more quickly thanCB M1 or a ault (up to the 30kA available ault current) sensed byboth devices.

Using this graphical method, it may be stated that to achieve selectivecoordination between two devices, they must have no time-currentcurve overlap up to the available ault current where their idealprimary protective zones meet. This concept is illustrated in Fig. 5.The act that CB M1 and CB F1 will both sense an overcurrentcondition at the primary protective zone boundary, along with thetime-current coordination between the two, establishes the actualprimary protective zone boundary at the location shown, which in thiscase coincides with the ideal boundary location.

2.. How is selective coordinationachieved?

Fig. : Typical Time-Current Coordination Plot

8/8/2019 600V and Below

7/36




The act that time is used to coordinate the operation o protectivedevices in series has an important, and unortunate, drawback: Thecloser to the source o power, the slower the protective device mustbe to coordinate with downstream devices. This means that or aultsclose to the source o power, ault clearing will be slower than it could

be i coordination were not a consideration. This has importantimplications or equipment damage and arc-fash hazards, both owhich must be taken in to consideration in an over-all system design.It also has important implications or the backup protection describedabove, since ault clearing will be slower i the closest upstreamdevice ails to operate or clear the ault. Techniques to mitigate theseproblems, such as Zone Selective Interlocking (ZSI), are available.

Fig. : Primary Protective Zones or the System o Fig. 1,Showing the Available Fault Current Reerenced in Fig.

UTILITY SERVICE

30kA

Available

Fault

OvercurrentCB M1

CB F1

CB PM1

CB B1

CB M1 PRIMARY

PROTECTIVE

ZONE

CB F1 PRIMARY

PROTECTIVE

ZONE


CB B1 PRIMARY PROTECTIVE ZONE

To illustrate how miscoordination o devices aects the protectivezones, consider the coordination between CB F1 and CB PM1 perFig. 6. CB F1 and CB PM1 have been deliberately selected so as tomiscoordinate or purposes o illustration. Note that coordinationbetween CB F1 and CB PM1 exists up to 21.6kA. There is, however,25kA available ault current at the line terminals o CB PM1 (because

protective devices generally do not present signicant impedance inthe circuit, the available ault current at either the line or loadterminals o a protective device is the same. The line side o thecircuit breaker is reerenced by convention, although the idealprotective zone boundaries meet at the load terminals). This has theeect o causing the primary protective zones or CB F1 and CB PM1to overlap to the point in the system where the available ault currentis 21.6kA. This is illustrated in Fig. 7. Similarly, the primary protectivezones or CB PM1 and CB B1 overlap to the point in the system

8/8/2019 600V and Below

8/36




where the available ault current is 2kA. It can be readily seen that theprimary protective zones in Fig. 7 are not the ideal primary protectivezones per Fig. 2.

Fig. 6: Time-Current Plot showing lack o selectivecoordination between CB F1 and CB PM1

Fig. : Protective Zones or Time-Current Plot o Fig. 6

30kA

Available

Fault

25kA

Available

Fault

21.6 kA Available Fault

2kA Available Fault

UTILITY SERVICE

CB F1

CB M1 PRIMARY

PROTECTIVE

ZONE

CB M1

CB PM1

CB B1

CB F1 PRIMARY

PROTECTIVE

ZONE


CB B1 PRIMARY

PROTECTIVE ZONE

CB F1, CB PM1 Coordinate through 21.6kA

CB PM1, CB B1 Coordinate through 2kA

10 100

1K

10K

100K

10100

1K

10K

100K

0.01 0.01

0.10 0.10

1 1

10 10

100 100

1000 1000

CURRENT IN AMPERES

TIMEIN

SECONDS

CB M1

CB F1

CB PM1

CB B1

8/8/2019 600V and Below

9/36




From the discussion above that it is apparent that it becomes moredicult to coordinate two overcurrent protective devices as the aultcurrent increases. This is an important concept in light o the statisticspresented earlier: The requency o occurrence o high-magnitudebolted aults is much less than that o lower-magnitude aults, such as

arcing ground aults.

Equipment protection is an important part o the coordination process.Time-current curves such as those shown above may be used toshow protection or cables, transormers and other equipment.Essentially, the damage curve or the equipment in question issuperimposed upon the time-current characteristic curve(s) or thedevice(s) that protect it. Equipment damage curves which all to theright and above the protective device curves with sucient margin areconsidered to be protected by the device(s). Equipment damagecurves which all on top o or to the let and below the protectivedevice curves are considered not to be protected by the device(s).

Because this paper ocuses on protective device coordination, deviceprotection is only addressed where it helps illustrate why a particularprotective device is set at a given level. However, it should beunderstood that device protection is important. Reerence [2] is anexcellent reerence both or equipment protection and protectivedevice coordination.

2.. What about EquipmentProtection?

The 2005 NEC requirements or selective coordination are, atpresent, more stringent that ever beore (like all code requirements,however, they are subject to interpretation). These requirements areas ollows. Code text is in italics [1]:

240.12 Electrical System Coordination.Where an orderly shutdownis required to minimize the hazard(s) to personnel and equipment, asystem o coordination based upon the ollowing two conditions shallbe permitted:

(1) Coordinated short-circuit protection.

(2) Overload indication based on monitoring systems or devices.

Where an orderly shutdown is required, short-circuit protection mustbe present, but overload protection can be indicating only. This is inlieu o ull coordinated overload protection and is intended to minimizethe risk o unintentionally shutting down part o a system automaticallydue to an overload condition where a lack o coordination can causehazards to personnel and equipment. An overload condition cangenerally be tolerated or a longer period o time than a ault; theoverload indication must be acted upon by operating personnel, butthe time can be taken or an orderly, rather than an abrupt, shut-downo the aected equipment.

2.6. NEC Requirements orSelective Coordination

2.6.1. Coordinated Short-CircuitProtection/Overload IndicationPermitted When Orderly Shutdownis Required (NEC 20.12)

8/8/2019 600V and Below

10/36




2.6.2. Elevators, Dumbwaiters,Escalators, Moving Walks,Wheelchair Lits, and StairwayChair Lits (NEC 620.62)

2.6.. Emergency Systems (NEC 00.2)

620.62 Selective Coordination.Where more than one drivingmachine disconnecting means is supplied by a single eeder, theovercurrent protective devices in each disconnecting means shall beselectively coordinated with any other supply side overcurrentprotective devices.

This requirement has been in the NEC or some time and is intendedto prevent an overcurrent condition in one elevator, escalator, etc.,motor rom de-energizing the entire eeder which supplies otherelevator(s), escalator(s), etc., which is important or re ghter accessduring a re.

