selective coordination

Selective Coordination

In-Depth Presentation of Short CircuitSelective Coordination with Low-Voltage Circuit Breakers

IEEE IAS San Francisco and Santa Clara Chapter

Ed Larsen, Senior MemberIndustry Standards ManagerSquare D/Schneider Electric

http://ewh.ieee.org/soc/ias/cms

http://www.ieee.org/portal/site/iportals

Introduction

Definition

NEC Selective Coordination Requirements

The Challenge

Circuit Breaker Principles

Resources from the Manufacturers

Challenges Meeting the NEC

Design Guidelines

Example

Summary

SF IAS Sel Coord.ppt 2

Definition


The Challenge




Design Guidelines

Example

Summary


Definition: What is selective coordination?

NEC Article 100 defines selective coordination as…Coordination (Selective).Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings.

In other words…Only the overcurrent protective device (OCPD) nearest to a fault should clear the fault


Definition


The Challenge




Design Guidelines

Example

Summary



Article 240 Overcurrent Protection240.12 Electrical System Coordination.Where an orderly shutdown is required to minimize the hazard(s) to personnel and equipment, a system of coordination based on the following two conditions shall be permitted: (1) Coordinated short-circuit protection(2) Overload indication based on monitoring systems or devices FPN: The monitoring system may cause the condition to go to alarm, allowing corrective action or an orderly shutdown, thereby minimizing personnel hazard and equipment damage.

Unchanged



Article 517 Health Care Facilities517.17 Ground-Fault Protection.(C) Selectivity. Ground-fault protection for operation of the service and feeder disconnecting means shall be fully selective such that the feeder device, but not the service device, shall open on ground faults on the load side of the feeder device. A six-cycle minimum separation between the service and feeder ground-fault tripping bands shall be provided. Operating time of the disconnecting devices shall be considered in selecting the time spread between these two bands to achieve 100 percent selectivity.FPN: See 230.95, fine print note, for transfer of alternate source where ground-fault protection is applied.Unchanged



Article 620 Elevators, Dumbwaiters, Escalators, Moving Sidewalks, Wheelchair Lifts, and Stairway Lift Chairs620.62 Selective Coordination.Where more than one driving machine disconnecting means is supplied by a single feeder, the overcurrent protective devices in each disconnecting means shall be selectively coordinated with any other supply side overcurrent protective devices.

Unchanged


2005 NEC Selective Coordination Requirements

Article 700 Emergency Systems

700.27 Coordination.

Emergency system(s) overcurrent devices shall 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 supply side overcurrent protective devices.

New

New



Article 517 Health Care Facilities

517.26 Application of Other Articles.

The essential electrical system shall meet the requirements of Article 700, except as amended by Article 517.

New


Typical Health-Care Facility Electrical System (2005 NEC FPN Figure 517.30)



Article 700 Emergency Systems

II. Circuit Wiring

700.9 Wiring, Emergency System.

(B) Wiring.

Exception to (5)(b): Overcurrent protection shall be permitted at the source or for the equipment, provided the overcurrent protection is selectively coordinated with the downstream overcurrent protection.

New



Articles 700 Emergency Systems and 701 Legally Required Standby Systems

The following exception was added to sections 700.27 and 701.18

Exception: Selective coordination shall not required in the following circuits:(1) Between transformer primary and secondary overcurrent protective devices, where only one overcurrent protective device or set of overcurrent protective devices exist(s) on the transformer secondary, or(2) Between overcurrent protective devices of the same size (ampere rating) in series.

New



SECONDARY CB

PRIMARY CB

LV TRANSFORMER

Making these two breakers coordinate with one another does not enhance system selectivity!



PANEL 1

PANEL 2

CB 1

CB 2

G

CB 1

CB 1

ENGINE-GENERATOR SET

SWITCHBOARD

Making these breakers coordinate with one another does not enhance system selectivity!



