Construction Method

You learned that low voltage power circuit breakers are essentially an assembly of parts on a metal frame or in an encased housing of insulating material. Because the makeup of both approaches was adequately discussed, the details will not be repeated. One point, however, that should be repeated centers around what applicable standards are required relative to the construction method. The frame construction used must hold all the circuit breaker parts in place and be capable of withstanding the tremendous physical forces and severe heating effects a power circuit breaker could be subjected to while performing its function. Standards do not specify the exact nature of the construction or the construction material. Those decisions are left to the circuit breaker manufacturer.

Obviously, the construction method and materials used must result in a strong, rigid design. For many years, the preferred approach was the open type metal-frame which had a number of pieces welded and/or bolted together. With the significant technological strides made in the areas of insulating material and molding processes, versatile rigid frame housings of high strength engineered thermoset composite resins have become available. Not only does the rigid frame housing type low voltage power circuit breaker meet the stringent requirements of ANSI, it exceeds them in a number of instances.

The Magnum DS circuit breaker uses a 3-piece construction:

A 2-piece engineered thermoset composite resin case completely encloses the current paths and arc chambers.
The operating mechanism sits on the front of the case and is electrically isolated from the current contact structures. It is in turn covered by an insulating front cover.



The rigid frame construction results in a more compact, lighter weight low voltage power circuit breaker. Previous circuit breaker designs with many parts that were once produced and attached individually to the frame can now be molded as an integral part of the rigid frame. The overall strength and rigidity of the engineered thermoset composite resin design can even contribute to higher performance capabilities by the power circuit breaker. This was the result with Magnum DS which has higher short time ratings (withstand) than previously available power circuit breakers, along with higher interrupting capabilities. In the way of review, remember that the short time rating consists of the following two components:

• Short delay current component (kA)
• Short delay time component (cycles)
In addition to the improved performance characteristics just mentioned, the rigid frame housing type low voltage power circuit breaker has individual arc chambers that (Figure 43):
• Insulate and isolate Arcing from other poles, the rest of the circuit breaker, and personnel
• Provide support for the current path pole assembly

Standards and Testing

The standards that are applicable to low voltage power circuit breakers and the testing involved to prove compliance by a specific low voltage power circuit breaker design are in Module 5 and previously in this module. You learned from those discussions that standards and testing go to the heart of the matter. This is true from three very important standpoints:
1. This is the industry's determination as to whether or not a particular circuit breaker design is capable of meeting a wide range of published operational and physical requirements.
2. The proven and stated compliance to specific standards tells potential users that the equipment from the manufacturers under consideration all meet certain basic standards, which makes the user's evaluation process much simpler. Once this determination is made, a particular manufacturer can still gain an evaluated advantage by offering additional unique features and/or an operational design approach preferred by the user.
3. It is a solid way of defining specific types of circuit breakers within a larger general grouping. For example: The larger general grouping is "Low Voltage Circuit Breakers." Specific types within the Low Voltage Circuit Breaker grouping would be "Low Voltage Power, Insulated Case, Molded Case and Miniature."
As you can see, when a specific type circuit breaker is specified, such as a low voltage power circuit breaker, the specifier already knows what the base expectations are from each manufacturer.
You will recall, from both Module 5 and previous sections, a map of the world showing the standards most influential in different parts of the world (Figure 37). It bears revisiting the map again to emphasize the importance, in today's global economy, of having flexible designs capable of complying with all major standards around the world. In this module the emphasis will be primarily on ANSI and IEC Standards. You should never lose sight of the fact, however, that there are a number of other standards that can play a critical role in determining what equipment is acceptable for application in a given area of the world. Even local and/or individual city codes and requirements may have to be considered.