700.27 Coordination.Emergency system(s) overcurrent devicesshall be selectively coordinated with all supply side overcurrentprotective devices.

The denition o an emergency system is a system legally required

and classed as emergency by municipal, state, ederal, or othercodes, or by any governmental agency having jurisdiction. Thesesystems are intended to automatically supply illumination, power, orboth, to designated areas and equipment in the event o ailure o thenormal supply or in the event o accident to elements o a systemintended to supply, distribute, and control power and illuminationessential or saety to human lie. The requirement or emergencysystem protective device selective coordination is new to the 2005 NEC.

Health Care acilities in Florida have long been subject to the activeoversight o the Florida Agency or Health Care Administration(Florida AHCA). Depending upon the jurisdiction, Florida AHCA inthe past has required coordination only down to the 0.1s level

(i.e., ignoring short-circuit coordination). The advent o the newlanguage above and in NEC 701.18 below will undoubtedly have aneect on this, however as o the time o writing the disposition o thisissue with Florida AHCA is unknown.

Note that selective coordination is reerenced in terms o devicesrather than as system selective coordinationas discussed herein.This can have important consequences or engineers trying tomeet the requirements o this Code section, as discussed inurther detail below.

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

The denition o a legally required standby system is a systemconsisting o circuits and equipment intended to supply, distribute,and control electricity to required acilities or illumination or power, orboth, when the normal electrical supply or system is interrupted. Therequirement or legally required standby system selective coordinationis new to the 2005 NEC (see comments above).

2.6.. Legally Required StandbySystems (NEC 01.1)

8/8/2019 600V and Below

11/36




2.6.. Service Ground-FaultProtection or Equipment(NEC 20.)

230.95 Ground-Fault Protection of Equipment.Ground-aultprotection o equipment shall be provided or solidly grounded wyeelectrical services o more than 150 volts to ground but not exceeding600 volts phase-to-phase or each service disconnect rated 1000amperes or more. The grounded conductor shall be connected

directly to ground without inserting any resistor or impedance device.The rating o the service disconnect shall be considered to be therating o the largest use that can be installed or the highestcontinuous current trip setting or which the actual overcurrent deviceinstalled in a circuit breaker is rated or can be adjusted.

Exception No. 1: The ground-ault protection provisions o this sectionshall not apply to a service disconnect or a continuous industrialprocess where a nonorderly shutdown will introduce additional orincreased hazards.

Exception No. 2: The ground-ault protection provisions o this sectionshall not apply to fre pumps.

(A) Setting.The ground-ault protection system shall operate tocause the service disconnecting means to open all ungroundedconductors o the aulted circuit. The maximum setting o the ground-ault protection shall be 1200 amperes, and the maximum time delayshall be one second or ground-ault currents equal to or greater than3000 amperes.

(B) Fuses. I a switch and use combination is used, the usesemployed shall be capable o interrupting any current higher than theinterrupting capacity o the switch during a time that the ground-aultprotective system will not cause the switch to open.

(C) Performance Testing.The ground-ault protection system shall

be perormance tested when frst installed on site. The test shall beconducted in accordance with instructions that shall be provided withthe equipment. A written record o this test shall be made and shall beavailable to the authority having jurisdiction.

Electrical services o 1000A or greater, with over 150V to ground and600V or less phase-to-phase (such as 480Y/277V systems), requireground-ault protection at the service. This protection must be set topick up at no more than 1200A and with a maximum time delay o 1second at 3000A or greater. Exceptions apply to continuous industrialprocesses and re pumps. This has a direct bearing on coordinationwith downstream devices, as explained below.

215.10 Ground-Fault Protection of Equipment.Each eederdisconnect rated 1000 amperes or more and installed on solidlygrounded wye electrical systems o more than 150 volts to ground,but not exceeding 600V phase-to-phase, shall be provided withground-ault protection o equipment in accordance with theprovisions o 230.95

2.6.6. Feeder Ground-FaultProtection or Equipment(NEC 21.10)

8/8/2019 600V and Below

12/36




Exception No. 1: The provisions o this section shall not apply toa disconnecting means or a continuous industrial process or wherea nonorderly shutdown will introduce additional or increased hazards.

Exception No. 2: The provisions o this section shall not apply

to fre pumps.

Exception No. 3: The provisions o this section shall not apply iground-ault protection o equipment is provided on the supplyside o the eeder.

Feeders disconnects rated 1000A or more on systems with more than150V to ground and 600V or less phase-to-phase require ground-aultprotection with the same requirements or services as stated in NEC230.95. Exceptions apply to continuous industrial processes and repumps, just as or NEC 230.95. In addition, i ground-ault protectionis provided on the supply side o the eeder (such as a eedersupplied rom a service with ground-ault protection) the ground-aultprotection is not required.

517.17 (B) Feeders.Where ground-ault protection is provided oroperation o the service disconnecting means or eeder disconnectingmeans as specifed by 230.95 or 215.10, an additional step oground-ault protection shall be provided in all next level eederdisconnecting means downstream toward the load. Such protectionshall consist o overcurrent devices and current transormers or otherequivalent protective equipment that shall cause the eederdisconnecting means to open.

The additional levels o ground-ault protection shall not beinstalled as ollows:

(1) On the load side o an essential electrical system transer switch

(2) Between the on-site generating unit(s) described in 517.35(B) andthe essential electrical system transer switch(es)

(3) On electrical systems that are not solidly-grounded wye systemswith greater than 150 volts to ground but not exceeding 600 voltsphase-to-phase

517.17 (C) Selectivity.Ground-ault protection or operation o theservice and eeder disconnecting means shall be ully selective suchthat the eeder device, but not the service device, shall open onground aults on the load side o the eeder device. A six-cycle

minimum separation between the service and eeder ground-aulttripping bands shall be provided. Operating time o the disconnectingdevices shall be considered in selecting the time spread betweenthese two bands to achieve 100 percent selectivity.

2.6.. Ground-Fault Protectionin Heath Care Facilities(NEC 1.1)

8/8/2019 600V and Below

13/36




Note that NEC 517.17 applies to hospitals and other buildings withcritical care areas or utilizing electrical lie support equipment, andbuildings that provide the required essential utilities or services or theoperation o critical care areas or electrical lie support equipment.