Article 708 Critical Operations Power Systems708.1 Scope. The provisions of this article apply to the installation, operation, monitoring, control, and maintenance of the portions of the premises wiring system intended to supply, distribute and control electricity to designated critical operations areas (DCOA) in the event of disruption to elements of the normal system.Critical operations power systems are those systems so classed by municipal, state, federal, or other codes, by any governmental agency having jurisdiction, or by facility engineering documentation establishing the necessity for such a system. These systems include but are not limited to power systems. HVAC. fire alarm. security. communications and signaling for designated critical operations areas.

Note: This NEC article was renumbered from 585 to 708

New




Article 708 Critical Operations Power Systems708.1 Scope. FPN No. 1: Critical Operations Power Systems are generally installed in vital infrastructure facilities that, if destroyed or incapacitated, would disrupt national security, the economy, public health or safety; and where enhanced electrical infrastructure for continuity of operation has been deemed necessary by governmental authority.

New

•Air traffic control centers•Central station service facilities (fire and security system monitoring)•Chemical, petrochemical, and hazardous material (including biohazard) handling facilities•Communications centers, telephone exchanges, cellular tower sites•Emergency evacuation centers•Financial, banking, business data processing facilitiesNote: This list was in an early draft of the article but is not be in the final text

•Fuel supply pumping stations (i.e. natural gas distribution and delivery infrastructure)•Hospitals and associated support facilities•Municipal infrastructure – water and sewer treatment facilities•911 centers•Offices and facilities deemed critical to continuity of government•Police, fire, civil defense facilities including power for radio repeater operations•Radio and television stations•Transportation infrastructure – airports, rail stations, seaports


Article 708 Critical Operations Power Systems708.54 Coordination. Critical operations power system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.

Note: No exceptions!

New


Definition


The Challenge




Design Guidelines

Example

Summary


Selectivity is typically achieved with circuit breakers by coordinating the time-current curve (TCC) characteristics of the devices to be coordinated

Coordinated in the overload zone

Seemingly not coordinated in the short circuit zone

Can circuit breakers comply with the Code?

10

1010

010

0

1K1K

10K

10K

100K

100K

0.01 0.01

0.10 0.10

1 1

10 10

100 100

1000 1000

CURRENT IN AMPERES

TIME IN

SE

CO

ND

S

CB M1

CB F1

CB PM1

CB B1

CB M1

CB F1

CB PM1

CB B1

The Challenge


Definition


The Challenge




Design Guidelines

Example

Summary



How the Instantaneous Trip Function WorksT-M Breakers

– Fixed instantaneous– Factory set– Must hold/trip values in the Digest reflect the

TCC tolerance

– Adjustable instantaneous– Factory set low– Final adjustment subject to +30%/-20%

tolerance per UL 489



How the Instantaneous Trip Function WorksElectronic Trip Breakers– Adjustable instantaneous

– Factory set low– Final adjustment subject to +/-10% tolerance

– Selective override– Factory set for breaker self-protection– Usually +/-10% tolerance

– Making current release (discriminator)* – Factory set for breaker self-protection– +/-10% tolerance– Turned off by a timer or switch after mechanism

is latched* Typically LVPCBs only


Circuit Breaker Principles Selective Override5. For a withstand circuit breaker, instantaneous can be turned OFF. See 613-7 for instantaneous trip curve. See 613-10 forinstantaneous override values.

10

10

100

100

1K1K

10K

10K

100K

100K

0.01 0.01

0.10 0.10

1 1

10 10

100 100

1000 1000

CURRENT IN AMPERESTIM

E IN

SEC

ON

DS

Current Scale X 10^0 Reference Voltage: 480

TCC view


Circuit Breaker Principles Making Current Release


The MCR results in a close and latch rating


Factors Impacting Short Circuit Selective CoordinationUsing time-current curves alone sometimes leads to the determination of a short circuit selective coordination level that is lower than can actually be achieved. Factors to consider…

How Time-Current Curves are Developed

Current Limiting Properties of Circuit Breakers

Dynamic Characteristics of Circuit Breakers



Selective Coordination with 2 Circuit Breakers in Series –Time-Current Curves (TCCs) vs. Tested Levels

