During the past revision cycle for the 2005 NEC, a new definition was created in Article 100 for selective
coordination. The need for this definition can be taken
from its creation and shows the expansion of selective
coordination requirements throughout the NEC. One of
the basic requirements for the creation of a definition in
Article 100 is the use of the term in two or more sections of
the Code. The 2005 NEC now has multiple sections
containing requirements for selective coordination of
overcurrent protective devices and most are contained in
sections pertaining to protection requirements for critical
circuits involving life safety. Compliance with these
selective coordination requirements is achieved through the
selection of overcurrent protective devices with appropriate
operating characteristics. The choice of overcurrent
protective devices with the proper operating characteristics
is not difficult. However, if proper review is not given to
the overcurrent protective device used, it is easy to have
systems that are not compliant and, thus, systems that do not
provide the proper safety of human life demanded in critical
circuits.
One of the areas containing requirements
for selective coordination of overcurrent protective devices
is elevator circuits in 620.62 of the 2005 NEC. This
article will cover the requirements for OCPDs used in elevator
circuits, provide background of the critical nature of these
requirements and discuss other considerations for the proper
selection of OCPDs.
Selective
Coordination and the NEC
The 2005 NEC contains definitions and requirements for
selective coordination of overcurrent protective devices in
the following sections:
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Article 100 Definition
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517.17 Health Care
-
517.26 Health Care Essential Electrical
System
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620.62 Multiple Elevators on single feed
-
700.27 Emergency Systems
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701.18 Legally Required Standby Systems
Selective
Coordination of Elevator Circuits
As can be seen from the list above most applications requiring
selectively coordinated overcurrent protective devices are
critical electrical circuits or systems. Selective
coordination of overcurrent protective devices fits well with
requirements that focus on these critical circuits. Selective
coordination provides overcurrent protection that keeps these
critical electrical systems operating as intended when needed
and maximizes the portion of the system that is operational
during an emergency. Putting this into the context of elevator
circuits it is important to provide power for elevators as
much as possible due to their relation to means of egress and
use by emergency personnel. This is particularly important for
multiple elevators supplied by a single feeder as an
overcurrent on a single elevator circuit could easily cascade
up to the feeder and take down the whole bank of elevators.
Therefore NEC 620.62 contains a requirement for
selective coordination of overcurrent protective devices where
more than one elevator is supplied by a single feeder that
reads as follows:
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.
What Is Selective
Coordination?
Selective coordination is defined in the 2005 NEC Article
100 as: "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."
In other words, isolating an overloaded or
faulted circuit from the remainder of the electrical system by
having only the nearest upstream overcurrent protective
device open (figures
1 and 2).
Figure
1 shows a system that does not contain overcurrent
protective devices that are selectively coordinated. The flow
of fault current is shown by the blue arrow. The reason is
that the overcurrent protective device nearest to the fault is
too slow, so the upstream overcurrent protective device(s)
also open. 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 is 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 entire 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.
Comparing this to the requirements in
620.62, note that there are three levels of feeders in the
systems depicted in figures
1 and 2. It is important to note that 620.62 requires
selective coordination of the branch overcurrent protective
device with all supply side overcurrent protective devices,
from the branch levels through all three feeders up to and
including the service overcurrent protective device. If
emergency power is provided through a transfer switch the
overcurrent protective devices in the emergency system
including any supplied with a generator need to be selectively
coordinated as well.
Important
Considerations for Section 620.62
A common error in elevator circuit applications is when
multiple circuits are run to the elevator equipment room in an
effort to bypass the selective coordination requirements
contained in 620.62. It is important to note that even in this
situation, there will be a single feeder supplying multiple
elevator loads and consideration of the service or main
overcurrent protection in relation to the separate feeders
needs to be reviewed
Figure
3.
In figure
4, observe that each of the feeders supplies its own
elevator. This does not bypass the requirements of 620.62,
which requires selective coordination when there is more than
one driving machine being fed from a single feeder. According
to Article 100, a feeder is considered to be all circuit
conductors between the service equipment and the
branch-circuit overcurrent protective device. This would mean
that the load-side conductors from M1 would be a single feeder
to multiple driving machines. This would require F1, F2, F3,
and F4 to be selectively coordinated with M1 in order to
comply with 620.62. These situations would require selective
coordination through to the main overcurrent protective device
in the building. Failure to use overcurrent protective devices
that are not selectively coordinated may compromise safety. A
fault on a branch may cause the main overcurrent protective
device to operate; so all the elevators may be rendered
inoperative, negatively affecting building egress or use by
emergency personnel.
Selectively
Coordinating OCPDs
Selectively coordinating overcurrent protective devices is
achieved by reviewing the operating characteristics of the
upstream overcurrent protective devices in relation to the
downstream overcurrent protective devices. Important
considerations include:
· Speed of operation under fault or
short-circuit conditions for phase-to-phase, three-phase, and
ground-fault conditions
· Assuring that the clearing time of the
downstream overcurrent protective device is less than the
melting or unlatching time of the upstream overcurrent
protective device(s).
