The rigid frame housing also provides for the rigid mounting of circuit breaker components which:
• Improves the mechanism's life by eliminating frame deflection
• Provides for the consistent operation of mechanism
• Eliminates the need for mechanism adjustments
Some of the most useful information about a low voltage power circuit breaker can be found in its ratings table. Ratings tables were briefly covered during the Circuit Breaker Selection discussion. The ratings published in these tables are backed up by testing as outlined in applicable standards. These ratings tell quite a story. They indicate how a particular circuit breaker will perform during a given set of application circumstances. In a similar regard, the ratings indicate what circuit breaker should be selected for a specific application.
Although ratings tables from different manufacturers often look similar and even reflect many of the same ratings, it is not a good idea to assume that all low voltage power circuit breakers have the same ratings. Get into the habit of reviewing different tables and comparing the published ratings. There is no room for unexpected surprises in circuit breaker selection.
To demonstrate the value of making these comparisons, let's once again look at a partial ANSI ratings table, contrasting the ratings for one type low voltage power circuit breaker (Type XYZ) with those of the Magnum DS low voltage power circuit breaker (Figure 44). Once you have taken the time to look the table over a bit, several comparison examples will be discussed. Keep in mind that this table only presents excerpts and is not intended to show the ratings of all available circuit breakers for an entire line. It can be assumed that both circuit breakers used in the examples meet all applicable ANSI standards for low voltage power circuit breakers.
The comparisons to be discussed are identified in the table by a circled letter, for example.
A - Notice first that the ratings given for the Magnum DS power circuit breaker apply for all system voltages. Ratings for other power circuit breakers can vary by voltage. Uniform ratings across all application voltages for the 3200 ampere frame circuit breakers and below simplifies the selection process. Interrupting ratings higher than 100,000 amperes are available with Magnum DS frame sizes above 3200 amperes for certain application voltages. Therefore, the uniformity is not totally maintained on the larger frame circuit breakers.
B - The short time rating (withstand) for Magnum DS once again applies across the entire range of application voltages, while the other is only applicable to 480 volts. In addition, notice that Magnum DS has higher short time current ratings than does the other power circuit breaker. A broader range and higher short time ratings allow Magnum DS to be applied in systems with higher available short circuit currents while maintaining full selectivity.
C - An interrupting rating of 100,000 amperes is available with 3200 ampere frame and below Magnum DS circuit breakers at all application voltages. This rating is not at all available with the other power circuit breaker. Notice further that the short time rating is 85,000 amperes (3200 amperes and below). What does this accomplish?
1. These ratings mean that this particular Magnum DS rating has a hefty short time (withstand) capability and can withstand a short circuit of 85,000 amperes for the full 30 cycles required by ANSI.
2. It also means that this Magnum DS rating can be applied on a system that could experience a short circuit as high as 100,000 amperes. The circuit breaker would be selective up to 85,000 amperes, and would open instantaneously from 85000 to 100,000 amperes.
D - One more area on a ratings table to be alert to is with and without instantaneous trip. It is possible for some circuit breakers to have higher interrupting ratings if they can trip instantaneously. In those instances, the interrupting rating might be lower if the circuit breaker is required to provide short time protection. This is not the case, however, with Magnum DS at interrupting ratings of 85,000 amperes and below.
As a general rule, the ratings for most ANSI rated low voltage power circuit breakers are similar where system requirements are less stringent. When the application requirements become more stringent and/or special, the capabilities of different power circuit breakers begin to differ. Look at ratings tables closely. They have a story to tell.
Extended Interrupting Ratings: Interrupting ratings can be extended up to 200,000 amperes by providing a combination of a low voltage power circuit breaker connected in series with current limiting fuses (current limiters). This combination is provided on systems where the overload and switching functions of the circuit breaker are required, and available fault currents could exceed the interrupting rating of the circuit breaker by itself. For smaller frame circuit breakers, the current limiters usually are integrally mounted in the circuit breaker (Figure 45). For larger frame circuit breakers, the current limiters are usually mounted in a separate drawout truck and positioned adjacent to the circuit breaker.