In previous modules, references other than ANSI and IEC were made with respect to standards and testing, such as UL, IEEE. There is a strong relationship between ANSI, UL and IEEE. As a matter of fact, you will notice in manufacturer's publications for low voltage power circuit breakers and even low voltage metal enclosed switchgear references made to all. The following two samples are typical statements you might encounter when reading publications for both the power circuit breaker and the metal enclosed switchgear:
Typical Low Voltage Power Circuit Breaker Statement: "Type XYZ low voltage power circuit breakers are UL listed, and built and tested to applicable NEMA, ANSI, IEEE and UL standards (ANSI C37.50, C37.13, UL 1066)."
Typical Low Voltage Metal Enclosed Switchgear Statement: "Type XYZ low voltage metal enclosed switchgear conforms to NEMA SG3, NEMA SG5, ANSI C37.20.1, ANSI C37.51 and UL1558."
It may seem to you like a confusing web at this point. Once the relationship is understood, it will be clear as to why these references are made. There will be no detailed discussion of the standards relating to low voltage metal enclosed in this module, only those relevant to the power circuit breaker. Keep in mind, however, it works the same way. The standards state different requirements for the different pieces of equipment, but the intent is the same - an uncompromised piece of equipment with proven performance capabilities.
For the purpose of this section, let's identify the key players as a minimum and elaborate on a couple. This should not be considered as a substitute for the standards themselves. For a full explanation of any standard, consult the standard itself for details and proper conformance instructions.
IEEE (Institute of Electrical and Electronic Engineers)
• IEEE is an objective technical organization made up of manufacturers, users, and other general interest parties.
• IEEE defines technical definitions, technical requirements, temperature limits, altitude correction, insulation limits, and service conditions. For electrical equipment, including switchgear, it supplies the test requirements for the low voltage power circuit breaker construction and test standards, namely ANSI C37.13 and ANSI C37.50.
NEMA (National Electrical Manufacturers Association)
• NEMA is an electrical equipment manufacturer only organization, such as Cutler-Hammer, General Electric, and Square D. NEMA defines preferred ratings, related requirements, and application recommendations.
• NEMA Standards normally cover additional information about a product of specific interest to the manufacturing community, which the American National Standards Committee does not include in its scope. NEMA votes on the suitability of standards for ANSI designation and adopts, by reference, the appropriate American National Standards.
• The applicable low voltage power circuit breaker NEMA Standard is SG-3, and it adopts ANSI C37.16 in its entirety.
UL (Underwriters Laboratories Inc.)
• UL is an independent, non profit, third party testing and certification company headquartered in Northbrook, Illinois. It functions to develop standards and to insure that equipment meets relevant published standards.
• UL also adopts otherwise recognized standards, and, in some instances, develops their own independent certification tests. In the case of low voltage power circuit breakers, the UL Standard is UL1066, which was previously mentioned. UL1066, entitled "Standard for Low Voltage AC and DC Power Circuit Breakers Used in Enclosures," calls for testing to demonstrate compliance with ANSI/IEEE C37.13 without change. A UL Label is affixed to the circuit breaker to indicate successful compliance.
CSA (Canadian Standards Association)
• The Canadian Standards Association is in the category of a major international standard. Its design and testing requirements are essentially the same as required by UL. In fact, harmonization programs between UL and CSA are ongoing to close the gap and/or eliminate differences. The Canadian Standards Association standard most associated with low voltage power circuit breakers is CSA 22.2-31 for Switchgear Assemblies.
ANSI (American National Standards Institute)
You were briefly introduced to ANSI. Now let's take the time to get to know ANSI much better because ANSI is the key to low voltage power circuit breakers. It is the recognized North American Authority on equipment standards.
ANSI's Purpose - ANSI is a nonprofit, privately-funded membership organization that coordinates the development of U.S. voluntary national standards, called American National Standards. It is also the U.S. member body to the non-treaty international standards bodies, such as the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC). ANSI serves both the private and public sectors' need for voluntary standardization.
ANSI's History - The institute was founded in 1918. It was prompted by the need for an umbrella organization to coordinate the activities of the U.S. voluntary standards system and eliminate conflict and/or duplication in the development process. The institute serves a diverse membership of over 1300 companies, 250 professional, technical, trade, labor and consumer organizations, and some 30 government agencies.
A simple yet very typical example of why ANSI came into existence can be related to the low voltage power circuit breaker. In the early days of low voltage power circuit breaker development, manufacturers and users were building and applying equipment with little thought given to uniform performance or design standardization. The C37 standard was developed and implemented to establish minimum performance standards for the circuit breaker and its physical design features.
The standard was meant to address even the smallest detail. A close button, for example, might not say close on it or it varied in color from one manufacturer to the next. These inconsistencies in design made products confusing for use by customers. This might seem to be one trivial point, but you can imagine how big the problem would be when compounded with every aspect of a low voltage power circuit breaker.
ANSI's Functions - ANSI functions to:
• Coordinate the self-regulating, due process consensus voluntary standards system
• Administer the development of standards and approve them as American National Standards
• Provide the means for the U.S. to influence development of international and regional standards
• Disseminate timely and important information on national, international and regional standards activities to U.S. industry
These standards are intended to provide guidance, direction, and requirements. Compliance to these standards does not, nor is it meant to limit manufacturers in construction, materials, or the technology used.
Specifically relating to power circuit breakers, ANSI standards are written by either the IEEE Switchgear Committee or NEMA. The electrical standards written by both of these organizations are reviewed and clarified by the Accredited Standards Committee (ASC) for power switchgear and power circuit breakers. The ASC standards group is entitled C37.
ANSI Defined Standards for Low Voltage Power Circuit Breakers - Although there are a multitude of ANSI standards relating to many different types of equipment, only those standards relating to low voltage power circuit breakers are outlined here. The intent is just to make you aware of just how many are applicable to just one category of electrical equipment. You will notice that each standard is followed by a specific year. As additions or changes are made to a standard, the year is altered to indicate the latest version. Obviously, staying on top of the latest version is an ongoing process. You should also note that each standard is given a broad word definition.
1. ANSI/IEEE C37.13-1990, "Low Voltage AC Power Circuit Breakers Used in Enclosures"
2. ANSI C37.16-1997, "Preferred Ratings Related Requirements and Application Recommendations for Low Voltage Power Circuit Breakers and AC Power Circuit Protectors"
3. ANSI C37.17-1997, "Trip Devices for AC and General Purpose DC Low Voltage Power Circuit Breakers"
4. ANSI C37.50-1989, "Test Procedures for Low Voltage AC Power Circuit Breakers Used in Enclosures"
5. IEEE Standard C37.100-1992, "IEEE Standard Definitions for Power Switchgear"
6. IEEE C37.20.1-1993, "Standard for Metal-Enclosed Low Voltage Power Circuit Breaker Switchgear"
7. ANSI C37.51-1989, "Standard for Switchgear - Metal-Enclosed Low Voltage AC Power Circuit Breaker Switchgear Assemblies - Conformance Test Procedures"
8. NEMA SG-3-1981, "Low Voltage Power Circuit Breakers"
9. UL1066-1993, "Standard for Low Voltage AC and DC Power Circuit Breakers Used in Enclosures"
This lengthy list gives you some indication why it is a matter of practicality when a manufacturer states that a piece of equipment is built and tested to all applicable NEMA, ANSI, IEEE and UL standards. It was also mentioned that a great deal of referencing to other standards takes place within the body of a specific standard. Successful testing and compliance with respect to one standard often includes automatic compliance with other standards. It is worth repeating one of the examples given.
Example: ANSI C37.13 details the physical attributes, such as Stored Energy, that a low voltage AC power circuit breaker must have to comply. At the same time, ANSI C37.50 references C37.13 and details how the described circuit breaker should be tested. The key here is that successful testing in keeping with ANSI C37.50 brings with it compliance to C37.13. There is no need to mention C37.13, when it is stated that the circuit breaker complies with C37.50.
IEC (International Electrotechnical Commission)
IEC presides over the standardization of equipment for a number of other parts of the world. In view of today's global markets, there is a significant amount of interaction between the organizations just discussed and IEC.
IEC 947-2 is a multi-part international testing standard covering a variety of devices, including circuit breakers of all types. It is entitled "Low Voltage Switchgear and Controlgear."
As far as IEC is concerned, every device tested to IEC 947-2 must be subjected to several test sequences in order to be approved. Because IEC 947-2 covers both low voltage power circuit breakers and low voltage molded case circuit breakers, the exact test sequences performed are not necessarily the same. They depend on the category of the device.
Category A Device - In general, this is a device without a short time Withstand Rating, such as a molded case circuit breaker.
Category B Device - This is a device with a short time withstand rating, such as a power circuit breaker and certain molded case circuit breakers. Typically, these devices are referred to as Air Circuit Breakers or just ACBs.
IEC 947-2 was developed with assistance from members of the U.S. National Committee. Still, a number of significant differences exist between IEC 947-2 and applicable ANSI standards. In particular, the various ratings of a circuit breaker can differ when tested to each standard. Therefore, any product comparisons made between products tested to these different standards (domestic versus international) should only be made with a thorough understanding of the differences.
Standards Conclusion
This might seem to be a monumental amount of information about standards. It is only the tip of the iceberg. This is not to imply that you must be an expert on standards to deal with power circuit breakers. You can, however, begin to appreciate just how much effort, investment, and plain hard work goes into being able to legitimately print in a document a statement such as:
"Magnum DS Low Voltage Power Circuit Breakers are UL Listed and built and tested to all applicable ANSI Standards." Keep in mind that all these standards establish minimum requirements. There is nothing prohibiting a manufacturer from exceeding standards by offering additional features and/or using newer and improved operational techniques for more efficient and higher levels of performance. Magnum DS does just that in a number of areas.
The majority of the remaining discussions in this module will be presented as they relate to applicable ANSI Standards. Remember, however, that other standards do exist in other parts of the world. They must be complied with to participate in the international segment (Figure 38).
Figure 38. Circuit Breaker Identification
Low Voltage Power Circuit Breaker C37.50 Testing
Testing of a low voltage power circuit breaker in keeping with required ANSI Standards provides the first glimpse at what makes a low voltage power circuit breaker unique. Remember, low voltage power circuit breakers are applied at or below their nameplate ratings. That nameplate rating is a result of having successfully completed a series of rigorous tests. This is referred to as a 100% rating.
Although every detail of the testing will not be covered here, you will have an appreciation for just how demanding these ANSI defined tests are for low voltage power circuit breakers. The tests will be described as four test sequences. It should be pointed out here that all tests are performed using a Drawout circuit breaker in its enclosure for each frame size.
The first three test sequences are similar in many ways to the tests performed on other types of low voltage circuit breakers, such as a molded case circuit breaker. It is the fourth test sequence performed on a low voltage power circuit breaker that differentiates the power circuit breaker from other types of circuit breakers.
Note that all the following test sequences, except for Test Sequence 4 in which the circuit breaker has no Trip Unit, are preceded and followed by a calibration test and dielectric check.
Test Sequence 1 - This test sequence consists of a temperature rise test, an overload switching test, and then a short circuit test. The circuit breaker is equipped with an instantaneous trip.
1. The circuit breaker is loaded to 100% of the maximum rating of the frame size (in normal enclosure) until the temperature is constant. The standard lists the maximum permissible temperature rises at various parts of the circuit breaker.
2. The circuit breaker is then subjected to number of opening operations on over-load switching.
3. The circuit breaker is then given a 3-phase short circuit test at its maximum voltage rating, which is 635 volts for this 600 volt rated circuit breaker. The short circuit current in this case can be no less than the 600 volt interrupting capacity of the circuit breaker being tested. The three maximum voltages used during testing and typically listed on the nameplate are listed below along with their corresponding application voltages:
Maximum Voltage Application Voltage
635 volts 600 volts
508 volts 480 volts
254 volts 240 volts
4. The short circuit test consists of initiating current through the closed circuit breaker, causing it to trip. After 15 seconds, the circuit breaker is re-closed on the fault, and then allowed to trip open to clear the fault. This is known as an O-CO (open-close open) test.
5. The short circuit test is followed by a calibration check and a dielectric test.
Test Sequence 2 - This test sequence consists of a series of short circuit tests on a circuit breaker equipped with selective tripping (no instantaneous).
1. Once again, all short circuit tests are preceded by a dielectric test and a calibration test. After the interruptions, the circuit breaker is given another dielectric test and the calibration is again checked.
2. Each short circuit test is an O-CO test meaning that the circuit breaker interrupts the full fault current twice.
3. After the 3-phase short circuit tests are completed at different prescribed voltages, single phase tests are performed. A new circuit breaker may be used for each test, or each test may be done on different poles of the same circuit breaker.
4. One single-phase test is done at each of the same three maximum voltage ratings used for the three-phase tests (635, 508 and 254 volts) at the appropriate Interrupting Rating for that voltage.
Test Sequence 3 - This test sequence includes tests of mechanical and electrical endurance.
1. A circuit breaker is calibrated, given a dielectric test, and subjected to a large number of operations. Some of the operations are at no load and some at full load.
2. The required number of operations varies by circuit breaker frame size.
3. After the endurance test, the same circuit breaker is given a full 3-phase O-CO short circuit test at 635 volts and a dielectric withstand test.
The required number of operations for the endurance test just described is about the same for larger frame power circuit breakers and somewhat higher for smaller frame power circuit breakers compared to molded case circuit breaker endurance tests. For the sake of comparison, refer to the two endurance ratings tables, one for power circuit breakers and one for molded case circuit breakers .