NEC 517.17 (B) requires an additional level o ground-ault protectionor health care acilities where a service or eeder disconnectingmeans is equipped with ground-ault protection. This additional levelo ground-ault protection must be at the next level o protectivedevices downstream rom the service or eeder. In NEC 517.17 (C),not only is it stated that selectivity must be achieved, but the amounto selectivity (6 cycles) is specied.

Note that NEC 517.17(B) eectively prohibits the use o ground-aultprotection on the essential electrical system. The result is a confictbetween NEC 517.17(B) and NEC 700.27 and NEC 701.18. This willbe discussed in urther detail below.

The only true method or achieving selective coordination andequipment protection, and documenting with certainty the actthat these have been achieved, is via a coordination study. Thecoordination study, also known as a time-current coordination study,compares the timing characteristics o the protective devicesused with each other and with the damage characteristics oequipment to be protected. For electronic-trip circuit breakers, theappropriate settings or the breaker trip units are developed in thecoordination study.

Because the short-circuit currents available at dierent points in thesystem is a concern, a coordination study is usually perormed in

conjunction with a short circuit study. The short-circuit study evaluatesthe short-circuit currents available in the system.

Note that the new, stringent 2005 NEC requirements mentionedabove or emergency and standby power systems do not in any wayexempt the power system engineer rom perorming a coordinationstudy. In act, in order to t in with the competitive bidding processor equipment the timing o the study may need to be perormedsooner in the project timeline than previously, in order to avoidcostly mistakes in protective device selection. This is discussed inmore detail below.

2.. The Coordination Study

8/8/2019 600V and Below

14/36




For all low-voltage use classes, the basic timing characteristics canbe classied in the same manner. Fuses are typically assigned aminimum melting characteristicand a total clearing characteristicbytheir manuacturer. These dene the boundaries o the use time-current characteristic band. For currents with time durations belowand to the let o the time current characteristic band, the use will not

blow or be damaged. For currents with time durations within the time-current characteristic band, the use may or may not blow or bedamaged. For currents with time durations above and to the right othe time-current characteristic band, the use will blow with a minimummelting time given by the minimum melting time characteristic and atotal clearing time given by the total-clearing time characteristic.Alternatively, the use may be assigned an average melting time

Fig. : Fuse Timing Illustration

Overcurrent condition initiated.Fuse element begins to melt

Fuse element is melted.Arcing begins.

Overcurrent is cleared.

Melting Time Arcing Time

Total Clearing Time

Overcurrent coordination is infuenced heavily by the characteristicso the overcurrent protective devices themselves. For systems 600Vand under, the two primary types overcurrent protective devices arecircuit breakers and uses. The characteristics o each, as they applyto overcurrent coordination, are discussed below.

Fuses are the simplest o all overcurrent protective devices. As such,they oer the least amount o adjustability o any overcurrentprotective device. A use consists o a melting elementwhich meltswith a pre-determined time-current characteristic or overcurrents.Low-voltage uses are divided into classes based upon theircharacteristics. Some uses are classied current-limiting. By strictdenition, a current-limiting use will interrupt currents in its current-limiting range within cycle or less, limiting the current to a valueless than that which would be available i the use were replaced by aconductor o the same impedance.

Fuse timing response to a given level o overcurrent may beseparated into melting time, which is the time required to melt thecurrent-responsive element, and arcing time, which is the timeelapsed rom the melting o the current-responsive element to thenal interruption o the circuit. The arcing time is dependent upon thecircuit characteristics, such as the voltage and impedance o thecircuit. The total clearing time is the sum o the melting time and thearcing time, as shown in Fig. 8.

. Protective DeviceCharacteristics

.1. Fuses

8/8/2019 600V and Below

15/36

8/8/2019 600V and Below

16/36




Fig. 10: Fuse Coordination Example

FU1 & FU2 Coordinate to 8200A Above 8200A Coordination mustbe established by fuse ratio tables

10

10

100

100

1K

1K

10K

10K

100K

100K

0.010.01

0.10 0.10

1 1

10 10

100 100

1000 1000

CURRENT IN AMPERES

TIMEIN

SECONDS

FU 1

FU 2

UTILITY BUS

FU 1

FU 2

FU 1

FU 2

.2. Circuit Breakers Circuit breakers oer many advantages over uses or theprotection o low-voltage power systems. These advantages will notbe elaborated upon here, however it should be noted that or thisreason circuit breakers are the prevalent orm o overcurrentprotection or low-voltage power systems. Successul selectivecoordination with circuit breakers is thereore a vital topic orsuccessul power system design.

Circuit breakers may be subdivided into two basic categories:Molded-caseand low-voltage powercircuit breakers. Molded-casecircuit breakers may be generally divided into thermal-magneticandelectronictripping types. Molded-case electronic-trip circuit breakersmay be generally be urther divided into two categories: those withtwo-step stored energy mechanisms, oten reerred to as insulatedcase circuit breakers(not a UL term, but does appear in the IEEEBlue Book [5]) and those without.

From a coordination standpoint, o particular importance is the ratedshort-time withstand current. This is dened as ollows [5]:

Rated Short-Time Withstand Current:(A)The maximum RMStotal current that a circuit breaker can carry momentarily withoutelectrical, thermal, or mechanical damage or permanent deormation.The current shall be the RMS value, including the DC component, at

8/8/2019 600V and Below

17/36




the major peak o the maximum cycle as determined rom theenvelope o the current wave during a given test time interval. (IEEEC37.100-1992) (B) That value o current assigned by themanuacturer that the device can carry without damage to itsel, underprescribed conditions. (NEMA AB1 1993) Syn: withstand rating;

short-time rating

All circuit breakers which have inherent time-delay characteristics(which is essentially every circuit breaker that is not an instantaneous-only circuit breaker) have a short-time withstand capability. Thiscapability may or may not be published as a short-time withstandrating, however it will maniest itsel in the time-current characteristicsor the circuit breaker since a circuit breaker must be designed so thatit will not be damaged or ault currents up to its interrupting rating.Table II gives a summary o the various low-voltage circuit breakertypes with respect to typical levels o short-time withstand capability.Because the inormation given in Table II is general in nature, specicmanuacturers data must be consulted or a given circuit breaker.

Table II: Low-Voltage Circuit Breaker Types1

CircuitBreaker Type

Standard Tripping Type Short-time WithstandCapability2

Molded-Case UL 489

Thermal-magnetic Typically much lowerthan interrupting rating

Electronic Typically lower thaninterrupting rating

Electronic(insulated case)3

Oten comparable tointerrupting rating

Low-Voltage

Power

ANSI C37.13

UL 1066

Electronic Typically comparable

to interrupting rating

1 Other circuit breaker types, such as molded-case circuit breakers with instantaneous-only tripunits, are available or specic applications, such as short-circuit protection o motor circuits

2 Short-time current is dened by ANSI C37.13 as the designated limit o available(prospective) current at which the circuit breaker is required to perorm a duty cycle consistingo two -second periods o current fow separated by a 15s interval o zero current. For UL489-rated circuit breakers short-time withstand is not dened and the duty cycle may vary.

