In Module 5, you learned that all circuit breakers have a number of design and functional characteristics in common:
• Compliance with Specific Standards
• Set of Open/Close Contacts
• Means to Open and Close the Contacts
• Means to Extinguish an Arc
• Means to Respond to Overcurrents/Commands
• Method for Enclosing Circuit Breaker Components
• Method For Mounting Circuit Breaker
Specific methods used for mounting and using low voltage power circuit breakers will be covered in the next section. In this section, the concentration will be on how low voltage power circuit breaker operate to accomplish their tasks and what accessory items are available to enhance their capabilities.
Basic low voltage power circuit breakers are generally composed of:
• Frame or Chassis
• Primary Contacts
• Arc Extinguishers
• Operating Mechanism
• Integral Trip Unit
• Accessory Items
Let's take a look at each.
Frame or Chassis
You will recall from Module 5 that all circuit breakers utilize some method to hold all the parts that make up a circuit breaker, usually called the frame or chassis. A low voltage power circuit breaker chassis today will be one of two types (Figure 14 and 15):
• Open Type Metal-Frame (Older Designs)
• Molded Frame of Engineered Thermoset Composite Resins (Newer Designs)
The open type metal-frame has a number of pieces welded and/or bolted together on which the different circuit breaker components are assembled. Components have a tendency to be larger, heavier, and may need to be adjusted.
in the workplace
The new Magnum DS power circuit breaker utilizes a rigid frame molded from engineered thermoset composite resins.
Molding improves the structural rigidity of the frame, allowing for higher interrupting and short time ratings.
Many individual circuit breaker parts are molded as integral assemblies. This improves the design by making it smaller and stronger with fewer individual parts, unlike the metal-frame approach.
Primary Contacts
Primary open/close contacts in a low voltage power circuit breaker provide a means for isolating or connecting a part of a circuit from or with the rest of the system. The design of the primary contacts is one of the most critical design considerations relative to the efficiency and overall effectiveness of any low voltage power circuit breaker. These contacts are used to carry or break the main continuous load current associated with the system in which the circuit breaker is applied. Each phase has an associated primary contact. A three-phase low voltage power circuit breaker, for example, would have three sets of primary contacts. Keep in mind that primary contacts come in a wide variety of designs and appearances. All designs do not use the same number of parts nor are all designs equally efficient. However, all designs act to provide the same general service.
Low voltage power circuit breaker primary contacts usually have separate arcing and main current carrying parts. This does not mean that they are necessarily separate pieces. They could both be part of the same component piece, although the arcing and main contacts act as individual pieces and perform distinctly different functions.
In Module 5, the discharge of electric current crossing a gap between two contacts was discussed . This phenomenon, on a small scale, can occasionally be observed when pulling a plug from a wall socket.
Arcing also occurs when opening and closing low voltage power circuit breakers under load, except to a much larger degree. The primary contact design challenge is to ensure that the arcing is dealt with first to protect the surface of the main contacts from arc damage. For this reason, primary contacts are mechanically designed such that on closing of the circuit breaker, the arcing contacts touch (make) before the main contacts. Also on opening of the circuit breaker, the main contacts part (break) before the arcing contacts. This construction ensures that arcing takes place on the heat resistant arcing contacts. Usually, primary contacts are replaceable on low voltage power circuit breakers, which can be needed in time if the operating duty of the breaker is severe enough.
A primary contact assembly is composed of:
• fixed (stationary) part
• moving part
A rigid insulating piece through a pushing or pulling motion is used to operate the moving part of the primary contact assembly.
The fixed and moving main and arcing portions of the assembly can be in any number of configurations, some more efficient than others (Figures 18 and 19). Usually the designs for a particular type circuit breaker are the same. The only variable is the number of parts used to handle the amount of current available. Larger circuit breaker frames require more and/or larger arcing and main contact pieces.
Keep in mind that the design goal is to efficiently handle arcing through the heat resistant arcing contacts so that the main contacts are protected from unnecessary damage. This approach permits the main contacts to be made from low resistance materials, such as silver or silver alloys to minimize the heat developed during normal operation.
Finally, it was pointed out in Module 5 that some newer low voltage power circuit breaker designs take full advantage of certain natural facts of physics to assist with the opening process. You will recall that the concept centers around magnetic fields established in conductors when current is flowing in the conductors.
The low voltage power circuit breaker design takes full advantage of this electromagnetic force to assist with opening and keeping the circuit breaker closed. In certain configurations, the force and also the insulator are used to help hold the contacts closed temporarily during a fault condition, which is where a power circuit breaker's short time rating comes from. Circuit breaker designs taking advantage of this concept can be smaller and lighter and still maintain the higher withstand (short-time) capabilities associated with low voltage power circuit breakers. When it is time for the contacts to open, this same force can be used in the opposite direction to speed the opening action.
Think about the concept of electromagnetic assistance with opening and closing of contacts in the following fashion . A door could be viewed as the movable contact. Our super-hero can be considered the rigid insulator used to push closed or pull open the door (contact). Assistance from the wind (electromagnetic force) in the proper direction would help our super-hero open or keep the door closed.
