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SELECTIVE COORDINATION — RESPONSIBILITIES OF THE AHJ
Selective coordination restricts outages
to the circuit or equipment
affected, ensuring
reliability of electrical power.
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.December 2007 IAEI NEWS
November
Selective
Coordination
SELECTIVE COORDINATION — RESPONSIBILITIES OF THE AHJ
Responsibilities for the AHJ
by Mark Hilbert
S
elective coordination ensures proper isolation and
localization for all possible overcurrents (from
overloads to maximum short-circuit currents at
the point of application) to the nearest upstream overcurrent protective device and prevents unnecessary loss
of power to other loads, especially loads essential for life
safety. The Code requires selective coordination for all
supply-side overcurrent protective devices in the circuits
to a limited number of life-safety related loads. Selective coordination is just one of many requirements in
the Code that ensures that these vital loads that are essential for safety of human life remain energized without interruption. This is especially important in times
of emergency such as might occur during fires, natural
disasters, man-caused disasters, building failures, loss
of utility power, and other situations. The investment
of an alternate power source can be negated by the application of overcurrent protective devices that are not
selectively coordinated. The life safety benefits of the
selective coordination requirements include, but are
not limited to:
• Increased system reliability by maintaining power
availability to vital loads
• Reduced unnecessary power outages
• Increased safety for building inhabitants
• Quicker power restoration when a fault occurs
As with many Code requirements, a proactive approach by the authority having jurisdiction (AHJ) is
the key factor as to whether circuits supplying these vital loads employ overcurrent protective devices that are
selectively coordinated for all eventualities. There are
four important considerations to keep in mind:
1. It concerns the integrity of the life safety system
2. Three code panels have now included selective coordination requirements in four Code articles
3. Selective coordination can be achieved using circuit
breaker systems, fusible systems, or various combinations.
4. To enforce the requirements, the AHJ only needs
to know the requirements and understand the concept
of selective coordination. The AHJ does not have to be
an expert at device coordination—that is the responsibility of engineering and design.
Requirements
Selective coordination requirements for life safety are
not a new concept for the Code. There has been a Code
requirement to coordinate selectively the overcurrent
protective devices for elevator circuits since 1993. The
overcurrent protection selectivity in 620.62 is crucial for
a few specific reasons such as not stranding passengers
for normal operation and for emergency egress as well
as keeping elevators in use for emergency firefighting
operations. The original elevator requirement evolved
from the Canadian Code where the substantiation recognized that selective coordination ensures faster restoration of power (the electrician does not have to search
for upstream panels where the overcurrent devices unnecessarily cascaded open).
NEC-2005 expanded the selective coordination requirements for emergency system loads and legally required
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SELECTIVE COORDINATION — RESPONSIBILITIES OF THE AHJ
standby system loads via 700.27 and 701.18. Also, the
definition coordination (selective) was moved to Article
100 from Article 240. NEC-2008 includes a selective
coordination component in the exception to 700.9(B)(5).
Section 517.26 includes selective coordination for the essential electrical systems in health care facilities since it is
required to comply with Article 700 requirements, unless
otherwise amended in Article 517. Examples of loads that
require selectively coordinated overcurrent protective devices in the supplying circuit paths would be emergency
and egress lighting for the safe evacuation from a building,
and to assist in crowd and panic control. Also, in many
cases, ventilation, pressurization systems, automatic fire
detection systems, voice/alarm communication systems,
smoke control systems, fire command center systems, fire
alarm systems, elevators, elevator car lighting, fire pumps,
public safety communications, and continuous processes
may be classified to be supplied by an emergency or legally required standby system by the authority having jurisdiction or the locally adopted building code. Where hazardous materials are manufactured, processed, dispensed,
or stored, ventilation, treatment systems, temperature
control, alarm, detection, or other electrically operated
systems may be classified to be supplied by emergency or
legally required standby systems. The essential electrical
systems of health care facilities include the loads on the
critical branch, life safety branch, and equipment branch.