3 Insulated-case circuit breakers exceed the UL 489 standard. The term insulated case isnot a UL term.

The typical time-current characteristic band o a thermal-magneticmolded-case circuit breaker is shown in Fig. 11. The time band is, bynecessity, quite large; or example, the UL 489 standard allows the

instantaneous trip characteristic or a circuit breaker with anadjustable instantaneous characteristic to vary rom -20% to +30% othe marked instantaneous trip current setting. The long-time portion othe trip characteristic is established by a thermal element and is usedor overload and low-level ault protection. The instantaneouscharacteristic is oten adjustable, as shown in Fig. 12, and is used orshort circuit protection.

.2.1. Thermal-Magnetic Molded-CaseCircuit Breakers

8/8/2019 600V and Below

18/36




Fig. 11: Typical Thermal-Magnetic Molded-Case Circuit BreakerTime-Current Characteristic Band

Thermal(long-time)Characteristic

Magnetic(instantaneous)Characteristic

100

1K

10K

100K

TIMEIN

SECONDS

10

100

1K

10K

100K10

0.01 0.01

0.10 0.10

1 1

10 10

100 100

1000 1000

CURRENT IN AMPERES

Fig. 12: Thermal-Magnetic Circuit Breaker Time-CurrentCharacteristic showing adjustable instantaneouscharacteristic

Magnetic(instantaneous)CharacteristicHI setting

Magnetic(instantaneous)CharacteristicLO setting

100

100

1K

1K

10K

10K

100K

100K

TIMEIN

SECONDS

10

10

0.01 0.01

0.10 0.10

1 1

10 10

100 100

1000 1000

CURRENT IN AMPERES

8/8/2019 600V and Below

19/36




.2.2. Electronic-Trip Circuit Breakers Electronic-trip circuit breakers typically are equipped with trip unitswhich give the circuit breakers the general characteristics per Fig. 13.The adjustable long-time pickup sets the trip rating o the circuitbreaker. The adjustable long-time delay, short-time pickup, short-timedelay, and instantaneous pickup allow the circuit breakers tripping

characteristics to be customized to the application. The trip unitrepresented by Fig. 13 is reerred to as an LSI trip unit, since it isequipped with long-time, short-time, and instantaneous tripcharacteristics. Trip units without a short-time setting are reerred toas LI trip units, and units without an instantaneous characteristic arereerred to as LS trip units. In most cases, the instantaneouscharacteristic on an LSI trip unit can be turned o i necessary.A trip unit which includes ground ault protection is denoted witha G, i.e., LSIG.

O particular importance to the tripping characteristic is theinstantaneous selective override level. For currents above thisoverride level, even i the instantaneous characteristic is turned o

the circuit breaker will trip instantaneously. The override level isactory-set to protect the circuit breaker according to its short-timewithstand capability. Thereore, the higher the withstand level, thehigher the override is set. This is an extremely important concept andoten determines whether two circuit breakers in series selectivelycoordinate. Note also that the tripping times or the instantaneouscharacteristic and or currents above the override level are non-adjustable. Further, as is the case or the circuit breaker representedin Fig. 13, there can be a dierence in tripping time when the circuitbreaker is operating in the instantaneous region below the overridelevel vs. above the override level.

8/8/2019 600V and Below

20/36




Fig. 1: Typical Time-current characteristics or electronic-tripcircuit breaker (molded-case circuit breaker with lowshort-time withstand shown)

100

1

00

1K

1

K

10K

1

0K

100K

1

00K

TIMEIN

SECONDS

10

10

0.01 0.01

0.10 0.10

1 1

10 10

100 100

1000 1000

CURRENT IN AMPERES

Instantaneous Override Value (24kA -10%)

Instantaneous Timingis Non-Adjustable

Instantaneous OverrideTiming is Non-Adjustable

Adjustable Long-Time Pickup

Adjustable Long-Time Delay

AdjustableShort-Time Pickup

Adjustable

Short-Time Delay

AdjustableInstantaneous Pickup

Like uses, circuit breakers can be designed to limit the fow oprospective short-circuit current. Similar to a current-limiting use,

a current-limiting circuit breaker limits the let-through I2t to a valuethat is less than its prospective value. Circuit breakers which arecurrent-limiting are typically shown with instantaneous characteristicsin which the tripping time decreases with current, as shown in Fig. 14.

It is worthy o note that, in some cases, even though the circuitbreaker is not ocially classied as current-limiting, a degree ocurrent-limitation may exist [3]. This results in the circuit breakerexhibiting time-current characteristics similar to those shown inFig. 14, although the instantaneous characteristic is shown as ahorizontal band.

.. Current-LimitingCircuit Breakers

8/8/2019 600V and Below

21/36




Fig. 1: Typical Time-Current Characteristic orCurrent-Limiting Circuit Breaker

A relatively new, but important, concept in the coordination olow-voltage circuit breakers is the concept o dynamic impedance.Simply stated, a circuit breaker, when it begins to open, serves tolimit the prospective fow o current, even i it is not UL listed as a

current-limiting circuit breaker [3]. The impedance presented to thecircuit by the circuit breaker during opening changes with time as thecircuit breaker opens, hence the term dynamic. This impedance canincrease the level coordination between two circuit breakers in seriesby limiting the current that the upstream circuit breaker sees or aault downstream o both circuit breakers when the downstreambreaker is opening.

Taking the dynamic impedance characteristics o circuit breakers intoaccount or selective coordination leads to an important new tool orthe coordination o circuit breakers: Short-Circuit Coordination Tables.Similar to use ratio tables, these show the level o coordination

between two circuit breakers in series, as determined by test.Because o the dynamic impedance eects o ordinary circuitbreakers, oten the level o coordination between two circuit breakersin series is greater than their time-current characteristic bandswould indicate. As an example, in Fig. 6 the coordination level

0.5 1 10100

1K

10K

100K

1 10 100

1K

10K

100K

0.01 0.01

0.10 0.10

1 1

10 10

100 100

1000 1000

CURRENT IN AMPERES

TIMEIN

SECONDS

.. Circuit Breakers in Series: TheDynamic Impedance Concept

..1. Short-Circuit Coordination Tables

8/8/2019 600V and Below

22/36




between CB F1 and CB PM1 was established graphically via thetime-current bands as 21.6kA. However, testing shows that these twocircuit breakers in series, as manuactured by one specicmanuacturer, coordinate up to 35kA! So, even though the time-current bands do not refect this, CB F1 and CB PM1 do coordinate

up to the available ault current o 25kA, as illustrated in Fig. 15. Thislevel o extra time-current coordination can oten make a largedierence, as in this case.

