selective coordination justin schroeder, p.e.. introduction definition nec coordination requirements...

Selective Coordination

Justin Schroeder, P.E.

Introduction

Definition NEC coordination requirements

2011 and 2014 editions NFPA 99-2012 requirements NFPA 110-2013 requirements Circuit breaker principles Resources and product solutions Challenges meeting the NEC requirements Design guidelines Summary

Definition: What is selective coordination?

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 selection and installation of overcurrent protective devices and their ratings or settings for the full range of available overcurrents, from overload to the maximum available fault current, and for the full range of overcurrent protective device opening times associated with those overcurrents.

In other words…

Only the overcurrent protective device (OCPD) nearest to a fault should clear the fault

Change

Selective Coordination Definition

The concept of protective zonesprotective zones is a useful tool to visualize this.UTILITY SERVICE

CB M1

CB F1

CB PM1

CB B1

CB M1 PRIMARYPROTECTIVEZONE

CB F1 PRIMARYPROTECTIVEZONE

CB PM1 PRIMARY PROTECTIVE ZONE

CB B1 PRIMARY PROTECTIVE ZONE

Fault in this zone CB M1 Trips

Fault in this zone CB F1 Trips

Fault in this zone CB PM1 Trips

Fault in this zone CB B1 Trips

NEC Selective Coordination Requirements

Article 517 Health Care Facilities 517.17(B) Feeders.

– Deleted the prohibition of a second step of GFPE on the generator feeder

– 2 levels of GFPE when main has GFPE per 230.95(C)– No GFPE on load side of ATS

517.17(C) Selectivity.– Deleted the requirement for 6 cycles of separation– GFPE must be selectively coordinated.

These changes should make coordination easier to achieve

Change


Article 517 Health Care Facilities

517.26 Application of Other Articles.

The life safety branch of the essential electrical system shall meet the requirements of Article 700, except as amended by Article 517 and NFPA 99 Chapter 6.

Informational Note No. 1: For additional information see NFPA 110-2010, Standard for Emergency and Standby Power Systems.

Informational Note No. 2: For additional information see 517.30 and NFPA 99 Chapter 6.

Change

Typical Health-Care Facility Electrical System (Informational Note Figure 517.30)

Must meet Article 700 per 2005-2011 editions

Must meet Article 700 per 2014 edition

Deleted in 2014 edition

Change

Changed to “branch” in 2014

edition

Definitions

The equipment system (Equipment Branch) – Meant for essential equipment loads

Feeds pumps, control systems, vacuums, essential heating, one elevator.

Can be fed by an alternate source at appropriate time-lag intervals.

The emergency system Life safety loads (Life safety Branch) – Meant for egress

Exit signs Elevators Automatic doors

Critical loads (Critical Branch) – Meant for patient care Select receptacles, power circuits related to patient

care, task illumination

Change


Article 517 Health Care Facilities

517.30 Essential Electrical Systems for Hospitals.

(G) Coordination. Overcurrent protective devices serving the essential electrical system shall be coordinated for the period of time that a fault’s duration extends beyond 0.1 second.

Exception No. 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.

Exception No. 2: Between overcurrent protective devices of the same size (ampere rating) in series.

Informational Note: The terms coordination and coordinated as used in this section do not cover the full range of overcurrent conditions.

New

Exception No 1

150A FEEDERSGHAFrame 400 ASensor/Trip 150.0 A

S

P 75 KVA TRANSFORMER75 kVAPri 480 VSec 208 VZ = 5.00 %

250A MAINSFLAFrame 250 ASensor/Trip 250.0 A

PANEL208 V

Exception No. 2

400A FEEDERHJXD6-AFrame 400 ASensor/Trip 400.0 A

MAIN PANEL480 V

400A MAINHJ6Frame 400 ASensor/Trip 400.0 A

PANEL480 V

CBL100 ft1 per phase500 AWG/kcmilTHHNMagneticCopper

Cable Riser Example


Article 620 Elevators, Dumbwaiters, Escalators, Moving Sidewalks, Wheelchair Lifts, and Stairway Lift Chairs

620.62 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. Selective Coordination shall be selected by a licensed professional engineer or other qualified persons engaged primarily in the design, installation, or maintenance of electrical systems. The selection shall be documented and made available to those authorized to design, install, inspect, maintain, and operate the system.