The emergency system of a hospital is made up of two
branches of the electrical system that are essential for life
safety and for the health and welfare of patients receiving
critical care and life support. These two branches are the
life safety branch and the critical system branch. In the 2008 Code process, every selective coordination requirement was challenged. With plenty of pro
and con proposals and comments submitted, after
presentations at the code panel meetings by interested
parties on both sides of the argument and after deliberate and thorough discussion by the respective code
panels, the existing requirements were overwhelmingly
retained; a clear message by the NEC technical committees. Panel 12 voted unanimously (11–0) to retain the
requirement for selective coordination in elevator circuits. Panel 13 voted 11–2 to add exceptions to 700.27
and 701.18 for two devices of the same ampere rating
in series and single devices on the primary and secondary of a transformer. (It was felt that neither exception
reduced life-safety because no additional parts of the
electrical system would be shut down unnecessarily.)
These exceptions were added by comments 13-185 and
13-238. In addition, the new Article 708 Critical Opwww.iaei.org
erations Power Systems included selective coordination
requirements via 708.54. Article 708 concerns itself
with systems powering loads vital to public health and
safety, national security, and the economy. The threat
of events such as 9/11 and similar natural disasters as
Hurricane Katrina were the catalyst for the creation
of Article 708. There were four comments to reduce
or eliminate the selective coordination requirement for
critical operations power system loads. Panel 20 voted
16–0 (three times) and 15–1 (one time) to reject all
attempts to reduce or eliminate this key life safety requirement.
In the 2008 Code cycle, Proposal 13-135 proposed
the elimination of the selective coordination requirement
for 700.27 and moving the language to a fine print note.
Code Panel 13 rejected this proposal 9–4. The panel
statement to this proposal sums up the matter well:
Panel Statement: This proposal removes the
selective coordination requirement from the
mandatory text and places it in a non-mandatory FPN. The requirement for selective
coordination for emergency system overcurrent devices should remain in the mandatory
text. Selective coordination increases the reliability of the emergency system. The current
wording of the NEC is adequate. The instantaneous portion of the time-current curve is
no less important than the long time portion.
Selective coordination is achievable with the
equipment available now.
Code Panel 20, which was responsible for the new
Article 708, summed up the need for selective coordination in their statement to Comment 20-13, which proposed deletion of the selective coordination requirement.
This comment was rejected 16–0. The actual panel statement to Comment 20-13:
Panel Statement: The overriding theme of
Articles 585 (renumbered to 708) is to keep
the power on for vital loads. Selective coordination is obviously essential for the continuity of service required in critical operations
power systems. Selective coordination increases the reliability of the COPS system.
Following are the 2008 Code requirements for selective coordination:
Article 100 Definitions
Coordination (Selective)
Localization of an overcurrent condition to restrict
outages to the circuit or equipment affected, accom-
.December 2007 IAEI NEWS
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SELECTIVE COORDINATION — RESPONSIBILITIES OF THE AHJ
plished by the choice of overcurrent protective devices
and their ratings or settings.
Article 708 Critical Operations Power Systems
708.54 Selective Coordination
Critical operations power system(s) overcurrent devices shall be selectively coordinated with all supplyside overcurrent protective devices.
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. So, What is Selective Coordination?
Article 100 provides the Code definition. Here is anothArticle 620 Elevators, etc.
er approach to describe selective coordination: “For the
620.62 Selective Coordination (2005)
full range of possible overcurrents, the act of isolating
Where more than one driving machine disconnecting means is supplied by a single feeder, the overcurrent an overloaded or faulted circuit from the remainder of
devices in each disconnecting means shall be selectively the electrical system, thereby eliminating unnecessary
coordinated with any other supply-side overcurrent pro- power outages.” The circuit causing the overcurrent is
isolated by the selective operation of only that overcurtective devices
rent protective device which is the closest upstream to
the overcurrent condition.
Article 700 Emergency Systems
700.9(B)(5)(b), Exception
Overcurrent protection shall be permitted at
the source or for the equipment, provided the
overcurrent protection is selectively coordinated
with the down stream overcurrent protection.