TCCs– Developed by testing a circuit breaker by itself– In the short circuit region may not be valid for two

circuit breakers connected in seriesTested Levels

– Take into account the current limiting properties and dynamic impedance of circuit breakers

– Developed by comparing the actual let-through current of the downstream circuit breaker with minimum instantaneous trip of the upstream circuit breaker

– Same principle as fuse ratio tables

10

10

100

100

1K1K

10K

10K

100K

100K

0.01 0.01

0.10 0.10

1 1

10 10

100 100

1000 1000

CURRENT IN AMPERES

TIME IN

SECO

ND

S



What is Needed for Good SelectivityWithstand Capability – Ability to withstand a high level of current– Dependent on many factors, such as...

– Current path geometry– Contact pressure springs– Mechanism

Trip System– Ability to utilize the withstand capability of the

breaker– Dependent on many factors, such as...

– Accuracy of the sensors– Adjustments available to the user

Upstream breakers must have both for the best selective coordination!


Definition


The Challenge




Design Guidelines

Example

Summary



Short Circuit Selective Coordination TablesPresent short circuit selective coordination data for various tested combinations of low-voltage circuit breakers

May yield higher levels of short circuit selective coordination than the TCCs indicate

TCC studies still have to be made in order address overload and other protection issues



Short Circuit Selective Coordination TablesEaton

– Selective Coordination Industry Application (IA01200002E)– Selective Coordination Breaker Application Chart For

Molded Case Circuit Breakers fed by Distribution Transformers

General Electric– GE Overcurrent Device Instantaneous Selectivity Tables

(DET-537)Siemens

– Selective Trip Coordination with Molded Case Circuit Breakers

– Selective Trip Coordination with Molded Case Circuit Breakers



Short Circuit Selective Coordination TablesSquare D

– Short Circuit Selective Coordination for Low Voltage Circuit Breakers (0100DB0501)

– Selectivity Guidelines for Square D Panelboards(0100DB0604)

– Enhancing Short Circuit Selective Coordination with Low Voltage Circuit Breakers (011DB0403)


Design and Application GuidesEaton

– Selective Coordination

Square D– Guide to Overcurrent Coordination 600V and

Below (0100DB0603)



Other ResourcesGeneral Electric

– InstaPlan™ 2.0, a tool for planning power system instantaneous selective coordination

Eaton– 2005 NEC Selective Coordination Design Issues– Selective Coordination Calculator




Square D Assumptions

Circuit Breaker Contact PositionAll upstream breakers are in the closed (ON) position when the fault occurs

Instantaneous Trip SettingThe instantaneous trip setting on all upstream breakers, if adjustable, will be set to the highest position (OFF if available – select LSI or LSIG trip units)


Using Short Circuit Selective Coordination for Low Voltage Circuit Breakers (0100DB0501)


Basic Information Needed

System One-line Diagram

System Voltage

Circuit Ampacity

Available Short Circuit CurrentAdd motor contribution (if necessary)Adjust for X/R (if necessary)From the normal source to the lowest point in the systemFrom the alternate source to the first bus below the ATS


Available Short Circuit Current


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

Normal source SCA

Alternate source SCA

This assumes the alternate source SCA < the normal source SCA

Example #1 - System Description

System208Y/120 Vac system with 10 kSCA available at the lighting panelboard

EquipmentNQOD 225A main lugs lighting panelboard with single pole 20A QOB 10 kAIR rated circuit breakers fed from a QDA32225 circuit breaker with a 25 kA interrupting rating located in an upstream I-Line power panelboard


Example #1 - Determining the Level of Short Circuit Selective Coordination

1. Find the 240Vac table listing QO downstream circuit breakers in Appendix A Table 1 on page 6

2. Find the column for the QO downstream circuit breaker to be studied

3. Go down the column until the row listing the 225 A QD upstream circuit breaker is found

4. Read the selective coordination level at the intersection of the column and row, namely 2.0 kA. This means that the QD upstream circuit breaker is selectively coordinated with downstream QO circuit breakers up to 2,000 amps.