Change

Inspection Form

http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/solution-center/electrical_inspector_tools/BUS_Ele_Selective_Coord_Req_ChkList.pdf

Inspection form helps assure the AHJ that the equipment supplied and installed meets code requirements.


Article 645 Information Technology Equipment

645.2 Definitions.

Critical Operations Data System. An information technology equipment system that requires continuous operation for reasons of public safety, emergency management, national security, or business continuity.

645.27 Selective Coordination. Critical operations data system(s) overcurrent protective devices shall be selectively coordinated with all supply-side overcurrent protective devices.

New


Article 695 Fire Pumps

695.3 Power Source(s) for Electric Motor-Driven Fire Pumps.

(C) Multibuilding Campus-Style Complexes.

(3) Selective Coordination.

The overcurrent protective device(s) in each disconnecting means shall be selectively coordinated with any other supply-side overcurrent protective device(s).

New


Article 700 Emergency Systems

700.28 Selective Coordination. Emergency system(s) overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices.

Article 701 Legally Required Standby Systems

701.27 Selective Coordination. Legally required standbysystem(s) overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices.

Exception to Both

Exception: Selective coordination shall not be required between two overcurrent devices located in series if no loads are connected in parallel with the downstream device.


Articles 700 Emergency Systems and 701 Legally Required Standby Systems

Added to 700.28 and 701.27

Selective coordination shall be selected by a licensed professional engineer or other qualified persons engaged primarily in the design, installation, or maintenance of electrical systems. The selection shall be documented and made available to those authorized to design, install, inspect, maintain, and operate the system.

Change


Article 708 Critical Operations Power Systems

Added to 708.52(D) GFPE selectivity

Separation of ground-fault protection time-current characteristics shall conform to the manufacturer’s recommendations and shall consider all required tolerances and disconnect operating time to achieve 100 percent selectivity.

Same text as 517.17(C) GFPE selectivity for health care facilities

Change


Article 708 Critical Operations Power Systems

708.54 Selective Coordination. Critical operations powersystem(s) overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices.Selective coordination shall be selected by a licensed professional engineer or other qualified persons engaged primarily in the design, installation, or maintenance of electrical systems. The selection shall be documented and made available to those authorized to design, install, inspect, maintain, and operate the system.Exception: Selective coordination shall not be required between two overcurrent devices located in series if no loads are connected in parallel with the downstream device.

Change

Where is it required again?

Article 620 Elevators, Dumbwaiters, Escalators, Moving Sidewalks, Wheelchair Lifts, and Stairway Lift ChairsArticle 645 Information Technology Equipment (Critical Operation Data Systems)Article 695 Fire Pumps – campus style building layoutsArticle 700 Emergency SystemsArticle 701 Legally Required Standby SystemsArticle 708 Critical Operations Power Systems (COPS)

2014 NEC Errata: Deleted Figures

Oops!

State Adoptions of NFPA -70 (NEC)

http://www.nema.org/Technical/Code-Alerts/pages/default.aspx

GA adopted the 2014 edition at the beginning of 2015 FL, SC, NC, AL are all still on 2011 TN 2008 with 2009 amendments

How does this affect you? Different states will have different requirements and amendments to

different code cycles so be aware! Currently in GA, the 2014 code has been adopted.

NEC-2014 went to great lengths to standardize their terminology and definitions which may mean slightly different things to older code cycles.

NFPA 99-2012 Health Care Facilities Code

Added three new sections* to read:

Overcurrent protective devices serving the essential electrical system shall be selectively coordinated down to 0.1 seconds.

Added three new annex sections** to read:

It is important that the various overcurrent devices be coordinated, as far as practicable, to isolate faulted circuits and to protect against cascading operation on short circuit faults. In many systems, however, full coordination could compromise safety and system reliability…

* 6.4.2.1.2.1, 6.5.2.1.1.1, 6.6.2.1.1.1 ** A.6.4.2.1.2, A.6.5.2.1.1, A6.6.2.1.1

Change

NFPA 110-2013 Standard for Emergency and Standby Power Systems

6.5 Protection.6.5.1 General. The overcurrent protective devices in the EPSS shall be coordinated to optimize selective tripping of the circuit overcurrent protective devices when a short circuit occurs.6.5.2 Short Circuit Current. The maximum available short circuit current from both the utility source and the emergency energy source shall be evaluated for the ability to satisfy this coordination capability.