Fig. 1: Time-Current Curve o Fig. 6, Showing Eects oDynamic Impedance and Current-Limiting on Level oSelective Coordination Between CB F1 and CB PM1

CB PM1, CB B1Coordinate through 2kA

21.6kA

10

100

100

1K

1K

10K

10K

100K

100K

TIMEIN

SECONDS

10

0.01 0.01

0.10 0.10

1 1

10 10

100 100

1000 1000

CURRENT IN AMPERES

CB F1 and CB PM1coordinate up to theavailable faultcurrent of 25kA,despite whattime-current bandsshow, due todynamic impedanceeffects (for onespecificmanufacturerscircuit breakers)

CB M1

CB F1

CB PM1

CB B1

As with use ratio tables, these tables must be developed by themanuacturer. It is extremely important that the levels o short-circuitcoordination in the short-circuit coordination tables, i dierent romthe levels determined rom the time-current bands, be determined bytest. The present state o the art does not lend condence tocalculated values.

.. Ground-Fault Protectiono Equipment

Ground-ault protection o equipment is designed to provide sensitiveprotection or ground-aults, typically set below the level o phaseovercurrent protection. Typically, ground-ault protection is built intothe trip unit o an electronic-trip circuit breaker or, in the case o a

8/8/2019 600V and Below

23/36




Fig. 16: Typical Ground-Fault Protection Characteristic

0.5 1 10

100

1K

10K

100K

1 10 100

1K

10K

100K

0.01 0.01

0.10 0.10

1 1

10 10

100 100

1000 1000

CURRENT IN AMPERES

TIMEIN

SECONDS

GF CHAR (TYP)

. Tying it All Together Design Philosophies

and Guidelines

From a system perormance standpoint, it is easy to take the positionthat selective coordination between overcurrent protective devices isalwaysbenecial, regardless o the circumstances However, on apractical basis ull selective coordination may not always beachievable or desirable. Various industry standards recognize thisact. Compromises may be required between selectivity andequipment protection to achieve the desired results. Further,economic trade-os are oten requently encountered, as well ascode issues. Some examples o wording rom various industrystandards regarding selective coordination are given in Table III.

thermal-magnetic circuit breaker or uses, can be supplied via aseparate ground relay. Note that i uses are used a separatedisconnecting means with shunt-trip capability is required. A typicaltime-current characteristic is given in Fig. 16.

The current-sensing arrangement or ground-ault protection mayconsist o a simple residual connection o current sensors/CTs,a single zero-sequence sensor/CT, or may be a complex aairwith dierential connections o the sensors/CTs, known as amodifed-dierential ground-aultarrangement. The application othe sensors/CTs is beyond the scope o this paper but the engineerresponsible or coordination should be cognizant o the requirementsand potential application issues.

8/8/2019 600V and Below

24/36




Table III: Selective coordination requirements/comments per various industry standards

Standard Requirement/Comment

NFPA 110 Standard orEmergency and Standby

Power Systems [6]

6. Protection

6..1* General. The overcurrent protective devices in the EPSS shall be coordinated to optimize

selective tripping o the circuit overcurrent protective devices when a short circuit occurs.Annex A

A.6..1 It is important that the various overcurrent devices be coordinated, as ar as practicable, toisolate aulted circuits and to protect against cascading operation on short circuit aults. In manysystems, however, ull coordination is not practicable without using equipment that could beprohibitively costly or undesirable or other reasons. Primary consideration also should be given toprevent overloading o equipment by limiting the possibilities o large current inrushes due toinstantaneous reestablishment o connections to heavy loads.

IEEE Std. 141 IEEERecommended Practiceor Electric PowerDistribution or IndustrialPlants (Red Book) [4]

Chapter Application and Coordination o Protective Devices

.1. Importance o Responsible PlanningProtection in an electric system is a orm o insurance. It pays nothing so long as there is no ault orother emergency, but when a ault occurs it can be credited with reducing the extent and durationo the interruption, the hazards o property damage, and personnel injury. Economically, thepremium paid or this insurance should be balanced against the cost o repairs and lost production.Protection, well integrated with the class o service desired, may reduce capital investment by

eliminating the need or equipment reserves in the industrial plant or utility supply system..2 Analysis o System Behavior and Protection Needs

.2.1 Nature o the ProblemOperating records show that the majority o electric circuit aults begin as phase-to-groundailures

IEEE Std. 241 IEEERecommended Practice orElectric Power Systems inCommercial Buildings(Gray Book) [9]

Chapter System Protection and Coordination

. Selective Coordination

..1 Coordination o Protective DevicesOn all power systems, the protective device should be selected and set to open beore thethermal and mechanical limitations o the protected components are exceeded.

.. Mechanics o Achieving CoordinationQuite oten, the coordination study will not demonstrate complete selective coordination becausea compromise has to be made between the competing objectives o maximum protection andmaximum service continuity.

IEEE Std. 242 IEEERecommended Practice orProtection and Coordinationo Industrial and CommercialPower Systems(Bu Book) [2]

Chapter 1 First Principles

1.1.2.2 Equipment damage versus service continuityWhether minimizing the risk o equipment damage or preserving service continuity is the moreimportant objective depends upon the operating philosophy o the particular industrial plant orcommercial business. Some operations can avoid to limited service interruptions to minimize thepossibility o equipment repair or replacement costs, while others would regard such an expenseas small compared with even a brie interruption o service.

In most cases, electrical protection should be designed or the best compromise betweenequipment damage and service continuity

Chapter 1 Overcurrent coordination

1.1 General discussionIn applying protective devices, it is occasionally necessary to compromise between protection andselectivity. While experience may suggest one alternative over the other, the preerred approach isto avor protection over selectivity. Which choice is made, however, is depended upon theequipment damage and the aect on the process.