Example #1 - Improving the Level of Short Circuit Selective Coordination

Using the same example, determine if a higher level of selective coordination can be achieved by following these steps:

1. Move down the column for the QO downstream circuit breaker to be studied, looking for upstream breakers that will yield a higher level of selective coordination

2. When the desired level of selective coordination is found, read across the row to find the upstream breaker that will yield this level. In this case, an LA-MC circuit breaker will yield a level of selective coordination of up to 18,000 amps. Footnote 10 refers to Appendix B.



Referring to Appendix B Table 12 on page 38

1. Find the column for the QO downstream circuit breaker to be studied

2. Go down the column until the row listing the 225 A LA-MC upstream circuit breaker is found

3. Read the selective coordination level at the intersection of the column and row, namely 18 kA for a series rated application

4. Footnote 2 indicates that for a fully rated application the level of selective coordination is 10 kA. This means that the LA-MC upstream circuit breaker is fully selectively coordinated with downstream QO circuit breakers up to 10,000 amps




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Name: PD-0001 Manufacturer: *SQUARE D Type: LA, LH/MC Frame/Model: 250A Trip: 225 AVoltage: 240 VSettings: Phase Fixed

Name: PD-0006 Manufacturer: *SQUARED Type: QO, 1P Frame/Model: 20A Trip: 20 AVoltage: 240 VSettings: Phase Fixed (730-3)

Name: PD-0001 Manufacturer: *SQUARE D Type: LA, LH/MC Frame/Model: 250A Trip: 225 AVoltage: 240 VSettings: Phase Fixed

Name: PD-0006 Manufacturer: *SQUARED Type: QO, 1P Frame/Model: 20A Trip: 20 AVoltage: 240 VSettings: Phase Fixed (730-3)

Coordinates to 18kA per DataBulletin 0100DB0501Coordinates to 18kA per DataBulletin 0100DB0501

Current Scale x 1 Reference Voltage: 240

There is an overlap on the TCC

However, these two circuit breakers coordinate to 10 kA, the level of short-circuit current at the downstream circuit breaker

They are fully coordinated

A text note is used to delineate that this breaker combination coordinates above the level shown on the TCC

Example #2 - System Description

System480Y/277 Vac system with 25 kSCA available at the lighting panelboard

EquipmentNF 250A main lugs lighting panelboard with single pole EG 35 kAIR rated circuit breakers fed from a JGA36250 circuit breaker with a 35 kA interrupting rating located in an I-Line power panelboard


Example #2 - Determining the Level of Short Circuit Selective Coordination

1. Find the 480Vac table listing EG downstream circuit breakers in Appendix A, Table 6: on page 20

2. Find the column for the EG downstream circuit breaker to be studied

3. Go down the column until the row listing the 250 A JG upstream circuit breaker is found

4. Read the selective coordination level at the intersection of the column and row, namely 2.4 kA. This means that the JG upstream circuit breaker is selectively coordinated with downstream EG circuit breakers up to 2,400 amps.



Using the same example, determine if a higher level of selective coordination can be achieved by following these steps:

1. Move down the column for the EG downstream circuit breaker to be studied, looking for upstream breakers that will yield a higher level of selective coordination

2. When the desired level of selective coordination is found, read across the row to find the upstream breaker that will yield this level. In this case, a PG circuit breaker will yield a level of selective coordination of 35,000 amps. This means that the upstream PG circuit breaker is fully selective with downstream EG circuit breakers.




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Name: PD-0006 Manufacturer: *SQUARE D Type: EG Frame/Model: 20A Trip: 20 AVoltage: 480 VSettings: Phase Fixed

Name: PD-0001 Manufacturer: SQUARED Type: POWERPACT P-Frame, 3.0 & 3.0A Frame/Model: PG Trip: 250 AVoltage: 480 VSettings: Phase LTPU/LTD (A 0.4-1.0 x S) 1 (250A); 0.5 INST PG 250-1200 (1.5-12 X S) 6 (1500A)

Name: PD-0006 Manufacturer: *SQUARE D Type: EG Frame/Model: 20A Trip: 20 AVoltage: 480 VSettings: Phase Fixed