Test Sequence 4 - This test sequence includes a short time current (withstand) test. Molded case and insulated case circuit breakers are not usually subjected to this type of test and, therefore, have no full 30 cycle Short Time Rating. This is one of the key differences.
1. For this test, the circuit breaker does not have a trip unit or the trip unit is disconnected. During the short-circuit testing, the circuit breaker can be tripped instantaneously by a shunt trip.
2. The circuit breaker is closed and then energized at its full short time current rating. The short time rating is usually equal to the 600 volt short circuit current rating.
3. The current is left on for 30 cycles (1/2 second), then off for 15 seconds, then back on for another 30 cycles. The circuit breaker remains closed during this test sequence.
4. After the short time current (withstand) tests, the same circuit breaker is given a 3-phase short circuit test sequence at full short circuit current rating and 635 volts. The circuit breaker is opened as quickly as possible by the shunt trip, which is energized at the same instant the power is applied. The intent is to force the circuit breaker to open during the worst case conditions (full current asymmetry). This is where the short time rating for the circuit breaker comes from.
5. After the final test, the circuit breaker calibration is checked and given a dielectric withstand test.
During Test Sequence 4, the circuit breaker is subjected to tremendous physical forces from the magnetic fields and to severe heating effects from the current. Think about it. Both the magnetic forces and the heating vary with the square of the current (Figure 41). For example, a 4000 ampere frame low voltage power circuit breaker at 85,000 amperes is subjected to forces and heating more than 450 times normal for each 30 cycle test. It is quite awesome.