IEEE Std. 446 1995 IEEERecommended Practice orEmergency and StandbyPower Systems orIndustrial and CommercialApplications(Orange Book) [7]

Chapter 6 Protection

6.2 Short Circuit ConsiderationsCareul planning is necessary to design a system that assures optimum selectivity andcoordination with both power sources

8/8/2019 600V and Below

25/36




The earlier in the design process selective coordination is considered,the less painul achieving selective coordination will be. The needor a coordination study, even a preliminary study, early in the designprocess is increasingly becoming recognized as a need i selectivecoordination is to be achieved without costly re-designs.

Working with overcurrent protective device manuacturers early in thedesign process generally makes the eort to achieve selectivecoordination go much more smoothly. In some cases this will requirechanges to the way projects are contracted and managed, sinceworking with a particular manuacturer generally means staying withthat manuacturer or the protective devices considered.

Good data is essential to the selective coordination eort. The utilityavailable ault current, impedance data or the generator units to beused, motor ault current contribution, and good estimates o cablerun lengths are all crucial. The earlier this inormation is obtained, theeasier the coordination eort will generally be. When obtaining the

utility available ault current, avoid innite bus calculations, even onthe primary side o a service transormer. Real world ault currentvalues will be lower than those which rely on innite-bus assumptions.While innite bus assumptions have long been recognized as beingconservative or short-circuit and coordination studies, coordinationper the 2005 NEC requirements and arc-fash concerns bothnecessitate obtaining actual ault current values rom the utility.Typically, obtaining both a maximum available ault current value oruse with the short-circuit and coordination studies and a minimumavailable ault current value or use with arc-fash studies is preerred(and is an acknowledgement o the electric utility industrys assertionthat available ault current values can change over time due to systemchanges), although this is typically a challenge due to the industrys

reliance on innite bus calculations.

The wording o 2005 NEC 700.27 and NEC 701.18 leaves an openissue. Although selective coordination is dened in NEC 100 aslocalization o an overcurrent condition to restrict outages to thecircuit or equipment aected, accomplished by the choice oovercurrent protective devices and their ratings or settings, NEC700.27 and NEC 701.18 contain the wording shall be selectivelycoordinated with all supply side overcurrent protective devices. Whatabout scenarios where two devices that are eectively in seriesprotect a given piece o equipment?

Such a scenario is given in Fig. 17. The transormer shown isprotected or short-circuits by the primary circuit breaker, and oroverloads by the secondary circuit breaker. For a ault where theprotective zones overlap, does it matter whether the primary orsecondary circuit breaker trips? The answer is, o course, no.However, because o the wording o NEC 700.27 and NEC 701.18 thetwo circuit breakers would need to be selectively coordinated with

.1. Consider SelectiveCoordination Early inthe Design Process

.2. Recognize the Conicts andIssues with the 200 NEC

.2.1. Selective Coordination What is it?

8/8/2019 600V and Below

26/36




each other, even though it has no bearing on the perormance o thesystem. So long as there are no connections to other devicesbetween the two circuit breakers, the system may be selectivelycoordinated even though these two circuit breakers themselves donot coordinate. This is a crucial dierence between selective

coordination o devices and system selective coordinationasdescribed in section 2.3 above.

Fig. 1: Typical Low-Voltage Transormer Protection Scenario

SECONDARY CB

PRIMARY CB

TRANSFORMER

PRIMARY CB

PROTECTIVE ZONE

SECONDARY CB

PROTECTIVE ZONE

Note that or transormers, such as the transormer shown in Fig. 17,removal o the secondary overcurrent protective device may not bepossible due to restrictions in NEC 450. Removal o this device mayalso hinder transormer protection. For these and other scenarios inwhich two overcurrent protective devices in series must be utilized,the local Authority Having Jurisdiction should be consulted to providea waiver.

Other possible scenarios or this issue are given in Fig. 18. In bothcases, selective coordination o CB 1 and CB 2 is not required orover-all system coordination, since there are no additional devicesbetween the two. Both devices could be the same size device with thesame settings.

8/8/2019 600V and Below

27/36




What can be done about this issue? For the short-term, the solution isto minimize occurrences o overcurrent protective devices in series,as discussed below. Long-term actions may include the submission ochange proposals or consideration in a uture code cycle. The moreproposals that are made on this issue, the more likely the issue is tobe recognized and corrected.

From the inormation in the preceding sections, a confict in the 2005NEC with respect to health care acilities can be recognized. To dothis, consider the typical hospital electrical system per Fig. 19. PerNEC 700.27, all emergency system devices must be selectivelycoordinated with all supply-side devices. Taken literally, this orcescoordination o emergency system protective devices up to thealternate power source and to the utility service. For services meetingthe criteria o NEC 230.95 (such as a 480Y/277V utility service 1000Aor greater), ground-ault protection is required at the service. Thisground-ault protection must be set at no greater than 1200A pickupand a time delay o no more than 1s at 3000A or greater. NEC517.17(B) requires an additional level o ground-ault protection in

health-care acilities, and NEC 517.17(C) requires the two levels oground-ault protection to coordinate with no less than a six-cycle(0.1s) margin between the two.

NEC 517.26 requires the essential electrical system to meet therequirements o NEC 700, which includes NEC 700.27. NEC 700.27does not specically require coordination o ground-ault protection.

Fig. 1: Other Examples Where Selective Coordination oDevices Is Not Required or Selective Coordinationo the System:

a.) Engine-Generator Set with Circuit BreakerFeeding Switchboard with Main Circuit Breaker

b.) One Panelboard Feeding Another Panelboardwith a Main Circuit Breaker

PANEL 1

PANEL 2

CB 1

CB 2

G

CB 1

CB 2

ENGINE-GENERATOR SET

SWITCHBOARD

a.) b.)

.2.2. Ground-Fault Protection inHealth-Care Facilities

8/8/2019 600V and Below

28/36




To achieve selective coordination or ground-ault protection, thelowest level o ground-ault protection would have to coordinate withthe phase time-current characteristics o the next lower downstreamdevice. Most oten, this will require additional levels o ground-aultprotection to supplement the two required levels per NEC 517.17(B).

However, there is a problem: NEC 517.17(B) also eectively prohibitsthe use o additional levels o ground-ault protection in the essentialelectrical system! Thereore, selective coordination o ground-aultprotection when the essential electrical system is supplied by thenormal (utility) source cannot be achieved, in most instances,without violating NEC 517.17(B). As stated above, ~95% o allsystem aultsare ground aults, thereore this is an issue withimportant consequences: A ground ault on a given branch o theessential electrical system, when it is supplied rom the normalsource, can cause that branch to be taken o-line, orcing a transerto the alternate (generator) source. The response o the generator(s)would be a unction o the ground ault current magnitude.