Name: PD-0001 Manufacturer: SQUARED Type: POWERPACT P-Frame, 3.0 & 3.0A Frame/Model: PG Trip: 250 AVoltage: 480 VSettings: Phase LTPU/LTD (A 0.4-1.0 x S) 1 (250A); 0.5 INST PG 250-1200 (1.5-12 X S) 6 (1500A)

Coordinates to 35ka per DataBulletin 0100DB0501Coordinates to 35ka per DataBulletin 0100DB0501

Current Scale x 1 Reference Voltage: 480

There is an overlap on the TCC

However, these two circuit breakers coordinate to 35 kA, above the level of short-circuit current at the downstream circuit breaker

They are fully coordinated

A text note is used to delineate that this breaker combination coordinates above the level shown on the TCC

Using Enhancing Short Circuit Selective Coordination with Low Voltage Circuit Breakers (011DB0403)


Using Enhancing Short Circuit Selective Coordination

How to Use Appendix AFind the table with the correct system voltageFind the breaker to be studiedFind the maximum instantaneous or selective override trip level

SuggestionUse this data bulletin when the desired combination cannot be found in Short Circuit Selective Coordination for Low Voltage Circuit Breakers


Using Enhancing Short Circuit Selective Coordination

Example – Coordination Across a LV TransformerTo select a primary circuit breaker that will coordinate with secondary branch circuit breakers:

1. Determine the available SCA downstream (assume 1 kA)2. Calculate the fault current that will be seen on the

primary: 208 V / 480 V = 0.433 x 1000 A = 433 A3. Find a circuit breaker with the proper voltage, interrupting

and continuous current ratings in Appendix A that also has an instantaneous trip of at least 433 A

4. Check to make sure that the primary circuit breaker meets the NEC transformer protection requirements

Enhancing Short Circuit Selective Coordination does not present new information, it just compiles it in one easy to use location!


Definition


The Challenge




Design Guidelines

Example

Summary



What to do?Understand the issues with the NEC requirements

Know the AHJ’s interpretation of NEC 700.27 and 701.18

Know how NEC 700.27 and 701.18 is enforced in your area (or if it will be enforced, state or local exceptions, etc.)

– Will selective coordination be required up to both the normal and alternate sources, or only up to the alternate source?

– What about existing equipment (such as an old switchboard feeding a new panelboard)?



Is selective coordination required up to both the normal and alternate sources, or only up to the alternate source?

700.1 Scope.The provisions of this article apply to…emergency systems…intended to supply, distribute, and control electricity…when the normal electrical supply or system is interrupted…These systems are intended to automatically supply illumination, power, or both…in the event of failure of the normal supply...The scope seems to imply only up to the alternate source




Appendix B.1 of NFPA 110 also seems to imply only up to the alternate source




But read 700.27 carefully

700.27 Coordination.Emergency system(s) overcurrent devices shall be selectively coordinated with all supply sideovercurrent protective devices.Normal system OCPDs are on the supply side of the emergency system, and thus are included (Square D interpretation)



Examples of Code ModificationsState of Washington

– 027 Coordination.The requirements for selective coordination described in NEC 700.27 are not required where the emergency system was installed prior to June 1, 2006. For new emergency systems that are supplied from an existing emergency system installed prior to June 1, 2006, the new portion of the emergency system must comply with NEC 700.27. The ground fault sensing function of overcurrent protective devices will only be required to selectively coordinate with the ground fault sensing functions of other overcurrent protective devices.

– A similar modification was made to 701.18


http://www.lni.wa.gov/


Examples of Code ModificationsState of Massachusetts 2008 MECAdded the following to 700.27 and 701.18Exception No. 2: Where the emergency system design is under the control of a licensed professional engineer engaged in the design or maintenance of electrical installations, the selection of overcurrent protective devices shall be permitted to coordinate to the extent practicable. The design shall be documented, stamped by the professional engineer, and made available for review by the authority having jurisdiction.FPN: Overcurrent protective devices used for emergency circuit protection, where coordinated to optimize selective operation ofthe circuit overcurrent protective devices when a short circuit or ground fault occurs, increase overall reliability of the system.