A.6.5.1 It is important that the various overcurrent devices be coordinated, as far as practicable, to isolate faulted circuits and to protect against cascading operation on short circuit faults. In many systems, however, full coordination is not practicable without using equipment that could be prohibitively costly or undesirable for other reasons…

Do NFPA 99-2012 and NFPA 110-2013 conflict with the NEC? No! NFPA 70 (NEC)

“This Code covers the installation of electrical conductors, equipment,…”

NFPA 99

“Chapter 6 covers the performance, maintenance and testing of electrical systems…”

NFPA 110

“This standard covers performance requirements for emergency and standby power systems…”

Standards Council

“NFPA 99 is considered the performance requirement and the NEC is considered the installation requirement.” (Similar statement made regarding NFPA 110)

Circuit Breaker Principles

Time-Current Curves (TCCs)

Developed by testing a single circuit breaker by itself

Two regions– Overload region: where the circuit

breaker has an inverse time characteristic

– Instantaneous region: where the circuit breaker operates “instantaneously”

“The instantaneous region is typically difficult to coordinate for two OCPDs connected in series”

Source: IEC/TR 61912-2

CURRENT

TIME

Instantaneous Region

Overload Region

Circuit Breaker Principles

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 instantaneous region may not be valid for two

circuit breakers connected in series Tested 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

SEC

ON

DS

Resources

Manufacturer references?Comparable selective coordination tables and tools are available from Schneider Electric, Eaton, GE and Siemens.

3rd Party System Analysis SoftwareSelective coordination information available in SKM, ETAP and Easypower products

NEMA ABP 1-2010 Selective Coordination White Paper Provides guidance on how to comply with selective

coordination requirements for design engineers and AHJs.

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

Challenges Meeting the NEC


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



700.28 Coordination.

Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.

Some interpret “all” to mean up to both sources



2011 NEC proposal 13-197 panel statement:

“…devices in the normal source…are outside the scope of Article 700.”

Use NEC Selective Coordination—Up to Which Source? (0600DB0902) to convince the AHJ that total coordination is required only up to the alternate source

Level of coordination up to the normal source is a matter of engineering judgment

In the 2014 NEC revision cycle CMP13 reversed itself, stating that selective coordination is required up to both sources (ROP 13-124 & ROC 13-83) March 8, 2012


Selective Coordination Update

Common misunderstandings and pitfalls Terminology review – 0.1s versus “total”

– We don’t say 0.01s, 0.05s, 0.001s, etc.– 0.1s – selectivity determined by TCCs – accounts for all initial

arcing and ground faults– Total – selectivity determined by tables comparing tested load-

side let-throughs and line-side instantaneous trips* (AND TCCs) – “selective for the entire operating ranges of the protective devices and for all levels of available fault current”

– 0.01s – terminology used, we believe mistakenly, intended for total coordination

* - SC selectivity based on test information

Selective Coordination Update

Common misunderstandings and pitfalls

Why we don’t say 0.01s, 0.05s, 0.001s, etc.

– If the fault current exceeds their current limiting point, current limiting fuses operate in less than 0.01s

– Current limiting fuses that coordinate down to 0.01s may not be coordinated at higher fault currents

– Current limiting fuse TCCs cut off at 0.01s because only the ratio tables can be used to evaluate total coordination below that point

Cautions Make sure automatic transfer switches have an

adequate withstand rating (30 cycle ratings vs. 3 cycle ratings)

– May need to move the switch away from the source, or

– May need to increase the frame size of the switch, or

– May need to specify a switch with a higher withstand rating (more than 3 cycles)

Make sure busway has adequate withstand ratings


Cautions Make sure the generator protection devices

coordinate with the downstream circuit breakers

Some manufacturers provide a time delay

Generator fault currentSCAgen = Gen FLC / x”d

x’’d => generator subtransient reactance

Gen FLC = kVA / (kVL-L * √3)


Short-circuit Decrement Curve of 250 kW Generator Plotted on 300 A LA curve

Cautions Mixing Overcurrent Protective Devices

– TCCs may be used so long as the fault current does not exceed the instantaneous trip point of the upstream circuit breaker or current limiting point of the upstream fuse

– OCPDs from different manufacturers or of different types usually cannot be mixed if the fault current is outside the parameters described above


Cautions Arc Flash

– Selective coordination may impact arc flash incident energy levels

– May be possible to reduce with…– Zone Selective Interlocking– Use of breakers LSI rather than

the traditional thermal magnetic breaker.