700.27 Coordination
Emergency system(s) overcurrent devices
shall be selectively coordinated with all supplyside overcurrent protective devices.
Exception: Selective coordination shall not be
required in (1) or (2):
(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,
(2) Between overcurrent protective devices of
the same size (ampere rating) in series.
Article 701 Legally Required Standby Systems
701.18. Coordination
Legally required standby system(s) overcurrent devices shall be selectively coordinated with
all supply-side overcurrent protective devices.
Exception: Selective coordination shall not be
required in (1) or (2):
(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,
(2) Between overcurrent protective devices of
the same size (ampere rating) in series.
IAEI NEWS November.December 2007
Figure 1
Figure 2
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SELECTIVE COORDINATION — RESPONSIBILITIES OF THE AHJ
each scenario. Some disruption is necessary due
to a fault. However, non-selective coordination
causes unnecessary power disruption to some
loads.
Key Aspects to Selective Coordination
Figure 3
Figure 4
Three important conditions for selective coordination:
• Achievable for the full range of possible overcurrents for the system involved
• Only the closest upstream overcurrent protective
device opens
• No other upstream (larger ampacity) overcurrent
protective devices open
Probably the best way to understand the concept of
selective coordination is with graphics. There are multitudes of possible scenarios and these represent a few.
Figures 1 and 2 illustrate selective coordination: figure
1 for a branch-circuit fault and figure 2 for a feeder-circuit fault. Figures 3, 4, 5, and 6 illustrate non-selective
coordination for both branch-circuit and feeder-circuit
faults. The most important aspect to visualize is the affect on the availability of power to individual loads for
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1. With today’s many options, it is possible to
design and install either fusible or circuit breaker systems that comply with the Code requirements for selective coordination. This article will
not get into how to use fuses or circuit breakers
to comply. If the reader is interested, contact the
fuse or the circuit breaker vendors. The fuse industry has published selectivity ratio tables for
decades. Since the 2005 Code requirements became effective, the circuit breaker vendors have
published materials to facilitate designing selectively coordinated circuit breaker systems. This
includes tables for circuit breakers that provide
the maximum available short-circuit current for
which specific circuit breakers are selectively coordinated.
2. These requirements are about reliability of
the power system supplying the designated vital
loads thus ensuring as much load availability as
possible. This is the case especially in emergency
situations or when the building or electrical system may be imperiled due to unusual or catastrophic conditions. The selective coordination
requirement (which also includes 517) in Articles 700, 701, and 708 encompasses the overcurrent protective devices in the circuit paths of
the normal source supplying these vital loads as
well as the circuit paths of the alternate source
supply. The requirements state, “overcurrent devices shall
be selectively coordinated with all supply-side overcurrent
protective devices.” These loads can be powered by the
normal source or the alternate source. So, the fuses or
circuit breakers must all be selectively coordinated from
the vital load branch-circuit devices up through the circuit paths to the main overcurrent protective device of
the normal source supply as well as from the vital load
branch-circuit devices up through the circuit paths to
the alternate source supply.
Why care about this? (see figure 7). If there is a fault
on the load side of the transfer switch at X1 or X2, only
the closest upstream circuit breaker or fuse (EB for X1
and EF1 for X2) is to open. Circuit breaker or fuse EF2
or the main fuse or circuit breaker is not to open (blue
path on left). The overcurrent protective devices in the
normal path must be selectively coordinated when fed
.December 2007 IAEI NEWS
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SELECTIVE COORDINATION — RESPONSIBILITIES OF THE AHJ
from the normal source, because if they are not
selectively coordinated, vital loads can be unnecessarily “left in the dark” (power loss). Let’s look
at a couple of scenarios:
Scenario (A): If a fault occurs on a branch
circuit at X1, and feeder overcurrent device EF1
unnecessarily opens, the ATS will not transfer
to the alternate source because power is still
available on the ATS’ line side. Yet many vital
loads will be unnecessarily without power.
