For certain vital circuits, the 2005 NEC requires that all overcurrent protective devices be
selectively coordinated with all supply side overcurrent
protective devices in the system. In addition, the definition
for selective coordination has been added in Article
100. This commentary includes an overview of the requirements,
what selective coordination means, the rationale for the new
requirements, system requirements to comply, and the role of
designers, contractors and inspectors.
Compliance with these new selective
coordination requirements is about the proper choice of
overcurrent protection. It is not difficult to choose the
proper overcurrent protective device to comply. However, if
attention is not given to overcurrent protective device
characteristics and proper choices, it is easy to have systems
that are not compliant and, thus, systems that do not provide
the proper safety of human life.
What Are the New
Requirements?
Three new sections in three different articles constitute the
changes to the 2005 NEC discussed in this article.
Article 700 Emergency Systems
700.27 Coordination. Emergency system(s) overcurrent
devices shall be selectively coordinated with all supply side
overcurrent protective devices.
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.
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.
Since there is not an amendment on this
subject in Article 517, 517.26 requires overcurrent protective
devices in essential electrical systems of health care
facilities to be selectively coordinated.
The systems covered by these three new
sections have normal power sources and alternate power sources
in order to maintain power to vital loads. The selective
coordination requirement is applicable to all overcurrent
protective devices that are part of the conduction path for
these systems: that is, from the branch-circuit overcurrent
protective devices up through and including all supply-side
overcurrent protective devices on both the normal source path
and the alternative (emergency) source path. This will be
mentioned later in this article. Figures 6 and 7 at the end
of the article illustrate this point.
What Is Selective
Coordination?
Selective coordination can be defined as isolating an
overloaded or faulted circuit from the remainder of the
electrical system by having only the nearest upstream
overcurrent protective device open.
The following was added to the 2005 NEC Article 100 Definitions: "Coordination (Selective).
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."
One of the better ways to understand
selective coordination is to first examine a system that does
not provide selective coordination. Figure
1 illustrates a system with overcurrent protective devices
that are not selectively coordinated for all possible
overcurrents the system can deliver. In this case, a fault
occurs on the load side of a branch-circuit overcurrent
protective device. The blue arrows represent the fault-current
flow from the source to the fault. The branch-circuit
overcurrent protective device clears this fault—depicted by
the solid red box. In this example, the upstream feeders and
main overcurrent protective devices represented by the solid
red boxes will also open due to the fault current. The reason
is that the overcurrent protective device nearest to the fault
is too slow, so the upstream overcurrent protective device(s)
also open. However, opening of the other upstream overcurrent
protective devices is unnecessary and disruptive. The result
is an unnecessary power outage to all the loads with the
hashed red and white boxes. This figure
1 system would not be compliant where selective
coordination as required.
Figure
2 illustrates a system with overcurrent protective devices
that are selectively coordinated for all the possible
overcurrents that can occur in the system. In this example,
the fault occurs on the load side of a branch circuit. The
blue arrows represent the fault-current flow from the source
to the fault. The branch-circuit overcurrent protective device
clears this fault—depicted by the solid red box. However, in
this example, no other upstream overcurrent protective devices
open. All the feeders and main overcurrent protective devices
represented by the boxes with solid white remain in operation.
All loads, other than the faulted branch circuit, remain
energized and there are no unnecessary load power outages.
This is the result of the overcurrent protective devices being
properly chosen so that they are selectively coordinated for
the range of overcurrents that may occur on this system.
For a system to be selectively coordinated,
each overcurrent protective device in each branch circuit,
feeder and main must be analyzed for selective coordination
with the other overcurrent protective devices in the system. A
fault that unnecessarily opens one or more upstream
overcurrent protective devices is not selectively coordinated.
So if the branch-circuit overcurrent protective device and the
next level feeder overcurrent protective device can open on a
fault, the system is not selectively coordinated. Also, if
the fault occurs on a feeder circuit, to be selectively
coordinated, only the overcurrent protective device protecting
that feeder circuit shall open; no other upstream overcurrent
protective devices shall open. This is similar to the
517.17 requirement for health care facilities where if there
are two levels of ground-fault protection, the ground-fault
relays on the main and feeders shall be coordinated.
Rationale for the
New Requirements
Under Article 700 Emergency Systems
700.1 Scope. …These systems are intended
to automatically supply illumination, power, or both to
designated areas and equipment in the event of failure of the
normal supply or in the event of accident to elements of a
system intended to supply, distribute, and control power and
illumination essential for safety to human life….
The substantiations for these new proposals
center on reliability and continuity of power to vital loads.
