Selective
Coordination
SELECTIVE COORDINATION FOR CRITICAL SYSTEMS |
by Dan Neeser
Introduction
Selective coordination has been required for multiple elevators, supplied from a single feeder, since
the 1993 National Electrical Code (NEC®). This
requirement was based on a similar requirement
in the Canadian Electrical Code. In the 2005 NEC,
requirements for selective coordination were added
to 700.27 for emergency (700.28 in 2014 NEC) and
701.18 for legally required standby systems (701.27
in the 2014 NEC). These requirements additionally
applied to essential electrical systems in healthcare
facilities, since these systems were required to follow
the requirements of emergency systems. In addition,
a definition of Coordination (Selective) was modified
and moved from Article 240 to Article 100 to better
define and emphasize the objective of this requirement. This definition was important to identify the
“level” of coordination when compared to traditional
“coordination and short-circuit studies”. The definition of selective coordination indicated localization of
all overcurrent conditions. If one looks at the definition of overcurrent in Article 100, it indicates that this
includes overloads, ground faults and short-circuits.
Therefore, selective coordination is considered
“total” coordination. Traditional coordination and
short-circuit studies, did not necessarily assure “total”
coordination, simply the recommended settings for
the devices installed to achieve the best “level” of
coordination possible with the installed overcurrent
protective devices.
In subsequent editions of NEC, the requirements
for selective coordination have been expanded to
include Critical Operation Power Systems (COPS)
in the 2008 NEC and Fire Pumps in Multibuilding
Campus-Style Complexes in the 2011 NEC.
Over the years, the biggest challenge for the electrical inspector with regard to selective coordination
was assuring code-compliant selectively coordinated
systems were being installed. This was based on
two concerns. First, was the definition of selective
coordination properly interpreted as “total” coordination? Second, were the overcurrent protective
devices properly selected, documented, and installed
to meet the definition of selective coordination? This
article will discuss changes that have occurred in the
for Critical
Systems per
the 2014 NEC
66 IAEI NEWS May . June 2014
www.iaei.org
www.iaei.org
May . June 2014 IAEI NEWS
67
SELECTIVE COORDINATION FOR CRITICAL SYSTEMS |
2014 NEC to address these two concerns as well as
examples of documentation that must be submitted
to support compliance with selective coordination.
Definition of Selective Coordination
The first change in the 2014 NEC is to the definition of Coordination (Selective). This definition was
revised to:
Coordination (Selective). Localization of
an overcurrent condition to restrict outages
to the circuit or equipment affected, accomplished by the choice 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.
These revisions clarified the intent of the definition. The first change was to replace “choice” with
“selection and installation”. This helps clarify that
it is not a “choice” but rather the proper “selection
and installation” of overcurrent devices (types,
ratings and settings) that achieves selective coordination. The additional text added to the end of the
definition clarifies that the term selective coordination applies to the full range of overcurrents, from
overload to the maximum available fault current
and all operating times of overcurrent devices for
these overcurrent conditions. This added language
makes it clear that “partial or limited coordination,”
such as coordination for operating times of 0.1
seconds or longer would not meet the definition of
selective coordination.
Selective Coordination
Documentation Requirements
A key concern for selective coordination is to provide
proper documentation of compliance. The 2014
NEC has addressed this concern by adding new text
to several sections of the Code where selective coordination is required. This new text has been added
to 620.62; 700.28 (previously 700.27); 701.27; and
708.54 and is shown below:
“Selective coordination shall be selected by a licensed professional engineer or other qualified
person 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.”
This change makes it clear that persons competent
for the task must select overcurrent protective devices that will selectively coordinate and the selection
must be documented. The documentation provides
the detail on the selection of each overcurrent protective device and substantiates that all the overcurrent
protective devices are selectively coordinated. This
includes detailing each overcurrent protective device
type, ampere rating, and settings. The documentation must be made available to the inspector, contractor, and system owner. This is especially important
during the installation phase of the electrical equipment. When electrical equipment is shipped from the
manufacturing location, the circuit breakers are not
set as indicated in the selective coordination study
and fuses of the correct type and ampacity rating
are not installed. As such, the installer must use this
documentation to assure the proper circuit breaker
(types, setting and ratings) and fuses (types and ratings) are installed. This new language, which was proposed by IAEI International, will help improve the
process for the enforcement authorities. In addition,
the documentation provides the future maintainers
with the details so that the system can be maintained
as a selectively coordinated system.
From the inspector standpoint, use of “checklists”
or “inspection forms” can be useful tools to communicate important Code requirements to electrical
designers, manufacturers and installers. With regard
to selective coordination, the electrical designer may
not always be the person who selects the overcurrent
protective devices to comply with the requirements
for selective coordination. Some electrical designers
will specify a “basis of design” to achieve selective
coordination. Other electrical designers may simply
indicate that the electrical equipment manufacturer
or installer must select and provide documentation
68 IAEI NEWS May . June 2014
www.iaei.org
in series with each other. Also plotted is
the calculated maximum available fault
current. This is an example of traditional
time-current curves. This TCC curve
for a circuit breaker system is not selectively coordinated (as indicated by the
overlap in circuit breaker time-current
curves for each fault current value).
