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Power topic #6044 | Technical information from Cummins Power Generation
Selective coordination standards
and design challenges
■ White Paper
By Rich Scroggins, Technical Advisor, Sales Application Engineering, Cummins
Power Systems
Selective coordination is one of the
most widely debated and discussed
subjects in the electrical engineering
community. There is no consensus
on what constitutes an appropriately
coordinated system. Two of the most
commonly applied standards —
NFPA 70 (National Electrical Code
[NEC]) and NFPA 99 (Health Care
Facilities Code) — give seemingly
contradictory definitions.
With that in mind, let’s look at the
tradeoffs involved with designing a
coordinated system and outline the
technical and regulatory case for
applying the NFPA 99 definition of
selective coordination.
The NEC defines selective coordination as:
“Localization of an overcurrent condition to
restrict outages to the circuit or equipment
affected, accomplished by the choice of
overcurrent protective devices and their ratings
or settings.”
In a selectively coordinated system, a fault
is cleared by the protective device nearest
upstream from that fault and does not result in
unnecessary power loss to other downstream
loads. Selective coordination is typically
achieved by using an Overcurrent Protective
Device (OCPD) with an instantaneous trip
setting at the bottom of the distribution and
progressively longer time delays higher in the
system; in the event of a fault only the OCPD
closest to that fault will trip.
Without Selective Coordination
OPENS
OPENS
NOT AFFECTED
UNNECESSARY
POWER LOSS
With Selective Coordination
Fault
NOT
AFFECTED
Fault
Figure 1: Faults with and without selective coordination
Designing a selectively coordinated system presents
a challenge to engineers; safety and equipment
protection, as well as fault isolation, need to be
addressed. The effect of using time delays on OCPDs
is to expose downstream equipment and cable to
fault current for a long period of time. When timedelayed trip settings are used, it is critical that the
entire system must be designed for the same minimum
level of fault protection. This requirement is mandated
by NEC 110-10:
“The overcurrent protective devices, the total
impedance, the component short circuit ratings,
and other characteristics of the circuit to be protected
shall be selected and coordinated to permit the circuit
protective device used to clear a fault to do so without
extensive damage to the electrical components of
the circuit.”
NEC 110-10 requires that cable and equipment
be sized to handle the available fault current for
the duration of the OCPD time delay. Table 1
was published by the Insulated Cable Engineers
Association and provides an illustration of how an
extended fault clearing time delay will require the
use of larger cable in an application.
02 | Power Topic #6044
From Table 1 we can see that a circuit fed from a
source with 40,000 amps of available fault current
should use 3/0 cable if the OCPD will clear the fault
in 50 msec (3 cycles at 60 Hz). However, if the OCPD
has a 500 msec time delay (30 cycles), 500 kcmil cable
would be required to safely handle the fault.
Oversizing the cable not only increases the cost of the
system, but also results in a higher level of arc flash
incident energy at downstream equipment, as the
cable resistance is reduced and allows more current to
pass through.
Table 1: Allowable short circuit currents for insulated copper
conductors
Table 2 — derived from NFPA 70E, Table H.4(a) —
illustrates that arc flash energy is a function of both
fault clearing time and the available fault current.
Increasing these parameters increases the arc flash
incident energy and may require a higher level of
PPE to operate and maintain equipment. Mandating
coordination for all levels of fault current is at odds with
the stated purpose of the NEC, which is “the practical
safeguarding of persons and property from hazards
arising from the use of electricity” (NEC 90.1(A))
Fault
Clearing
Time (sec)
0.05
0.1
0.2
0.33
0.5
Max 3 Phase Bolted Fault Current at
480Vac
2
8 cal/cm PPE
68 kA
32 kA
15 kA
8 kA
Not Recommended
2
40 cal/cm PPE
200 kA
183 kA
86 kA
50 kA
32 kA
Table 2: Arc flash energy is a function of both fault clearing time and
the available fault current
Selective Coordination
Definitions
The NFPA 70 definition of selective coordination refers
to “the full range of available overcurrent, from overload
to the maximum available fault current, and for the
full range of overcurrent protective device opening
times…”
It is for this reason that NFPA 99 defines selective
coordination as achieved if it is demonstrated for all
time durations greater than 0.1 seconds. The specific
text (section 6.4.1.2.1) says “Overcurrent protective
devices serving the essential electrical system shall
selectively coordinate for the period of time that a
fault’s duration exceeds 0.1 second.” In Annex A NFPA
99 clearly states the justification for this requirement
(A.6.4.2.1.2) “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 cases,
however, full coordination could compromise safety
and system reliability.”
