Tìm hiểu chi tiết về Molded Case Circuit Breakers - MCCB, Cấu tạo và Ứng dụng thực tế (Video)
Moulded Case Circuit Breakers hay MCCB được thiết kế để cung cấp bảo vệ mạch cho các hệ thống phân phối điện áp thấp hay còn gọi là điện hạ thế. Chúng sẽ bảo vệ các thiết bị kết nối chống lại cả quá tải và ngắn mạch. Chúng thường được sử dụng trong các tủ - bảng điện điều khiển khi chúng được gắn cố định, mặc dù một số MCCB lớn có sẵn có thể có sẵn trong thiết kế lắp khung.
Định nghĩa và chức năng của một CB khối MCCB- Molded Case Circuit Breaker
Một CB khối, viết tắt là MCCB, là loại thiết bị đóng cắt bảo vệ điện có thể được sử dụng cho nhiều điện áp, và tần số của cả 50 Hz và 60 Hz. Sự khác biệt chính giữa CB khối MCCB và CB tép MCB là MCCB có thể có dòng định mức lên tới 2.500 ampees, và các cài đặt trip của nó thường được điều chỉnh. Sự khác biệt nữa là MCCBs có khuynh hướng lớn hơn MCBs. Giống như hầu hết các loại máy cắt, một MCCB có ba chức năng chính:
- Bảo vệ chống quá tải - dòng điện vượt quá giá trị được đánh giá cao hơn thời gian sử dụng bình thường.
- Bảo vệ chống lại lỗi điện - Trong một lỗi như ngắn mạch hoặc lỗi đường dây, có dòng điện cực cao phải được ngắt ngay lập tức.
- Bật và tắt mạch - Đây là một chức năng ít phổ biến hơn của bộ ngắt mạch, nhưng chúng có thể được sử dụng cho mục đích đó nếu không có một bộ chuyển đổi bằng tay phù hợp.
Một loạt các dòng định mức có sẵn từ các MCCB cho phép chúng được sử dụng trong một loạt các ứng dụng. MCCBs có sẵn với các dòng định mức, từ các giá trị thấp như 15 ampees, đến các mức xếp hạng công nghiệp như 2,500 ampe. Điều này cho phép chúng được sử dụng trong cả hai ứng dụng điện hạ thế và điện cao thế.
CB khối (MCCBs) là thiết bị ngắt mạch được chấp nhận UL 489, các bộ phận, cấu trúc và thiết bị trip đều được chứa hoàn toàn trong một vỏ bọc bằng vật liệu cách điện. MCCBs có sẵn trong các kích cỡ khung khác nhau với mức ngắt khác nhau cho mỗi kích thước khung. MCCBs là một trong hai loại cơ bản của mạch điện áp thấp.
Moulded Case Circuit Breakers MCCB được thiết kế để cung cấp bảo vệ mạch cho các hệ thống phân phối điện áp thấp. Chúng sẽ bảo vệ các thiết bị kết nối chống lại cả quá tải và ngắn mạch. Chúng thường được sử dụng trong các tủ - bảng điện điều khiển, được gắn cố định, mặc dù một số MCCB lớn được lắp riêng trong một cabinet.
Một số MCCBs có các tính năng đặc biệt phù hợp để bảo vệ các mạch điện động cơ, khi sử dụng kết hợp với một thiết bị bảo vệ quá tải riêng biệt. Khi được sử dụng trong các ứng dụng như vậy, chúng thường được gọi là MCPs bảo vệ mạch.
Cơ chế hoạt động
Bên trong lõi, cơ chế bảo vệ được sử dụng bởi MCCBs dựa trên cùng một nguyên tắc vật lý được sử dụng bởi tất cả các loại thiết bị ngắt mạch bằng nhiệt.
Bảo vệ quá tải được thực hiện theo cơ chế nhiệt. MCCB có một bimetallic contact what expands and contracts với sự thay đổi nhiệt độ. Trong điều kiện hoạt động bình thường, tiếp điểm cho phép điện qua MCCB. Tuy nhiên, ngay khi dòng điện vượt quá giá trị điều chỉnh, đầu tiếp xúc sẽ bắt đầu nóng và mở rộng cho đến khi mạch bị ngắt hoàn toàn. Việc bảo vệ nhiệt chống quá tải được thiết kế với thời gian trễ để cho phép quá dòng trong thời gian ngắn, đây là một phần hoạt động bình thường đối với nhiều thiết bị. Tuy nhiên, bất kỳ điều kiện quá dòng nào kéo dài nhiều hơn những gì thường xảy ra là quá tải, và MCCB sẽ trip để bảo vệ thiết bị và người vận hành.
Mặt khác, bảo vệ lỗi được thực hiện bằng cảm ứng điện từ, và đáp ứng ngay lập tức. Dòng lỗi phải được gián đoạn ngay lập tức, cho dù thời gian của chúng là ngắn hay dài. Bất cứ khi nào xảy ra lỗi, dòng điện cực cao tạo ra từ trường trong một cuộn dây solenoid nằm bên trong bộ phận ngắt điện - cảm ứng từ tính này truyền đến một tiếp điểm và dòng điện bị gián đoạn. Là một bổ sung cho cơ chế bảo vệ từ tính, các MCCB có các biện pháp tản nhiệt bên trong để tạo sự gián đoạn.
Cũng như tất cả các loại máy cắt, MCCB bao gồm một bộ ngắt kết nối được sử dụng để tự động ngắt bộ ngắt. Nó được sử dụng bất cứ khi nào việc cung cấp điện phải được ngắt kết nối để thực hiện công việc thực địa như bảo trì hoặc nâng cấp thiết bị.
Cấu hình của một CB khối MCCB - Molded Case Circuit Breaker
Một CB khối MCCB được cấu thành bởi 5 thành phần chính. Đó là: Khung định hình, cơ chế hoạt động, bộ phận triệt tiêu hồ quang, tiếp điểm và thiết bị ngắt (trip).