As shown in figure 2, for a fault at the
furthest bus downstream on the load
side of the 20-A circuit breaker, all three
overcurrent protective devices see a
fault current of up to 6,500 amps which
exceeds the instantaneous pickup of all
three overcurrent protective devices.
Therefore, all three circuit breakers
may trip. If the trip curves are the only
method being used to determine selectivity, these devices do not selectively
coordinate.
There are options available to achieve
selective coordination for the system
shown in figure 2. The first and most
cost effective method is to leverage
circuit breaker selective coordination
tables to show selectivity even though
Figure 1: Example of a selective coordination inspection form for
electrical inspectors.
there is an overlap of curves at the
specified fault current. Another method is to select
of compliance with selective coordination. In either
circuit breakers based on TCC curves, circuit breakcase, a Selective Coordination Inspection Form, as
ers that incorporate higher instantaneous trips and/
shown in figure 1, is an example of an inspection
or short-time delay settings that avoid overlap of
tool that can be used to make sure this selection and
the downstream circuit breaker time-current curves
documentation are provided and the person responsible for this selection has signed a document indicat- for the specified levels of fault current. Selective
coordination tables published by the circuit breaker
ing compliance with these requirements.
manufacturer indicate the maximum level of available fault current to which pairs of circuit breakers
Compliance with
achieve selective coordination for all values of time
Selective Coordination Requirements
(even below 0.01 seconds as shown traditional timeIn order to verify compliance with requirements for
current curves). Circuit breaker manufacturers also
selective coordination, time-current characteristic
publish coordination tables for 0.1 seconds which
(TCC) curves can be constructed to show the openonly confirm separation of trip curves for times being and clearing times of the overcurrent devices for
yond 0.1 seconds (overload conditions only).
all values of overcurrents up to the maximum availWhen applying fuse type overcurrent protective
able fault current. In order to achieve selective coordevices, such as current-limiting fuses, selective codination, there can be no overlap of TCC curves at
ordination is achieved in a similar, but not identical,
the fault current, which the devices will see. Figure 2
manner. Figure 3 shows an example of time-current
is a plot of the TCC curves for three circuit breakers
www.iaei.org
May . June 2014 IAEI NEWS
69
SELECTIVE COORDINATION FOR CRITICAL SYSTEMS |
Figure 2: Example time-current curves for circuit breakers where selective coordination is not achieved by the overcurrent devices at the calculated values of available fault current due to overlap in the time-current curves.
curves for a fuse system that is selectively coordinated.
Figure 3 also shows a manufacturer specific selective
coordination ampacity ratio table for specific types of
current-limiting fuses. As shown in the Eaton’s Bussmann Business fuse ampacity ratio table, Low-Peak®
fuses have a minimum of 2:1 to achieve selective coordination. These tables assure selective coordination for all
values of times (even below 0.01 seconds) and all values
of fault current levels, up to the interrupting rating of the
current-limiting fuses, or 200,000 amperes, whichever
is less. A major difference between what was discussed
above for circuit breakers and that which is discussed
here for fuses is that the fuse tables make it possible to
determine selectivity without having to plot time current curves up to the available fault currents.
It is important to note that fuse tables and circuit
breaker tables only apply to the products of the
manufacturer who published them.
Essential Electrical Systems
in Healthcare Facilities
The first change in the 2014 NEC for essential electrical systems in healthcare facilities was the deletion
of “Emergency System.” In the previous edition of the
NEC, the emergency system comprised the life safety
branch and the critical branch. This change was made to
correlate with the 2012 NFPA 99 Health Care Facilities
Code, and to avoid the misconception that the critical
branch of a healthcare facility is considered a part of the
emergency system of a facility and must comply with
Article 700. Additionally, NEC 517.26 was revised to
affirm that only the life safety branch of the essential
electrical system must comply with Article 700. A new
informational note was also added to 517.26 to refer the
user to the requirements of essential electrical systems
for hospitals per 517.30 and NFPA 99.
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.
Informational Note No. 2: For additional
information, see 517.30 and NFPA 99-2012,
Health Care Facilities Code.
The second change to Article 517 addressed the
requirements of selective coordination for the essential electrical system in hospitals. In the 2011 NEC,
70 IAEI NEWS May . June 2014
www.iaei.org
Figure 3: Time-current curves showing selective coordination of current-limiting fuses and manufacturer specific
selective coordination minimum ampacity ratio table for manufacturer specific fuses.
the entire essential electrical system was required to
comply with Article 700 per 517.26. Because of this,
the essential electrical system was required to comply
with requirements for selective coordination per NEC
700.27. In the 2014 NEC, new 517.30(G) was added
to correlate with the requirements from NFPA 99.