Altitude and Ratings: Low voltage power circuit breakers are applicable at their full voltage and current ratings up to a maximum altitude of 6600 feet (2000 meters) above sea level. When a circuit breaker is installed at higher altitudes, the ratings are subject to correction factors in accordance with ANSI Standards. Fortunately, you are not often faced with this situation. Even if you are, it probably has already been taken into account by the specifier. It is good, however, to be aware of such application exceptions.
Current Waveforms and Ratings: The subject of current waveforms and their effect on circuit breaker ratings will not be discussed in great detail in this module. Once again this consideration is usually made by the specifier and taken into account when a particular circuit breaker is specified. You should be somewhat familiar with the concepts. If for no other reason, it could help to explain why a certain circuit breaker rating selection was made for a particular application. It will also be good background information later in this module when applications are discussed. You noticed in the ratings table just discussed that interrupting ratings and short time ratings were expressed in symmetrical amperes. For most discussions and selections involving low voltage power circuit breakers, this will be the case. Without getting into too much detail, it is important to know that there are two common ways to rate protective devices in amperes. These ratings are symmetrical and asymmetrical. The symmetrical and asymmetrical ampere ratings can be quite different for the same device. To clarify the significance of the two terms, let's briefly discuss each individually.
The graphic shown of a Symmetrical AC Current represents a fault current flowing in a circuit (Figure 46). The fault current has a sine wave shape and is symmetrical with respect to the horizontal axis. That is, the current rises and falls equally above and below the horizontal axis, so the shapes are symmetrical.
The graphic shown of an Asymmetrical Current becoming Symmetrical represents an offset fault current wave (Figure 47). It rises above the horizontal axis considerably more than it goes below for the first few cycles. This wave is said to be offset from or asymmetrical with respect to the horizontal axis. This condition occurs in circuits containing reactance which are short-circuited at some time other than when the current is passing through the zero point on the cycle. It occurs in all 3-phase circuits in one or more phases. When this happens, a DC current is superimposed on top of the AC current causing asymmetry. The DC component actually decays to zero within a short time after the fault occurs. The final decay of the DC component signifies a change from asymmetrical to symmetrical. How fast this actually happens depends upon the quantity relationship of reactance and resistance in the circuit, the X/R ratio. The more resistance in the circuit, the faster the DC component decays, or the larger the X/R ratio, the longer the decay time.
Asymmetrical Current Becoming Symmetrical as DC Components Decay
Continuing this discussion of current waveforms, let's take a look at some background information. Low voltage power circuit breakers typically part their Contacts after several cycles of fault current, assuming there is no intentional time delay. In short, contact parting only begins once time delays are expired. The circuit breaker can be called upon to interrupt more than the symmetrical value of fault current, as calculated from the impedance of the circuit. This is because of the presence of the DC component just discussed.
We will not discuss here why the degree of asymmetry can be different, just know that the degree can vary. What is of real importance is the rate at which the DC component decays and the change to symmetrical takes place. This rate, and hence the current value, can be related directly back to the ratio of circuit reactance to circuit resistance or the X/R ratio.
For this reason, the X/R ratio is a significant and specified factor for standards testing. It is also an important ratio to know when selecting circuit breakers for application on a system. As long as the X/R ratio for the system does not exceed the tested X/R for the circuit breakers, you are home free.
If the system X/R ratio exceeds the tested X/R for the circuit breakers, the circuit breakers would have to have their published interrupting capacity and short delay current capability de-rated. A circuit breaker with a higher interrupting capacity and higher short delay current rating might be needed to accommodated the de-rating factor.
As previously mentioned, these types of system determinations are normally made well ahead of time by the specifier or consultant, which is when the equivalent system short circuit rating is normally specified.