Advanced Low Voltage Power Circuit Breakers

The Magnum DS Family of low voltage power circuit breakers is not an extension of any other low voltage design (Figure 36). It is at the forefront of technology and development. For this reason, it is an excellent design to discuss when certain specific examples are required in this module. Keep in mind, however, all low voltage power circuit breakers do not necessarily offer as many features or use the same advanced technology as Magnum DS. Even though this might be the case, it does not mean that another design does not qualify as a low voltage power circuit breaker, or that there are not other capable low voltage power circuit breakers.


Magnum DS is a low voltage power circuit breaker. It is built and tested to all applicable ANSI Standards for low voltage AC power circuit breakers and Underwriter's Laboratories Listed. Because of its flexible design, an International Electrotechnical Commission rated version of Magnum DS is also available to address international requirements. This IEC version is called Magnum. Everything that is expected of an ANSI rated low voltage power circuit breaker is delivered by Magnum DS, and then some. If you think this sounds a bit biased, it is. Cutler-Hammer is justifiably proud of Magnum DS, and as you learn more and more detailed information about low voltage power circuit breakers, you will most certainly agree.
You will recall a discussion of the areas that might set a low voltage power circuit breaker apart from other types of low voltage circuit breakers.
Namely:
• Method used to make and break circuits
• Ratings
• Construction/Maintainability
• Integrally Mounted Trip Units
• Operating Mechanisms
• Testing

You were initially introduced to the primary factors that make the low voltage power circuit breaker unique. Other factors and methods that were rather common with low voltage power circuit breakers but not necessarily unique were also discussed. You will revisit a number of the areas just mentioned. Each topic discussed, however, will be presented in more detail with special emphasis placed on those factors that set the low voltage power circuit breaker apart from other types of low voltage circuit breakers, such as molded case and insulated case circuit breakers. The general topics to be discussed are:

• Standards and Testing
• Construction Methods
• Ratings and Performance
• Operational Techniques
• Integral Trip Unit
• Applications
• Low Voltage Power Circuit Breaker Summary

The last section reiterates many of the facts learned with special attention given to the unique factors associated with low voltage power circuit breakers. This summary can serve as a review and a future quick reference.

Testing

The testing required and the standards that must be met by a low voltage power circuit breaker depend on the area of the world where the circuit breaker is applied. To play a major international role, low voltage power circuit breakers should be able to meet the requirements of ANSI, UL and IEC .

As you continue through this module, you will become well aware that the required testing is the key factor in defining the type of circuit breaker. In a very general and simplistic way, low voltage power circuit breakers undergo a sequence of four rigorous tests.

1. The first sequence consists of a temperature rise, an overload, and a short-circuit test.
2. The second sequence is a series of short-circuit tests.
3. The third sequence is an endurance test.
4. The fourth is a momentary rating test.

Molded case circuit breakers, for example, are subjected to tests similar to numbers 1, 2 and 3. The fourth test sequence, momentary rating test, is specific to power circuit breakers and to some IEC molded case circuit breakers.
Specific testing details will not be covered in this module. It should be pointed out, however, that the momentary rating test just mentioned (test sequence 4) subjects the circuit breaker to tremendous physical forces and severe heating effects. Very simply speaking, the circuit breaker is subjected to its full short time current rating for two (2) time periods up to 30 cycles each. The short time rating indicates what magnitude of current the circuit breaker can stand with its contacts closed for a short period without being damaged. The circuit breaker's short time rating is often equal to its 600 volt interrupting capacity. A low voltage power circuit breaker must be strong enough to survive this test and function properly after completion.

Helping the Customer
Selection of the proper low voltage power circuit breaker for a specific application is not a difficult process. There are some important questions, however, you must be able to answer. Fortunately, the most difficult part of the job has already been done by other qualified individuals when they determined the requirements of the system.
This includes determining things like:
• Circuit Breaker type required
• Application voltage
• Maximum fault current system could see
• Continuous current for the system and each branch
• System frequency
• Types of trip unit capabilities
• Programmable functions
• Accessory needs
• Mounting configuration
• Special requirements

Your job is to make sure these types of questions are answered. The more familiar you are with what a particular circuit breaker line has to offer, the easier the task. Let's start by looking at what circuit breaker manufacturers do to help.
Manufacturers normally provide a great deal of assistance in the way of printed material, computer accessible information and direct contact. This does not mean, however, you should not put forth that extra effort to know personally what is available. Learn to use all the information provided.
Most selection factors fall into one of two categories:

• Standard selection factors
• Special selection factors

Standard Selection Factors

Standard selection factors normally are associated with the circuit breaker's ratings/standards, operation method, accessory items, and how the breaker will be mounted. The most common points to consider will be discussed.
1. Standards - Applicable standards were discussed in this module and earlier modules. You should be told or it will appear in a written specification what standards the circuit breakers must meet. Newer low voltage power circuit breakers meet a wide array of standards which will make them acceptable in most parts of the world. In addition, make sure you are aware of any special local requirements and/or standards.
2. Ratings - This is a critical part of the selection process. You should already know what is required. Now you must determine what specific circuit breakers will meet the rating requirements. Manufacturers normally provide easy to read tables outlining the ratings of every circuit breaker frame. Keep in mind there could be more than one table. This is especially true for newer circuit breaker designs that meet both ANSI and IEC requirements. A manufacturer might choose to present it as one combined table or two tables. If you know what is required, you will be able to make a selection from the tables under normal circumstances.