Frame: The Frame, also known as the molded case, provides an insulated housing to mount to mount all of the circuit breaker components. This will often be made of a glass-polyester material or thermoset composite resin that combines ruggedness and high dielectric strength in a compact design. A frame designation is assigned to each different type and size of molded case. This designation is used to describe the breaker's characteristics including maximum voltage and current ratings.
Operating Mechanism: The Operating Mechanism handles the opening and closing of the contacts. The speed that the contacts open or close is independent of how fast the handle is moved. This is known as "quick-make, quick-break". The breaker cannot be prevented from tripping by holding the handle in the on position. This is known as "trip-free". The position of the handle indicates the status of the contacts - whether they are closed, open, or tripped. The handle will be in a midway position when the contacts are tripped, for example. In the event of a trip, the handle must first be moved to the off position from its center-tripped position, and then to the on position. When breakers are mounted in a group such as in a panelboard, the distinct handle position will clearly indicate the faulted circuit. Some breaker designs may also incorporate a push-to-trip mechanism which allows for a manual means to trip the breaker and test the mechanism.
Arc Extinguisher: Khi hồ quang xuất hiện, bộ phận ngắt mạch sẽ ngắt dòng điện. Công việc của arc extinguisher là hạn chế và chia cắt hồ quang, do đó triệt tiêu nó. Nó thường được làm bằng một chồng các tấm thép được xếp với nhau bằng hai tấm cách điện. Khi có gián đoạn dòng điện xảy ra và các tiếp điểm tách rời, dòng điện đi qua vùng ion hóa của các tiếp điểm tạo ra một từ trường hồ quanh và bộ phận triệt tiêu hồ quang. Các luồng từ trường tạo ra xung quanh hồ quang và lực của nó làm cho hồ quang bị thu vào các tấm thép. Khí này sau đó đi qua deionization và chia nhỏ hồ quang, và bị triệt tiêu. MCCB tiêu chuẩn sử dụng dòng điện tuyến tính qua các tiếp điểm. Trong các điều kiện ngắn mạch, một dòng điện nhỏ được tạo ra, giúp mở các tiếp điểm. Phần lớn hành động mở đầu là từ năng lượng cơ học được lưu trữ trong cơ chế trip. Điều này là do dòng trong cả hai đầu tiếp điểm đi theo cùng một hướng và thu hút nhau. Thiết kế mới hơn sử dụng một vòng lặp đảo ngược của dòng về cơ bản là đường đối diện. Điều này tạo ra một hành động repulsion và kết quả trong một lực lượng thổi lớn hơn. Lực này hỗ trợ cho việc dập tắt hồ quang nhanh bằng cách làm cho tiếp xúc mở nhanh hơn. Lực lượng này tỉ lệ thuận với kích cỡ dòng điện lỗi. Lỗi càng lớn, lực càng lớn, và các tiếp xúc càng mở nhanh.
Trip Unit: là bộ não của máy cắt. Chức năng của nó là để kích hoạt cơ chế ngắt mạch khi xuất hiện ngắn mạch hoặc quá tải dòng liên tục kéo dài. MCCB truyền thống sử dụng các trip cơ điện. Cơ chế bảo vệ hoạt động bằng cách kết hợp một thiết bị nhạy nhiệt với thiết bị điện từ nhạy dòng, cả hai đều hoạt động cơ học trên cơ chế ngắt. Các khối ngắt điện tử sẵn sàng bảo vệ và giám sát phức tạp. Hầu hết các MCCB được sử dụng một hoặc nhiều kiểu ngắt khác nhau để bảo vệ mạch cho các ứng dụng khác nhau. Chúng chống lại sự quá tải nhiệt, các mạch ngắn và các arcing ground fault. MCCB thông thường có một đơn vị trip cơ điện cố định hoặc có thể hoán đổi cho nhau. Nếu một mức ngắt mới được yêu cầu cho một máy cắt cố định, toàn bộ bộ phận ngắt phải được thay thế. Với một đơn vị trip có thể hoán đổi cho nhau, chỉ cần thay đổi đơn vị trip đến mức dòng định mức tối đa của khung máy cắt. Các đơn vị chuyến đổi được cũng được gọi là phích cắm định mức. Một số máy cắt cho phép trao đổi trao đổi giữa các đơn vị cơ điện và điện tử trong cùng một khung.
Tripping Characteristics
To provide short-circuit protection, electromechanical trip circuit breakers have adjustable magnetic elements. To provide overload protection, electromechanical trip circuit breakers contain thermal trip elements. Breakers that use a combination of magnetic elements and thermal elements are often called thermal magnetic breakers. Increasingly, molded case circuit breakers with conventional thermal magnetic trip units are being replaced by breakers with electronic trip units. These units provide increased accuracy and repeatability. Additionally, some units have built-in ground fault protection, removing the need for separate ground fault relays and shunt trips. Some units can also provide system monitoring, data gathering and communication to energy management systems.
In general, electronic trip systems are made up of three components:
A current transformer (sensor) is used on each phase to monitor the current. It also reduces the current to the proper level for input to a printed circuit board.
Electronic circuitry (printed circuit board) that interprets the input and makes a decision based on predetermined values. A decision to trip results in sending an output to the next component.
A low power flux-transfer internal shunt trip that trips the breaker. This is typically a mechanical, spring loaded device held in place by a permanent magnet.
When a tripping signal is received from the electronic circuitry, the effects of the permanent magnet are momentarily counteracted by the tripping pulse, allowing the mechanical tripping action to take place. There is no need for an external source of tripping power, since the entire tripping system has very low power requirements.
Types of MCCB Circuit Breaker by Application
Molded case circuit breakers can have very high current ratings, which allows them to be used in heavy duty applications. The following are some typical uses of an MCCB:
Main electric feeder protection – The electric feeder circuits that supply power to large distribution boards normally have very high currents, of hundreds of amperes. In addition, if more circuits are added to the system in the future, it may be necessary to adjust the circuit breaker trip settings. Therefore, a molded-case circuit breaker is required.
Capacitor bank protection – Capacitor banks are a very important component of commercial and industrial electrical systems, since they allow power factor correction – reducing line currents and preventing fees from the electric utility company. Large capacitor banks may draw high currents and will require MCCB protection.
