The 2005 National
Electrical Code introduces a change regarding series
ratings for circuit breakers that deserves careful review
before applying an engineered series rating. The additional
paragraph 240.86 (A) reads:
(A) Selected Under
Engineering Supervision in Existing Installations. The series
rated combination devices shall be selected by a licensed
professional engineer engaged primarily in the design or
maintenance of electrical installations. The selection shall
be documented and stamped by the professional engineer. This
documentation shall be available to those authorized to
design, install, inspect, maintain and operate the system.
This series combination rating, including identification of
the upstream device, shall be field marked on the end use
equipment.
Notice that this new
paragraph deals exclusively with existing installations and
provides for a licensed professional engineer to determine the
series rating in these installations. Any series rating
applied in a new installation or where the equipment is
replaced in an existing installation would need to comply with
paragraph (B), which requires the application of tested
combinations. Several issues need to be understood related to
the new paragraph (A):
• Why is this provision
just for existing installations?
• Is an added series
rating a solution for the situation for which this is
intended?
• What information does a
licensed professional engineer need and what information does
he/she have that will permit him/her to determine an effective
series rating?
Existing
Installations
The substantiation for permitting engineered series ratings
only on existing installations is to address conditions where
available fault current has been increased after installation
due to an upgrade in the electrical distribution system. With
the modification, the existing equipment and overcurrent
protective devices are underrated for the higher fault
current. Under the NEC revision, an overcurrent
protective device (OCPD) having an interrupting rating at
least as high as the increased fault current can be added to
protect the installed system, if a licensed professional
engineer determines that a series rating exists using this
device and the installed OCPDs.
Utilities and other service operators
occasionally revise their systems to provide for increased
energy demand, for power quality and other reasons. When these
system changes result in a higher available fault current that
exceeds the rating of installed equipment, a serious hazard
exists to the facility and to those working with or near the
under-rated electrical equipment. Options for correcting this
situation include replacing the equipment with properly rated
equipment, replacing OCPDs with higher-rated OCPDs, or
applying higher impedance transformers or reactors and similar
equipment modifications to reduce the available fault current.
Some of these options can be complex and costly, so it is
reasonable that a simple and low cost solution would be
desired. However, even with simple improvements the true cost
could be high if the improvements do not provide the expected
protection and give a false sense of security.
Attempted
Corrections with Series Ratings
This definition of a series rating appeared in a 1994
article by the National Electrical Manufacturers Association (NEMA):
Series rating: A short-circuit
interrupting rating assigned to a combination of two or more
overcurrent protective devices which are connected in series
and in which the rating of the downstream device(s) in the
combination is less than the series rating. 1
From the definition, we see that the series
rating will be higher than the rating of the downstream
circuit breaker(s). By locating the higher-rated circuit
breaker or fuse electrically ahead or upstream of the
lower-rated circuit breaker, the higher-rated device will
protect itself and the lower-rated downstream circuit breaker,
if the series is properly selected. So, in the case of
installed equipment that becomes under-rated, adding a fully
rated fuse or circuit breaker electrically ahead of it might
be considered as a method of protecting it. The challenge is
how to select the OCPD to be added to be sure that it will
provide the required protection. Let’s look at two methods—the
Selection of Certified Rating Method and the Analytical Method—as
possible ways to select this device.
Selection of Certified Rating Method
Let’s examine the simple 3-tier system of figure
1, typical of a number of commercial installations. Then,
consider adding series devices as in figure
2.
Suppose we added a rated OCPD at location A
(device A), on the supply side of the main. We would have to
determine that the added device A will protect both the 1200 A
main circuit breaker and equipment in which it is installed.
This step, if successful, would be an improvement in
protecting the main and equipment, but it does not necessarily
help us in protecting the feeders or the downstream system
protected by the feeders.
The first step is to determine whether a
tested series rating already exists between device A and the
installed 1200 A main circuit breaker. If the rating is marked
on the equipment, we know that by installing device A, the
1200 A main circuit breaker and the equipment in which it is
installed would be protected. If a component recognized series
rating is found to exist, but it does not appear on the
equipment label, we know only that the overcurrent protective
devices will perform acceptably together under special test
conditions, but we do not know whether they will perform
acceptably in the end use equipment, such as the switchgear or
switchboard. The integrity of the enclosure, busing, and
insulators in the end use equipment must be considered as is
required for application of tested series ratings.