ANSI Example
Let's take a look at a typical type ANSI table for a low voltage power circuit breaker and see what it has to offer (Figure 35). The table used in this example will not cover every circuit breaker rating for a particular design.
Enough of the table is presented to give you a good working knowledge on how to proceed. Each area of the table that is discussed is identified by a circled letter to simplify the discussion. One last important point should be made before beginning. Always read footnote references carefully. They provide important information and could be critical to the proper selection.



A: The "Breaker Type" is usually the name given to the circuit breaker by the manufacturer along with some general information about the ratings of that specific circuit breaker type. In the example ratings table shown, XYZ-508 is the first circuit breaker listed. The XYZ is the circuit breaker's name. The first number "5" gives you a general idea what the interrupting rating is of the XYZ-508 circuit breaker at a voltage of 480 volts. This is a common presentation method because the widest used application voltage domestically is 480 volts. The last two numbers, "08" in this case, usually tell you the maximum continuous current rating of the circuit breaker. XYZ-508 can, therefore, carry 800 amperes continuously, and interrupt 50,000 amperes at 480 volts.
B: This column outlines specifically the maximum continuous current the circuit breaker will carry. Notice that circuit breaker type XYZ-616 in the example table will carry a maximum continuous current of 1600 amperes. Notice also that the last two numbers of the circuit breaker type XYZ-616 ("16") give you the same information, with 16 meaning 1600. Take the time to make this same comparison with circuit breaker type XYZ-632.
C: Notice that the rest of this table is devoted to the interrupting capabilities in amperes of the different circuit breaker types at different application voltages. Also notice that the application voltages listed are:
• 208-240 volts
• 480 volts
• 600 volts
The nominal voltage range for the ANSI market is 208 to 600 Volts AC at a frequency of 50 or 60 hertz. Get comfortable with seeing these voltages when talking about ANSI rated low voltage power circuit breakers.

D: You will notice that these two columns are labeled differently. The first column entitled "With Instantaneous Trip" outlines the interrupting capabilities of each circuit breaker frame at the different application voltages. These ratings are applicable when the circuit breaker's trip unit provides instantaneous protection. In other words, the circuit breaker can be applied to safely handle faults of the magnitudes shown.

You will also notice in the column entitled "Without Instantaneous Trip" that some of the interrupting ratings are somewhat lower than the left column under 208-240 volts. These ratings are the magnitudes that the circuit breaker can tolerate safely for a short delay period of time (30 cycles) before opening at the short delay current ratings shown. This might sound like a contradiction. It really is not for a number of reasons. Consider the following points.

1. You will recall from material presented earlier that a low voltage power circuit breaker's short time rating is normally the same as its interrupting rating. The key word here is normally. The partial ratings table being considered here already indicates that there are some very limited times when a low voltage power circuit breaker could have a higher interrupting rating if it has instantaneous protection versus just short time protection and no instantaneous. This was probably the result of a conscious decision to meet some very specific application requirement for a particular customer or industry, knowing the fact that a circuit breaker had to have instantaneous to be applied at these somewhat higher ratings.

2. The fact still remains that low voltage power circuit breakers must be and are only applied in keeping with their nameplate rating. This, in almost all cases, shows the interrupting rating and the short time rating to be the same. When electrical systems are being considered, fault calculations are done to determine the maximum fault current a system can experience. Low voltage power circuit breakers are then selected with ratings that are able to deal successfully and safely with the worst case fault scenario calculated. In other words, if a low voltage power circuit breaker with an adequate short delay current rating is applied, it will stay closed for the appropriate short time no matter what. This is true because it will not see (experience) more that it was designed to safely handle. End of that part of the story.

3. On the other hand, a low voltage power circuit breaker, which is already in the open position, will trip open instantaneously if an attempt is made to close the breaker on an existing fault. This safety feature prevents damage that could result from closing on a fault. Today, this feature is normally accomplished through circuitry which is part of the trip unit. This self protecting circuitry is often called a discriminator circuit or may be called a making current release in newer designs like Magnum DS. Its purpose has nothing to do with a circuit breaker that is already closed and functioning.

For now, how this feature is technically accomplished will not be discussed. Just be aware that such a feature exists with low voltage power circuit breakers. Future training material specific to a particular low voltage power circuit breaker design will discuss just how it is accomplished.
Remember:

• Low voltage power circuit breakers are applied at their nameplate ratings.
• Low voltage power circuit breakers are sized and selected for application to handle the maximum fault that could be encountered where they are applied.
• Low voltage power circuit breakers are provided with a means to trip (open) instantaneously if they are closed in on an existing fault.

E: Let's just briefly take a look at the footnote. It tells you that these ratings are also the short time ratings of the circuit breaker. The material in D was discussed as though we already knew these were short time ratings, and we did. Suppose we did not know that fact and failed to read the footnote. We would not be as informed as we should be for the proper circuit breaker selection. It could be like making the selection blindfolded. Be sure to read the footnotes.

IEC Example

IEC Example - An IEC example similar to the one just presented will not be offered here. Ratings tables and their appearance as to how data is presented can change from country to country and even manufacturer to manufacturer. The information presented, however, is usually similar. You should be aware of some of the noticeable differences in the presented data, and start now to become familiar with IEC rated breakers. For now, consider the following to get started:
• The voltage range for the international market is 380 through 690 Volts AC at a frequency of 50 or 60 hertz.
• The general continuous current range for low voltage power circuit breakers is 800 through 6300 amperes.
• The voltage and current abbreviations and names are different, such as:
Ue - application voltage, such as 380 or 690 volts.
In - rated current such as 800 or 6300 amperes.
Ics - rated service short circuit breaking capacity.
Icu - rated ultimate short-circuit breaking capacity.
Icw - rated short time withstand current (similar to the ANSI short time rating and the circuit breaker is expected to function properly again after having dealt with the Icw).

