Molded case circuit breakers are a type of electrical protection device that is commonly used when load currents exceed the capabilities of miniature circuit breakers. They are also used in applications of any current rating that require adjustable trip settings, which are not available in plug-in circuit breakers and MCBs.
Molded Case Circuit Breaker Definition and Function
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 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 over current, which is a normal part of operation for many devices. However, any over current 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.
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.
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:
· Visual inspection
· Lubrication
· Cleaning
· Testing –as above
Visual inspection
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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