All o this can transpire even i the system complies with thewording o NEC 700.27!

Fig. 1: Typical Health-Care Facility Electrical System(Source: NEC 200 FPN Figure 1.0)

Automaticswitchingequipment

Delayedautomaticswitchingequipment

Nonessentialloads

Normalsource

Alternatepower source

Equipmentsystem

Life safetybranch

Criticalbranch

Emergency system

Essential electrical system

Normalsystem

What can be done about this issue? For the short-term, bringing theissue up to the local Authority Having Jurisdiction or resolution is theonly recourse. Long-term actions may include the submission ochange proposals or consideration in a uture code cycle. The moreproposals that are made on this issue, the more likely the issue is tobe recognized and corrected.

8/8/2019 600V and Below

29/36




As mentioned in 2.2 above, the requency o occurrence o high-magnitude bolted aults is much lower than that o lower-magnitudeaults, such as arcing ground aults. Also, the higher the current levelto which two overcurrent protective devices are coordinated, the moredifcult the coordination eort becomes. The impact o this act upon

system protection and selective coordination are twoold, namely:

1.) It diminishes the practical need or selective coordination up to theavailable ault current in avor o practicable coordination to alower level o ault current.

2.) It reinorces the need or coordinated ground-ault protection.

The wording o the 2005 NEC ignores the statistical evidence o therequency o occurrence o high-level bolted aults. In reality, theseaults are most common during the commissioning phase o theelectrical system in a acility, when damage to cable insulation andother application and installation issues are corrected. During thenormal lietime o the system, these types o short-circuits are rareindeed, especially at lower levels in the system. One practical way toaddress selectivity in emergency and standby systems might be to setan established limit o 50% o the bolted ault current as the level ocoordination or overcurrent devices below a given level (or example,400A or below); this is an approximate worst-case or the calculatedvalue o the arcing ault current or a 480V system when calculatedper the empirical equations in IEEE-1584 IEEE Guide or PerormingArc-Flash Hazard Calculations[8]. Selective coordination up to such alimit would be justiable on a practical basis. However, no code orstandard presently sets this limit.

Arc-fash perormance o the system is also a actor. In some cases,arc-fash perormance, particularly at the lower levels o the system,may be impaired by orcing selectivity up to the available bolted aultcurrent. The reason or this is that the arc-fash incident energy levelis directly proportional to the time duration o an arcing ault, whichis the clearing time or the overcurrent protective device whichclears the ault.

Also, as described above the NEC eectively prohibits coordinatedground-ault protection in health care acility essential electricalsystems, even though ~95% o all system aults are ground aults.

From the oregoing discussion in 4.2.1, in many cases it is possibleto meet the wording o NEC 700.27 and NEC 701.18 by avoiding

the use o overcurrent protective devices in series with no equipmentin between. Two examples o this are shown in Fig. 18 above.Fig. 20 shows the examples re-designed to eliminate redundantprotective devices.

.2.. Is Coordination up to theAvailable Fault current Justifedon a Practical Basis?

.2.. Avoid Placing Protective Devices

in Series with No EquipmentBetween Them

8/8/2019 600V and Below

30/36

8/8/2019 600V and Below

31/36




or the selectivity o downstream devices. As an illustration o theeects o this lack o selectivity, consider the system o Fig. 22, whichis the same system rom Fig. 21 expanded to show the primaryprotective zones o the overcurrent protective devices. Note thatalthough CB 3 and CB 6 selectively coordinate, the required settings

o CB 1 and CB 2 or generator protection cause their primaryprotective zones to completely overlap the CB 3 protective zone andextend into the CB 6 protective zone. One method to prevent this is todesign the system with a larger number o smaller-size generators, asshown in Fig. 23. This is a gross simplication, but it does illustratethe concept. In reality, reliability concerns will, in many cases, orceadditional generators to be added or redundancy; this is much moreeconomically easible or the system o Fig. 22 than or the system oFig. 23. The addition o 51V or 51C voltage restrained/controlledrelays can oten improve the generator protection, but will notimprove coordination.

Fig. 21: Application with Paralleled Generators

G

CB 1

G

CB 2

AUTOXFER

SW

CB 4

TO NORMAL SOURCE

EN

CB 3

AUTOXFER

SWE

N AUTOXFER

SW

CB 5

EN

Fig. 22: System o Fig. 21 Expanded to Show PrimaryProtective Zones

G

CB 1

G

CB 2

AUTOXFERSW

CB 4

TO NORMAL SOURCE

E N

CB 3

AUTOXFERSW

E N AUTOXFERSW

CB 5

E N

CB 6

CB 1 PROTECTIVE ZONE CB 2 PROTECTIVE ZONE

CB 3

PROTECTIVE

ZONE

CB 6 PROTECTIVE ZONE

8/8/2019 600V and Below

32/36

8/8/2019 600V and Below

33/36




In Fig. 24, the dierential relay 87B would typically be o the high-impedance type, and would trip CB 1, CB 2, CB 3, CB 4, and CB 5.A ault between CB 1/CB 2 and CB 3/CB 4/CB 5 will cause this relayto trip, and, i it is set appropriately, it will operate aster than the tripunit settings o CB 1 or CB 2, providing short-circuit protection or the

generators in this protective zone as well as providing short-circuitprotection or the paralleling switchgear bus. Generator overloadprotection would still be provided by CB 1 and CB 2. Note thatgenerator dierential protection is not shown; it could be provided toprovide additional protection or the generator, but would not be anaid to selectivity. Generator dierential relays, i used, should be othe percentage-dierential type rather than impedance type. Notealso that lockout relays, while recommended, are not shown. Thecircuit breakers which must be tripped by the dierential relays mustbe suitable or external relay tripping (suitable insulated case circuitbreakers or ANSI power circuit breakers are recommended, but aretypically used in this application anyway). Economic concerns (costo dierential relays and CTs and the extra wiring required) must, ocourse, be taken into account when considering this approach.

A more in-depth treatment o generator protection or emergency andstandby power systems is given in a separate paper, Protection oLow-Voltage Generators Considerations or Emergency andStandby Power Systems.

Circuit breakers are the de-acto standard or low-voltageovercurrent protection, or various reasons. As discussed above,circuit breakers need not be ANSI power circuit breakers to havea short-time withstand capability. Contrary to popular belie, circuitbreakers also need not be electronic trip breakers to have a

short-time withstand capability. When speciying circuit breakers,remember, however, that the UL 489 standard to which molded-casecircuit breakers are designed and tested does not require a short-timewithstand capability.

