Selective Coordination

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Selective Coordination
Justin Schroeder, P.E.
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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

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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
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Selective Coordination Definition

• The concept of protective zones is a useful tool
  to visualize this.

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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
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NEC Selective Coordination Requirements

• 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
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Typical Health-Care Facility Electrical System
(Informational Note Figure 517.30)
Change
Must meet Article 700 per 2014 edition
Changed to branch in 2014 edition
Deleted in 2014 edition
Must meet Article 700 per 2005-2011 editions
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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
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NEC Selective Coordination Requirements

• 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 faults
  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
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Exception No 1
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Exception No. 2
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Cable Riser Example
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NEC Selective Coordination Requirements

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

• http//www.cooperindustries.com/content/dam/public
  /bussmann/Electrical/Resources/solution-center/ele
  ctrical_inspector_tools/BUS_Ele_Selective_Coord_Re
  q_ChkList.pdf

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NEC Selective Coordination Requirements
New

• 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.

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NEC Selective Coordination Requirements

• 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
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NEC Selective Coordination Requirements

• 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
  standby
• system(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.

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NEC Selective Coordination Requirements

• 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
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NEC Selective Coordination Requirements

• 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
  manufacturers 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
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NEC Selective Coordination Requirements

• Article 708 Critical Operations Power Systems
• 708.54 Selective Coordination. Critical
  operations power
• system(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
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Where is it required again?

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

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2014 NEC Errata Deleted Figures
Oops!
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State Adoptions of NFPA -70 (NEC)

• http//www.nema.org/Technical/Code-Alerts/pages/de
  fault.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.

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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
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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

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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)

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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

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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

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Resources

• Manufacturer references?
• Comparable selective coordination tables and
  tools are available from Schneider Electric,
  Eaton, GE and Siemens.
• 3rd Party System Analysis Software
• Selective 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.

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Challenges Meeting the NEC

• 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
  toemergency systemsintended to supply,
  distribute, and control electricitywhen the
  normal electrical supply or system is
  interrupted
• These systems are intended to automatically
  supply illumination, power, or bothin the event
  of failure of the normal supply...
• The scope seems to imply only up to the
  alternate source

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Challenges Meeting the NEC

• Is selective coordination required up to both the
  normal and alternate sources, or only up to the
  alternate source?
• Appendix B.1 of NFPA 110 seems to imply only up
  to the alternate source

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Challenges Meeting the NEC

• Is selective coordination required up to both the
  normal and alternate sources, or 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

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Challenges Meeting the NEC

• Is selective coordination required up to both the
  normal and alternate sources, or only up to the
  alternate source?
• 2011 NEC proposal 13-197 panel statement
• devices in the normal sourceare outside the
  scope of Article 700.
• Use NEC Selective CoordinationUp 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

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Selective Coordination Update

• Common misunderstandings and pitfalls
• Terminology review 0.1s versus total
• We dont 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

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Selective Coordination Update

• Common misunderstandings and pitfalls
• Why we dont 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

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Challenges Meeting the NEC

• 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

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Challenges Meeting the NEC

• Cautions
• Make sure the generator protection devices
  coordinate with the downstream circuit breakers
• Some manufacturers provide a time delay
• Generator fault current
• SCAgen Gen FLC / xd
• xd gt generator subtransient reactance
• Gen FLC kVA / (kVL-L v3)

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Challenges Meeting the NEC

• 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

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Challenges Meeting the NEC

• 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

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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

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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

Downstream Available Short Circuit Current
42
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 breaker

Downstream Available Short Circuit Current
43
Design Guidelines

• Lighting Panelboard Recommendations
• Dont 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
• Dont daisy chain panel risers

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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

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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 transformers100 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

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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, 11 or higher impedance
  transformer or reactors

47
Design Guidelines

• What if the AHJ requires selective coordination
  up to both the alternate and normal sources?
• Conduct a preliminary short circuit study from
  the source that can potentially produce the
  highest SCA to the bottom of the system
• Conduct a preliminary selective coordination
  study on that part of the system
• Conduct a preliminary short circuit study from
  the other source to the first bus on the load
  side of the transfer switch(es)
• Conduct a preliminary selective coordination
  study from the other source to the transfer
  switch feeder(s)

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Design Guidelines

• Field Adjustment
• Dont 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

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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

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Summary
System Coordination to Coordination to Reference(s)
System Which Source What Level Reference(s)
Healthcare essential electrical Alternate Optimize as far as practicable 0.1s NEC-2011 700.1, 700.27 NFPA 110 6.5.1 NEC-2014 517.30(G), NFPA 99-2012
Healthcare GFPE Normal alternate Total NEC 517.17(C)
Elevator Normal Unspecified Total 1. NEC-2011 620.62 2. NEC-2014 620.62
Fire pump feeders in multi-building campus Normal Unspecified Total 1. NEC-2011 695.3(C)(3) 2. NEC-2014 695.3(C)(3)
Emergency Alternate Optimize as far as practicable Total NEC-2011 700.1, 700.27 NFPA 110 6.5.1 NEC-2014 700.28
Legally required Alternate Optimize as far as practicable Total NEC-2011 701.1, 701.27 NFPA 110 6.5.1 NEC-2014 701.1, 701.27
COPS Alternate Optimize as far as practicable Total NEC-2011 708.1, 708.54 NFPA 110 6.5.1 2. NEC-2011 708.1, 708.54
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Questions??? Thank you!!!Justin Schroeder,
P.E.Schneider Electric USA2979 Pacific Drive,
Suite E Norcross, GA 30071Justin.Schroeder_at_schnei
der-electric.comPhone 770-734-1367