Generator protection – Large electrical generators may provide an output of hundreds of amperes. In addition, gen-sets are normally very expensive. The high current ratings of molded case circuit breakers allow them to provide reliable protection in this application.
Welding applications – Some welding machines may draw very high currents that exceed the capabilities of miniature circuit breakers, requiring the use of an MCCB.
Low current applications that require adjustable trip settings – MCCBs are not only for high current applications. There are models rated below 100 amperes for when low current equipment requires the adjustable trip settings provided by MCCBs.
Motor protection – The reliable protection capabilities of MCCBs make them an adequate choice for motor protection. A molded case circuit breaker can be adjusted to provide overload protection without tripping during the inrush current of an electric motor.
In summary, an MCCB offers adequate protection whenever an application requires a high current rating, adjustable trip settings, or a combination of both factors.
MCCB Ratings
Molded case circuit breaker manufacturers provide technical specifications for every circuit breaker model. It is very important to understand these ratings in order to select the correct MCCB for every application:
Rated Frame Current (Inm) is the maximum current value for which the MCCB is designed, and it also determines the physical dimensions of the device. The rated frame current defines the upper limit of the adjustable trip current range.
Rated Current (In) is the current value above which overload protection is tripped. For an MCCB, the rated current is an adjustable range instead of a fixed value. The rated frame current defines the upper limit of the rated current range.
Rated insulation voltage (Ui) is the maximum voltage that the MCCB can resist according to laboratory tests. It is higher than the rated working voltage, in order to provide a margin of safety during field operation.
Rated working voltage (Ue) is the continuous operation voltage for which the MCCB is designed. This value is typically equivalent or close to a standard system voltage.
Operating short-circuit breaking capacity (Ics) is the highest fault current that the MCCB can trip without being damaged permanently. The MCCB will be reusable after interrupting a fault, as long as it doesn’t exceed this value.
Ultimate short-circuit breaking capacity (Icu) is the maximum possible fault current that the MCCB can clear. If the fault current exceeds this value, the MCCB will be unable to trip and another protection mechanism with a higher breaking capacity must operate. If a fault above the Ics but below the Icu occurs, the MCCB can interrupt it successfully but will most likely need a replacement due to the damage suffered.
The mechanical life of an MCCB is the number of times the device can be operated manually before failure.
On the other hand, the electrical life refers to the amount of times the MCCB can trip before failure.
Adequate understanding of these terms allows selection of an MCCB that can provide reliable protection according to the voltage and current ratings of the application, as well as the expected fault currents.
MCCB Sizing
Molded case circuit breaker sizing is always carried out according to the expected operating current of the application, as well as the possible fault currents. The main aspects to consider when selecting an MCCB are the following:
The rated working voltage of the MCCB must match the system voltage of the application.
The MCCB must be adjustable to the adequate trip value, calculated according to the current drawn by the load.
The operating breaking capacity of the MCCB must be higher than the expected fault currents in the system.
These are the three main conditions that must be met to ensure adequate selection of an MCCB. It is recommendable to hire the services of a qualified professional in order to know the exact trip settings and breaking capacity that the MCCB must have. An adequate selection process must never be overlooked, especially in the case of molded-case circuit breakers due the high operating currents that are commonly involved.
Selecting an MCCB
When selecting an appropriate circuit breaker for an application for an application, the ratings and environment need to be considered. The voltage rating of a circuit breaker is determined by the maximum voltage that can be applied across the terminals, the type of distribution system and how the breaker is being applied in the system. The 480Y/277V is the voltage system most commonly found in commercial and institutional buildings. It has a solidly grounded neutral. This system is also very prevalent in industrial plants and some high-rise residential buildings.
When a breaker is applied in a panelboard, it is important that is has the lowest possible voltage rating that will do the job and meet the specifications. It can save the customer a lot of money if the correct breaker is chosen. The continuous current rating of a molded case circuit breaker is the amount of current that it is designed to carry in open air. The breaker has a specific ampere rating and is ambient compensated. Most manufacturers calibrate their breakers for a 40°C (107°F) ambient. The National Electric Code (NEC) allows a breaker to be applied to a maximum of 80% of the breaker's continuous current rating. Some manufacturers offer breakers that can be used at 100% if they are specifically designed and tested for such use. They are also required to specify the minimum size enclosure, ventilation needs and conductor size for the application.
The interrupt rating of a molded case circuit breaker is the amount of fault current it can safely interrupt without damaging itself. The interrupt rating must be equal to or greater than the amount of fault current available at the point in the system where the breaker is applied. The interrupt rating always decreases as the voltage increases. The interrupt rating is one of the most critical factors in the breaker selection process.
Most molded case circuit breakers retain the same tripping characteristics whether they are applied to a 50 Hz or 60 Hz system. On higher frequency systems, the breaker may need to be specially calibrated or derated. A molded case circuit breaker that has a thermal magnetic trip unit may not have the same thermal or magnetic performance at a higher frequency than 60 Hz. MCCBs with electronic trip units require special derating factors and cables or bus at higher frequencies.
The number of poles of a molded case circuit breaker is determined by the type of distribution system in which it is applied. Except in certain in certain special applications, each hot conductor is considered a pole. For single-phase applications with a grounded neutral, a single-pole breaker can be used. Two-pole and three-pole breakers are used in three-phase systems.
Environments for Molded Case Circuit Breakers
Thermal magnetic breakers can be affected by large differences in the ambient temperature. At ambient temperatures below 40°C, the breaker will carry more current than its continuous current rating. The mechanical operation of the breaker carries more current than its continuous current rating. The mechanical operation of the breaker could be affected if the temperature is significantly below the 40°C standard. The breakers will carry less current than their continuous rating if the temperature is above 40°C, and could cause nuisance tripping. It could also cause unacceptable temperature conditions at the terminals of the breaker.