Examples of Code ModificationsCity of Tucson

– Section 700.27 Coordination. DELETE this section in its entirety.

– Section 701.18 Coordination. DELETE this section in its entirety.

State of Florida– Healthcare facilities come under the jurisdiction of

AHCA for plan check and inspection– Requires selective coordination to 0.1 sec.



Examples of Code ModificationsState of California

– Healthcare facilities come under the jurisdiction of OSHPD for plan check and inspection

– Proposals– 700.27 Coordination. Emergency system(s)

overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices. [Not permitted for OSHPD 1, 2, 3, & 4]

– 701.18 Coordination. Legally required standby system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices. [Not permitted for OSHPD 1, 2, 3, & 4]


http://www.oshpd.ca.gov/index.htm


Examples of Code ModificationsState of Wisconsin

– Proposals– 517.26 Application of Other Articles.

The essential electrical system shall meet the requirements of Article 700, except as amended by Article 517. Essential electrical system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices for faults with a duration of 0.1 seconds and longer. The selection and coordination of the overcurrent devices shall be documented and stamped by a professional engineer and approved by the engineer of record for the project. This study and all associated documentation shall be made available for review by the authority having jurisdiction.



Examples of Code ModificationsState of Wisconsin

– Proposals– 700.27 Coordination.– Emergency system(s) overcurrent devices shall be selectively coordinated

with all supply side overcurrent protective devices for faults with a duration of 0.1 seconds and longer.

– 701.18 Coordination.– Legally required standby system(s) overcurrent devices shall be selectively

coordinated with all supply side overcurrent protective devices for faults with a duration of 0.1 seconds and longer.

– 708.54 Coordination.– Critical operations power system(s) overcurrent devices shall be selectively

coordinated with all supply side overcurrent protective devices for faults with a duration of 0.1 seconds and longer.

– Exceptions: – (1) Between transformer primary and secondary overcurrent protective

devices, where only one overcurrent protective device or set of overcurrent protective devices exists on the transformer secondary,

– (2) Between overcurrent protective devices of the same size (ampere rating) in series.



Examples of Code ModificationsNew York City

– Question (from a consulting engineer dated 9/26/07)“The NEC sections 700.27 and 701.18 require that ‘Emergency System(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.’ Does this mean that all the emergency system overcurrent devices, from the smallest branch to the power supply, must be selectively coordinated?”

– Answer (from the NYC Electrical Code Revision and Interpretation Committee dated 9/15/07)“Selective coordination requirements for short-circuit conditions are defined in section 240.12. Emergency system(s) overcurrent devices, per section 700.27, shall be selectively coordinated for overcurrent conditions only.”



Examples of Code ModificationsNew York CityNYC Electrical Code (based on the 2005 NEC)Section 240.12 - Revise to read as follows:240.12 Electrical System Coordination. Rating and arrangement of service overcurrent devices, which have a rating above 601 amperes, shall be selectively coordinated. Such coordination shall provide a system of selective short circuit and overload protection between the service overcurrent protection and the second level overcurrent protection point. Where an orderly shutdown is required to minimize the hazard(s) to personnel and equipment, an additional overcurrent protection level is permitted. A system of coordination based on the following two conditions shall be permitted:

(1) Coordinated short-circuit protection(2) Overload indication based on monitoring systems or

devices.



Examples of Code ModificationsNew York CityNew York City Electrical Code (based on the 2005 NEC)Section 240.12 - Revise to read as follows:240.12 Electrical System Coordination. Exception No. 1: Service overcurrent devices which supply single loads (i.e., motors) shall not require coordination.Exception No. 2: Coordination between the service overcurrent device and distribution main shall not be required where the service disconnecting means supplies a single main overcurrent device for a single distribution panel or switchboard. However,selective coordination shall be required between distribution branch devices, and between the service equipment and the main panel. Exception No. 3: The provisions of this Section shall not apply to the operation of ground fault protection equipment.