· Assuring that no overlap exists in the
performance curves or time-current curves for overloads and
low-level fault conditions.
· Assuring documentation is provided by
the overcurrent protective device manufacturer relating to
selective coordination under high-fault or short-circuit
conditions as these operating times are often beyond what is
covered by a traditional time-current curve. (Important note,
this one condition is not in and of itself sufficient to
verify selective coordination. The other conditions must be
met as well.)
For fuses, selective coordination can be
achieved by following the selective coordination ratios
provided by the fuse manufacturers. These ratios provide the
level of separation needed between upstream and downstream
fuses to assure an overcurrent is cleared by the downstream
fuse before the upstream fuse starts to melt. (See figure
5 for an example of applying fuse coordination ratios).
To ensure circuit breakers selectively
coordinate with each other consideration needs to be given to
the type of circuit breakers used (fixed trip molded case,
adjustable trip molded case, insulated case, or power circuit
breakers) and the settings or options needed to provide
selectivity. One method of providing selective coordination
with circuit breakers is by the use of zone interlocking. In
this type of setup the circuit breakers "talk" to
each other in an effort to localize overcurrents to the
circuit breaker nearest the fault. Another method of achieving
selective coordination with circuit breakers is by using
circuit breakers with short time delay upstream and applying
appropriate settings to achieve selectivity between all the
circuit breakers in the system. Insulated case and power
circuit breakers provide this option. Yet another option in
systems with low available fault current levels is to provide
circuit breakers with adjustable trip settings upstream, which
can be set above the available short-circuit current levels
and therefore removing the instantaneous trip operation from
the upstream circuit breakers. It is important to note that
depending upon the system design and instantaneous trip
settings of the circuit breakers in the system it does not
always require the use of the more expensive or complex type
of circuit breakers. The simple rule to follow when looking to
achieve selective coordination with circuit breaker-to-circuit
breaker systems is to keep the instantaneous trip portion of
the upstream circuit breakers beyond the available fault
current levels. (See figures 6, 7, and 8 for examples of achieving selective coordination with circuit
breakers).
Other
Considerations for Elevator Circuits
Multiple Standards to comply with
In addition to the selective coordination
requirements in 620.62 there are additional requirements
surrounding elevator circuits in Article 620. The requirements
for the local elevator motor disconnect are located in Section
620-51, which can be summarized into four main requirements.
1. The disconnecting means must be a listed
device.
2. The disconnecting means must have the
capabilities of being locked in the "open" position.
3. The disconnecting means must be a
fusible disconnect or a circuit breaker.
4. The disconnecting means is allowed to
have shunt trip capabilities.
In addition to this list, there is a fine
print note, which refers to the elevator safety code ASME/ANSI
A17.1. If we look at 102.3 (c)(3), we can explore the
conditions mandated for the elevator motor disconnecting
means. The rules follow the same guidelines as the NEC,
however, in ASME/ANSI A17.1: "The shunt trip capability
of the disconnect is required, when the elevator shaft
and/or machine room is sprinkled."
The shunt trip in the disconnecting means
is typically initiated by the closure of a set of dry contacts
in the fire alarm system. This contact can be wired to close
via heat detector activation or by a flow switch that operates
due to the flow of water in the pipes in the sprinkler system.
The importance of this consideration lies
in the fact that the disconnecting means for the overcurrent
protective devices requiring selective coordination need to
provide shunt trip capabilities and comply with the additional
codes and standards. There are various product offerings and
solutions available to comply with this multitude of
requirements (figure
9). Further discussion on this topic can be found on the
Bussmann website at http://www.bussmann.com/products/powermod/ .
Inspector’s Role
During Plan Review
þ Assure proper
documentation is provided showing selective coordination of
overcurrent protective devices used in elevator circuits where
multiple elevators are supplied through a single feeder.
Ensure this selective coordination is achieved for all levels
of overcurrents even at fault levels that cause opening times
below 0.01 seconds. Note any settings that are required or
ratios that are used in achieving this compliance.
During Final Inspection
þ Assure that the
overcurrent protective devices called out and reviewed during
plan review where installed. Assure that any and all
appropriate settings are correct and documentation is provided
to ensure these settings will be known in future.
þ Use fuse
coordination ratios to verify selective coordination in the
field where fuses are used. Use the simple method of circuit
breaker amp rating x instantaneous trip setting for molded
case circuit breakers in the field. Ensure that the feeder
instantaneous trip setting is above the available
short-circuit current level at the elevator branch disconnect.
For example, if a 400-amp circuit breaker is feeding a 60-amp
circuit breaker, make sure the available short-circuit current
does not exceed 4000 amps or 400x10.
Todd Lottmann is an electrical
engineer employed by Cooper Bussmann, Inc. focusing on
codes and standards. Todd is a principle member of NEC
Code-Making Panel 12, a member of the UL 508A Standards
Technical Panel, member of NEMA 1IS Industrial Controls
section, and involved with the NFPA79 technical
committee. Todd is the Bussmann IAEI representative
participating in the national section meetings and
various chapter meetings around the country.
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