For your information, the ANSI tested X/R ratio for power circuit breakers is 6.6. This 6.6 ratio was not always the case. As a matter of fact, power circuit breakers were not always rated in symmetrical terms. That is history and we will not discuss why a change was made from asymmetrical to symmetrical.
At some point, the industry made the change and determined that an X/R ratio of 6.6 for power circuit breakers was typical, and would be a good base to work with for testing and application. At least all the manufacturers were working with the same standardized starting point. Now, when the calculated short circuit current X/R ratio for a particular system is higher than the standard 6.6 X/R ratio for low voltage power circuit breakers, a table can be consulted to determine what de-rating factor should be applied to the circuit breaker's interrupting rating to insure proper circuit breaker sizing and selection.
To give you an idea of what these de-rating factors look like, refer to the partial table of low voltage power circuit breaker de-rating factors (Figure 48). Keep in mind that these considerations and decisions must be made for all types of circuit breakers, not just low voltage power circuit breakers. Before this discussion is concluded, let's take a look a one simple example.
Example: The total available fault current from all sources that this low voltage power circuit breaker could see was calculated to be 48,000 amperes symmetrical. For this example, we are only considering what circuit breaker could be used to deal instantaneously with a potential fault of this magnitude. From some selection chart you might select a circuit breaker with a 50,000 ampere interrupting capability.
In this example that would not be a good selection because it has also been determined that the system X/R ratio is 9.94. You can see from the de-rating table that a 9.94 ratio equates to a de-rating factor of 0.937. This factor used on the circuit breaker's interrupting capability of 50,000 amperes reduces it to 46,850 amperes, not sufficient to deal with the potential fault current of 48,000 amperes [50,000 x 0.937 = 46,850].
Obviously, a circuit breaker with a higher interrupting capability must be selected for this application. If a circuit breaker with an interrupting capability of 65,000 amperes is selected, you can see from the calculation that it would do the job [65,000 x 0.937 = 60,905].
Labels:
Circuit Breaker
• Improves the mechanism's life by eliminating frame deflection
• Provides for the consistent operation of mechanism
• Eliminates the need for mechanism adjustments
Some of the most useful information about a low voltage power circuit breaker can be found in its ratings table. Ratings tables were briefly covered during the Circuit Breaker Selection discussion. The ratings published in these tables are backed up by testing as outlined in applicable standards. These ratings tell quite a story. They indicate how a particular circuit breaker will perform during a given set of application circumstances. In a similar regard, the ratings indicate what circuit breaker should be selected for a specific application.
Although ratings tables from different manufacturers often look similar and even reflect many of the same ratings, it is not a good idea to assume that all low voltage power circuit breakers have the same ratings. Get into the habit of reviewing different tables and comparing the published ratings. There is no room for unexpected surprises in circuit breaker selection.
To demonstrate the value of making these comparisons, let's once again look at a partial ANSI ratings table, contrasting the ratings for one type low voltage power circuit breaker (Type XYZ) with those of the Magnum DS low voltage power circuit breaker (Figure 44). Once you have taken the time to look the table over a bit, several comparison examples will be discussed. Keep in mind that this table only presents excerpts and is not intended to show the ratings of all available circuit breakers for an entire line. It can be assumed that both circuit breakers used in the examples meet all applicable ANSI standards for low voltage power circuit breakers.
The comparisons to be discussed are identified in the table by a circled letter, for example.
A - Notice first that the ratings given for the Magnum DS power circuit breaker apply for all system voltages. Ratings for other power circuit breakers can vary by voltage. Uniform ratings across all application voltages for the 3200 ampere frame circuit breakers and below simplifies the selection process. Interrupting ratings higher than 100,000 amperes are available with Magnum DS frame sizes above 3200 amperes for certain application voltages. Therefore, the uniformity is not totally maintained on the larger frame circuit breakers.