Electronic trip circuit breakers often have a wider temperature range (-20°C - 55°C) and so are less susceptible to ambient temperature fluctuations. At very low temperatures, the mechanical parts of the trip unit could require special lubrication. At very high temperatures, the electronic circuitry components could be damaged. Some MCCBs with electronic trip units have special self-protection circuitry to trip, should the internal temperature rise to an unsafe level. An atmosphere with high moisture content or the presence of corrosive elements should be avoided. Electrical equipment should be mounted in clean and dry environments. If moist conditions cannot be avoided, special fungus treatments may be necessary. While the glass-polyester molded cases may not support the growth of fungus, terminals and other parts may. If changes in temperature create condensation, space heaters in the enclosures may be required.
Because air is thinner at higher altitudes, the cooling and dielectric characteristics are reduced compared to those found in the denser air found in lower latitudes. Circuit breakers must be derated for voltage, current and interrupting ratings at altitudes above 6,000 feet. Special shock resistant breakers must be used for installations subject to high mechanical shock. Special installed anti-shock devices hold the trip bar latched under shock conditions, but don't inhibit the proper functioning of the breaker for short circuits or overload conditions.
Mounting an MCCB
Generally, molded case circuit breakers can be mounted in any position. Mounting them up, down, horizontal or vertical does not affect the tripping interrupting characteristics of the breaker. However, mounting them in a vertical position with the ON position as anything other than up is in violation of National Electric Code. In some cases because of the physical arrangement of a panelboard or switchboard, it is necessary to reverse feed of a circuit breaker. The circuit breaker must be tested and listed accordingly for this type of application. Only breakers that have fixed trips can be used, and they often have sealed covers. They often do not have "Line" and "Load" marked on the cover, so the powder source can be connected to either the line or the load terminations.
In addition to being mounted in motor control centers, switchboards and panelboards, molded case circuit breakers are mounted individually in separate enclosures. The National Electric Code and local electric codes determine the proper selection of an enclosure type for a particular application. The National Electrical Manufacturers Association (NEMA) and the International Electro-technical Commission (IEC) have set standards for the protection of devices in various environmental situations. Enclosure types are rated to withstand water, dust, oil, and other environmental conditions. NEMA assigns Type classifications to enclosures. When an enclosure is rated a particular type, it means it is made of the specified materials and has passed specific tests. IEC also has tests and standards that enclosures must conform to. They assign an "IP" classification.
The most commonly-used types of enclosures are:
NEMA Type 1: These enclosures conform to IP40 standards and are designed for indoor applications. They are suitable for installations where unusual conditions do not exist, but where a measure of protection from accidental contact is required. They are commonly used in commercial buildings and apartment buildings. They are often referred to as general purpose enclosures.
NEMA Type 3R: These enclosures conform to IP52 standards and are designed for outdoor use where falling rain, sleet or external ice may form. They use a gasket on the cover to keep water out. Some versions may have a top hinged front cover which must be opened to gain access to the circuit breaker handle. Other versions may have an external side operated handle mechanism. These enclosures are often referred to as "raintight" enclosures.
NEMA Type 4: This type of enclosure conforms to IP65 standards and is suitable for either indoor or outdoor use. They provide protection against splashing water, wind-blown dust or rain. They even protect the circuit breaker from hose-directed water. They are well suited for application in dairies, breweries, paper mills, food processing plants and other process industries. These enclosures are often referred to as watertight enclosures.
NEMA Type 4X: These enclosures conform to IP65 standards and are very similar to type 4 enclosures except they are composed of gasketed, stainless steel. In some designs, they are made of a nonmetallic material. They provide better resistance to corrosion than the type 4. Industries dealing with a high amount of corrosive liquids, require a high measure of hose-down cleaning, or are in a saltwater environment will typically use these enclosures. They are often referred to as corrosion-proof enclosures.
NEMA Type 12: These enclosures conform to the IP62 series of standards and are designed for indoor use in dirty and dusty applications. They are constructed of sheet metal and provide protection from dripping liquids (non-corrosive), falling dirt and dust. A special NEMA 12K version provides knockouts for conduits. These enclosures are often referred to as dust-tight enclosures.
This enclosure from Fibox is rated NEMA 4, 4X, 6 and 12. It is designed to withstand both indoor and outdoor use.
Molded Case Circuit Breakers and Motors
Special considerations need to be made when using circuit breakers with motors. Most faults on a motor circuit are caused by a breakdown of the insulation within the motor windings. The initial fault current is usually low when compared to the overall system capacity. However, because it causes an arcing condition, it could cascade and short out more and more of the motor windings. If the fault is allowed to continue, serious motor and starter damages occur, increasing repair costs. While fusible switches and thermal magnetic breakers can provide motor brand circuit protection, the level of protection is not as effective against this type of fault.
For this reason, the motor circuit protector was developed. A motor circuit protector (MCP) operates on a magnetic only principle. It has a specially designed current sensing coil in each of its three poles to provide sensitive low level protection. It can clear a fault faster than a fusible device. However, it does not provide overload protection for the motor. As a result, a contactor with an overload relay or motor starter must be used in conjunction with the motor circuit protector. Motor circuit protectors can be used in combination starter units within a motor control center. They allow for protection against both low and high level fault currents without requiring current limiters. They can also be applied in standalone combination starters.
When properly sized, they can provide short circuit protection for resistance welding devices. The normal high welding currents can flow, but the HMCP trips instantaneously if a short circuit develops. HMCPs can be used in panelboards. You can have both distribution branch circuit protection and protection of the motor circuits within the same enclosure.
Molded Case Circuit Breaker Testing
The three main tests that are carried out as part of MCCB maintenance are described below.
Insulation Resistance
This test involves disconnecting the MCCB and testing the insulation between phases and across the supply and load terminals. If insulation resistance has dropped below the values recommended by circuit breaker manufacturer, the MCCB will not be able to provide reliable protection.
Insulation resistance varies according to the MCCB model, but for a unit in good conditions it will be in the megohms range (millions of ohms).
Contact Resistance
This test consists on testing the resistance of the electrical contact. As in insulation testing, the measured values must be compared with manufacturer provided values. Under normal conditions, contact resistance is a very low value since the MCCB must allow operating current through with a minimal voltage drop.