Examples of EnforcementState of Washington

– Selective coordination must be assured by a state licensed PE

– The Department of Labor and Industries is interpreting the Code to mean that selective coordination is required only to the alternate source

City of Tacoma– Selective coordination must be assured by a state

licensed PE. A coordination study need not be submitted. (unconfirmed)

City of Denver– Selective coordination required only to the alternate

source (unconfirmed)



CautionsMake sure automatic transfer switches have adequate withstand ratings

– May need to relocate the switch, or

– May need to increase the frame size of the switch

Make sure busway has adequate withstand ratings

Make sure the generator protection devices coordinate with the downstream circuit breakers

Total ground fault selective coordination may not be possible, or may be difficult, due to other Code requirements [517.17(B)(1) and (2)]



CautionsMixing Overcurrent Protective Devices

– Mixing OCPDs from different manufacturers or mixing fuses and circuit breakers requires using TCCs only

– Fuse ratio or circuit breaker tables cannot be used

Arc Flash

– Selective coordination impact on arc flash PPE levels needs to be considered

– May be possible to reduce with Zone Selective Interlocking


Definition


The Challenge




Design Guidelines

Example

Summary


Design Guidelines

Conduct a Selective Coordination Study FirstBefore letting a job out for bid, conduct a selective coordination study first as it may affect the system design

Work from the Bottom UpStarting from the bottom of the system, coordinate the branch lighting panels first, then the power distribution panels, then the switchboard or switchgear


Design Guidelines

Nest CurvesThe time-current curve of a thermal-magnetic circuit breaker can sometimes be nested underneath the time-current curve of an upstream electronic trip circuit breaker

Name: PD-0006 Manufacturer: *SQUARE D Type: EG Frame/Model: 20A Trip: 20 AVoltage: 480 VSettings: Phase

Fixed

Name: PD-0001 Manufacturer: SQUARED Type: POWERPACT P-Frame, 3.0 & 3.0A Frame/Model: PG Trip: 250 AVoltage: 480 VSettings: Phase

LTPU/LTD (A 0.4-1.0 x S) 1 (250A); 0.5 INST PG 250-1200 (1.5-12 X S) 6 (1500A)

Name: PD-0006 Manufacturer: *SQUARE D Type: EG Frame/Model: 20A Trip: 20 AVoltage: 480 VSettings: Phase

Fixed

Name: PD-0001 Manufacturer: SQUARED Type: POWERPACT P-Frame, 3.0 & 3.0A Frame/Model: PG Trip: 250 AVoltage: 480 VSettings: Phase

LTPU/LTD (A 0.4-1.0 x S) 1 (250A); 0.5 INST PG 250-1200 (1.5-12 X S) 6 (1500A)


Design Guidelines

Use the Short Circuit Selective Coordination Tables Rather than the Time-Current Curves

Feeding Lighting Panelboards

Don’t feed lighting panelboards from lighting panelboards unless there is a transformer in between

Other Lighting Panelboard Recommendations

Better levels of selective coordination are available with 225A and larger panelboards

Consider using main lugs panels


Design Guidelines

Increase the Frame Size of the Upstream Circuit Breaker

The upstream circuit breaker should be at least one frame size larger than the downstream circuit breaker. This may necessitate increasing the size of panelboards and feeder conductors.

Very high levels of short circuit selective coordination may be achieved by using high amp frame electronic trip circuit breakers with low amp sensors and/or lower ampere rating adjustments


Design Guidelines


Rarely needed, but as a last resort...

Change the Upstream Circuit Breaker Type

Insulated case circuit breakers or low voltage power circuit breakers

Reduce the Voltage

If the desired level of selective coordination cannot be achieved using a 480Y/277Vac panelboard, consider feeding a 208Y/120Vac panelboard through a transformer

Split Up Some of the Loads (multiple smaller transformers)

Insert Impedance

Longer run of wire, 1:1 or higher impedance transformer or reactors

Design Guidelines

Generator ProtectionSelective coordination is sometimes difficult or impossible while providing adequate generator protectionBe wary of circuit breakers supplied with engine-generator sets

– They are often thermal-magnetic or electronic trip with LI protection

– They may need to be electronic trip with LS protection and high withstand if possible, or ANSI LV power circuit breakers

Make sure generator protective relays or controls will coordinate with the downstream devices


Design Guidelines

Challenge: CB1 and CB2 must both be selective with CB3, CB4, CB5 and all downstream breakers (CB6…)

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


Design Guidelines

One solution: More, smaller generators without paralleling

G

CB 1

G

AUTOXFERSW

TO NORMAL SOURCE

E NAUTOXFERSW

E N AUTOXFERSW

E N

CB 6

G Expensive!