– Other means


Design Guidelines

Conduct Preliminary Short-Circuit & Selective Coordination Studies First

Reduce the number of levels (buses) that need to be coordinated to 3 or 4 if possible

Before letting a job out for bid, conduct preliminary short circuit and selective coordination studies first as they may affect the system design

Consider 3-phase and ground fault currents

If a lack of coordination seems to exist using the TCCs, use the Short Circuit Selective Coordination Tables

Work from the Bottom Up

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

Design Guidelines

Overlapping Curves If there is no overlap of the curves at any

point below the available short circuit current at the downstream panel, the circuit breakers are totally coordinated

If there is overlap in the short circuit region, use the short circuit selective coordination tables

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)

Downstream Available Short Circuit Current

Design Guidelines

Nest Curves

The time-current curve of a thermal-magnetic circuit breaker can sometimes be nested underneath the time-current curve of an upstream electronic trip circuit breakerName: 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)

Downstream Available Short Circuit Current

Design Guidelines

“Lighting” Panelboard Recommendations Don’t feed “lighting” panelboards from “lighting”

panelboards unless there is a transformer in between Better levels of selective coordination are available with

225 A and larger panelboards, (larger selection of breakers)

Consider using main lugs panels, particularly at 277 V Don’t daisy chain panel risers

Design Guidelines

Ground Fault Make sure system is selectively coordinated for ground

faults Make sure the GFPE also coordinates with the

downstream phase overcurrent devices, not just the downstream GFPE

Design Guidelines

Transformers

Use the higher protection limits in Article 450 (e.g. 250% instead of 125% FLA on the primary)

Consider 30 to 75 KVA transformers…100 to 225 amp panelboards with Mission Critical Mains

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/277 Vac panelboard, consider feeding a 208Y/120 Vac 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

What if the AHJ requires selective coordination up to both the alternate and normal sources?1. Conduct a preliminary short circuit study from the source

that can potentially produce the highest SCA to the bottom of the system

2. Conduct a preliminary selective coordination study on that part of the system

3. Conduct a preliminary short circuit study from the other source to the first bus on the load side of the transfer switch(es)

4. Conduct a preliminary selective coordination study from the other source to the transfer switch feeder(s)

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

Summary

Use the correct terminology! Terms to use

Coordination

“System to be coordinated to 0.1 s” Selective coordination

“System to be selectively coordinated”

DO NOT use “coordination to 0.01s” when selective coordination is desired

Summary

System

Coordination to

Reference(s)Which Source What Level

Healthcare essential electrical Alternate 1. Optimize as far as practicable

2. 0.1s

1. NEC-2011 700.1, 700.27; NFPA 110 6.5.1

2. NEC-2014 517.30(G), NFPA 99-2012

Healthcare GFPE Normal & alternate Total NEC 517.17(C)

Elevator Normal 1. Unspecified2. Total

1. NEC-2011 620.622. NEC-2014 620.62

Fire pump feeders in multi-building campus

Normal 1. Unspecified2. Total

1. NEC-2011 695.3(C)(3)2. NEC-2014 695.3(C)(3)

Emergency Alternate 1. Optimize as far as practicable

2. Total

1. NEC-2011 700.1, 700.27; NFPA 110 6.5.1

2. NEC-2014 700.28

Legally required Alternate 1. Optimize as far as practicable

2. Total

1. NEC-2011 701.1, 701.27; NFPA 110 6.5.1

2. NEC-2014 701.1, 701.27

COPS Alternate 1. Optimize as far as practicable

2. Total

1. NEC-2011 708.1, 708.54; NFPA 110 6.5.1

2. NEC-2011 708.1, 708.54

Questions???

Thank you!!!

Justin Schroeder, P.E.Schneider Electric USA

2979 Pacific Drive, Suite ENorcross, GA 30071

[email protected]: 770-734-1367

selective coordination justin schroeder, p.e.. introduction definition nec coordination requirements...

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