ANSI and IEC Example
Let's make a quick comparison from a presentation standpoint. Keep in mind, the important things are:
1. Will the circuit breaker being considered do the job?
2. Will the circuit breaker being considered meet the standards in effect where the circuit breaker is to be used?
It is not possible to simply take a product designed and tested to one standard (ANSI or IEC) and certify it to the other standard. A manufacturer must undertake a concerted design effort to satisfy both standards individually.

This is by far not an all inclusive example. It is only intended to draw some simple ANSI and IEC comparisons between some of the most common selection points that must be considered when selecting low voltage power circuit breakers. You can see that although not exactly the same, it is primarily a matter of familiarization.

3. Operation Method - As discussed earlier, low voltage power circuit breakers are either manually or electrically operated. You must always specify the method of operation. At some point, you will need to know the secondary control voltage being used for an electrically operated circuit breaker. Even if the circuit breakers are manually operated, it is a good idea to find out the secondary control voltage. The control voltage is necessary for the final selection of a number of items, not just electrically operated circuit breakers.
4. Accessory Items - Many of the common accessory items associated with low voltage power circuit breakers were discussed earlier. You have to be alert for these items in a specification or ask the customer if any are required. A determination can then be made if a compatible accessory is available to meet the need. Make a list of the required accessories and the specific requirements that are appropriate for them, such as control voltage, number and types of contacts and overall function.
5. Mounting Method - You will need to know whether the breakers will be fixed mounted or drawout. Always check to see if there are any special requirements for either configuration.
Special Selection Factors
There may not be special conditions to consider, although this should be determined as soon as possible. You may be able to deal with certain special conditions and others might call for assistance from the manufacturer. Do not hesitate to ask for help. Some conditions or requirements to look for that might not be considered standard are:

• High or low ambient temperatures
• Moist or corrosive atmospheres
• Altitude
• High shock conditions
• Unusual circuit breaker mounting conditions

Governing Standards

You will recall from Module 5 that circuit breakers are designed, built and tested in accordance with one or more specific sets of standards. In this module, you will be introduced to the standards specific to low voltage power circuit breakers. The intent here is not to present and study the different applicable standards word for word. That type of undertaking would be a course unto itself. Our goal is to understand a little about low voltage power circuit breaker related standards, where they were, and where they are today.



You will hear many people repeat specific standards designations. Many of those same people do not have an intimate knowledge of what the standards actually say, nor are we saying they should. The actual product selection based on standards compliance should be left to the experts. It is helpful, however, to know what specific standards your products comply with and what general topic a specific standard addresses.

Keep in mind that a standard exists for almost everything. There are specific standards for circuit breakers and others for the structural assembly. Compliance with these exacting standards ensures customers of the very best possible product selection with a high degree of comfort. There is no room for compromise when performance, quality and safety are involved.
A number of years ago, low voltage power circuit breakers and most other types of equipment were designed and built primarily with only domestic standards in mind. This approach also was taken by foreign suppliers. A manufacturer would offer a circuit breaker designed, tested and manufactured in keeping with applicable standards for that part of the world or even particular country. Trying to play a significant role in other world markets was, at best, extremely difficult. If manufacturers today expect to be global participants, they must offer products that comply with the standards applicable to a variety of different markets around the world. This will require that you become familiar with both domestic and international nomenclature, ratings, procedures and governing standards. The task is greater, but so is the reward.
Some of the lines separating different types of low voltage circuit breakers were at times blurred in the past. Low voltage metal-frame power circuit breakers were built and tested to certain ANSI and UL specifications, while some low voltage encased circuit breakers were tested to UL specifications specific to molded case circuit breakers. The newest low voltage power circuit breakers today, like Magnum DS, are tested to specific low voltage power circuit breaker standards, like ANSI. They are also tested to standards that cover a much broader product scope, like IEC. The primary testing standards associated with low voltage power circuit breakers today are:

ANSI
The American National Standards Institute's ANSI C37.50 is a specific North American testing standard entitled "Low Voltage AC Power Circuit Breakers Used In Enclosures." This standard specifies rigorous tests for product performance. There are additional C37 standards which govern power circuit breaker and trip unit construction, such as C37.13 and C37.17 respectively.

UL
Underwriter's Laboratories Incorporated's UL1066, for the most part, calls for testing to demonstrate compliance with ANSI C37.50 just mentioned. A UL Label is affixed to the breaker to indicate successful compliance.

IEC
The International Electrotechnical Commission IEC 947-2 is a more general international testing standard covering a variety of devices, including circuit breakers of all types, and is entitled "Low Voltage Switchgear and Controlgear."

Closing Comments on Standards
Before concluding this section, it might help to minimize confusion if you remember that there is often a great deal of referencing to other standards that takes place within a specific standard. Successful testing with respect to one standard often includes automatic compliance with other standards.

Example 1 - ANSI C37.13 details the physical attributes, such as stored energy, that a low voltage AC power circuit breaker must have, while ANSI C37.50 references C37.13 and details how the described breaker should be tested. The key here is that successful testing in keeping with ANSI C37.50 brings with it compliance to C37.13.
Example 2 - In a similar fashion, IEC 947-2 references IEC 947-1 (General Rules). Compliance with IEC 947-2, therefore, brings with it IEC 947-1 compliance.

Mounting Methods

As briefly discussed earlier, low voltage power circuit breakers are usually available in the two following mounting configurations:

• Fixed
• Drawout

Total usage of low voltage power circuit breakers today is dominated by the drawout configuration because it provides for easier maintenance and continuity of service. Most circuit breaker manufacturers, however, offer both types.