Analytical Method
If a tested series rating is not available, we might
attempt an analytical method. From an analytical viewpoint the
performance of two OCPDs together for a series rating should
be considered acceptable if all of the following four criteria
are met by the upstream OCPD:
1. It reduces the let-through current to a
value below the interrupting rating of the downstream circuit
breaker.
2. It clears the circuit at a time before
the contacts of the downstream circuit breaker begin to open.
3. Items (1) and (2) are true for all
current levels from the rating of the downstream circuit
breaker through the series rating of the combination (not just
at the maximum current level of the system).
4. It has an interrupting rating at or
above the engineered series rating.
The question an engineer will have to
answer is how to know whether the first three criteria are
met.
While these criteria identify the ability
of the upstream OCPD to protect the downstream circuit
breaker, they do not determine whether they will perform
acceptably in the end use equipment, such as the switchgear or
switchboard. One way of predicting whether the equipment is
also protected is to estimate the let-through current and
duration under which the equipment would have been tested. If
these values are lower with the added device in the circuit,
the equipment is most likely protected. But how would an
engineer determine the conditions under which the equipment
would have been tested?
Downstream Circuit
Breakers
The analysis above relates to protecting the main circuit
breaker and its end use equipment by adding another device at
location A. It has not addressed protection of the equipment
on the load side of the main including the feeders and
branches. Looking at figure 2 again, we see that available
fault current has increased for both the feeder and branch
locations and exceeds the interrupting ratings of circuit
breakers in these positions. A fault on the load terminals of
the 400 A feeder will be seen by the feeder, the main, and the
added device A. The four criteria listed above for this
situation must once again be considered, but here there are
two sets of contacts that may open and add dynamic impedance.
In this case, if the contacts of either the feeder, or the
main open before device A clears, the protection is unknown.
By taking energy, the load-side device causes the supply-side
device to open later than it would by itself, or perhaps to
not open at all.
Protection of branch-circuit breakers adds
yet another set of contacts to consider. The likelihood that a
device with a continuous current rating of the main would be
adequate for protecting the branch-circuit breakers becomes
even lower.
In order to protect feeder and
branch-circuit breakers in the system, it may be necessary to
add OCPD devices at locations B and C in figure 2 to provide
sufficient protection for the system including all
disconnects. These added devices would need to be fully rated
for the fault current available at their specific location in
the system.
Tested Series
Ratings
It is important to understand that prior to this addition in
the 2005 NEC, 240.86 permitted only those series
ratings that were marked on the end use equipment. In other
words, the series rating was evaluated by the certifying
organization prior to the original installation of the
equipment. The means of certifying these ratings for
molded-case circuit breakers (MCCBs) was by several rigorous
series of tests detailed in UL 489, Standard for
Molded-Case Circuit Breakers, Molded-Case Switches and
Circuit-Breaker Enclosures, and UL 67, Standard for
Panelboards. Under this certification process, the
performance requirement and application of series ratings is
clear.
Are Analytical
Methods Effective?
If the four criteria in the method above are rigorously met
and the end use equipment can be found to be satisfactory, the
analysis should be effective. However, meeting the criteria
with real equipment and devices may not be practical.
Obtaining information related to the first three criteria may
be difficult since it is not published and will be unique to
the conditions present.
One analytical method used in the past for
estimating some series ratings was the Up-Over-Down method for
protection of some equipment by current-limiting fuses.
However, as related to series ratings, it was found that if
the downstream circuit breaker contacts were to open before
the upstream fuse clears, the protection is unknown. The
reason protection cannot be predicted in this way is that the
two devices are sharing the energy of interruption. Contact
separation is the beginning of circuit interruption by the
downstream circuit breaker. Remember this breaker is not rated
for the circuit with the increased fault current that it is
attempting to clear. This action by the circuit breaker
introduces dynamic impedance in the circuit and takes some of
the energy of interruption from the upstream device, which
slows clearing by the upstream device. The result is that the
downstream circuit breaker often attempts to interrupt more
energy than it is designed to take and it is therefore not
protected. Publications in endnote references 2, 3, and 4
describe this interaction.