Tripping Test
This test consists on simulating overcurrent and fault conditions, and observing the response of the MCCB. Since this test involves high current and the MCCB will heat, it must be carried out last – otherwise the insulation and contact resistance measurements will be altered by temperatures. Normally, this test consists of two parts:
Thermal protection is tested by submitting the MCCB to a large current, for example 300% of the rated value. If the breaker doesn’t trip correctly, thermal protection is failing.
Magnetic protection is tested with short pulses of very high current that simulate a fault. Short pulses are used due to the fact that an electric fault is extremely dangerous. However, since magnetic protection is instant, short pulses can be used for testing purposes.
Molded Case Circuit Breaker Maintenance
Since molded case circuit breakers protect important loads that typically draw high currents, periodic maintenance and testing are fundamental in order to guarantee reliable operation. Normally, maintenance procedures for MCCBs include:
As its name implies, visual inspection involves looking for signs of damage to the MCCB. Some specific signs to look out for include:
- Deformed contacts or cracks in the casing and insulation, which is a sign of overheating
- Burns in the contacts or casing, which is a sign of electric arcing
It is important to note that some MCCB models can be opened, while others are factory sealed. If the model is openable, it is recommended to carry out a visual inspection of the internal components as well.
Lubrication
Since this procedure requires opening the MCCB, it is only possible for models that allow it. Adequate lubrication ensures that the manual disconnection switch and the internal moving parts will operate smoothly. This is very important during a fault, where the internal mechanism must operate within a short timeframe.
Cleaning
Dirt can cause a deterioration of MCCB components over time. If dirt includes a conducting material, there is also the risk of creating a path for current and causing an internal fault. Normally, a vacuum cleaner is used to remove dirt from an MCCB.
Adequate cleaning helps with visual inspection, since damage signs may not be so obvious if the molded case circuit breaker is dirty.
Conclusion
Molded case circuit breakers are a fundamental component of electrical protection for high-current applications, or whenever adjustable trip settings are required. Adequate sizing and maintenance of an MCCB are key elements in order to guarantee safe and reliable operation in the long term.
(TheoInternet)
Định nghĩa và chức năng của một CB khối MCCB- Molded Case Circuit Breaker
A molded case circuit breaker, abbreviated MCCB, is a type of electrical protection device that can be used for a wide range of voltages, and frequencies of both 50 Hz and 60 Hz. The main distinctions between molded-case and miniature circuit breaker are that the MCCB can have current ratings of up to 2,500 amperes, and its trip settings are normally adjustable. An additional difference is that MCCBs tend to be much larger than MCBs. As with most types of circuit breakers, an MCCB has three main functions:
Protection against overload – currents above the rated value that last longer than what is normal for the application.
Protection against electrical faults – During a fault such as a short circuit or line fault, there are extremely high currents that must be interrupted immediately.
Switching a circuit on and off – This is a less common function of circuit breakers, but they can be used for that purpose if there isn’t an adequate manual switch.
The wide range of current ratings available from molded-case circuit breakers allows them to be used in a wide variety of applications. MCCBs are available with current ratings that range from low values such as 15 amperes, to industrial ratings such as 2,500 amperes. This allows them to be used in both low-power and high-power applications.
Molded case circuit breakers (MCCBs) are UL 489-approved circuit breakers whose current-carrying parts, mechanisms and trip devices are all completely contained within a molded case of insulating material. MCCBs are available in various frame sizes with various interrupting ratings for each frame size. MCCBs are one of the two basic low voltage classes of circuit breakers.
Molded Case Circuit Breakers are designed to provide circuit protection for low voltage distribution systems. They will protect connected devices against both overloads and short circuits. They are most-commonly-used in panelboards and switchboards where they are fixed mounted, though some of the larger MCCBs available may be available in a drawout mount design.
MCCBs are available with special features which make them suitable for the protection of motor circuits when used in conjunction with a separate overload protection device. When used in such applications, they are often referred to as motor circuit protectors (MCPs).
Molded Case Circuit Breaker Operating Mechanism
At its core, the protection mechanism employed by MCCBs is based on the same physical principles used by all types of thermal-magnetic circuit breakers.
Overload protection is accomplished by means of a thermal mechanism. MCCBs have a bimetallic contact what expands and contracts in response to changes in temperature. Under normal operating conditions, the contact allows electric current through the MCCB. However, as soon as the current exceeds the adjusted trip value, the contact will start to heat and expand until the circuit is interrupted. The thermal protection against overload is designed with a time delay to allow short duration overcurrent, which is a normal part of operation for many devices. However, any overcurrent conditions that last more than what is normally expected represent an overload, and the MCCB is tripped to protect the equipment and personnel.
On the other hand, fault protection is accomplished with electromagnetic induction, and the response is instant. Fault currents should be interrupted immediately, no matter if their duration is short or long. Whenever a fault occurs, the extremely high current induces a magnetic field in a solenoid coil located inside the breaker – this magnetic induction trips a contact and current is interrupted. As a complement to the magnetic protection mechanism, MCCBs have internal arc dissipation measures to facilitate interruption.
As with all types of circuit breakers, the MCCB includes a disconnection switch which is used to trip the breaker manually. It is used whenever the electric supply must be disconnected to carry out field work such as maintenance or equipment upgrades.
Molded Case Circuit Breaker Components
Molded Case Circuit Breakers are composed of five main components. These are: Molded case/frame, operating mechanism, arc extinguishers, contacts and trip units.
Frame: The Frame, also known as the molded case, provides an insulated housing to mount to mount all of the circuit breaker components. This will often be made of a glass-polyester material or thermoset composite resin that combines ruggedness and high dielectric strength in a compact design. A frame designation is assigned to each different type and size of molded case. This designation is used to describe the breaker's characteristics including maximum voltage and current ratings.
Operating Mechanism: The Operating Mechanism handles the opening and closing of the contacts. The speed that the contacts open or close is independent of how fast the handle is moved. This is known as "quick-make, quick-break". The breaker cannot be prevented from tripping by holding the handle in the on position. This is known as "trip-free". The position of the handle indicates the status of the contacts - whether they are closed, open, or tripped. The handle will be in a midway position when the contacts are tripped, for example. In the event of a trip, the handle must first be moved to the off position from its center-tripped position, and then to the on position. When breakers are mounted in a group such as in a panelboard, the distinct handle position will clearly indicate the faulted circuit. Some breaker designs may also incorporate a push-to-trip mechanism which allows for a manual means to trip the breaker and test the mechanism.