In an emergency, safety of human life depends on power to each
emergency load. Selective coordination of overcurrent
protective devices fits well with existing requirements in
Articles 700, 701 and 517 that focus on these vital circuits
operating as intended when needed and having as much of the
system as possible be operational during an emergency. For
instance:
700.4 requires initial and periodical
testing of the complete emergency system. The system is to be
maintained in proper operating condition. This focuses on
providing a reliable emergency system that will be operational
when called upon for its use.
700.9(B) requires emergency circuits to be
physically separated from normal supply circuits. This helps
to ensure that emergency circuits will operate when called
upon.
700.9(C) requires emergency circuit wiring
specifically to be designed and located to minimize failures
due to fire, icing or other adverse conditions. This helps to
ensure that emergency circuits will operate when called upon.
700.16 requires that for emergency
illumination, failure of one component must not result in a
condition where a means of egress will be in total darkness.
700.26 has an allowance to not require
ground-fault protection for equipment for the alternative
source. This demonstrates the preference for system operation
in the event of an emergency and the fact that if a ground
fault occurs, the whole system should not go down creating an
unsafe situation for human life.
The objective of these requirements is
safety of human life by ensuring maximum system uptime during
emergencies and thus a more reliable emergency system. The
exact nature of the threat during an emergency cannot be
anticipated. The cause may be due to loss of utility power or
it may be due to major physical damage to the building
structure caused by fire, earthquake, etc. If some event
negatively affects a portion of the emergency system, the
criterion is to minimize the impact to the emergency system.
Selective coordination of overcurrent protective devices in
these vital circuits is another element to minimize the impact
of a negative event to the emergency system. Selective
coordination is another means to ensure adequate safety to
human life in case of an emergency. If there is a fault on one
circuit, it is critical to isolate only that portion of the
system that must be removed and ensure all other circuits
remain powered. Having overcurrent protective devices that are
selectively coordinated provides a more reliable emergency
system.
The code panel for emergency systems and
legally required standby systems made this statement to these
proposals: The panel agrees that selective coordination of
emergency system overcurrent devices with the supply side
overcurrent devices will provide for a more reliable emergency
system.
The code panel made a similar statement for
legally required standby systems.
System
Requirements to Comply
With proper device selection and analysis, it is possible for
both fusible systems and circuit breaker systems to be
selectively coordinated. However, the proper equipment choice,
analysis and interpretation of characteristics are important.
Fusible systems
Selective coordination with fuses can be achieved by using
the fuse manufacturers’ published fuse selective
coordination ratios; a full short-circuit and coordination
study is not necessary to verify selective coordination. Figure
3 shows a simple example. The table in figure
3 is an excerpt from one manufacturer’s Fuse
Selectivity Ratio Guide. If the system is designed with
fuses that maintain fuse type and ampere rating ratios that
are equal to or greater than the published ratios, then
selective coordination is achieved. There is no need to draw
or plot fuse time current curves in order to analyze fuse
systems for selective coordination. Just use the ratio guide.
For a fusible system, in order to achieve
selective coordination for the entire system requires fusible
branch-circuit lighting panelboards. There has not been an
abundance of fused lighting panelboards on the market for many
years. However, with these new requirements, fused lighting
panelboards for these vital systems are now available. Photo
1 illustrates one manufacturer’s new fusible lighting
panelboard.
Circuit breaker systems
Selective coordination with circuit breakers depends on
their characteristics and settings, as well as the circuit
parameters for the specific application. It is generally
difficult to achieve selective coordination with circuit
breakers that incorporate instantaneous trip settings. The
commonly used molded case circuit breaker has instantaneous
trip characteristics that may not be suitable for these
critical circuits. A fault on one portion of the circuit can
unlatch upstream circuit breakers before the circuit breaker
closest to the fault can clear. Typically circuit breakers
with short-time delay settings or zone selective interlock
features may be necessary. If circuit breakers are to be
considered, a full short-circuit current and coordination
study must be done with proper analysis and interpretation.
Figure
4 illustrates a circuit breaker system that is selectively
coordinated if the available short-circuit current at
the 20-A branch circuit does not exceed 900 A. For a fault on
the 20-A branch circuit less than 900 A, only the 20-A circuit
breaker will open. The 100-A and 800-A upstream circuit
breakers will not open under this circumstance. Also, if a
fault occurs on the 100-A feeder circuit, only the 100-A
circuit breaker will open since the 800-A circuit breaker has
a short time delay. So in this case, the 100-A and 800-A
circuit breakers are selectively coordinated.
In Figure
4, if the available short-circuit current does exceed 900
A at the 20-A branch circuit, then the upstream feeder circuit
breaker may open for a fault on the 20-A branch circuit; this
is not a selectively coordinated system. A
short-circuit current study and coordination study can
determine whether this type circuit breaker design is
selectively coordinated for the specific system parameters. If
this type system is found suitable today, future system
changes that increase the available short-circuit current may
negate their suitability.
