There are both low voltage DC power circuit breakers and low voltage AC power circuit breakers. The interruption of direct current is distinctly different from the interruption of alternating current, and generally more difficult at comparable voltages and currents. Large quantities of low voltage AC power circuit breakers are used throughout industry in comparison to very small numbers of DC devices. For this reason and the fact that this is an introduction to low voltage power circuit breakers, only AC designs will be covered. Keep in mind, however, low voltage DC power circuit breakers do exist and are used in a number of specialty applications, such as rapid transit.
Circuit Breakers are often classified by certain modifying words, such as low voltage power. Low voltage AC power is considered to be for applications at 1000 volts AC and below. For comparison reasons then, medium voltage AC power is considered to be for application above 1000 volts AC. In general, however, low voltage power circuit breakers are viewed as 600 volt circuit breakers applied at a number of different voltage levels, such as 240 or 480 volts.
Sound confusing? Let's try to clear it up a bit by taking a brief look at why a low voltage power circuit breaker might be used along with some background information.
Why use a low voltage power circuit breaker over another type of low voltage circuit breaker? Most often the determination is made by the specific application. Let's consider a number of the more prominent reasons why a low voltage power circuit breaker is ideally suited for certain applications. Keep these reasons in mind as you proceed through this module. You will learn about the features and requirements that support and further explain the following reasons for applying low voltage power circuit breakers:
• Continuity of Service - Continuity of service allows for the maximum up time and minimum down time of equipment. A low voltage power circuit breaker has a significant Short Time Rating (also: "withstand rating"). This means that the low voltage power circuit breaker has the strength to withstand the stresses of a fault for up to 1/2 second or 30 cycles, instead of opening immediately. This ability to delay opening allows for a circuit breaker nearest the fault to clear the fault. This helps to prevent facility outages or a wide shutdown of facility equipment.
• Maintainability - A low voltage power circuit breaker is designed to be maintained in the field. This extends the useful service life of the circuit breaker. Especially for heavy, repetitive duty applications, maintenance of the circuit breaker is quite an important feature. Low voltage power circuit breakers allow for the inspection and replacement of parts on site.
• Safety - Low voltage power circuit breakers are tested as drawout devices in an enclosure. As such, four distinct circuit breaker positions relative to its enclosure are provided for maximum operator safety. The four drawout circuit breaker positions allow for the following uniquely different functions:
Connected Position: The circuit breaker is fully connected and functional.
Test Position: The circuit breaker's primary connections are disconnected. Secondary connections are not disconnected and testing can be safely performed because the circuit breaker is not energized. This is not possible with a circuit breaker that is permanently mounted.
Disconnect Position: Neither the primary nor secondary electrical connections of the circuit breaker are made. This position is often used as a storage position for the circuit breaker within its enclosure.
Withdrawn Position: In this position, the circuit breaker has no electrical connections. It is far enough out of its enclosure, usually on some type of integral extension rails, to permit inspection and maintenance without turning the power off to an entire assembly of equipment.
Reliability - Low voltage power circuit breakers are tested for and must be able to meet high electrical and mechanical endurance ratings. Electrical endurance is the number of operations at rated continuous current and maximum system voltage. Mechanical endurance is the number of operations with no voltage applied.
Remote Operation and Reclosing - Low voltage power circuit breakers are designed for operation remotely. They have two-step Stored Energy mechanisms which permit circuit breakers to rapidly reclose after a fault. The two-step stored energy mechanism makes multiple charge-close operations possible, such as the operating sequence: charge-close-recharge-open-close-open.
Custom has led to using phrases such as low voltage power circuit breaker, low voltage metal-frame circuit breaker, low voltage air circuit breaker, and 600 volt power circuit breaker. Although these circuit breakers fall into the classification of 1000 volts and below, real world applications are usually 600 volts and below, thus the 600 volt reference. In general, such a device must be built and tested in accordance with a very specific set of standards, such as ANSI Standards. A low voltage power circuit breaker is a device with both an Interrupting Rating and a short time rating, where the short time rating is composed of two components:
• Short Delay Current (expressed in kA)
• Short Delay Time (expressed in cycles)
This is the primary differentiating feature between a power circuit breaker and a molded case circuit breaker. The importance of this difference will be discussed a number of times later in this module.
For many years, low voltage power circuit breakers were essentially an assembly of parts on a welded metal frame, thus the phrase metal-frame circuit breaker. Distinguishing one low voltage circuit breaker from another at that point was rather simple. If it was a metal-frame circuit breaker, it was probably a power circuit breaker. If the circuit breaker parts were enclosed by an insulating material, it was called a molded case circuit breaker.
Certain hybrid low voltage circuit breakers were later developed and quite successful in certain markets. These circuit breakers had their parts encased in an insulating material, not unlike a molded case circuit breaker. From a performance standpoint, however, they performed more like a power circuit breaker. They had several of the same physical attributes as the power circuit breaker, but were never able to achieve the short time ratings of a power circuit breaker or pass all the power circuit breaker test standards.
This type of circuit breaker, although not tested to all the same standards as a power circuit breaker, found its application niche to be similar to traditional power circuit breakers. This design became known as a low voltage insulated case circuit breaker .
