Selective Coordination
Schneider Electric
1
Objectives
Upon completion of this presentation you should be able to:
● Identify:
●The NFPA 70, 99 and 110 requirements for coordination
●The instantaneous trip functions of circuit breakers
●The limitations of using time-current curves to achieve
selective coordination
● Determine how to:
●Use short circuit selective coordination data
●Optimize system designs for selective coordination
2
Selective Coordination – Definitions
●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
3
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 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.
4
NEC Selective Coordination
Requirements
ENGINE-GENERATOR SET
PANEL 1
G
CB 1
PRIMARY CB
CB 1
PANEL 2
CB 2
LV TRANSFORMER
SECONDARY CB
SWITCHBOARD
CB 1
Making these circuit breakers coordinate with one
another does not enhance coordination!
5
Selective Coordination –
2014 Changes
● 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 110-2013, Standard for Emergency and Standby
Power Systems.
Informational Note No. 2: For additional information see
517.30 and NFPA 99-2012, Healthcare Facilities Code.
6
NEC Selective Coordination
Requirements
● Articles 700 and 701 Emergency Systems and Legally Required
Standby Systems
700.28 and 701.27 Selective Coordination
Emergency system (700.28) and legally required standby system
(701.27) 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: Not required between devices in series if no loads are
connected in parallel downstream
7
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.
8
NEC Selective Coordination
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.
9
NEC Selective Coordination Requirements
 Article 708 Critical Operations Power Systems


708.54: “…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.
911 centers, police/fire stations, government buildings
10
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).
11
Selective Coordination – Other NFPA
References
●Other Standards
● NFPA 99-2012 Standard for Health Care Facilities
● Three sections* read:
Overcurrent protective devices serving the essential electrical
system shall be selectively coordinated down to 0.1 seconds.
● Annex sections**
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.…
The terms coordination and coordinated do not cover the full
range of overcurrent conditions.
* 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
12
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…
13
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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Code Requirements Summary
Coordination to
System
Healthcare essential electrical
Healthcare GFPE
Which Source
What Level
Reference(s)
Alternate
0.1s
NEC 517.2, NFPA 99-2012
Normal & alternate
Total
NEC 517.17(C)
Elevator
Normal
1.
2.
Unspecified
Total
1. NEC-2011 620.62
2. NEC-2014 620.62
Fire pump
Normal
1.
2.
Unspecified
Total
1. NEC-2011 695.3(C)(3)
2. NEC-2014 695.3(C)(3)
Emergency
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
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
15
But aren’t all electrical systems coordinated?
 There are different levels of coordination corresponding to
the different types of overcurrent
 Overloads – properly designed systems are always
coordinated for overloads
 Short Circuits – systems may or may not be coordinated
for short circuits (bolted faults)
 Ground Faults - systems may or may not be coordinated
for ground faults
 A system that is coordinated for all three can be said to be
selectively (totally) coordinated
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The Selective Coordination Challenge
CB M1
CB F1
CB PM1
CB B1
 Coordination is typically achieved with circuit
breakers by adjusting the time-current curve
(TCC) characteristics of the devices to be
coordinated
 Coordinated in the overload region
 Seemingly not coordinated in the
instantaneous region
“Most hospitals are built using circuit
breaker circuit protection (which are
impossible to coordinate in the
instantaneous region due to their
mechanical design)…”1
 Is this true, or can selective coordination
be achieved with circuit breakers?
1
Circuit Protection in Hospitals, Consulting-Specifying Engineer, June/July 2011, pg. 22
17
Circuit Breaker Principles
 How the Instantaneous Trip Function Works
 T-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
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Circuit Breaker Principles
 How the Instantaneous Trip Function Works
 Electronic Trip Breakers
– Adjustable instantaneous
– Factory set low
– Final adjustment subject to +/-10% tolerance
– Instantaneous override
– Factory set for breaker self-protection
– Usually +/-10% tolerance
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100K
1000
1000
100
100
10
10
1
1
0.10
TIME IN SECONDS
5. For a withstand circuit breaker,
instantaneous can be turned OFF.
See 613-7 for instantaneous trip
curve. See 613-10 for
instantaneous override values.
CURRENT IN AMPERES
10K
10
1K
Instantaneous Override
100
Circuit Breaker Principles
0.10
TCC view
100K
10K
1K
0.01
10
100
0.01
Current Scale X 10^0
Reference Voltage: 480
20
Circuit Breaker Principles
● Factors Impacting Selective Coordination
Using time-current curves alone sometimes leads to the determination
of a 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
21
Circuit Breaker Principles
Overload
Region
Instantaneous
Region
T
I
M
E
CURRENT
● 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
22
1000
100
100
10
10
1
0.10
TIME IN SECONDS
1
0.10
100K
10K
1K
0.01
100
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
23
Using Short Circuit Selective Coordination
for Low Voltage Circuit Breakers
(0100DB0501)
24
Basic information needed