Figure
5 illustrates a circuit breaker system that is selectively
coordinated for the full range of overcurrents up to the
interrupting rating of each respective circuit breaker. The
branch-circuit breaker is a standard 20-A molded case circuit
breaker (with instantaneous trip) and the feeder and main
circuit breakers have short-time delay settings with no
instantaneous trip settings.
Figures 6 and 7 at the end
of this article illustrate selective coordination and
non-coordination through the normal power supply and the
emergency power source.
The Cooper Bussmann SPD Selecting
Protective Devices publication (download from
www.bussmann.com) has an in-depth discussion on selective
coordination analysis with published fuse selectivity ratios,
some simple rules for evaluating coordination of instantaneous
trip circuit breakers, and an illustration of short-time delay
circuit breakers.
New Requirement
Compliance
Achieving overcurrent protective device selective coordination
for a system requires the proper engineering, specification
and installation of the required devices; and, in addition,
knowledgeable plan review and inspection are required to
ensure compliance. The designer, contractor, and plan
review/inspector each has his or her role in compliance to
selective coordination in order to ensure safety to human
life.
Role of designers
For these vital systems, the designer must select,
specify, and document overcurrent protective devices that
achieve selective coordination for the full range of possible
overcurrents and for faults at all possible points in the
system (faults can occur in the branch circuits, sub-feeders,
and feeders).
For fusible systems, it is a matter of
selecting the fuse types and ampere ratings that adhere to the
manufacturer’s selectivity ratio guide. This ensures
selective coordination for overcurrents up to the interrupting
rating of the fuses, which is typically 200,000 A or 300,000
A. The plan specifications must detail the specific type fuses
and ampere ratings that are designed into the system to
achieve selective coordination for the system. This is
important so that the contractor quotes and installs the
proper type and ampere rating fuses to achieve selective
coordination.
For circuit breaker systems, a
short-circuit current study and coordination study are needed
that detail the system available short-circuit currents, the
specified circuit breaker types and settings, and analysis
that selective coordination is achieved for the full range of
available overcurrents. It is important that the plan
specifications detail the type circuit breakers and specific
circuit breaker settings to achieve selective coordination.
This information is necessary so that the contractor quotes,
installs and sets the circuit breakers as designed.
Role of contractors
Contractors must install the system as designed to ensure
selective coordination for the system. If a fusible system,
install the fuse types and ampere ratings as called for in the
design. If a circuit breaker system, install the circuit
breaker types with the specified settings.
If the contractor opts to suggest a value
engineering option for these vital systems, it is critical
that the engineer evaluates and approves of any changes. For
instance, value engineering with overcurrent protective
devices that are series rated combinations may reduce the
cost, but series rated combinations may be difficult to
selectively coordinate. Hence, extra analysis may be required
if series rated combinations are considered for use in
emergency circuits, legally required standby circuits, and
essential circuits in health care facilities.
Role of plan review/inspectors
During the plan review process, the engineer must provide
the substantiation in the form of documentation that his
design achieves selective coordination for these vital
circuits. The plan reviewer should not have to prove or
disprove that selective coordination is achieved. The engineer’s
documentation should be clear enough to demonstrate that the
design work included the proper selective coordination
analysis and that the plans and specifications clearly
articulate the required details on type of overcurrent
protective devices, ampere ratings, and settings (if circuit
breakers). The engineer’s documentation should clearly state
that the plans specify overcurrent protective devices that
achieve selective coordination for these vital systems.
During the inspection process, prior to
energization, the field inspection should include verifying
that the overcurrent protective devices have been installed as
specified. If circuit breakers are used, the settings should
be verified as per plan.
Summary
These new requirements for selective coordination of
overcurrent protective devices in emergency systems, legally
required standby systems, and health care facility essential
electrical systems provide better system reliability for the
safety of human life. Selective coordination can be defined as
isolating an overloaded or faulted circuit from the remainder
of the electrical system by having only the nearest upstream
overcurrent protective device open. It is possible for both
fusible systems and circuit breaker systems to be selectively
coordinated with proper selection and analysis. Achieving the
proper overcurrent protective device selective coordination
for a system requires proper engineering analysis,
specification and installation of the required devices. During
the plan review process, it is the design engineer’s
responsibility to provide documentation that verifies the
overcurrent devices are selectively coordinated for the full
range of overcurrents that can occur in the system. And the
site inspection should verify the overcurrent protective
devices are installed as specified to achieve selective
coordination.
Tim Crnko, training & technical
services manager for Cooper Bussmann, has MSEE and is a member
of IAEI, IEEE and NFPA.
tcrnko@CooperBussmann.com |