At that point, the line between frame material to identify the type of circuit breaker became blurred. All this said, the differentiating feature still remains the device's ability to meet power circuit breaker test standards, not the frame's type of construction.
Circuit Breakers are often classified by certain modifying words, such as low voltage power. Low voltage AC power is considered to be for applications at 1000 volts AC and below. For comparison reasons then, medium voltage AC power is considered to be for application above 1000 volts AC. In general, however, low voltage power circuit breakers are viewed as 600 volt circuit breakers applied at a number of different voltage levels, such as 240 or 480 volts.
Sound confusing? Let's try to clear it up a bit by taking a brief look at why a low voltage power circuit breaker might be used along with some background information.
Why use a low voltage power circuit breaker over another type of low voltage circuit breaker? Most often the determination is made by the specific application. Let's consider a number of the more prominent reasons why a low voltage power circuit breaker is ideally suited for certain applications. Keep these reasons in mind as you proceed through this module. You will learn about the features and requirements that support and further explain the following reasons for applying low voltage power circuit breakers:
• Continuity of Service - Continuity of service allows for the maximum up time and minimum down time of equipment. A low voltage power circuit breaker has a significant Short Time Rating (also: "withstand rating"). This means that the low voltage power circuit breaker has the strength to withstand the stresses of a fault for up to 1/2 second or 30 cycles, instead of opening immediately. This ability to delay opening allows for a circuit breaker nearest the fault to clear the fault. This helps to prevent facility outages or a wide shutdown of facility equipment.
• Maintainability - A low voltage power circuit breaker is designed to be maintained in the field. This extends the useful service life of the circuit breaker. Especially for heavy, repetitive duty applications, maintenance of the circuit breaker is quite an important feature. Low voltage power circuit breakers allow for the inspection and replacement of parts on site.
• Safety - Low voltage power circuit breakers are tested as drawout devices in an enclosure. As such, four distinct circuit breaker positions relative to its enclosure are provided for maximum operator safety. The four drawout circuit breaker positions allow for the following uniquely different functions:
Connected Position: The circuit breaker is fully connected and functional.
Test Position: The circuit breaker's primary connections are disconnected. Secondary connections are not disconnected and testing can be safely performed because the circuit breaker is not energized. This is not possible with a circuit breaker that is permanently mounted.
Disconnect Position: Neither the primary nor secondary electrical connections of the circuit breaker are made. This position is often used as a storage position for the circuit breaker within its enclosure.
Withdrawn Position: In this position, the circuit breaker has no electrical connections. It is far enough out of its enclosure, usually on some type of integral extension rails, to permit inspection and maintenance without turning the power off to an entire assembly of equipment.
Reliability - Low voltage power circuit breakers are tested for and must be able to meet high electrical and mechanical endurance ratings. Electrical endurance is the number of operations at rated continuous current and maximum system voltage. Mechanical endurance is the number of operations with no voltage applied.
Remote Operation and Reclosing - Low voltage power circuit breakers are designed for operation remotely. They have two-step Stored Energy mechanisms which permit circuit breakers to rapidly reclose after a fault. The two-step stored energy mechanism makes multiple charge-close operations possible, such as the operating sequence: charge-close-recharge-open-close-open.
Custom has led to using phrases such as low voltage power circuit breaker, low voltage metal-frame circuit breaker, low voltage air circuit breaker, and 600 volt power circuit breaker. Although these circuit breakers fall into the classification of 1000 volts and below, real world applications are usually 600 volts and below, thus the 600 volt reference. In general, such a device must be built and tested in accordance with a very specific set of standards, such as ANSI Standards. A low voltage power circuit breaker is a device with both an Interrupting Rating and a short time rating, where the short time rating is composed of two components:
• Short Delay Current (expressed in kA)
• Short Delay Time (expressed in cycles)
This is the primary differentiating feature between a power circuit breaker and a molded case circuit breaker. The importance of this difference will be discussed a number of times later in this module.
For many years, low voltage power circuit breakers were essentially an assembly of parts on a welded metal frame, thus the phrase metal-frame circuit breaker. Distinguishing one low voltage circuit breaker from another at that point was rather simple. If it was a metal-frame circuit breaker, it was probably a power circuit breaker. If the circuit breaker parts were enclosed by an insulating material, it was called a molded case circuit breaker.
Certain hybrid low voltage circuit breakers were later developed and quite successful in certain markets. These circuit breakers had their parts encased in an insulating material, not unlike a molded case circuit breaker. From a performance standpoint, however, they performed more like a power circuit breaker. They had several of the same physical attributes as the power circuit breaker, but were never able to achieve the short time ratings of a power circuit breaker or pass all the power circuit breaker test standards.
This type of circuit breaker, although not tested to all the same standards as a power circuit breaker, found its application niche to be similar to traditional power circuit breakers. This design became known as a low voltage insulated case circuit breaker .
At that point, the line between frame material to identify the type of circuit breaker became blurred. All this said, the differentiating feature still remains the device's ability to meet power circuit breaker test standards, not the frame's type of construction.
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