Arc Extinguisher: An arc is created whenever a circuit breaker interrupts a current flow. The Arc Extinguisher's job is to confine and divide that arc, thereby extinguishing it. Arc extinguishers are typically made of a stack of steel plates held together by two insulator plates. When an interruption occurs and the contacts separate, the current flow through the ionized region of the contacts induces a magnetic field around the arc and the arc extinguisher. The lines of magnetic flux created around the arc and its force drives the arc into the steel plates. The gas then goes through deionization and the arc divides, allowing it to cool. Standard MCCBs use a linear current flow through the contacts. Under short-circuit conditions, a small blow-apart force is created, which helps open the contacts. The majority of the opening action comes from the mechanical energy stored in the trip mechanism itself. This is because the current in both contacts are going in the same direction and attract each other. Newer design breakers use a reverse loop of current flowing in essentially opposite paths. This creates a repulsion action and results in a greater blow-apart force. This force assists with rapid arc extinguishing by causing the contact to open faster. The force is directly proportional to the size of the fault current. The greater the fault, the greater the force, and the faster the contacts open.
Trip Unit: The Trip Unit is the brain of the circuit breaker. The function of the trip unit is to trip the operating mechanism in the event of a short circuit or a prolonged overload of current. Traditional molded case circuit breakers use electromechanical trip units. Protection is provided by combining a temperature-sensitive device with a current sensitive electromagnetic device, both of which act mechanically on the trip mechanism. Electronic trip units are now available and they can provide much more sophisticated protection and monitoring. Most molded case circuit breakers utilize one or more different trip elements to provide circuit protection for different applications. These trip elements protect against thermal overloads, short circuits and arcing ground faults. Conventional MCCBs are available with either a fixed or interchangeable electromechanical trip unit. If a new trip rating is required for a fixed trip breaker, the entire breaker must be replaced. With an interchangeable trip unit, only the trip unit needs to be changed up to the maximum current rating of the breaker frame. Interchangeable trip units are also often called rating plugs. Some breakers offer interchangeability between electromechanical and electronic trip units within the same frame.
Tripping Characteristics
To provide short-circuit protection, electromechanical trip circuit breakers have adjustable magnetic elements. To provide overload protection, electromechanical trip circuit breakers contain thermal trip elements. Breakers that use a combination of magnetic elements and thermal elements are often called thermal magnetic breakers. Increasingly, molded case circuit breakers with conventional thermal magnetic trip units are being replaced by breakers with electronic trip units. These units provide increased accuracy and repeatability. Additionally, some units have built-in ground fault protection, removing the need for separate ground fault relays and shunt trips. Some units can also provide system monitoring, data gathering and communication to energy management systems.
In general, electronic trip systems are made up of three components:
A current transformer (sensor) is used on each phase to monitor the current. It also reduces the current to the proper level for input to a printed circuit board.
Electronic circuitry (printed circuit board) that interprets the input and makes a decision based on predetermined values. A decision to trip results in sending an output to the next component.
A low power flux-transfer internal shunt trip that trips the breaker. This is typically a mechanical, spring loaded device held in place by a permanent magnet.
When a tripping signal is received from the electronic circuitry, the effects of the permanent magnet are momentarily counteracted by the tripping pulse, allowing the mechanical tripping action to take place. There is no need for an external source of tripping power, since the entire tripping system has very low power requirements.
Types of MCCB Circuit Breaker by Application
Molded case circuit breakers can have very high current ratings, which allows them to be used in heavy duty applications. The following are some typical uses of an MCCB:
Main electric feeder protection – The electric feeder circuits that supply power to large distribution boards normally have very high currents, of hundreds of amperes. In addition, if more circuits are added to the system in the future, it may be necessary to adjust the circuit breaker trip settings. Therefore, a molded-case circuit breaker is required.
Capacitor bank protection – Capacitor banks are a very important component of commercial and industrial electrical systems, since they allow power factor correction – reducing line currents and preventing fees from the electric utility company. Large capacitor banks may draw high currents and will require MCCB protection.
Generator protection – Large electrical generators may provide an output of hundreds of amperes. In addition, gen-sets are normally very expensive. The high current ratings of molded case circuit breakers allow them to provide reliable protection in this application.
Welding applications – Some welding machines may draw very high currents that exceed the capabilities of miniature circuit breakers, requiring the use of an MCCB.
Low current applications that require adjustable trip settings – MCCBs are not only for high current applications. There are models rated below 100 amperes for when low current equipment requires the adjustable trip settings provided by MCCBs.
Motor protection – The reliable protection capabilities of MCCBs make them an adequate choice for motor protection. A molded case circuit breaker can be adjusted to provide overload protection without tripping during the inrush current of an electric motor.
In summary, an MCCB offers adequate protection whenever an application requires a high current rating, adjustable trip settings, or a combination of both factors.
MCCB Ratings
Molded case circuit breaker manufacturers provide technical specifications for every circuit breaker model. It is very important to understand these ratings in order to select the correct MCCB for every application:
Rated Frame Current (Inm) is the maximum current value for which the MCCB is designed, and it also determines the physical dimensions of the device. The rated frame current defines the upper limit of the adjustable trip current range.
Rated Current (In) is the current value above which overload protection is tripped. For an MCCB, the rated current is an adjustable range instead of a fixed value. The rated frame current defines the upper limit of the rated current range.
Rated insulation voltage (Ui) is the maximum voltage that the MCCB can resist according to laboratory tests. It is higher than the rated working voltage, in order to provide a margin of safety during field operation.
Rated working voltage (Ue) is the continuous operation voltage for which the MCCB is designed. This value is typically equivalent or close to a standard system voltage.
Operating short-circuit breaking capacity (Ics) is the highest fault current that the MCCB can trip without being damaged permanently. The MCCB will be reusable after interrupting a fault, as long as it doesn’t exceed this value.