• Compliance with Specific Standards
• Set of Open/Close Contacts
• Means to Open and Close the Contacts
• Means to Extinguish an Arc
• Means to Respond to Overcurrents/Commands
• Method for Enclosing Circuit Breaker Components
• Method For Mounting Circuit Breaker
Specific methods used for mounting and using low voltage power circuit breakers will be covered in the next section. In this section, the concentration will be on how low voltage power circuit breaker operate to accomplish their tasks and what accessory items are available to enhance their capabilities.
Basic low voltage power circuit breakers are generally composed of:
• Frame or Chassis
• Primary Contacts
• Arc Extinguishers
• Operating Mechanism
• Integral Trip Unit
• Accessory Items
Let's take a look at each.
Frame or Chassis
You will recall from Module 5 that all circuit breakers utilize some method to hold all the parts that make up a circuit breaker, usually called the frame or chassis. A low voltage power circuit breaker chassis today will be one of two types (Figure 14 and 15):
• Open Type Metal-Frame (Older Designs)
• Molded Frame of Engineered Thermoset Composite Resins (Newer Designs)
The open type metal-frame has a number of pieces welded and/or bolted together on which the different circuit breaker components are assembled. Components have a tendency to be larger, heavier, and may need to be adjusted.
in the workplace
The new Magnum DS power circuit breaker utilizes a rigid frame molded from engineered thermoset composite resins.
Molding improves the structural rigidity of the frame, allowing for higher interrupting and short time ratings.
Many individual circuit breaker parts are molded as integral assemblies. This improves the design by making it smaller and stronger with fewer individual parts, unlike the metal-frame approach.
Primary Contacts
Primary open/close contacts in a low voltage power circuit breaker provide a means for isolating or connecting a part of a circuit from or with the rest of the system. The design of the primary contacts is one of the most critical design considerations relative to the efficiency and overall effectiveness of any low voltage power circuit breaker. These contacts are used to carry or break the main continuous load current associated with the system in which the circuit breaker is applied. Each phase has an associated primary contact. A three-phase low voltage power circuit breaker, for example, would have three sets of primary contacts. Keep in mind that primary contacts come in a wide variety of designs and appearances. All designs do not use the same number of parts nor are all designs equally efficient. However, all designs act to provide the same general service.
Low voltage power circuit breaker primary contacts usually have separate arcing and main current carrying parts. This does not mean that they are necessarily separate pieces. They could both be part of the same component piece, although the arcing and main contacts act as individual pieces and perform distinctly different functions.
In Module 5, the discharge of electric current crossing a gap between two contacts was discussed . This phenomenon, on a small scale, can occasionally be observed when pulling a plug from a wall socket.
Arcing also occurs when opening and closing low voltage power circuit breakers under load, except to a much larger degree. The primary contact design challenge is to ensure that the arcing is dealt with first to protect the surface of the main contacts from arc damage. For this reason, primary contacts are mechanically designed such that on closing of the circuit breaker, the arcing contacts touch (make) before the main contacts. Also on opening of the circuit breaker, the main contacts part (break) before the arcing contacts. This construction ensures that arcing takes place on the heat resistant arcing contacts. Usually, primary contacts are replaceable on low voltage power circuit breakers, which can be needed in time if the operating duty of the breaker is severe enough.
A primary contact assembly is composed of:
• fixed (stationary) part
• moving part
A rigid insulating piece through a pushing or pulling motion is used to operate the moving part of the primary contact assembly.
The fixed and moving main and arcing portions of the assembly can be in any number of configurations, some more efficient than others (Figures 18 and 19). Usually the designs for a particular type circuit breaker are the same. The only variable is the number of parts used to handle the amount of current available. Larger circuit breaker frames require more and/or larger arcing and main contact pieces.
Keep in mind that the design goal is to efficiently handle arcing through the heat resistant arcing contacts so that the main contacts are protected from unnecessary damage. This approach permits the main contacts to be made from low resistance materials, such as silver or silver alloys to minimize the heat developed during normal operation.
Finally, it was pointed out in Module 5 that some newer low voltage power circuit breaker designs take full advantage of certain natural facts of physics to assist with the opening process. You will recall that the concept centers around magnetic fields established in conductors when current is flowing in the conductors.
The low voltage power circuit breaker design takes full advantage of this electromagnetic force to assist with opening and keeping the circuit breaker closed. In certain configurations, the force and also the insulator are used to help hold the contacts closed temporarily during a fault condition, which is where a power circuit breaker's short time rating comes from. Circuit breaker designs taking advantage of this concept can be smaller and lighter and still maintain the higher withstand (short-time) capabilities associated with low voltage power circuit breakers. When it is time for the contacts to open, this same force can be used in the opposite direction to speed the opening action.
Think about the concept of electromagnetic assistance with opening and closing of contacts in the following fashion . A door could be viewed as the movable contact. Our super-hero can be considered the rigid insulator used to push closed or pull open the door (contact). Assistance from the wind (electromagnetic force) in the proper direction would help our super-hero open or keep the door closed.
Posts
No comments:
Post a Comment