Molded Case Circuit Breakers
MCCB Components and 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).
Side view of the construction of an ABB TMAX Series Molded Case Circuit Breaker.
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.
Selecting an MCCB
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.
This enclosure from Fibox is rated NEMA 4, 4X, 6 and 12. It is
designed to withstand both indoor and outdoor use.
- 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.
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 Video Overview