Ultimate short-circuit breaking capacity (Icu) is the maximum possible fault current that the MCCB can clear. If the fault current exceeds this value, the MCCB will be unable to trip and another protection mechanism with a higher breaking capacity must operate. If a fault above the Ics but below the Icu occurs, the MCCB can interrupt it successfully but will most likely need a replacement due to the damage suffered.
The mechanical life of an MCCB is the number of times the device can be operated manually before failure.
On the other hand, the electrical life refers to the amount of times the MCCB can trip before failure.
Adequate understanding of these terms allows selection of an MCCB that can provide reliable protection according to the voltage and current ratings of the application, as well as the expected fault currents.
MCCB Sizing
Molded case circuit breaker sizing is always carried out according to the expected operating current of the application, as well as the possible fault currents. The main aspects to consider when selecting an MCCB are the following:
The rated working voltage of the MCCB must match the system voltage of the application.
The MCCB must be adjustable to the adequate trip value, calculated according to the current drawn by the load.
The operating breaking capacity of the MCCB must be higher than the expected fault currents in the system.
These are the three main conditions that must be met to ensure adequate selection of an MCCB. It is recommendable to hire the services of a qualified professional in order to know the exact trip settings and breaking capacity that the MCCB must have. An adequate selection process must never be overlooked, especially in the case of molded-case circuit breakers due the high operating currents that are commonly involved.
Selecting an MCCB
When selecting an appropriate circuit breaker for an application for an application, the ratings and environment need to be considered. The voltage rating of a circuit breaker is determined by the maximum voltage that can be applied across the terminals, the type of distribution system and how the breaker is being applied in the system. The 480Y/277V is the voltage system most commonly found in commercial and institutional buildings. It has a solidly grounded neutral. This system is also very prevalent in industrial plants and some high-rise residential buildings.
When a breaker is applied in a panelboard, it is important that is has the lowest possible voltage rating that will do the job and meet the specifications. It can save the customer a lot of money if the correct breaker is chosen. The continuous current rating of a molded case circuit breaker is the amount of current that it is designed to carry in open air. The breaker has a specific ampere rating and is ambient compensated. Most manufacturers calibrate their breakers for a 40°C (107°F) ambient. The National Electric Code (NEC) allows a breaker to be applied to a maximum of 80% of the breaker's continuous current rating. Some manufacturers offer breakers that can be used at 100% if they are specifically designed and tested for such use. They are also required to specify the minimum size enclosure, ventilation needs and conductor size for the application.
The interrupt rating of a molded case circuit breaker is the amount of fault current it can safely interrupt without damaging itself. The interrupt rating must be equal to or greater than the amount of fault current available at the point in the system where the breaker is applied. The interrupt rating always decreases as the voltage increases. The interrupt rating is one of the most critical factors in the breaker selection process.
Most molded case circuit breakers retain the same tripping characteristics whether they are applied to a 50 Hz or 60 Hz system. On higher frequency systems, the breaker may need to be specially calibrated or derated. A molded case circuit breaker that has a thermal magnetic trip unit may not have the same thermal or magnetic performance at a higher frequency than 60 Hz. MCCBs with electronic trip units require special derating factors and cables or bus at higher frequencies.
The number of poles of a molded case circuit breaker is determined by the type of distribution system in which it is applied. Except in certain in certain special applications, each hot conductor is considered a pole. For single-phase applications with a grounded neutral, a single-pole breaker can be used. Two-pole and three-pole breakers are used in three-phase systems.
Environments for Molded Case Circuit Breakers
Thermal magnetic breakers can be affected by large differences in the ambient temperature. At ambient temperatures below 40°C, the breaker will carry more current than its continuous current rating. The mechanical operation of the breaker carries more current than its continuous current rating. The mechanical operation of the breaker could be affected if the temperature is significantly below the 40°C standard. The breakers will carry less current than their continuous rating if the temperature is above 40°C, and could cause nuisance tripping. It could also cause unacceptable temperature conditions at the terminals of the breaker.
Electronic trip circuit breakers often have a wider temperature range (-20°C - 55°C) and so are less susceptible to ambient temperature fluctuations. At very low temperatures, the mechanical parts of the trip unit could require special lubrication. At very high temperatures, the electronic circuitry components could be damaged. Some MCCBs with electronic trip units have special self-protection circuitry to trip, should the internal temperature rise to an unsafe level. An atmosphere with high moisture content or the presence of corrosive elements should be avoided. Electrical equipment should be mounted in clean and dry environments. If moist conditions cannot be avoided, special fungus treatments may be necessary. While the glass-polyester molded cases may not support the growth of fungus, terminals and other parts may. If changes in temperature create condensation, space heaters in the enclosures may be required.
Because air is thinner at higher altitudes, the cooling and dielectric characteristics are reduced compared to those found in the denser air found in lower latitudes. Circuit breakers must be derated for voltage, current and interrupting ratings at altitudes above 6,000 feet. Special shock resistant breakers must be used for installations subject to high mechanical shock. Special installed anti-shock devices hold the trip bar latched under shock conditions, but don't inhibit the proper functioning of the breaker for short circuits or overload conditions.
Mounting an MCCB
Generally, molded case circuit breakers can be mounted in any position. Mounting them up, down, horizontal or vertical does not affect the tripping interrupting characteristics of the breaker. However, mounting them in a vertical position with the ON position as anything other than up is in violation of National Electric Code. In some cases because of the physical arrangement of a panelboard or switchboard, it is necessary to reverse feed of a circuit breaker. The circuit breaker must be tested and listed accordingly for this type of application. Only breakers that have fixed trips can be used, and they often have sealed covers. They often do not have "Line" and "Load" marked on the cover, so the powder source can be connected to either the line or the load terminations.
In addition to being mounted in motor control centers, switchboards and panelboards, molded case circuit breakers are mounted individually in separate enclosures. The National Electric Code and local electric codes determine the proper selection of an enclosure type for a particular application. The National Electrical Manufacturers Association (NEMA) and the International Electro-technical Commission (IEC) have set standards for the protection of devices in various environmental situations. Enclosure types are rated to withstand water, dust, oil, and other environmental conditions. NEMA assigns Type classifications to enclosures. When an enclosure is rated a particular type, it means it is made of the specified materials and has passed specific tests. IEC also has tests and standards that enclosures must conform to. They assign an "IP" classification.