Fixed Circuit Breaker
Fixed low voltage power circuit breakers usually have fixed primary conductor stabs protruding from the rear of the circuit breaker. The circuit breaker is bolted in position within its assembly compartment, and the rear conductor stabs are bolted to primary bus connections (Figure 30). Secondary connections are also made manually. Power must be turned off to the assembly to connect a fixed circuit breaker into the system or to remove it from the system.


Drawout Circuit Breaker
Drawout low voltage power circuit breakers have a levering device to move the circuit breaker from one compartment position to the next. Usually part of the levering mechanism is on the circuit breaker with a corresponding part is in its compartment. Working together, they provide the mechanical means required to move the circuit breaker. Drawout circuit breakers are designed to be removed and connected without cutting power to the entire assembly under load conditions because the circuit breaker, by design, automatically opens before racking can take place. This means that power to the assembly does not have to be turned off when a circuit breaker is removed from or inserted into the assembly, thus ensuring continuity of service.
Drawout circuit breaker compartments are provided with extension rails which, when not in use, are stored inside the compartment (Figure 31). The extension rails provide a means by which a drawout circuit breaker can be easily removed from its compartment for inspection, maintenance or movement to another area.


Primary electrical connections between the circuit breaker and the primary bus are automatically made or broken as the circuit breaker is moved into or out of the "Connected" position within the circuit breaker compartment. Primary connectors mounted to the back of the circuit breaker slide onto the primary bus connectors. These primary connectors, often called finger clusters or disconnect contacts, are frequently composed of a number of spring loaded fingers (contacts) . The number of fingers (contacts) used is dictated by the amount of current they will carry. Fingers (contacts) are made of an excellent conducting material or material combination, such as silver plated copper.


Secondary electrical connections are usually automatically made or broken as the circuit breaker is moved into and out of its compartment. As the circuit breaker is moved into the "Test" position from the "Disconnect" position, the secondary connections are made providing the required secondary power for testing or operating the breaker, but no primary power. The secondaries remain connected as the breaker moves into the "Connected" position. When the circuit breaker is moved out of the "Connected" position, the secondaries remain connected and stay connected until the circuit breaker is moved farther out of its compartment past the "Test" position. The graphics of the four positions presented earlier in the module demonstrate the movement and connections.

Arc Extinguishers

In Module 5, a number of ways or combinations of ways to extinguish an arc was discussed. Low voltage power circuit breakers use some type of Arc Extinguishers (arc chutes or arc chambers) mounted above and around the main contacts to extinguish arcs in air (Figures 21 and 22). This leads to the name low voltage power air circuit breakers.

Arc chutes, in some form, have been used to extinguish arcs for more than a half century. The primary purpose of an arc chute is to extinguish an arc each time a circuit breaker interrupts a current. This is accomplished by confining, dividing and cooling the arc. This accomplished, the arc is not able to sustain itself through current zero.

Not all arc extinguishers are created equal and, therefore, some are more efficient than others. Efficiency is very important because the amount of contact damage caused by arcing is directly related to how fast or efficiently arcs are extinguished. More efficiency leads to longer contact life.

During the arcing process, ionized gases are generated and normally vented, in some fashion, harmlessly away from the circuit breaker, breaker compartment, and any operator who might be in close proximity to the equipment. It is also known that the high pressure created by these gases, if controlled properly, can be put to good use during interruption.
To this end, the molded case low voltage power circuit breaker design, for example, utilizes this gas pressure to help with the interruption process while minimizing gas leakage back into the circuit breaker itself. This improvement is accomplished through the use of seals in the arc chamber and a close fit of pieces and parts. This can only be accomplished with molded frame designs. Obviously, the design and process is a bit more involved than just described. For now, the most important thing to remember is that the original arc extinguisher concept is still used today, but great strides have been taken to improve upon the original concept with significant improvements in overall efficiency.
Operating Mechanism
You learned in Module 5 that some type of a mechanism is provided with all circuit breakers for opening and closing. Low voltage power circuit breakers are no exception. A low voltage power circuit breaker operating mechanism is composed of a number of different parts, assemblies and accessories, all dedicated to ensuring that the circuit breaker opens and closes consistently.
The mechanism is a two-step spring charged stored energy type providing three basic functions:

• A means to charge the closing springs
• A means to close/open the circuit breaker using the stored energy of the closing and opening springs
• A means to perform an Open-Close-Open duty cycle
Two varieties of the mechanism exist:
• Manual
• Electrical (Motor Operated)

The manually operated circuit breaker has its closing springs charged manually through the use of some type of charging handle. The circuit breaker is closed using a manual close button which is a mechanical process. As the circuit breaker closes, a set of smaller opening springs are charged. The circuit breaker is opened using a manual trip (open) button, which is a mechanical process.

Safety interlocks, accessory items and trip units can also cause the circuit breaker to trip through mechanical means. Most manually operated power circuit breakers can be equipped with an optional device to electrically release the spring's stored energy, thus closing the circuit breaker.

Previously, it was not practical or even possible to convert manually operated low voltage power circuit breakers to electrically operated circuit breakers in the field. This is no longer impossible with newer low voltage power circuit breaker designs. Such designs permit manually operated circuit breakers to be converted to electrically operated circuit breakers by field installing UL Listed electrical operators.


An electrically operated circuit breaker can be operated every way a manually operated circuit breaker can be operated. In addition, a small electric motor is normally used to automatically charge the closing springs, and an electrical means to close or trip (open) the circuit breaker is provided.

Integral Trip Unit
For a circuit breaker to do its job, a means must be provided enabling the circuit breaker to perform automatically or in response to other commands. In short, the circuit breaker is a rather dumb device without a brain (intelligence source). This source of intelligence is the trip unit.
As required by ANSI Standards, low voltage power circuit breakers must be provided with an Integrally Mounted Trip Unit. This means that the trip unit must be inside of, or part of, the circuit breaker. Prior to the advent of the first solid state trip unit, electromagnetic type tripping devices, commonly called dual-overcurrent magnetic trips, were used with all low voltage power circuit breakers. In recent times, this type of tripping device on low voltage power circuit breakers has disappeared from the scene. For this reason, only microprocessor-based trip units will be discussed in this module.