The primary difficulty in meeting the
analysis criteria is the fact that circuit breaker contacts
open very rapidly, especially when the fault current is higher
than the rating of the circuit breaker. Some typical contact
separation times for MCCBs from series tests are listed in table
1. Even current-limiting circuit breakers and fuses do not
generally operate rapidly enough to clear before circuit
breaker contacts start separating. In such cases, the only way
to determine whether a series rating is satisfactory is by
test.
All of the circuit breakers represented in
table 1 are of designs from the 1980s or before. Even the
fastest current-limiting circuit breakers or fuses do not
clear faster than these contact separation times. Further, for
intermediate short circuit current levels, current-limiting
OCPDs operate slower than they do at these very high short
circuit levels. In general, MCCB contacts will always start
separating before any OCPD of the same rating in the circuit
is able to clear the circuit. It is not practical to
determine series ratings by analytical methods for MCCBS.
For decades, circuit breaker design
engineers have attempted to determine an analytical method
that will avoid the enormous amount of testing associated with
developing and maintaining series ratings by test. Even
sophisticated numerical analysis has not been successful to
date.
Low-Voltage Power
Circuit Breakers
Series ratings are not generally applied to the large,
low-voltage power circuit breakers (LVPCBs). Fused circuit
breakers have been developed to allow very high interrupting
ratings with the limiter fuse protecting the circuit breaker
above its normal interrupting rating. Since LVPCBs generally
operate more slowly than MCCBs, it would seem more practical
to engineer a series rating for LVPCBs than for MCCBs. The
same four-point analysis mentioned above would apply for
LVPCBs as well as for MCCBs.
An engineer attempting to analytically
determine such a series rating should be aware that some fused
LVPCBs have additional contacts and heavier operating springs
than the standard circuit breaker to allow them to withstand
the higher current they will see when subjected to a short
circuit above their rating. These design differences also
address higher temperatures they experience when operated
together with a fuse. For these reasons, ANSI/IEEE C37.13
paragraph 10.8 recommends against the application of cascade
ratings, another name for series ratings. Rather than adding a
device in the circuit, it may be more practical to replace the
under-rated circuit breaker with one of the proper rating.
Degrees of
Protection
Attempting to engineer a series rating may not be as practical
as the exercise would seem at first glance. As circuit breaker
engineers have found while developing series ratings by test,
there are many factors influencing the degree of protection
offered by one OCPD for a lower-rated circuit breaker in the
same circuit. No method other than testing has proven
successful.
However, the new 240.86(A) was added to the NEC in an attempt to provide some genuine help for
installations that are operating today with available fault
current well above the rating of the installed equipment. It
seems to make sense that adding a fully rated device in the
circuit ahead of the installed equipment would at least add a
degree of protection that is not available without more
extensive and costly improvements to the system. For someone
working in under-rated equipment, it is more assuring to know
that a properly rated OCPD is someplace in the system rather
than depending on protection by utility transformer
protection, which is usually not designed to protect the
low-voltage equipment. Members of NEC code-making panel
10 in their deliberations considered this new paragraph to add
a "degree of protection."
In testing for series ratings,
non-conforming results include: damaged internal parts of the
downstream circuit breaker that would not allow it to be
turned on again, bursting of the circuit breaker’s molded
case, or arcing from the circuit breaker to the end use
equipment enclosure. A long list of criteria exists with which
the series devices must comply. Those who write the standards
consider each criterion essential to the safe application of
the rating. If the same criteria are not expected for
application with existing installations, it is reasonable to
question whether the provisions of 240.86(A) are truly series
ratings. If a degree of protection is acceptable, as compared
to demonstrated compliance with all requirements, provision
for special protective measures for existing installations in
which the fault current has increased should be clarified and
should not be considered a series rating.
Summary
The rationale for adding 240.86(A) to the 2005 NEC is
to address a real problem facing a number of installations;
however, the solution in the revised wording may not be a
valid solution. The ideal solution is to have equipment
installed that has ratings suitable for the application and
that is the intent of the revised wording. Circuit breaker
manufacturers, based on many years of experience of designing
and testing circuit breakers, understand that a series rating
can only be determined by test. An analytical approach for
engineering series ratings is enumerated above that would be
effective if devices satisfy the specified criteria. However,
because of interaction between the devices involved,
engineering a series rating without testing is not practical
where MCCBs are involved and product standards recommend
against the application of series ratings using low-voltage
power circuit breakers.