B - The short time rating (withstand) for Magnum DS once again applies across the entire range of application voltages, while the other is only applicable to 480 volts. In addition, notice that Magnum DS has higher short time current ratings than does the other power circuit breaker. A broader range and higher short time ratings allow Magnum DS to be applied in systems with higher available short circuit currents while maintaining full selectivity.
C - An interrupting rating of 100,000 amperes is available with 3200 ampere frame and below Magnum DS circuit breakers at all application voltages. This rating is not at all available with the other power circuit breaker. Notice further that the short time rating is 85,000 amperes (3200 amperes and below). What does this accomplish?
1. These ratings mean that this particular Magnum DS rating has a hefty short time (withstand) capability and can withstand a short circuit of 85,000 amperes for the full 30 cycles required by ANSI.
2. It also means that this Magnum DS rating can be applied on a system that could experience a short circuit as high as 100,000 amperes. The circuit breaker would be selective up to 85,000 amperes, and would open instantaneously from 85000 to 100,000 amperes.
D - One more area on a ratings table to be alert to is with and without instantaneous trip. It is possible for some circuit breakers to have higher interrupting ratings if they can trip instantaneously. In those instances, the interrupting rating might be lower if the circuit breaker is required to provide short time protection. This is not the case, however, with Magnum DS at interrupting ratings of 85,000 amperes and below.
As a general rule, the ratings for most ANSI rated low voltage power circuit breakers are similar where system requirements are less stringent. When the application requirements become more stringent and/or special, the capabilities of different power circuit breakers begin to differ. Look at ratings tables closely. They have a story to tell.
Extended Interrupting Ratings: Interrupting ratings can be extended up to 200,000 amperes by providing a combination of a low voltage power circuit breaker connected in series with current limiting fuses (current limiters). This combination is provided on systems where the overload and switching functions of the circuit breaker are required, and available fault currents could exceed the interrupting rating of the circuit breaker by itself. For smaller frame circuit breakers, the current limiters usually are integrally mounted in the circuit breaker (Figure 45). For larger frame circuit breakers, the current limiters are usually mounted in a separate drawout truck and positioned adjacent to the circuit breaker.
Altitude and Ratings: Low voltage power circuit breakers are applicable at their full voltage and current ratings up to a maximum altitude of 6600 feet (2000 meters) above sea level. When a circuit breaker is installed at higher altitudes, the ratings are subject to correction factors in accordance with ANSI Standards. Fortunately, you are not often faced with this situation. Even if you are, it probably has already been taken into account by the specifier. It is good, however, to be aware of such application exceptions.
Current Waveforms and Ratings: The subject of current waveforms and their effect on circuit breaker ratings will not be discussed in great detail in this module. Once again this consideration is usually made by the specifier and taken into account when a particular circuit breaker is specified. You should be somewhat familiar with the concepts. If for no other reason, it could help to explain why a certain circuit breaker rating selection was made for a particular application. It will also be good background information later in this module when applications are discussed. You noticed in the ratings table just discussed that interrupting ratings and short time ratings were expressed in symmetrical amperes. For most discussions and selections involving low voltage power circuit breakers, this will be the case. Without getting into too much detail, it is important to know that there are two common ways to rate protective devices in amperes. These ratings are symmetrical and asymmetrical. The symmetrical and asymmetrical ampere ratings can be quite different for the same device. To clarify the significance of the two terms, let's briefly discuss each individually.
The graphic shown of a Symmetrical AC Current represents a fault current flowing in a circuit (Figure 46). The fault current has a sine wave shape and is symmetrical with respect to the horizontal axis. That is, the current rises and falls equally above and below the horizontal axis, so the shapes are symmetrical.