Scenario (B): If a fault occurs on a branch at
X1, and feeder overcurrent devices EB, EF1, and
EF2 all open, the ATS will transfer to the alternate source (there is no power to the line side
of the ATS due to EF2 unnecessarily opening).
This transfer is unnecessary and would not have
to occur if the overcurrent protective devices
are selectively coordinated, but vital loads will
still be blacked out because EF1 has opened unnecessarily. The unnecessary transferring of the
power from the normal source to the alternate
source reduces the reliability of the system and
can lead to unavailability of power to vital loads,
if, for instance, the generator does not start or
the transfer switch does not operate properly.
3. The definition and requirements for selective coordination are very clear, understandable, and enforceable. During the 2008 codemaking process, proposals and comments were
submitted to alter the selective coordination
requirement to phrases such as “optimize selective coordination tripping,” “selective coordination for overcurrent protective device
opening times of 0.1 seconds or greater,” “only
apply for faults times greater than 3 cycles,” “coordination shall not be required in the current-limiting or
instantaneous portions of the time-current curves,” or
other similar language. In reality, these proposals and
comments would have changed the requirement to coordination for only overloads or very low-level faults
and ignored selective coordination for the full range of
overcurrents. Moderate and higher level fault currents
can and do occur. Code Panel 13’s statement in rejecting Proposal 13-135 was clear in that the requirement
includes all possible overcurrent levels: “…the instantaneous portion of the time-current curve is no less important than the long time portion.” And well it should.
In time of fire, catastrophes, failure of components,
improper installation of components, and worker incidents, overcurrents can run the full range of values
up to and including the maximum three-phase bolted
Figure 5
Figure 6
fault current. High faults are less probable, but they
do occur. Another phrase that is unenforceable is “selective coordination where practicable.” (Where is it
delineated as to what or when selective coordination is
practicable? Is it impracticable because of cost? What
is the cost of a life? Is it impracticable because a selective coordination analysis may require more effort to
engineer? Is it impracticable if selectively coordinated
gear takes up more space?) The present selective coordination definition and requirements are very clear
and achievable. These proposals and comments to
change the requirement were thoroughly discussed by
the code panels and overwhelmingly rejected. Practical Approach for the AHJ
The AHJ needs to know the requirements and enforce
compliance. It is nice if the AHJ understands what se-
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SELECTIVE COORDINATION — RESPONSIBILITIES OF THE AHJ
lective coordination means. However, it is not necessary
for the AHJ to know how to achieve selective coordination at the engineering level. That requires specific knowhow best left to the engineering community. The AHJ
in carrying out his or her responsibilities, should require
engineers to submit a selective coordination analysis for
the circuit paths required to be selectively coordinated
per Articles 517, 620, 700, 701, and 708. The analysis
should have a summary statement clearly stating that
selective coordination has been achieved and the analysis should include the engineer’s seal. (Although not allinclusive, an example checklist is provided at the end of
this article. It simplifies the record-keeping of the AHJ
and puts the onus on the consulting engineer.) During
installation, the AHJ should verify (spot check) that
the specified overcurrent protective devices with correct
ratings or settings have been installed and match the
in a broader context, illustrated by requirements exengineering design.
panding the use of AFCIs and now requiring tamperproof receptacles. Selective coordination of overcurrent
Conclusion
In recent Code cycles, safety for persons is being taken protective devices is another requirement increasing
safety for persons. The requirements pertain to the
overcurrent protective devices in the circuit paths
supplying a limited number of loads which are vital
during emergencies or which have critical availability
requirements. Selective coordination requirements entered the Code in 1993 and the requirements have continued to expand where there are life safety concerns.
Circuit breaker or fusible systems can be designed and
installed to comply with the selective coordination requirements. The AHJ does not have to be an expert in
overcurrent protection coordination to enforce these
requirements. Just be sure the engineer provides suitable documentation that includes an analysis. The
analysis must encompass all the circuit paths supplyFigure 7
ing the designated loads, including the normal source
path and the alternate source path to those vital loads,
and for the full range of overcurrents possible.