A typical microprocessor-based trip unit used with low voltage power circuit breakers offers the following capabilities:
• Programming
• Monitoring
• Diagnostic
• Communication
• Testing


The capabilities of a particular trip unit depends on the trip unit design itself and system requirements. Some trip units can only offer basic features, while others can offer basic features or, if required by the system, additional sophisticated and highly advanced features.
The operating response of a trip unit is graphically represented by time-current characteristic curves. These curves show how and when a particular trip unit will act for given values of time and current. A characteristic curve is represented by a band created by a minimum and maximum value of time or current.

The programmable or adjustable features of a trip unit permit movement of its characteristic curve or parts of the curve . This movement can be done in both a horizontal and vertical direction. Some trip units even allow the shape of the curve to be changed.

Most trip units offer protection combinations of:

• (L) Long delay protection - protection against overloads and short circuits
• (S) Short delay protection - protection against short circuits
• (I) Instantaneous protection - protection against short circuits
• (G) Ground fault protection - protection against ground faults

A trip unit offering all four of these protection at one time is commonly called an LSIG Trip Unit. Other combinations are also available, such as LI, LS, LSI, LIG and LSG.
The long, short and ground functions would have programmable values of current and time. Obviously, instantaneous has no associated time because the trip is instantaneous (Figure 26). Trip units have these different programmable features programmed so they coordinate with one another and with the requirements of the system being protected to provide the closest possible system coordination and protection against all eventualities. This coordination discipline is where you start hearing phrases like curve shaping and close coordination. No attempt will be made in this module to get into the details of this discipline. It is quite specialized and best left to individuals with the proper training.



More advanced trip units are able to monitor and display currents, energy, power, power quality and power factor. They also may be able to diagnose problems and provide advance warnings of potential problems, such as harmonics. Two way communications for remote monitoring and control is also available. This affords the user a cost effective way to monitor and control expansive, multi-location facilities with a wide array of protective equipment and operational machinery.

Trip and no trip tests can usually be performed on the trip unit and circuit breaker utilizing integral testing capabilities or separate test kits. Normally, the tests can be performed with the circuit breaker in service and full protection provided during the testing. This type of testing is secondary testing. Primary testing involves specialty testing equipment and a testing expertise, and is not discussed in this module.

Accessory Items
Accessories used with low voltage power circuit breakers are usually added to the circuit breaker to provide additional features, such as status indication and remote operation. It is possible, however, that some accessories for some circuit breaker designs might be mounted remotely from the circuit breaker. These devices might be totally mechanical, totally electrical or a combination. The intent here is to briefly discuss the function of commonly used accessory items, although all low voltage power circuit breakers do not necessarily offer all of the devices being discussed, nor is this list all inclusive.
• Electrical Operator - This is an assembly of devices including a small spring charging motor that when added to a manually operated circuit breaker converts it to an electrically operated circuit breaker. This allows for remote operation (open/close) of the circuit breaker. The ability to field install this device is more common with newer low voltage power circuit breakers. Power circuit breakers normally use to be either manual or electrical by design, and could not be easily converted.
• Operations Counter - An operations counter is a counting device, usually linked in some fashion to the operating mechanism. It is used to count the open and close operations of the circuit breaker, and serves as a maintenance aid.
• Auxiliary Switch - An auxiliary switch consists of "normally open" (NO) and "normally closed" (NC) contacts (Figure 27). The contacts on some switches are convertible from NO to NC and vice versa. The contacts are frequently referred to as "a" or "b" contacts. The "a" being open when the circuit breaker is open and the "b" closed when the circuit breaker is open. In short, these auxiliary contacts change "state" when the circuit breaker main contacts change "state." An auxiliary switch is normally mounted on the circuit breaker. Contacts from these switches are frequently used for electrical operation of a circuit breaker, remote signaling, and electrical interlocking.


• Undervoltage Release (UVR) - An undervoltage release, normally a circuit breaker mounted electromechanical device, trips the circuit breaker when the voltage falls below a predetermined level.
• Shunt Trip (ST) - A shunt trip is an electromechanical device which is standard on most electrically operated power circuit breakers. When added to a manually operated circuit breaker, it provides for remote controlled electrical tripping.
• Spring Release (SR) - The spring release device is standard on most electrically operated power circuit breakers. When added to a manually operated circuit breaker, it permits the circuit breaker to be closed electrically from a remote location.
• Bell Alarm (OTS) - The bell alarm, frequently called an overcurrent trip switch (OTS) on a power circuit breaker, is normally circuit breaker mounted. Its function is to provide a signal to indicate that the circuit breaker has tripped open automatically (trip unit command). It will not operate if the circuit breaker is tripped open by other means, such as the use of a manual trip button, an electrical control switch, or the operation of an undervoltage release device.
• Locking Devices - Low voltage power circuit breakers normally have a wide array of mechanical locking devices to prevent unauthorized circuit breaker operation.


• Mechanical Interlocks - These devices provide a way to mechanically interlock two circuit breakers. A typical use for such a device is to prevent one circuit breaker from closing while another circuit breaker is already closed.
• Capacitor Trip Device - A capacitor trip device is normally mounted externally from the circuit breaker. It uses a small storage capacitor for AC control of the circuit breaker to ensure reliable tripping power during fault conditions.
• Lifting Device - Because some low voltage power circuit breakers can be sizable and heavy, a variety of devices is usually available to lift and move the circuit breaker once it is out of its compartment (Figure 29).
 Rail Mounted Lifting Device Being Used to Lift a Magnum DS Power Circuit Breaker


• Truck Operated Cell Switch (TOC) - A TOC switch is usually mounted in the circuit breaker compartment and is activated by movement of a drawout circuit breaker into and out of the "Connected" position. As the circuit breaker moves, the contacts are activated providing a means for remote indication of the circuit breaker's position.