Transcript Selective Coordination Overview of Short Circuit Selective

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?
Change
 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
Selective Coordination Definition
 The concept of protective zones is a useful tool to visualize this.
UTILITY SERVICE
Fault in this zone
 CB M1 Trips
CB M1
CB M1 PRIMARY
PROTECTIVE
ZONE
CB F1
Fault in this zone
 CB F1 Trips
CB F1 PRIMARY
PROTECTIVE
ZONE
Fault in this zone
 CB B1 Trips
CB PM1
Fault in this zone
 CB PM1 Trips
CB B1
CB B1 PRIMARY PROTECTIVE ZONE
CB PM1 PRIMARY PROTECTIVE ZONE
NEC Selective Coordination Requirements
Change
 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
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.
Change
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.
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
Definitions
Change
 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
NEC Selective Coordination Requirements
 Article 517 Health Care Facilities
517.30 Essential Electrical Systems for Hospitals.
New
(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.
Exception No 1
150A FEEDER
SGHA
Frame 400 A
Sensor/Trip 150.0 A
P
S
75 KVA TRANSFORMER
75 kVA
Pri 480 V
Sec 208 V
Z = 5.00 %
250A MAIN
SFLA
Frame 250 A
Sensor/Trip 250.0 A
PANEL
208 V
Exception No. 2
MAIN PANEL
480 V
400A FEEDER
HJXD6-A
Frame 400 A
Sensor/Trip 400.0 A
CBL
100 ft
1 per phase
500 AWG/kcmil
THHN
Magnetic
Copper
400A MAIN
HJ6
Frame 400 A
Sensor/Trip 400.0 A
PANEL
480 V
Cable Riser Example
NEC Selective Coordination Requirements
 Article 620 Elevators, Dumbwaiters, Escalators, Moving
Sidewalks, Wheelchair Lifts, and Stairway Lift Chairs
620.62 Selective Coordination.
Change
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.
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/Resourc
es/solutioncenter/electrical_inspector_tools/BUS_El
e_Selective_Coord_Req_ChkList.pdf
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.
NEC Selective Coordination Requirements
 Article 695 Fire Pumps
695.3 Power Source(s) for Electric Motor-Driven Fire
Pumps.
New
(C) Multibuilding Campus-Style Complexes.
(3) Selective Coordination.
The overcurrent protective device(s) in each disconnecting
means shall be selectively coordinated with any other supplyside overcurrent protective device(s).
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.
NEC Selective Coordination Requirements
 Articles 700 Emergency Systems and 701 Legally
Required Standby Systems

Change
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.
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 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
NEC Selective Coordination Requirements
Change
 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.
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)
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.
Change
 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
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
Overload
Region
Instantaneous
Region
 Time-Current Curves (TCCs)


T
I
M
E

CURRENT
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
1000
100
100
10
10
1
0.10
0.10
100K
10K
1K
100
0.01
TIME IN SECONDS
1
0.01
10
 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
100K
10K
CURRENT IN AMPERES
1K
10
1000
100
Circuit Breaker Principles
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.
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 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
 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
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
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 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 loadside 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
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
Challenges Meeting the NEC
Short-circuit
Decrement
Curve of
250 kW
Generator
Plotted on
300 A LA
curve
 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 / x”d
x’’d => generator subtransient reactance
Gen FLC = kVA / (kVL-L * √3)
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
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
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
Name: PD-0001
Manufacturer: *SQUARE D
Type: LA, LH/MC
Frame/Model: 250A
Trip: 225 A
Voltage: 240 V
Settings: Phase
Fixed
Name: PD-0006
Manufacturer: *SQUARED
Type: QO, 1P
Frame/Model: 20A
Trip: 20 A
Voltage: 240 V
Settings: Phase
Fixed (730-3)
Downstream Available Short Circuit Current
 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
Design Guidelines
Name: PD-0001
Manufacturer: SQUARED
Type: POWERPACT P-Frame, 3.0 & 3.0A
Frame/Model: PG
Trip: 250 A
Voltage: 480 V
Settings: 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 A
Voltage: 480 V
Settings: Phase
Fixed
Downstream Available Short Circuit Current
 Nest Curves
The time-current curve of a thermalmagnetic circuit breaker can
sometimes be nested underneath the
time-current curve of an upstream
electronic trip circuit breaker
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
Coordination to
System
Which Source
Healthcare essential electrical
Alternate
Healthcare GFPE
What Level
1. Optimize as far as
practicable
2. 0.1s
Normal & alternate
Total
Reference(s)
1. NEC-2011 700.1, 700.27; NFPA
110 6.5.1
2. NEC-2014 517.30(G), NFPA 992012
NEC 517.17(C)
Elevator
Normal
1.
2.
Unspecified
Total
1. NEC-2011 620.62
2. NEC-2014 620.62
Fire pump feeders in multibuilding campus
Normal
1.
2.
Unspecified
Total
1. NEC-2011 695.3(C)(3)
2. NEC-2014 695.3(C)(3)
Alternate
1.
Optimize as far as
practicable
Total
1.
Optimize as far as
practicable
Total
1. NEC-2011 701.1, 701.27; NFPA
110 6.5.1
2. NEC-2014 701.1, 701.27
Optimize as far as
practicable
Total
1. NEC-2011 708.1, 708.54; NFPA
110 6.5.1
2. NEC-2011 708.1, 708.54
Emergency
2.
Legally required
Alternate
1.
2.
COPS
Alternate
1.
2.
2.
NEC-2011 700.1, 700.27; NFPA
110 6.5.1
NEC-2014 700.28
Questions???
Thank you!!!
Justin Schroeder, P.E.
Schneider Electric USA
2979 Pacific Drive, Suite E
Norcross, GA 30071
[email protected]
Phone: 770-734-1367