Figure 8
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Mark Hilbert is the chief electrical inspector for the state of New
Hampshire, Bureau of Electrical Safety and Licensing and has been
employed by the office for 14 years. He holds all the IAEI electrical
inspector certifications. He holds a master electrician’s license in two
states and is a former electrical contractor in the states of New Hampshire and Massachusetts. Prior to opening his contracting business, he
had over seven years of industrial electrical experience. Mark represents
IAEI as an alternate member on Code Making Panel 4 and is a principle member on the NFPA 79 Committee, the Standard for Electrical
Equipment of Industrial Machinery. He is past president of the Granite State Chapter and the Eastern Section. He is the Eastern Section’s
delegate to the IO Board of Directors. Mark has taught electrical code
classes since 1994 and has over 35 years of experience in the electrical
industry. He has taught the National Electrical Code and promoted
electrical safety nationally and internationally.
.December 2007 IAEI NEWS
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SELECTIVE COORDINATION — RESPONSIBILITIES OF THE AHJ
Check List: Selective Coordination
Requirements for Life Safety Loads
Several sections in the Code require all supply side overcurrent protective devices to be selectively coordinated in
the circuits supplying life-safety-related loads. These loads
are those supplied by elevator circuits (620.62), emergency systems (700.9(B)(5)(b) Exception and 700.27), legally
required standby systems (701.18), and critical operations
power systems (708.54). These requirements have been
taken into account and the installation has been designed
to meet the following sections for both the normal and
alternate source circuit paths to the loads: (Check all that
apply.)
Article 620 Elevators, Dumbwaiters, Escalators, Moving Walks, Wheelchair Lifts, and
Stairway Chair Lifts
620.62 Selective Coordination
Where more than one driving machine disconnecting
means is supplied by a single feeder, the overcurrent
devices in each disconnecting means shall be selectively coordinated with any other supply side overcurrent
protective devices.
Article 700 Emergency Systems
Exception to 700.9(B)(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.
700.27 Coordination
Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices.
(See exception for single devices on the primary and
secondary of a transformer and two devices of the same
ampere rating in series.)
Article 701 Legally Required Standby Systems
701.18. Coordination
Legally required standby system(s) overcurrent devices
shall be selectively coordinated with all supply side overcurrent protective devices.
(See exception for single devices on the primary and
secondary of a transformer and two devices of the same
ampere rating in series.)
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.
Article 517 Healthcare Facilities
517.26 Application of Other Articles
The essential electrical system shall meet the requirements of Article 700, except as amended by Article 517.
(The essential electrical systems of health care facilities include the loads on the critical branch, life safety
branch, and equipment branch.)
In order to ensure the design meets the requirements
for selective coordination, we utilized industry-accepted
tool(s). Where time-current curves are utilized, we analyzed the curves to assure that there is no overlap of curves
for all available levels of overcurrent. [Check (below) all
tools that were utilized, and provide the supporting documentation. Appropriate documentation includes copies of
fuse and circuit breaker manufacturers’ selectivity charts/
tables or circuit breaker time-current curves for each circuit path, with available short-circuit current analysis for
applicable points in the system.]
Fuse System
Fuse Selectivity Ratio Guide (Short-circuit current
calculations are not necessary if available short-circuit
current is less than or equal to 200 kA.)
Circuit Breaker Systems
Time-current characteristic curves coordination
study: Includes integrating the short-circuit current analysis at necessary points in the system, plots of time-current curves, and analysis/interpretation of the curves to
ensure selective coordination is achieved. Short-circuit
current calculations were performed for all points where
circuit breakers are applied.
Selective Coordination Tables: Selective coordination is achieved by comparing the available short-circuit
currents to the specific circuit breaker to circuit breaker
selective coordination values in the manufacturers’ tables. Short-circuit current calculations were performed
for all points where circuit breakers are applied.
Signature Date PE Seal
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