The most commonly-used types of enclosures are:
NEMA Type 1: These enclosures conform to IP40 standards and are designed for indoor applications. They are suitable for installations where unusual conditions do not exist, but where a measure of protection from accidental contact is required. They are commonly used in commercial buildings and apartment buildings. They are often referred to as general purpose enclosures.
NEMA Type 3R: These enclosures conform to IP52 standards and are designed for outdoor use where falling rain, sleet or external ice may form. They use a gasket on the cover to keep water out. Some versions may have a top hinged front cover which must be opened to gain access to the circuit breaker handle. Other versions may have an external side operated handle mechanism. These enclosures are often referred to as "raintight" enclosures.
NEMA Type 4: This type of enclosure conforms to IP65 standards and is suitable for either indoor or outdoor use. They provide protection against splashing water, wind-blown dust or rain. They even protect the circuit breaker from hose-directed water. They are well suited for application in dairies, breweries, paper mills, food processing plants and other process industries. These enclosures are often referred to as watertight enclosures.
NEMA Type 4X: These enclosures conform to IP65 standards and are very similar to type 4 enclosures except they are composed of gasketed, stainless steel. In some designs, they are made of a nonmetallic material. They provide better resistance to corrosion than the type 4. Industries dealing with a high amount of corrosive liquids, require a high measure of hose-down cleaning, or are in a saltwater environment will typically use these enclosures. They are often referred to as corrosion-proof enclosures.
NEMA Type 12: These enclosures conform to the IP62 series of standards and are designed for indoor use in dirty and dusty applications. They are constructed of sheet metal and provide protection from dripping liquids (non-corrosive), falling dirt and dust. A special NEMA 12K version provides knockouts for conduits. These enclosures are often referred to as dust-tight enclosures.
This enclosure from Fibox is rated NEMA 4, 4X, 6 and 12. It is designed to withstand both indoor and outdoor use.
Molded Case Circuit Breakers and Motors
Special considerations need to be made when using circuit breakers with motors. Most faults on a motor circuit are caused by a breakdown of the insulation within the motor windings. The initial fault current is usually low when compared to the overall system capacity. However, because it causes an arcing condition, it could cascade and short out more and more of the motor windings. If the fault is allowed to continue, serious motor and starter damages occur, increasing repair costs. While fusible switches and thermal magnetic breakers can provide motor brand circuit protection, the level of protection is not as effective against this type of fault.
For this reason, the motor circuit protector was developed. A motor circuit protector (MCP) operates on a magnetic only principle. It has a specially designed current sensing coil in each of its three poles to provide sensitive low level protection. It can clear a fault faster than a fusible device. However, it does not provide overload protection for the motor. As a result, a contactor with an overload relay or motor starter must be used in conjunction with the motor circuit protector. Motor circuit protectors can be used in combination starter units within a motor control center. They allow for protection against both low and high level fault currents without requiring current limiters. They can also be applied in standalone combination starters.
When properly sized, they can provide short circuit protection for resistance welding devices. The normal high welding currents can flow, but the HMCP trips instantaneously if a short circuit develops. HMCPs can be used in panelboards. You can have both distribution branch circuit protection and protection of the motor circuits within the same enclosure.
Molded Case Circuit Breaker Testing
The three main tests that are carried out as part of MCCB maintenance are described below.
Insulation Resistance
This test involves disconnecting the MCCB and testing the insulation between phases and across the supply and load terminals. If insulation resistance has dropped below the values recommended by circuit breaker manufacturer, the MCCB will not be able to provide reliable protection.
Insulation resistance varies according to the MCCB model, but for a unit in good conditions it will be in the megohms range (millions of ohms).
Contact Resistance
This test consists on testing the resistance of the electrical contact. As in insulation testing, the measured values must be compared with manufacturer provided values. Under normal conditions, contact resistance is a very low value since the MCCB must allow operating current through with a minimal voltage drop.
Tripping Test
This test consists on simulating overcurrent and fault conditions, and observing the response of the MCCB. Since this test involves high current and the MCCB will heat, it must be carried out last – otherwise the insulation and contact resistance measurements will be altered by temperatures. Normally, this test consists of two parts:
Thermal protection is tested by submitting the MCCB to a large current, for example 300% of the rated value. If the breaker doesn’t trip correctly, thermal protection is failing.
Magnetic protection is tested with short pulses of very high current that simulate a fault. Short pulses are used due to the fact that an electric fault is extremely dangerous. However, since magnetic protection is instant, short pulses can be used for testing purposes.
Molded Case Circuit Breaker Maintenance
Since molded case circuit breakers protect important loads that typically draw high currents, periodic maintenance and testing are fundamental in order to guarantee reliable operation. Normally, maintenance procedures for MCCBs include:
As its name implies, visual inspection involves looking for signs of damage to the MCCB. Some specific signs to look out for include:
- Deformed contacts or cracks in the casing and insulation, which is a sign of overheating
- Burns in the contacts or casing, which is a sign of electric arcing
It is important to note that some MCCB models can be opened, while others are factory sealed. If the model is openable, it is recommended to carry out a visual inspection of the internal components as well.
Lubrication
Since this procedure requires opening the MCCB, it is only possible for models that allow it. Adequate lubrication ensures that the manual disconnection switch and the internal moving parts will operate smoothly. This is very important during a fault, where the internal mechanism must operate within a short timeframe.
Cleaning
Dirt can cause a deterioration of MCCB components over time. If dirt includes a conducting material, there is also the risk of creating a path for current and causing an internal fault. Normally, a vacuum cleaner is used to remove dirt from an MCCB.
Adequate cleaning helps with visual inspection, since damage signs may not be so obvious if the molded case circuit breaker is dirty.
Conclusion
Molded case circuit breakers are a fundamental component of electrical protection for high-current applications, or whenever adjustable trip settings are required. Adequate sizing and maintenance of an MCCB are key elements in order to guarantee safe and reliable operation in the long term.
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