Decreases reliability

Not always practical


Design Guidelines

Better solution:

Allow paralleling switchgear feeders to provide short-circuit protection

Supplement with bus-differential protection for the generator paralleling bus

Not a “cure-all”, but it does often help


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

87B PROTECTIVE ZONE

CB1 and CB2 set to provide overload, but not short-circuit, protection for the generators

These settings allow coordination with CBs on the level of CB3

Bus differential protection

provides short circuit

protection for the

generators for faults on

generator paralleling

bus

CBs on CB3 level provide short circuit protection for generators

Design Guidelines


Design Guidelines

Selective coordination requires an extremely high level of analysisOften not possible to achieve on conventional designs without major reconfiguration (cannot succeed with device selections alone)Expect significantly higher design time, space requirements, and equipment costsVendor-Specific DesignDifficult to delegate the design to a vendor by specifying

“vendor shall provide fully selective equipment” because of effects on equipment sizes, room sizes, system layout strategy, feeder sizes, etc.Bogue Waller, P.E., Principal Electrical Engineer, Nash LipseyBurch, LLC, Nashville, TN


Design Guidelines

Field Adjustment

Don’t neglect to properly adjust circuit breakers in the field as they are often shipped from the factory with all but the ampere-rating switch in the lowest position


Definition




Design Guidelines


Example

Summary


Example

Which of these circuit breakers need to be selectively coordinated?


Example


#3 must coordinate with #4 because it is a supply side device

Yes


Example


#1 does not need to coordinate with #3, assuming that they are both the same size, in accordance with Exception 2 in the 2008 NEC which states, "Exception: Selective coordination shall not be required in (1) or (2): (2) Between overcurrent protective devices of the same size (ampere rating) in series." #1 does need to coordinate with #4 and the other breakers on that bus. (Note: While this exception does not exist in the 2005 NEC, most engineers would come to this same conclusion, and most AHJs would probably agree.)

No

Yes


Example


#A does not need to coordinate with #1 or 3, however, it does need to coordinate with #4 because it is a supply side device

No

Yes


Example


#2 does not need to coordinate with #3, assuming that they are both the same size, but it does need to coordinate with #4 and the other breakers on that bus

No

YesYes


Example


#A, #B and #C are not required to be coordinated in accordance with the scope of Article 700 and drawing B.1 in NFPA 110 Annex B

This illustrates the problem with requiring coordination up to the normal source – if the normal loads are not coordinated, has the emergency system reliability really been improved?

No

No


Definition




Design Guidelines


Example

Summary


Summary

Understand the NECSelective coordination requirementsHow the AHJ will interpret and enforce them

Understand the Operating Characteristics of Circuit Breakers

Limitations of the TCCs in the short circuit zone


Summary

Understand the Design Characteristics of Circuit Breakers

Withstand Capability

Instantaneous Trip SettingField adjustable instantaneous adjustment can be turned OFF on LSI and LSIG trip units

Continuous Current Rating OverlapAllows for selecting a larger frame size breaker, particularly on electronic trip circuit breakers


Summary

Evaluation MethodologyConduct a short circuit study (from both sources)Make initial circuit breaker selectionsDetermine the selective coordination levels

Optimizing TechniquesUse the short circuit selective coordination tables rather than the TCCs if there is an overlap of the TCCs in the short circuit zoneConsider better system configurationsIncrease the frame size of the upstream breakersChange the upstream breaker typeAdd impedance to reduce short circuit current levels


Summary

Low voltage circuit breaker based systems can be selectively coordinated!

Thank You!

Questions?


selective coordination

Engineering