Requiring coordination only for time durations greater
than 0.1 second gives the engineer the flexibility to
use instantaneous trip settings for high levels of fault
current and use time-delayed trips on the lower level
fault currents. This way the system will be protected for
all levels of fault current up to the theoretical maximum
and will be coordinated for the likelier event of arcing
ground faults and overload conditions.
Figure 2 illustrates a system that is coordinated based
on the NFPA 99 definition. The trip curves for the two
breakers in this example overlap in the instantaneous
region; however, there is no overlap for longer time
durations, so this system would meet the NFPA 99
selective coordination requirements.
Figure 2: System showing overlapped breakers based on the NFPA
99 definition of selective coordination
There appears to be a conflict between NFPA 99
and NFPA 70 on whether a system needs to be
coordinated for all levels of fault current and fault
clearing times or only for the duration of a fault that
extends beyond 0.1 second. The NFPA has developed
a policy for resolving such conflicts. They determined
that NFPA 99 is responsible for the performance
of healthcare electrical systems and NFPA 70 is
responsible for the installation of these systems.
NFPA 70 must defer to NFPA 99 on performance
issues. Clearly, selective coordination describes
the performance of the system, so NFPA 99, the
performance standard, is the appropriate standard
to apply.
NFPA 110 (Standard for Emergency and Standby
Power Systems) has language similar to NFPA 99
regarding selective coordination. Annex A (A.6.5.1)
states: “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.”
NFPA 110 applies to all emergency and standby
power systems, and the language gives the engineer
discretion in determining the level of coordination.
The engineer may cite the NFPA 99 standard of
requiring coordination only in excess of 0.1 second
as a best practice. NFPA 70 Articles 700 (Emergency
Systems), 701 (Legally Required Standby Systems)
and 708 (Critical Operations Power Systems) all
state: “Selective coordination shall be selected by
a licensed professional engineer or other qualified
persons engaged primarily in the design, installation,
03 | Power Topic #6044
Maximum available fault current typically refers to
the current that would theoretically flow in the event
of a three phase bolted fault, although a three phase
bolted fault rarely occurs in practice. Single phase
arcing ground faults are much more common and
typically result in lower current levels for which delayed
interruption is more manageable. Designing a system
so that it will coordinate in the unlikely event of a three
phase bolted fault at the theoretical maximum level of
fault current results in a system that will allow higher
levels of fault current to flow in the more likely event of
a single phase fault. This could create higher levels of
arc flash incident energy at downstream equipment
and compromise system safety.
About the author
Rich Scroggins is a Technical Advisor in the
Application Engineering group at Cummins
Power Generation. Rich has been with Cummins
for 20 years in a variety of engineering and
product management roles. Rich has led product
development and application work with transfer
switches, switchgear controls and networking
and remote monitoring products and has developed
and conducted seminars and sales and service training
internationally on several products. Rich received his
bachelors degree in electrical engineering from the
University of Minnesota and an MBA from the University
of St. Thomas.
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.”
The language in these standards has been interpreted
as allowing a licensed PE discretion over system
performance. The PE is allowed to use engineering
judgment in determining that selectively coordinating
the system only for faults beyond 0.1 second and
allowing OCPDs to clear instantaneously in the
event of higher levels of fault current are the best
ways to balance system reliability and performance
considerations with the NEC purposes of safety and
fire prevention.
Summary
Designing a selectively coordinated system presents
challenges. The engineer needs to balance the risks of
temporarily cutting power to the system against safety
risks and the risk of permanent system damage due to
the increased level of arc flash energy.
Requiring coordination only for time durations greater
than 0.1 second gives the engineer the flexibility to use
instantaneous trip settings for very high levels of fault
current and use time-delayed trips on the lower level
fault currents, which are more likely to occur.
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GLPT-6044-EN (01/15)
04 | Power Topic #5590
A requirement for “total” selective coordination
means that OCPDs must be coordinated for all
faults, including a three-phase bolted short circuit, a
worst case scenario and one not commonly seen in
real applications. Delaying interruption of such high
levels of fault current results in exposing downstream
equipment and cables to excessively high levels of fault
energy and could increase the risk of arc flash.