The graphic shown of an Asymmetrical Current becoming Symmetrical represents an offset fault current wave (Figure 47). It rises above the horizontal axis considerably more than it goes below for the first few cycles. This wave is said to be offset from or asymmetrical with respect to the horizontal axis. This condition occurs in circuits containing reactance which are short-circuited at some time other than when the current is passing through the zero point on the cycle. It occurs in all 3-phase circuits in one or more phases. When this happens, a DC current is superimposed on top of the AC current causing asymmetry. The DC component actually decays to zero within a short time after the fault occurs. The final decay of the DC component signifies a change from asymmetrical to symmetrical. How fast this actually happens depends upon the quantity relationship of reactance and resistance in the circuit, the X/R ratio. The more resistance in the circuit, the faster the DC component decays, or the larger the X/R ratio, the longer the decay time.
Asymmetrical Current Becoming Symmetrical as DC Components Decay
Continuing this discussion of current waveforms, let's take a look at some background information. Low voltage power circuit breakers typically part their Contacts after several cycles of fault current, assuming there is no intentional time delay. In short, contact parting only begins once time delays are expired. The circuit breaker can be called upon to interrupt more than the symmetrical value of fault current, as calculated from the impedance of the circuit. This is because of the presence of the DC component just discussed.
We will not discuss here why the degree of asymmetry can be different, just know that the degree can vary. What is of real importance is the rate at which the DC component decays and the change to symmetrical takes place. This rate, and hence the current value, can be related directly back to the ratio of circuit reactance to circuit resistance or the X/R ratio.
For this reason, the X/R ratio is a significant and specified factor for standards testing. It is also an important ratio to know when selecting circuit breakers for application on a system. As long as the X/R ratio for the system does not exceed the tested X/R for the circuit breakers, you are home free.
If the system X/R ratio exceeds the tested X/R for the circuit breakers, the circuit breakers would have to have their published interrupting capacity and short delay current capability de-rated. A circuit breaker with a higher interrupting capacity and higher short delay current rating might be needed to accommodated the de-rating factor.
As previously mentioned, these types of system determinations are normally made well ahead of time by the specifier or consultant, which is when the equivalent system short circuit rating is normally specified.
For your information, the ANSI tested X/R ratio for power circuit breakers is 6.6. This 6.6 ratio was not always the case. As a matter of fact, power circuit breakers were not always rated in symmetrical terms. That is history and we will not discuss why a change was made from asymmetrical to symmetrical.
At some point, the industry made the change and determined that an X/R ratio of 6.6 for power circuit breakers was typical, and would be a good base to work with for testing and application. At least all the manufacturers were working with the same standardized starting point. Now, when the calculated short circuit current X/R ratio for a particular system is higher than the standard 6.6 X/R ratio for low voltage power circuit breakers, a table can be consulted to determine what de-rating factor should be applied to the circuit breaker's interrupting rating to insure proper circuit breaker sizing and selection.
To give you an idea of what these de-rating factors look like, refer to the partial table of low voltage power circuit breaker de-rating factors (Figure 48). Keep in mind that these considerations and decisions must be made for all types of circuit breakers, not just low voltage power circuit breakers. Before this discussion is concluded, let's take a look a one simple example.
Example: The total available fault current from all sources that this low voltage power circuit breaker could see was calculated to be 48,000 amperes symmetrical. For this example, we are only considering what circuit breaker could be used to deal instantaneously with a potential fault of this magnitude. From some selection chart you might select a circuit breaker with a 50,000 ampere interrupting capability.
In this example that would not be a good selection because it has also been determined that the system X/R ratio is 9.94. You can see from the de-rating table that a 9.94 ratio equates to a de-rating factor of 0.937. This factor used on the circuit breaker's interrupting capability of 50,000 amperes reduces it to 46,850 amperes, not sufficient to deal with the potential fault current of 48,000 amperes [50,000 x 0.937 = 46,850].
Obviously, a circuit breaker with a higher interrupting capability must be selected for this application. If a circuit breaker with an interrupting capability of 65,000 amperes is selected, you can see from the calculation that it would do the job [65,000 x 0.937 = 60,905].
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