The net eect o a high short-time withstand capability or a circuitbreaker is in its tripping perormance in the short-circuit region. Thiscan be seen by evaluating the time-current characteristics or a givencircuit breaker, although short-circuit coordination tables must beused to gain the ull advantage rom such circuit breakers due to thedynamic impedance and current-limiting eects described above. Inmany cases it will be necessary to increase the rame size o theupstream circuit breaker in order obtain short-time withstand levels

high enough to achieve total selective coordination.

For the service switchgear/switchboards, ANSI power circuit breakersor insulated case circuit breakers are essential, especially ormedium- to large systems.

.. Utilize Circuit Breakerswith High Short-TimeWithstand Capabilities

8/8/2019 600V and Below

34/36




A airly popular misconception is that when using electronic circuitbreakers with the instantaneous unction turned o, ANSI C37.20.1low-voltage power switchgear is required. The reason behind thismisconception is that UL 891 switchboard through-bus withstand testsare only required to be conducted or 3 cycles, whereas ANSI low-

voltage switchgear is required to have a short-time withstand rating o30 cycles. The exception, o course, would be where a manuacturertests a switchboard conguration to the ull 30-cycle withstand rating.In reality, the need or a short-time withstand rating or theswitchboard bussing is only a concern where ANSI low-voltage powercircuit breakers or insulated-case circuit breakers with high (or no)instantaneous override level is provided when the instantaneousunction is turned o. In most cases the circuit breakers provided withswitchboards have instantaneous overrides that cause the circuitbreaker to trip instantaneously above a given level even i theinstantaneous unction is turned o, and these are tested with theswitchboard to insure compatibility.

It must be stressed that the ewer the number o levels oovercurrent protective devices, the easier coordination becomes.Fig. 25 illustrates this point. In Fig. 25 a.), three panels arearranged so that three levels o selective coordination are required(CB 1 CB 2 CB 3). In Fig. 25 b.) the same number o panels hasbeen re-arranged so that only two levels o selective coordination arerequired (CB 1 CB 2 and CB 1 CB 3). Oten such anarrangement can be realized in a very economically easible manner.

.. Avoid Multiple Levelso Protective DevicesWhere Possible

Fig. 2: Illustration Showing Multiple Levels o Selectivity:a.) Three Levels

b.) Same Number o Panels Re-Arranged with Two Levels

PANEL 1

PANEL 2

CB 1

CB 2

a.) b.)

PANEL 3

CB 3

PANEL 1

PANEL 2

CB 1

CB 2

PANEL 3

CB 3

8/8/2019 600V and Below

35/36




Remember that transormer impedance will lower the available aultcurrent, and the smaller the kVA size o the transormer, the moredrastic the reduction. Where coordination at the 480V level, orexample, is not possible, coordination rom 480V to 208V through astep-down transormer may be. I loads can be converted to utilize the

lower voltage, this can be a way to achieve selectivity.

Although the smaller the transormer, the lower the available aultcurrent at the secondary, there may be cases where transormersmust be up-sized in order to achieve selective coordination. This isusually due to the rame size o the primary circuit breaker required tocoordinate with devices at the next level below the transormersecondary main. A careul balance between the required rame size othe primary circuit breaker and the available ault current at thetransormer secondary is usually required.

A popular misconception is that zone-selective interlocking (ZSI)between electronic-trip circuit breakers can orce otherwisemiscoordinated systems to coordinate. While it is true that ZSI canreduce the amount o energy let-through during a ault, it cannot beused to orce selective coordination. The reason or this is that ZSItypically uses the short-time or ground-ault pickup (or both) on adownstream circuit breaker to identiy that the circuit breaker detectsa ault; the downstream breaker then sends a signal to restrain thenext level upstream circuit breaker rom tripping instantaneously whileat the same time itsel tripping instantaneously to clear the ault.However, the upstream circuit breaker will still continue to time out onits time-current band, ultimately tripping i the downstream circuitbreaker ails to clear the ault in time. I the two circuit breakers aremiscoordinated, the upstream circuit breaker may trip beore thedownstream circuit breaker, even with ZSI in place.

Used or the right reasons, however, ZSI is a powerul tool orreducing equipment damage and arc-fash incident energy since, on acoordinated system, it orces the device closest to a given ault toopen in the minimum amount o time. Typically this time is somewhatlonger than the instantaneous characteristic o the circuit breaker, dueto the inherent time delay required or the ZSI logic operation.

Despite the most careul planning, selective coordination eorts canquickly come to nothing i the overcurrent protective devices are not

properly set on-site. Most manuacturers actory-set all but theampere rating switch or electronic-trip circuit breakers in their lowestpositions, or example. The coordination study should includetabulated settings or each overcurrent protective device whichrequires adjustment, such as electronic-trip circuit breakers, thermal-magnetic circuit breakers with adjustable instantaneouscharacteristics, ground-ault relays, etc.

.6. Utilize Step-DownTransormers to LowerFault Current

.. Increase Transormer SizesWhere Necessary

.. Zone-Selective Interlocking The Facts and theMisconceptions

.. Dont Forget On-Site

Adjustment Requirements

8/8/2019 600V and Below

36/36


S h id El t i N th A i O ti Di i i


. Reerences [1] The National Electrical Code, NFPA 70, The National FireProtection Association, Inc., 2005 Edition.

[2] IEEE Recommended Practice or Protection and Coordination oIndustrial Power Systems, IEEE Std. 242-2001, December 2001.

[3] Short Circuit Selective Coordination or Low Voltage CircuitBreakers, Square D Data Bulletin 0100DB0501, October 2005.

[4] IEEE Recommended Practice or Electric Power Distribution orIndustrial Plants, IEEE Std. 141-1993, December 1993.

[5] IEEE Recommended Practice or Applying Low-Voltage CircuitBreakers Used in Industrial and Commercial Power Systems,IEEE Std.1015-1997, October 1997.

[6] Standard or Emergency and Standby Power Systems, NFPA 110,The National Fire Protection Association, Inc., 2005 Edition.

[7] IEEE Recommended Practice or Emergency and Standby PowerSystems or Industrial and Commercial Applications, IEEE Std.446-1995, July 1996.

[8] IEEE Guide or Perorming Arc-Flash Hazard Calculations, IEEEStd. 1584-2002, September 2002.

[9] IEEE Recommended Practice or Electric Power Systems inCommercial Buildings, IEEE Std. 241-1990, December 1990.

600V and Below

Documents

Transcript of 600V and Below