System One-Line Diagram
System Voltage
Circuit Ampacity
Available Short Circuit and Ground Fault Current
 Add motor contribution (if necessary)
 Adjust for X/R (if necessary)
 From the alternate source to the lowest point in the
system
 Possibly from the normal source (more on this later)
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Assumptions
 Instantaneous Trip Setting
 The instantaneous trip setting on all upstream breakers, if
adjustable, will be set to the highest position

For Micrologic electronic trip units
– Model 5.0 (LSI functions) or 6.0 (LSIG functions) will
be used
– Adjustable instantaneous trip setting will be set to
OFF (LS or LSG functions)
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Example – System Description
● System
480Y/277 Vac system with 25 kA available at the
lighting panelboard
● Equipment
NF 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
27
Example – Check the TCC
 There is an overlap on the TCC
28
Example – Improving the Level of
Selective Coordination
1. Find the 480Vac table listing EG
downstream circuit breakers in
Appendix B, Table 12 on page 23
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 2.5 kA. This means that
the JG upstream circuit breaker is
selectively coordinated with
downstream EG circuit breakers up to
2.5 kA
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Example – Improving the Level of
Selective Coordination
Your text here
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 kA. This means
that the upstream PG circuit
breaker is fully selective with
downstream EG circuit breakers.
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Example – Documenting the Level of
Selective Coordination
Coordinates to 35ka
per Data Bulletin
0100DB0501
 The TCC shows a lack of
selective coordination
 But these two circuit breakers do
selectively coordinate to 35 kA,
above the level of short-circuit
current at the downstream circuit
breaker
 A text note is used to delineate
that this circuit breaker
combination selectively
coordinates above the level
shown on the TCC
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Circuit Breaker Principles
 Upstream circuit breakers must have one of the
following for good selectivity:
1. Ability to withstand a high level of current
– Current path: geometry, contact pressure
springs, mechanism
– Trip system: instantaneous trip level, accuracy of
the sensors, adjustments available to the user
– Must have both for the best coordination
2. Trip system with instantaneous delay
– Ability to delay instantaneous tripping for ½ cycle
to allow time for the downstream circuit breaker
to clear the fault
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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
33
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)
● 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
34
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
● 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
35
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*
● Consider using main lugs panels*, particularly at 277 V
● Don’t daisy chain panel risers
● Consider using NQ selectively Coordinated Panelboard
* Exception: NQ Selectively Coordinated Panelboard
36
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
37
Design Guidelines
● Transformers
● Use the higher protection limits in Article 450
● 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
38
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
39
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)
40
Selective Coordination to Both Sources
G
Conduct
short circuit
and
coordination
studies
CB 1
CB 3
AUTOXFER
SW
E
N
TO NORMAL SOURCE
G
CB 2
CB 4
E
N
CB 5
AUTOXFER
SW
E
N
AUTOXFER
SW
Conduct short
circuit and
coordination
studies
CB 6
This assumes the alternate
source SCA < the normal
source SCA
41
Challenges Meeting the NEC
 Cautions
 Make sure automatic transfer switches have an
adequate withstand rating
– 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
42
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)
43
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 cannot be mixed if the fault current is outside the
parameters described above
44
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 PowerPact H or L frame
Mission Critical circuit breakers
45
Thank you!
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