Appendix
Dynamic
Behavior
Overcurrent protective devices (OCPDs) exhibit
electrically dynamic behavior as they operate
automatically to open a circuit. They are dynamic in
that they open automatically when they detect an
overcurrent condition. As related to series ratings, the
dynamic behavior is the continuous changing of impedance
during the opening operation as they clear the circuit.
At the point of initiation of a short-circuit condition
in a circuit protected by the device, it is fully closed
and appears as a conductor to the circuit. Its impedance
is near zero. As a circuit breaker senses the short
circuit and begins to open the circuit, an arc will form
between the fixed and moving contacts. If the device is
a fuse, the arc is between segments of the fuse links.
The device clears by extinguishing the arc, at which
point the impedance of the device is infinitely large.
Let’s look at how these changing
circuit conditions impact overcurrent protective devices
connected in series. For this purpose it will be helpful
to think about the sequence of conditions that occur
during the interruption, which is outlined in figure
A.
Condition (a), short-circuit
initiation: As the short-circuit starts, the
contacts of both devices are closed. Both the
supply-side and load-side devices act as conductors in
the circuit carrying the short circuit. The sensing
elements (not shown in the diagram) are beginning to
detect an overcurrent condition.
Condition (b), first contact
separation: The load-side device begins to open
first. It is first because it has a lower continuous
current rating and will detect an overcurrent at a lower
value than the supply-side device. Also, it will have
smaller mechanical parts with lower mass, lower inertia
than those of the supply-side device and they will begin
moving first under influence of the same circuit forces.
When the supply-side device is a fuse, it has a thermal
mass or inertia that will cause it to remain closed as
the load-side circuit breaker contacts separate. By
opening first, this load-side circuit breaker is
attempting to interrupt the circuit alone, without the
help of the supply-side device. That is, the smaller
device takes all of the energy of interruption during
this period. It also has developed an internal impedance
that begins to limit short-circuit current. As a result
of this dynamic impedance, the supply-side device does
not see the same short circuit it would if it alone were
attempting to clear the circuit. By taking energy, the
load-side device causes the supply-side device to open
later than it would by itself, or perhaps to not open at
all.
If both devices have the same
continuous current rating, there will be a race for
first to begin opening. In this case, when the
supply-side device begins to open first, Condition (b)
is bypassed and the load-side circuit breaker will not
have to attempt to clear the short circuit alone.
However, it will see the entire short-circuit current
and will share interruption energy as in Condition (c).
Condition (c), second contact
separation: For series protection, eventually the
supply-side device opens to share the energy of
interruption. The degree of protection will depend on
the dynamic coordination of the devices. If the
supply-side device is a current-limiting device in its
current-limiting mode, it will develop interruption
voltage (impedance) rapidly and take most of the final
stages of interruption. If the supply-side device is not
current limiting or is a current-limiting device
operating below its threshold of current limitation, it
may develop voltage slowly and leave much of the final
interruption to the load-side device.
Condition (d), clearing: In the
figure A diagram, both devices are fully open. We
anticipate that the load-side circuit breaker will be
open. It is not essential that the supply-side device be
open at the end of the interruption, but to provide
series protection, it will most often be open.
There are
many variables involved in determining the degree of
sharing of energy of interruption. We can identify mass
of parts, spring forces, latch loads, thermal inertia
(mass) and similar physical variables involved with
either the upstream or downstream device. There are also
electrical variables such as power factor, electrical
angle of fault initiation, and system configuration that
will impact the degree of sharing. The problem of
determining performance of two OCPDs under these
conditions is not elementary and has not been solved for
ready application. |
1 National
Electrical Manufacturers Association, "Series
Ratings," IAEI News, Richardson, TX, March/April
1994, pp. 23-25.
2 Bernie
DiMarco and Steven Hansen, "Interplay of Energies in
Circuit Breaker and Fuse Combinations," IEEE
Transactions on Industry Applications, May/June 1993, pp.
557-561.
3 C. W. Kimblin
and Y. K. Chien, "Integrated Series Ratings of Protective
Devices," IEEE Conference Record of 1995 Annual Pulp
and Paper Industry Technical Conference, pp. 183-189.
4 G. D. Gregory
and W. Stoppelmoor, "Test for Series Connected Circuit
Breakers," IEEE Transactions on Industry Applications,
May/June 2003, pp. 605-611.
Used by permission of NEMA.
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