Because of the special “defend in place” requirements
in Article 517 and NFPA 99 that are intended to allow healthcare to “ride through” full or partial system
blackouts, the healthcare industry believes that it can
safely “ride through” any lack of selective coordination
(blackouts) due to short-circuits, ground faults and
arcing faults. This new section simply requires “coordination” for times of 0.1 seconds and greater. There
is no mention of requirements for “selective coordination” for the essential electrical system in hospitals. As
such, a system that does not comply with “selective
coordination,” as shown in figure 2, may comply with
“coordination” to 0.1 seconds as shown in figure 4. In
addition, exceptions were added similar to those
shown in 700.27 and 701.18 of the 2008 NEC. Lastly,
an informational note was added to clarify that “coordination” and “coordinated” do not cover the full range
of overcurrent conditions. The term coordination, as
per the informational note associated with 517.30(G)
www.iaei.org
which states that “The terms coordination and coordinated as used in this section do not cover the full range
of overcurrent conditions.”, essentially disregards the
available fault current and only seeks separation of trip
curves beyond 0.1 seconds (overloads only).
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.
May . June 2014 IAEI NEWS
71
SELECTIVE COORDINATION FOR CRITICAL SYSTEMS |
Photo 1. Selective coordination is required for multiple elevators supplied by a single feeder.
“A system that does not
comply with ‘selective
coordination’, may
comply with
‘coordination’
to 0.1 seconds.”
of hospitals, since it provides a similar purpose to
that of an emergency system for other facilities,
where selective coordination is required. In addition, selective coordination may be considered
by some electrical system designers as needed
for some or all parts of the critical branch in
hospitals to increase system reliability. As we’ll
see in this article, new requirements around who
performs these studies places more responsibility on the electrical system designer to make the
right decisions for safety.
The TCC Curve of figure 4 does not include
the plot of available fault current. As per the
informational note of this section, 0.1 second
coordination levels disregard current, only looking for separation of curves for the times above
0.1 seconds.
It is important to note that although selective
coordination is not required per the 2014 NEC
for the essential electrical systems in hospitals,
some electrical system designers may require
selective coordination for the life safety branch
Critical Operation Data Systems
In the 2014 NEC, a new requirement for selective
coordination was added to Article 645, which covers Information Technology Equipment. This new
section, 645.27, requires selective coordination for
“critical operation data systems.” Critical operation
data systems are similar to critical operation power
systems, where selective coordination is also required
per 708.54. The definition of critical operation data
system per NEC 645.2 and the requirements for selec-
72 IAEI NEWS May . June 2014
www.iaei.org
code cycle, the definition
of Coordination (Selective) has been revised to
clarify the intent of the
definition. Additionally,
documentation requirements have been added
for select critical systems
to aid enforcement and
compliance with the
requirements for selective coordination. For
electrical inspectors, a
selective coordination
inspection form may be
used to communicate
the requirements and
needed documentation
for systems that require
selective coordination.
To show compliance
with selective coordination, time-current curves
and/or manufacturer
Figure 4: Compliance with coordination of 0.1 seconds does not consider all overcurspecific selective corent conditions; some have coined this as only overload coordination.
ordination tables for
both
circuit
breakers
and
fuses can be used. This
tive coordination per 645.27 are shown below:
documentation must be provided to the designer,
installers, inspectors, maintainers and operators
647.2 Definitions
of the system. Finally, because of the “defend in
Critical Operations Data System. An inforplace” philosophy of healthcare facilities, the
mation technology equipment system that
requirements for essential electrical systems in
requires continuous operation for reasons of
healthcare
facilities have been lowered to only republic safety, emergency management, naquire “coordination” for times of 0.1 seconds and
tional security, or business continuity.
greater, although some electrical system designers may choose to apply selective coordination to
645.27 Selective Coordination. Critical opthese systems to increase reliability and safety.
erations data system(s) overcurrent protective
devices shall be selectively coordinated with
all supply-side overcurrent protective devices.
Conclusion
Selective coordination is a key requirement for
increasing system reliability and safety of critical
systems. Over the past several code cycles, the
requirements for selective coordination have
expanded to new types of systems. In the past
www.iaei.org
Dan Neeser is a field application engineer with Eaton’s
Bussman Business. He had been employed by this company since
1996 and specializes in training on the design and application of
overcurrent protective devices in electrical distribution systems in
accordance with the National Electrical Code®, and equipment in
accordance with the various product standards. He participates in
IEEE (Sr. Member) with Industrial and Commercial Power Systems, NEMA, NFPA (committee member for NEC CMP-13 and
NFPA 79), UL (508, 508A, and 508C) and IAEI activities.
May . June 2014 IAEI NEWS
73